
Ask HN: What scientific phenomenon do you wish someone would explain better? - qqqqquinnnnn
I&#x27;ve been studying viruses lately, and have found that the line between virus&#x2F;exosome&#x2F;self is much more blurry than I realized. But, given the niche interest in the subject, most articles are not written with an overview in mind.<p>What sorts of topics make you feel this way?
======
arkanciscan
Quantum Computers. Not like I'm five, but like I'm a software engineer who has
a pretty decent understanding of how a classical turing machine works. I can't
tell you how many times I've heard someone say "qubits are like bits except
they don't have to be just 1 or 0" without providing any coherent explanation
of how that's useful. I've also heard that they can try every possible
solution to a problem. What I don't understand is how a programmer is supposed
to determine the correct solution when their computer is out in some crazy
multiverse. I guess what I want is some pseudo code for quantum software.

~~~
__sy__
Since no one has listed it yet, please check out
[https://quantum.country/](https://quantum.country/)

It's by Andy Matuschak and Michael Nielsen, and it is excellent. Have fun!

~~~
cloogshicer
Yes! How come this isn't higher up in the list? This is one of the best pieces
of education I've ever seen. Absolutely wonderful.

------
rsp1984
Macroeconomics. Central banks are "creating" a trillion here, a trillion
there, like nobody's business. But what are the consequences? What is the
thought process that central bankers have gone through to make these
decisions?

Also why, exactly, are they buying the exact assets that they are buying
(govt. debt, high-yield bonds, etc..) and why not others (e.g. stocks or put
money into startups)? And then, what happens if a debtor pays back its debt?
Is that money consequently getting "erased" again (just like it's been
created)? What happens if a debtor defaults on its debt? Does that money then
just stay in the economy, impossible to drain out? What is the general
expectation of the central banks? What percentage of the debt is expected to
default and how much is expected to be paid back?

And specifically in the case of central banks buying govt. debt: Are central
banks considered "easier" creditors than the public? What would happen if a
country defaults on a loan given by a central bank? Would the central bank
then go ahead and seize and liquidate assets of the country under a bankruptcy
procedure to pay off the debt (like it would be standard procedure for
individuals and companies)?

~~~
dmichulke
One approach is reductio ad absurdum:

If Central Banks can create money without negative effects, then

\- why tax people?

\- why even work? Can't we just print enough money for everyone and live
happily ever after?

I realize these questions are quite provocative and their answering only
explains _if_ it will work but not how or when it will fail.

~~~
nicoburns
> \- why tax people?

Printing money is actually more or less equivalent to a tax, because it
reduces the value of the existing money supply.

> \- Can't we just print enough money for everyone and live happily ever
> after?

No, because printing money redistributes wealth, it doesn't create it.

~~~
odyssey7
Not just any tax, but a wealth tax affecting anybody who has US dollars,
regardless of their citizenship or location.

~~~
mtrovo
That's a really good point and I never thought about it this way. It just
makes me think how powerful is for US the world relying on dollars as an
international currency.

------
umvi
I would like to understand how cellular biology processes actually work. Like,
how do all the right modules and proteins line up in the right orientation
every time? Every time I watch animations, it seems like the proteins and such
just magically appear when needed and disappear when not needed [0]. Sometimes
it's an ultra-complex looking protein and it just magically flys over to the
DNA, attaches to the correct spot, does it's thing, detaches, and flies away.
Yeah right! As if the protein is being flown by a pilot. How does it really
work?

[0] [https://youtu.be/5VefaI0LrgE](https://youtu.be/5VefaI0LrgE)

~~~
JabavuAdams
They don't. This is a pet-peeve of mine, and it's reinforced by animation
after animation.

Everything is being jostled around randomly. The molecules don't have brains
or seeker warheads. They can't "decide" to home in on a target.

The only mechanisms for guidance are: diffusion due to concentration
gradients, movement of charged molecules due to electric fields, and molecules
actually grabbing other molecules.

It's all probabilities. This conformation makes it more likely that this thing
will stick to this other thing. You may have heard that genes can be turned on
or off. How? DNA is literally wound on molecular spools in your cell nuclei.
When the DNA is loosely wound other molecules can bump into it and transcribe
it -- the gene is ON. When the DNA is tightly spooled, other molecules can't
get in there and the gene is OFF for transcription. There's no binary switch,
just likelihoods.

Everything is probabilistic, but the probabilities have been tuned by
evolution through natural selection to deliver a system that works well
enough.

~~~
kcolford
Even diffusion isn't some magical force guiding chemicals through the medium.
It's just random movement that statistically results in the chemical being
spread out. This is the same principle that the 2nd law of thermodynamics is
based upon. There's nothing magic to it, it's just the statistically likely
end result over many particles.

~~~
JabavuAdams
Yes. It's interesting how powerful and clarifying this model of its-all-just-
atoms-bumping-into-atoms is. It's interesting how many people take science
courses, but don't really _get_ this.

In the context of Covid19, I see so many people wearing PPE, but failing to
act as though they understand that the actual goal is to prevent this tiny
virion dust from entering your orifices. Like wearing gloves and a mask, but
then picking up unclean item in store then using now unclean gloves to adjust
mask and make it unclean.

People seem to think of things as having essences or talismanic effects. Like
gloves give you +2 against covid and a mask gives you +5 when it's really all
about preventing those virus things from bumping into your cell things.

~~~
lifeformed
Masks are for keeping your own particles from spreading far, not the other way
around.

~~~
echelon
> Masks are for keeping your own particles from spreading far, not the other
> way around.

Masks are for keeping your own particles from spreading far _AND_ for lowering
the probability of virions found in the environment from entering your
respiratory system.

Masks lower the probability when all other variables are held constant. If
someone thinks wearing a mask grants invincibility and in turn chooses to
increase their exposure to high viral load individuals or environments,
they're putting themselves at risk.

~~~
sah2ed
> _Masks are for keeping your own particles from spreading far AND for
> lowering the probability of virions found in the environment from entering
> your respiratory system._

Both of you may be correct. I think the person you responded to may not have
been precise in their framing.

I suspect that you had _N95 masks_ in mind when you wrote masks, which doesn’t
negate the point of the person you responded to, if they had _surgical masks_
in mind when they wrote masks. Surgical masks are far more common than N95
masks since they are cheaper and do not provide protection against viral
particles for the wearer.

~~~
partyboat1586
Surgical masks do provide some level of protection against virus droplets and
aerosol for the wearer they just are not as effective as N95. Even a teacloth
or a scarf wrapped around your face will provide some level of protection to
the wearer from virus particles entering their mucus membranes.

------
qubex
I find most explanations of the Equivalence Principle that lies at the
foundation of General Relativity to be very lax.

To wit, the idea is that you cannot distinguish whether you are in an
accelerated frame or in a gravitational field; alternatively stated, if you’re
floating around in an elevator you don’t know whether you’re freefalling to
your doom or in deep sideral space far from any gravitational source (though
of course, since you’re in an elevator car and apparently freefalling... I
think we’d all agree on what’s most likely, but I digress).

Anyway, what irks me that this is _most definitely not true_ at the “thought
experiment” level of theoretical thinking: if you had two baseballs with you
in that freefalling lift, you could suspend them in front of you. If you were
in deep space, they’d stay equidistant; if you were freefalling down a shaft,
you’d see them move closer because of tidal effects dictated by the fact that
they’re each falling towards the earth’s centre of gravity, and therefore at
(very slightly) different angles.

Of course, they’d be moving slightly toward each other in both cases (because
they attract gravitationally) but the tidal effect presents is additional and
present in only one scenario, allowing one to (theoretically) distinguish,
apparently violating the bedrock Equivalence Principle.

I never see this point raised anywhere and I find it quite distressing,
because I’m sure there’s a very simple explanation and that General Relativity
is sound under such trivial constructions, but I haven’t been able to find a
decent explanation.

~~~
knzhou
You're right that this is glossed over in popular explanations, but the point
you make is exactly the starting point for all formal courses and textbooks.

The first part of the argument is that for single _point_ particles falling,
the effect of gravity is the same for all particles. This suggests that we
should model gravity as _something_ intrinsic to spacetime itself, rather than
as a field living on top of spacetime, which could couple to different
particles with different strengths.

The second part of the argument, which is what you point out, is that gravity
can have nontrivial tidal effects. (This had better be true, because if _all_
gravitational effects were just equivalent to a trivial uniform acceleration,
then it would be so boring that we wouldn't need a theory of gravity at all!)
This suggests that whatever property of spacetime we use to model gravity, it
should reduce in the Newtonian limit to something that looks like a tidal
effect, i.e. a _gradient_ of the Newtonian gravitational field. That leads
directly to the idea of describing gravity as the curvature of spacetime.

So both parts of the argument give important information (both historically
and pedagogically). Both parts are typically presented in good courses, but
only the first half makes it to the popular explanations, probably out of
simplification.

~~~
bollu
> it should reduce in the Newtonian limit to something that looks like a tidal
> effect, i.e. a gradient of the Newtonian gravitational field.

Can you please explain to me how you went from"looks like a tidal effect in
the Newtonian limit" to "a gradient of the Newtonian Graviational field"?

~~~
knzhou
"Tidal effects" are defined in terms of having different gravitational fields
in one place than another (i.e. the tidal bulge near to the moon occurs
because the moon's field is stronger there).

~~~
rocqua
That's not quite true, as illustrated by the tidal bulge opposite the moon.

Tidal forces occur much more due to the difference in the direction of gravity
than due to the difference in magnitude.

------
aazaa
Sort of meta, but I always shudder when someone says that science has "proven"
something.

What sets science apart from most other methods of seeking answers is its
focus on _disproof_. Your goal as a scientist is to devise experiments that
can _disprove_ a claim about the natural world.

This misconception rears its head most prominently in discussions at the
intersection between science and public policy. Climate change. How to handle
a pandemic. Evolution. Abortion. But I've even talked to scientists themselves
who from time to time get confused about what science can and can't do.

The problem with believing that science proves things is that it blinds its
adherents to new evidence paving the way to better explanations. It also leads
to the absurd conclusion that a scientific question can ever really be
"settled."

~~~
xyzzy99
Not to be rude, but given current daily attacks on science and the scientific
method, I can't let this stand - I think your meta intuition represents a
fundamental misunderstanding of how science works.

It is simply wrong to think that scientific questions can never be
definitively settled. Clearly there are some hypotheses that have been
difficult (and may be impossible) to prove, for example, Darwin's idea that
natural selection is the basis of evolution. There's ample correlative
evidence in support of natural selection, but little of the causal data
necessary for "proof" (until perhaps recently). In the case of evolution the
experiments required to prove that natural selection could lead to systematic
genetic change were technically challenging for a variety of reasons.

In the case of climate change, the problem again is that the evidence is
correlative and not causal. Demonstrating a causal link between human behavior
or CO2 levels and climate change (the gold standard for "proof") is
technically challenging, so we are forced to rely on correlations, which is
the next best thing. But, you are right, it is not "proof".

Establishing causality can be difficult but not impossible - the standard is
"necessary and sufficient". You must show necessity: CO2 increase (for
example) is necessary for global warming; if CO2 remains constant, no matter
what else happens to the system global temperatures remain constant. And you
must also demonstrate sufficiency: temperatures will increase if you increase
CO2 while holding everything else constant. Those are experiments that can't
be done. As a result, we are forced to rely on correlation - the historical
correlation between CO2 and temperature change is compelling evidence that CO2
increases cause global warming, but it is not proof. It then becomes a
statistical argument, giving room for some to argue the question remains
"unsettled".

My point is that there are plenty of examples in science where things have
been proven -- DNA carries genetic information, DNA (usually) has a double
stranded helical structure, V=IR, F=Ma, etc. And there are things that are
highly likely, but not "proven", e.g., human activity causes of climate
change.

While some of the issues you bring are remain unproven, what's really absurd
is to think that no scientific questions can be settled.

~~~
johnmorrison
> Establishing causality can be difficult but not impossible - the standard is
> "necessary and sufficient". You must show necessity: CO2 increase (for
> example) is necessary for global warming; if CO2 remains constant, no matter
> what else happens to the system global temperatures remain constant. And you
> must also demonstrate sufficiency: temperatures will increase if you
> increase CO2 while holding everything else constant. Those are experiments
> that can't be done.

No. What is the basis for these claims?

They're both wrong.

It's not true that CO2 increase is necessary for global warming. If the sun
got a lot hotter, global temperatures would rise. If non-CO2 GHGs increased,
global temperatures would rise. If the overall albedo of the planet changes,
global temperatures can rise. There are literally thousands of things that
could cause the temperature to rise.

It's also not true that CO2 increase, holding everything else constant, would
lead to long term or even medium term warning. We have no idea what the
ecosystem will do for any given change in CO2 levels, since there are
countless species both who are net producers and net consumers of atmospheric
CO2, all of whom have exponential growth and feedback loops.

Even still, even since both of those claims are wrong, CO2 increase _may
still_ cause global warming.

Furthermore, the things you claim are proven, are not proven, they are true by
definition. All molecules carry information, and the fact that DNA carries
genetic information is a direct consequence of the fact that it is DNA. V=IR
by definition. F=ma by definition. There's no such thing as a "force" or
"mass" or "acceleration" entity per se, these are metrics that are by
definition equal in a given physical framework.

There is no way to 'technically' prove _anything_ in science, and the reasons
are simple:

(1) The past is gone - you can't access it

(2) You can't see the future

(3) Your knowledge of the present is extremely limited and inaccurate

These are the limitations of the real world, and science does its best to
provide utility within that. It only focuses on making future predictions
using the observed past as evidence, because you only _can_ do that. You can't
check your model in the present, because you can't instantaneously observe
anywhere you aren't already observing. Checking your model on the past relies
on what you _think_ happened, i.e. what _allegedly_ happened, but there is
absolutely no way to truly know.

You can't even really prove anything 'novel' in mathematics, which is the only
place where you _can_ actually prove anything, but even there all proofs are
effectively just framing something that was already implied axiomatically in a
way that allows our limited human minds to see the relevant/useful patterns
that aren't immediately obvious to us.

My point is, acting as though you can _truly_ prove anything in science,

> what's really absurd is to think that no scientific questions can be settled

is not only wrong, but in my opinion is a distraction from what science is
actually for. It's not about settling questions. Science is never settled, and
that's part of what's beautiful about it. It's about reducing our own
ignorance and proving our past selves wrong, discovering patterns and models
that equip us with the knowledge to build a better world for ourselves and the
rest of humanity.

Why lie about being a great soccer player when you're already great at
basketball? Let's focus on the beauty of science as a great journey of growth
and exploration that accelerates the progress of humanity, instead of trying
to make it do something that isn't possible in the real world.

~~~
mistermann
> No. What is the basis for these claims?

"Science", as it is represented in the media, and in turn repeated _and
enforced_ (not unlike religion, interestingly) on social media and in social
circles.

As opposed, of course, to _actual science_.

 _" Perception is reality."_ \- Lee Atwater, Republican political strategist.

[https://www.cbs46.com/news/perception-is-
reality/article_835...](https://www.cbs46.com/news/perception-is-
reality/article_83572d66-9e74-53ec-b0b2-60610fda4e75.html)

[https://en.wikipedia.org/wiki/Lee_Atwater](https://en.wikipedia.org/wiki/Lee_Atwater)

 _" Sauron, enemy of the free peoples of Middle-Earth, was defeated. The Ring
passed to Isildur, who had this one chance to destroy evil forever, but the
hearts of men are easily corrupted. And the ring of power has a will of its
own. It betrayed Isildur, to his death."_

 _" And some things that should not have been forgotten were lost. History
became legend. Legend became myth. And for two and a half thousand years, the
ring passed out of all knowledge._"

[https://www.edgestudio.com/node/86110](https://www.edgestudio.com/node/86110)

Threads like this one, and _many_ others like it, well demonstrate the
precarious situation we are in _at this level_. Imagine the state of affairs
around the average dinner table. Although, it's not too infrequent to hear the
common man admit (which is preceded by _realization_ ) that they don't know
something. As one moves up the modern day general intelligence curve, this
_capability_ seems to diminish. What the exact cause of this is a bit of a
mystery (24 hour cable propaganda and the complex dynamics of social media is
my best guess) - hopefully someone has noticed it and is doing some research,
although I've yet to hear it mentioned anywhere. Rather, it seems we are all
content to attribute any misunderstanding that exists in modern society to Fox
News, Russia, QAnon, or the alt-right. I'm a bit concerned that this approach
may not be the wisest, but I imagine we will find out who's right soon enough.

------
harimau777
I don't know if this would be my "one question" if I could ask the most
brilliant minds in science, but something that always bothered me:

When I took physics they basically said "at first scientists were disturbed by
the fact that magnets imply that two objects are interacting without any
physical contact, but then Faraday came along and said 'the magnets are
actually connected by invisible magnetic field lines' and that resolved
everything."

How does saying "but what if there's invisible lines connecting them" resolve
anything? To be clear, I'm not objecting to any of the actual electromagnetic
laws or using field lines to visualize magnetic fields. It's just that I don't
get how invoking invisible lines actually explains anything about how objects
are able to react without physical contact.

(Also, it is not lost on me I that this question boils down to "fraking
magnets, how do they work?")

~~~
keldaris
I'm a physicist specifically working with magnetic systems, but I have very
little pre-graduate teaching experience, so take this attempt to answer the
question with a grain of salt.

The reason some people regard Faraday's original explanation of the eponymous
law (it is worth noting that at the time it was widely regarded as inadequate
and handwavy) as illuminating is because Faraday visualized his "lines of
force" as literal chains of polarized particles in a dielectric medium,
thereby providing a seemingly mechanistic local explanation of the observed
phenomena. Not much of this mindset survived Maxwell's theoretical program and
it has very little to do with how we regard magnetism today. Instead, the
unification of electricity and magnetism naturally arises from special
relativity, whereas the microscopic basis of magnetism requires quantum
mechanics. There isn't really any place for naive contact mechanics in the
modern picture of physics, so in that sense I would regard Faraday's view as
misleading.

Finally, I can't end any "explanation" of magnetism without linking the famous
Feynman interview snippet [1] where he's specifically asked about magnetism.
It doesn't answer your question directly, but it's worth watching all the more
because of it.

[1]
[https://www.youtube.com/watch?v=MO0r930Sn_8](https://www.youtube.com/watch?v=MO0r930Sn_8)

~~~
JabavuAdams
That interview is so good! What a dick, but what a teacher!

~~~
onionisafruit
What I see at the beginning of that video is somebody who doesn’t want to
spend the energy answering a complex question. Then, in the process of
dismissing the question he gets drawn in and can’t help himself from really
getting into it.

I don’t know anything about Feynman beyond vaguely associating his name with
science, but watching this makes me want to seek out more from him.

~~~
strgcmc
You're in for quite a treat then. It sounds like you might have more of an
interest in his technical work and scientific contributions and teaching
materials (of which there is plenty, and of high quality), but personally I
quite enjoyed this book of his as well:
[https://en.wikipedia.org/wiki/Surely_You%27re_Joking,_Mr._Fe...](https://en.wikipedia.org/wiki/Surely_You%27re_Joking,_Mr._Feynman)!

------
memset
Crypto and practical security. I get tired of the circular “don’t roll your
own crypto unless you’re qualified”. How does one become qualified? I don’t
feel like I know how to evaluate many of the arguments people make for or
against technologies people argue about on HN, such as Signal or different
password managers. I feel like “security through obscurity” is a bad thing,
and “layers of security” are a good thing, but isn’t all security obscuring
something, and how does one evaluate whether a layer is adequate? “Just use
bcrypt” - okay, help me understand!

~~~
Eridrus
The reason people say not to roll your own crypto is that there is no secret
answer to making things secure, we just have smart and creative people bash
their heads against a crypto protocol/implement for a long time and hope we
found all the problems.

So unless you have a good reason to do something else, and the budget to pay
experienced people to bash their heads against it, you should stick to an
implementation that has had this effort expended on it.

If you want an intro about common problems in custom cryptosystems, go look at
cryptopals or something, but don't get too cocky that you know everything.

~~~
regularfry
It's also easy to dramatically underestimate the order of magnitude of effort
involved in "the budget to pay experienced people to bash their heads against
it".

------
vmception
Has someone that thought they were taking LSD _ever_ turned into a permanent
schizophrenic zombie or in a mental institution, or is it all urban legend. If
someone that didn't know they were predisposed to mental illness, is it
applicable to dismiss their experience in order to maintain how safe LSD is?

If any of this is true, are there any sources aside from "my friend's friend's
brother took too much and now he is....", and what is the scientific
explanation and do we know enough about the mind at all?

I feel like LSD has a lot of contradictory information out there, and the
proponents feel the need to hand waive concerns away because it is 'completely
harmless and leaves your system in 10 hours'. But when nobody knows what
they're actually getting because it doesn't exist in a legal framework, then
it muddies the whole experience.

People say certain doses can't do more effect than lower doses after a certain
threshold. It seems like the same people say "omg man 1000ug you are going to
fry your brain!"

What is the truth? If it "just" had an FDA warning like "people with a family
history of schizophrenia should not take it", that would be wildly better than
what we have today.

Please no explanation about shrooms. Just LSD the 'research chems' distributed
as LSD.

~~~
gavinray
Permanent schizophrenic zombie, maybe a bit extreme, but severe and traumatic
long-lasting psychological damage is a not-uncommon phenomena.

I had a fling with psychedelics in my teens, and everything was great until
the one time it wasn't. I was taking psychedelics pretty much every weekend,
and by my count have tried over a dozen of them.

Had an experience with LSD which completely shook me to my core and gave me
such severe PTSD and trauma that every night I started to have massive panic
attacks and needed medical help. My entire worldview and perception of reality
was shattered, I wasn't able to "anchor" myself anymore and it all felt like a
sham. I was completely dissociated. I also got HPPD: to this day, everything
has a sharpened oil-painting type texture to it that increases based on my
anxiety level, and I'm sensitive to visual + aural stimuli (loud, brightly-
colored places are unpleasant). If I get too anxious, I start to dissociate.

It took ~2 years for the PTSD to subside for the most part, but still if I am
under a lot of stress I am liable to have a panic attack and get flashbacks
and need to go find somewhere quiet to sit somewhere alone to try to work
through it.

LSD being the particular substance has nothing to do with it, in my opinion. I
was young, dumb, reckless, and played with fire then got burned. It could have
happened with any of the other dozen psychedelics I took, but it just so
happened to be LSD the one time that it did.

But I want to add, that while giving me the most nightmarish, traumatizing
experience of my life, the best/most positively-profound experience has also
been on the same substance. I grew up in a pretty abusive household and didn't
do well forming relationships growing up, and had a lot of anger and
resentment in my worldview. After taking psychedelics (LSD, 2C-B, Shrooms) and
MDMA with the right group of people a few times, my entire perspective
shifted. For the first time in my life, it felt like I understand how it felt
to be loved, and what _" love"_ was, and how we're "all in this together" so
we may as well be good to each other while we're here.

It's been a long time since I've touched any of that stuff and I'm not sure I
ever will again, but I don't think it's inherently bad or good. Psychedelics
are like knives, they're neutral - can be used as a tool or cut the hell out
of you if you're reckless.

\---

Footnote: For context, this was probably due to life circumstances/psyche at
the time. I was in a relationship with a pretty toxic partner, and my mental
state wasn't the greatest. In hindsight, it seems like I was almost begging
for a "slap in the face" if you will.

~~~
kace91
If you don't mind me asking (and this is clearly a sensitive topic so feel
free not to reply): What do you mean by PTSD and flashbacks? As in, was the
trip so bad that remembering it creates anxiety, or are you reliving unrelated
traumatic memories that weren't an issue to live with before using the drug?

~~~
gavinray
It literally feels as if I'm being transported back to that same night,
starting to relive it all over again. It's entirely illogical but if you knew
what happened it might make more sense (happy to elaborate here and give a
brief description of what happened/why it messed me up so bad, I'm perfectly
okay to talk about it now).

~~~
coffeecat
I'd be interested to hear your story. I've never used psychedelic drugs, but I
find their effects fascinating.

~~~
gavinray
I took 300ug of LSD recklessly on a particularly bad day for me, in a
particularly uncomfortable setting.

Well, that night went bad. Really, really, life-alteringly bad. For the first
time, I had a bad trip. And not like, some mildly uncomfortable thoughts. I
got a bad feeling in my stomach from the moment I dosed, and I knew something
was going to be different this time.

As I started to come up, the bad feeling and a dark presence grew, and I
pulled out my phone. I started a timer, and I watched as the time slowed to a
point where it completely stopped. I started looping, I would get up off the
couch, walk a few feet, and be teleported back. Over and over.

I realized that I had gotten so high, that time was no longer moving. And if
time was not moving, I could maybe never come down. I was stuck here forever.
And then the hellish nightmare started.

I felt like I was losing control of myself, like something else was trying to
take over, and whoever won the battle, that is the consciousness that would
exist. The more I fought, the more painful things got. Pain the likes of which
I no one can physically imagine.

Went upstairs and laid down in my bed, began going out of body. I started
dying over and over in unimaginable ways in my head, trapped in loops. Pain
beyond anything I've ever felt in reality, there was no limit. It was tied to
my breath, I realized that it had been so long since I had breathed, I kept
forgetting who I was and what was going on, and then I would catch a slight
glimpse and remember and fight so hard to take another breath. And there was
so much pain in fighting to "survive" and hold on to who I was.

Eventually, the pain/struggle became too much, and I "gave in" and said "okay,
I give up, you win, I can't take it anymore, I'd rather die." And that's when
it's stopped. There appeared this giant shape of light/energy that was every
color at once, and colors we don't have words for, and it "touched me" (could
have been me moving towards it, or it towards me, there wasn't really a
concept of this).

When it "touched" me, what it "showed" me was something I later learned is
called an "Ouroboros", the snake eating it's tail. It showed me what
"infinity" really meant, and that was too much to handle and shattered my
psyche.

In that moment my body/mind/soul felt like it was obliterated to pieces by
some energy beam in the most excruciating, searing pain, and I woke up in my
bed having just pissed myself.

It took a long time to piece myself back together after that one.

\---

There are a lot of details I've omitted for brevity's sake, but this captures
the gist of it.

The majority of my trauma has to do with anything related to loops: think
Nietzsche's Eternal Return, general time-loops, fear of time-stopping, etc.

When I have panic attacks I have to stop myself from starting a stopwatch on
my phone to make sure time is still moving because it'll cause a feedback loop
and ratchet-up the panic, causing the time-dilation to increase in a vicious
cycle.

~~~
kharak
Holy. That sounds intense, to say the least. A part of me likes to experience
this, even though you made it abundantly clear, that it did not have a
positive impact on your life.

~~~
vmception
"ego death" is a common aspect of acid trips and the experience seems to come
down to your willingness to relinquish control. this reads like what was
described. if you were to look up that term you'll see others that will
feature similar features - with or without pain, with or without worry.

not having a reliable way to know exactly what you took can amplify the
anxiety, when your brain starts filling up with seratonin and whites
everything out just like people on their deathbeds report, are you supposed to
let go? when your sense of self has been obliterated and the next moment you
are in the body of another mammal lost and confused in the forest for an
entire lifetime before being transported back into your body and only a minute
has gone by - but your trip is to last another 9 hours, should you fight it?
Distinct neural networks in your mind that never communicate are now
connected, vestigial components of the mind are now being expressed, are you
being replaced in a firmware dump and flash?

a lot of people have a friend with them to guide them through an acid trip
because trips can be steered with sounds and words, simple chimes, melodies.

would it have helped? very hard to say. but as the author wrote, the bad day
and uncomfortable setting did not help. It is similar to a dream state (just
radically more intense), where the things on your mind and also happening
around you can affect the direction of your dreams.

~~~
gavinray
Yeah, I think it entirely had to do with my inability to relinquish control
and "just let go". Although in this context, that was literally what felt like
the fight to survive, instead of "being chill". Ego death commonly is either
the most horrendous or most nirvanic thing depending on how readily someone
gives in.

> when your sense of self has been obliterated and the next moment you are in
> the body of another mammal lost and confused in the forest for an entire
> lifetime before being transported back into your body and only a minute has
> gone by - but your trip is to last another 9 hours, should you fight it?

There was a lot of this, during that out-of-body-period. I existed in multiple
places/points in time at once as different people of various
ages/genders/nationalities and then occasionally as animals, and lived entire
simultaneous lifetimes. At one "time", in places + times A, B, C, D as
different living things. Really does a number on your sense of self for a bit,
heh.

~~~
vmception
and that had never happened to you in your other trips?

~~~
gavinray
No, was really strange, I was pretty experienced by then too. Was never the
same after.

~~~
vmception
Did you take 300ug before?

Sorry for the questions, we can talk about it somewhere else, just add an
email or protonmail account to your hackernews account I'll mail you there

------
robertakarobin
If I buy a stock, does the price at which I agreed to buy it become the new
share price on the stock exchange?

Every article on "Where do stock prices come from?" seems to just talk at a
high level about supply and demand.

But where does the price come from at a nitty-gritty level? Is it an average
of all existing offers or something?

Do different exchanges and stock-ticker websites have different formula for
calculating share price?

If a very low-volume stock is listed at $4, and then I offer to buy a share
for $100, does the NYSE suddenly start listing its price at $100?

~~~
atomicnumber3
Disclaimer: I used to work in HFT

Each exchange is basically its own world, with the exception of Reg NMS, which
I'll get to in a sec.

Let's task about order books. Each stock has its own order book. This might be
an example of the book for AAPL:

* SELLING 100 shares @ $10.02 * SELLING 200 shares @ $10.01 * SELLING 100 shares @ $10.00 * BUYING 100 shares @ $9.99 * BUYING 200 shares @ $9.98 * BUYING 100 shares @ $9.97

So if you want to buy some AAPL, you will want to go grab the cheapest shares
you can see, which here is the fellow selling 100 shares at $10.00. You submit
a limit order to buy at 10.00 and are matched with that guy. The book now
looks like this:

* SELLING 100 shares @ $10.02 * SELLING 200 shares @ $10.01 * BUYING 100 shares @ $9.99 * BUYING 200 shares @ $9.98 * BUYING 100 shares @ $9.97

Now let's suppose a market maker decides they think the price is going to
follow, so they go and fill in the hole by submitting an order to BUY 100
shares at 10.00. There's no more shares to buy at $10.00, so their order rests
on the book.

Now we have:

* SELLING 100 shares @ $10.02 * SELLING 200 shares @ $10.01 * SELLING 100 shares @ $10.00 * BUYING 100 shares @ $10.00 * BUYING 100 shares @ $9.99 * BUYING 200 shares @ $9.98 * BUYING 100 shares @ $9.97

Now that we've played out this scenario, let's go back to your original
question. What is the price of AAPL at any point in here? Well, it depends. At
the start, if you wanted to buy, you could say the price is $10.00. But if you
wanted to sell, the best you'd get is 9.99. So, hard to say.

It's worth noting that the prices you see in the book are only there because
people _aren 't_ agreeing on the prices. If they did agree, a trade would
happen, and the prices wouldn't be on the book. So, with that in mind, you
could say that really, the price of a stock is the last price people agreed
at: the last trade price. That's better, we're at least down to just one price
to think about.

That could be quite different from what the best bid/offer are right now,
though (some stocks don't trade very often) so even if (let's say) you last
saw AAPL trade at 9.50 before our example, obviously that price is long gone.
So even the last trade price is potentially not "the price of the stock".

So, in short, there's really no such thing as "the price of a stock". It'll
all depend on how sophisticated you want to be about the price at which you
buy your shares.

When people talk generally about the price of a stock, it's usually just up to
whatever site people are looking at, and usually markets are liquid enough and
trade enough that all the kinds of prices we just talked about are usually
only a penny different, so when people are just at the watercooler saying "Did
you see the price of AAPL?" they don't care about the pennies, and by the time
they've managed to say that question, the price has moved anyway, probably
lots of times. So it all gets a little hand-wave-y.

I want to mention two other things that might interest you. Reg NMS is what
ties all the exchanges together, so to speak. Let's say you want to buy AAPL
and NYSE has shares selling at $10.00 each, but NASDAQ has them for $9.99
each. It's actually illegal (against Reg NMS) to trade with that guy at $10.00
at NYSE because NASDAQ has the "NBBO" (national best bid/offer) right now.
Extra caveat: if you sent a special order to NYSE that says "I promise you,
I've also sent an order to NASDAQ to buy the shares for $9.99 and I've
determined you're the next best price at $10.00, let me buy them", it'll let
you. It's called an ISO (Intermarket Sweep Order) and if you lie about them or
mistakenly lie about them, you get fined. A lot.

The other interesting thing: Your last question was "If a very low-volume
stock is listed at $4, and then I offer to buy a share for $100, does the NYSE
suddenly start listing its price at $100?" There's actually a lot to unpack
here. Let's go through it.

If you're a registered broker-dealer and are connected directly to NYSE, and
you send a limit order for XYZ @ $100/share, what's going to happen is you're
going to get "price improvement" and you'll end up getting the shares at $4.
If you send an order for LOTS of shares at $100, you'll clear out a bunch of
price levels in one go. Ex:

Let's say this is the book for XYZ:

* [...] * SELLING 200 shares @ $110.00 * SELLING 1000 shares @ $5.00 * SELLING 200 shares @ $4.02 * SELLING 100 shares @ $4.01 * SELLING 500 shares @ $4.00 * BUYING 100 shares @ $3.99 * [...]

Usually when you get away from the middle of the book, liquidity dries up fast
and the prices get further apart. So let's say you send an order for 10000
shares at $100. You're going to get 500 at $4, 100 at $4.01, 200 at $4.02,
1000 at $5.00. Now the next price is 110, but your limit price is 100. So your
order will now actually rest partially-filled on the book. So now this is the
book:

* [...] * SELLING 200 shares @ $110.00 * BUYING 8200 shares @ $100.00 * BUYING 100 shares @ $3.99 * [...]

Neat, huh? That was a lot of price movement. So yes, if you can send for
enough shares and are willing to pay through a lot of price levels, you can
move the price of the stock. Remember Reg NMS though - if more stock exchanges
existed in our example, you'd also likely need to go get shares at them if
they have a better price than the exchange you just moved the price at.

But let's now suppose you're NOT a registered broker-dealer, but are instead
Joe A. Schmoe, a client of Charles Schwab Brokerage. You enter your order in
your web browser and hit trade. Schwab has a legal obligation to fill your
order, if possible, only at the NBBO. They could route your order right to an
exchange, but instead, they will send your order to their friend, Citadel, who
will have the opportunity to trade against your flow before it gets routed to
the stock exchanges. Generally, this is good for you: they might decide your
order represents good information and they want your shares. They could decide
to fill your order themselves and sell you all 10000 shares you want. They're
constrained by the NBBO though, so you get all 10000 shares at $4. For being
the source of this order, Citadel pays Schwab some money. Usually practically
a pittance, pennies, if that. Order flow is dirt cheap nowadays.

This is called "selling order flow" and lots of people find it scary, because
it's not really super intuitive why someone would want to buy or sell the
actual flow of orders. But it's actually pretty boring and more about high-
level statistics than anything actually interesting to Joe Schmoe, who would
get bored when he realized he's not really getting ripped off.

Sorry, I got a bit off-topic. But I love finance, so please forgive me.

~~~
nojvek
Love the detailed explanation. Thanks. Once I saw Boeing at $120. I thought to
myself. That’s dirt cheap and should buy. So I hit buy at market and lo behold
I bought for $130. Wait! Whaaa! Did the exchange lie to me ? That day I
learned a very hard lesson that there are infact two prices. Ask price and bid
price. I wasn’t paying attention.

Now I try to always put limit orders. I put sell for Boeing at $150 with “good
till cancelled” option. One morning I wake up to see they’ve been fulfilled.
Wohoo! But the price had dropped down to $140. So I cashed in on the spike.

The market is crazy. I still don’t understand if. P/E ratios for some
companies are through the roof (100+), why are people still investing in them
like crazy? We don’t have a cov2 vaccine, millions of people don’t have jobs,
why did the marker recover half it’s losses already? Shopify, Amzn, Zoom. WTF!
Their charts seem hyped. Or may be I’m just plain wrong and don’t understand
the fundamentals.

~~~
sah2ed
> _Shopify, Amzn, Zoom. WTF! Their charts seem hyped. Or may be I’m just plain
> wrong and don’t understand the fundamentals._

Since the outbreak of COVID-19, demand for the kind of services offered by
those 3 Internet businesses have in fact skyrocketed. Increasing demand imply
those businesses still have room to grow revenue. Shopify [1] for instance is
now seeing huge Black Friday-like traffic during the shelter-in-place and a
lot of these small businesses are first-timers on their platform who will
likely stick around after the pandemic.

1:
[https://mobile.twitter.com/jmwind/status/1250816681024331777](https://mobile.twitter.com/jmwind/status/1250816681024331777)

------
pjungwir
Quantum spin. Electrons aren't really spinning, right? But why do we call it
spin? I know it has something to do with angular momentum. What are the
possible values? Is it a magnitude or a vector? Is there a reason we call it
"spin" instead of "taste" or some other arbitrary name? How do you change it?
What happens to it when particles interact?

~~~
lisper
> Electrons aren't really spinning, right?

Correct.

> But why do we call it spin?

Because it is a physical quantity whose units are those of angular momentum,
and we have to call it _something_.

> What are the possible values?

+/\- h/4pi where h is Planck's constant. (It is usually written has h-bar/2
where h-bar is h/2pi.)

> Is it a magnitude or a vector?

It's a vector that always points in a direction corresponding to the
orientation of the apparatus you use to measure it.

> Is there a reason we call it "spin" instead of "taste" or some other
> arbitrary name?

Yes. See above.

> How do you change it?

You can change an electron spin by measuring it along a different axis than
the last time you measured it. The result you get will be one of two possible
values. You can't control which one you get.

> What happens to it when particles interact?

Their spins become entangled.

~~~
madhadron
> It's a vector

It's not exactly a vector...

~~~
johnmorrison
It only has direction + magnitude right? (±h/4π)e_i for some unit vector e_i.

So it can be written as a vector? No?

------
memset
Law. How much of it rests on technicalities, and how much do judges care about
the essence of the facts of a case? You only hear about the weird outcomes on
the news. For example, I was once working with an attorney because my landlord
didn’t supply heat in the apartment. I started keeping a temperature log, but
how would a judge know that the log was accurate? Do I need to prove that my
thermometer is accurate... etc. In practice, no, but how can I better reason
about what is “likely” vs not?

~~~
Waterluvian
I've begun to grok so much about how law _practically_ works by listening to
Popehat's All the President's Lawyers podcast.

Basically you get an educated layperson asking a veteran criminal lawyer
questions, usually around the First Amendment, always related to current
events. The lawyer (Ken White) explains in practical terms what is likely to
happen and why.

------
achenatx
I consider that there are 4 levels of scientific writing.

1) news articles/lay press - basically terrible and typically get things wrong

2) scientific lay press (scientific american, discover, science news) - get
things right, but generally no data/citations or nuance

3) journal summaries - get things right, citations and data for everything.
Good summary of the latest scientific thought on a topic. Tend to push a point
of view, which generally will be right, but that educated people can debate.
Dont always show the data, but at least refer to it. These help you to get up
to speed with the primary experiments that were used to establish current
thinking.

4) first source articles - typically make claims too broad for the actual
results. But has all data. Often times the claims don't follow from the data
at all. Generally have to work in the field to understand strengths and
weaknesses of methods and you cant just take the conclusions at face value.

As a PhD student, I used #3 a lot to get centered on a space. To understand 4,
I typically had to learn directly from my research advisor or other grad
students that specialized in an area.

My point here is that you can find these summary articles in journals
(microbiology, immunology,virology etc). They are published infrequently so
can be hard to find, but they exist and you should look for them.

~~~
Denzel
I was shoulder deep in primary source material (#4) on real-time fuzzy search
and wasn’t able to make heads or tails of where everything fit. Until I found
a #3 overview that referenced most of the primary sources and put them in
context. That paper was worth its weight in gold!

Do you have any tips for how to quickly find these #3 materials in other
spaces?

~~~
Thriptic
The name for what you and the parent described is a review paper. I can't
speak for Math or CS, but in life sciences the following techniques would
work:

1\. Search X and sort by citation count. High quality review papers get cited
A LOT, typically in introduction sections of primary research papers.
Alternatively, google "[X] review" or "best review paper on X".

2\. Look for review journals. Many fields will have journals who only publish
reviews. Nature has several such publications for example.

3\. Look for the top journals in the space (start by sorting by impact factor)
and see if they have review sections. If they do, try to search those
sections. Most journals will reach out to top labs in a space and request that
they write a review on a subject if the journal editors feel one is needed.

4\. Ask someone in the field. Any researcher should be able to immediately
point you to canonical reviews in their space.

------
anton_tarasenko
Subreddit /r/askscience does a good job at explaining science in plain words.
I usually google "site:reddit.com/r/askscience/ __QUESTION__".

The StackExchange sites have less coverage and answers tend to be more
technical.

University websites return reliable answers, but often neither short nor
accessible.

------
lpellis
Bell's theorem. It somehow proves that quantum physics is incompatible with
local hidden variables, but I could never see an understandable explanation
(for me at least) of just how it works.

~~~
sciolizer
Yudkowsky's explanation[1] is the first one that worked for me. I later found
Quantum mysteries for anyone[2] helpful. The latter has less soap-boxing.

1: [https://www.lesswrong.com/posts/AnHJX42C6r6deohTG/bell-s-
the...](https://www.lesswrong.com/posts/AnHJX42C6r6deohTG/bell-s-theorem-no-
epr-reality)

2:
[https://kantin.sabanciuniv.edu/sites/kantin.sabanciuniv.edu/...](https://kantin.sabanciuniv.edu/sites/kantin.sabanciuniv.edu/files/makale/mermin.pdf)

~~~
ShamelessC
I started to read the first one but his insistence that Many Worlds is true
was too frustrating. Many Worlds Theorem seems specifically useful at saying
"the variables aren't hidden because everything before wavefunction collapses
actually plays out in different worlds.

But, we specifically have no way of proving that theory. So now we're back to
the essence of the original question - if these things seem random why do we
know that they're in fact deterministic without any hidden variables?

~~~
kubanczyk
Well, I'd recommend to read the whole series. It's not so bad as it sounds.
There are so many steps from where you are to appreciating the utter
_weirdness_ of Bell's experimental result. Not the weirdness of any theory (or
an interpretation, which Many Worlds actually is) but of the basic
experimental result.

If you _are_ properly amazed by it, rejecting MWI or any crazy-ish borderline-
conspiracy theory seems suddenly a lot harder.

I feel the whole Yudkowsky's QM series in fact served to deliver that one
post.

~~~
sooheon
Why isn't the MWI another form of hidden variables (a supremely non-
parsimonious one at that), where the hidden variable is which of the many
worlds you happen to inhabit?

~~~
wizeman
I think you can make an argument for viewing it that way, depending on exactly
what you mean by "you".

But IIUC, one of the remarkable things about MWI is that it would be a _local_
hidden variable theory!

This is a very important property to have because the principle of locality is
deeply ingrained in the way the Universe behaves. Note that (almost?) no other
quantum interpretation is both realist and local at the same time.

Maybe you wonder, how is it possible that MWI can be considered a local hidden
variable theory if Bell's theorem precisely shows that local hidden variable
theories are not possible?

I think that it was Bell himself who said that the theorem is only valid if
you assume that there is only one outcome every time you run the experiment,
which is not the case in MWI.

This means that MWI is one of the few (the only?) interpretation we have that
can explain how we observe Bell's theorem while still being a local,
deterministic, realist, hidden variable theory.

~~~
sooheon
For it to be local (causality does not propagate faster than light), it must
be superdeterministic (all the many worlds that ever will be, already are).
For it not to be superdeterministic (many worlds decohere at the moment of
experimentation), it is also not local (the decoherence happens faster than
the speed of light, across the universe).

~~~
wizeman
I'm sorry but I don't follow.

If you take the Bell test experiment where Alice and Bob perform their
measurements at approximately the same time but very far apart, I think you
and I both agree that when Alice does a measurement and observes an outcome,
she will have locally decohered from the world where she observes the other
outcome.

But I don't see why the decoherence necessarily has to happen faster than the
speed of the light.

It makes sense that even if Alice decoheres from the world where she observes
the other outcome, the outcomes of Bob's measurement are still in a
superposition with respect to each Alice (and vice-versa).

And that only when Alices' and Bobs' light cones intersect each other will the
Alices decohere from the Bobs in such a way that the resulting worlds will
observe the expected correlations (due to how they were entangled or maybe
even due to the worlds interfering with each other when their light cones
intersect, like what happens in general with the wave function).

I admit I'm not an expert in this area, but is this not possible?

------
abiogenesis_123
Abiogenesis. I understand that this is not considered understood but I'd be
interested in hearing a qualified scientist talk about how we get from "dumb"
matter to self-replicating, goal-driven matter. The closest I've ever heard
anyone get (in a personal conversation) is that chemistry is about
transformation, so "dumb" matter isn't really dumb in the sense that it's
static. Still lots of hand waving to get from baking soda and vinegar
volcanoes to me typing this question, however.

What's the playing field look like for proto-life? How "smart" are the
simplest molecular interactions? What does almost-replication look like? Could
we use a computational model for this?

Not sure how much of this is known, but I'd love to hear an expert paint a
picture of their mental model of the subject.

~~~
javajosh
This question interests me as well, and I've done some thinking about it over
the years. In particular, what I'm interested in is a hypothetically simplest
object that can reproduce in a solution of simpler components, along with some
differentiating characteristic. For example, maybe you have toroids that pick
up particules, grow the torus until its too big, and then splits - and the
ends of both halves click together, forming a total of two toroids. Another
characteristic that very simple life must have (I believe) is some level of
circularity in the sense that the element is an "accumulation of experience"
\- we might say a reduction of its environment. In the same way Schordinger
was interested in life thermodynamically[1] I am interested in speculating
about the simplest possible mechanisms in the beginning. (NB I'd expect _none_
of these very simple machines to survive to present day - in fact, I'd imagine
there to be several generations of early life, each
obliterating/consuming/sublimating the ones before.)

1 -
[https://en.wikipedia.org/wiki/What_Is_Life%3F](https://en.wikipedia.org/wiki/What_Is_Life%3F)

~~~
abiogenesis_123
> NB I'd expect none of these very simple machines to survive to present day -
> in fact, I'd imagine there to be several generations of early life, each
> obliterating/consuming/sublimating the ones before.

That's another really interesting axis of this topic -- why was early Earth
special?

There may be no environments on Earth today that are like early Earth, but we
could probably recreate them. Wouldn't we then be able to witness abiogenesis?

Or, early Earth wasn't special and abiogenesis happens today. If so, where do
we look?

~~~
javajosh
_> Wouldn't we then be able to witness abiogenesis?_

It may happen quite infrequently, and only because it happened in a large,
lifeless (but 'nutrient' rich bath) did it have a chance to amplify. What's
interesting to me is that it only has to happen once.

Yes, Earth's specialness is interesting, too, and counts for what I believe
are the best reasons to believe in God. Earth has so many amazing qualities:
it is a cozy distance from the Sun (temp), tilted quite a bit (seasons), with
a molten core (cosmic ray protection) and a huge moon (tides, nocturnal
light). All of these may be necessary conditions for life to arise, and they
are all, as far as we know, quite rare individually, and astronomically
unlikely in combination.

------
ramboldio
Fourier Transforms. I'd wish I had a intuitive understanding of how they work.
Until then I'm stuck with just believing that the magic works out.

~~~
ivan_ah
The best way to understand the Fourier transformations is to think of them as
change-of-basis operations, like we do in linear algebra. Specifically a
change from the "time basis" (normal functions) to the "frequency basis"
(consisting of a family of orthonormal functions).

Here is the chapter on Fourier transforms from my linear algebra book that
goes into more details:
[https://minireference.com/static/excerpts/fourier_transforma...](https://minireference.com/static/excerpts/fourier_transformations.pdf)

As for the math, there really is no other way to convince yourself that sin(x)
and sin(2x) are orthogonal with respect to the product int(f,g,[0,2 _pi])
other than to try it
out[https://live.sympy.org/?evaluate=integrate(%20sin(x)*sin(2*x...](https://live.sympy.org/?evaluate=integrate\(%20sin\(x\)*sin\(2*x\)%2C%20\(x%2C0%2C2*pi\)%20\)%0A%23--%0A)
Try also with sin(3_x) etc. and cos(n*x) etc.

~~~
mkl
> As for the math, there really is no other way to convince yourself that
> sin(x) and sin(2x) are orthogonal with respect to the product
> int(f,g,[0,2pi]) other than to try it out
> [https://live.sympy.org/?evaluate=integrate(%20sin(x)*sin(2*x...](https://live.sympy.org/?evaluate=integrate\(%20sin\(x\)*sin\(2*x..).
> Try also with sin(3x) etc. and cos(n*x) etc.

I disagree with that. It's pretty easy to prove it in general by calculating
\int_0^{2\pi} sin(mx)sin(nx) dx etc. for m ≠ n.

~~~
ivan_ah
I would count an analytic solution as in the "trying out" category (actually
the best kind of trying out!).

The "no other way..." was referring to me not having an _intuitive_
explanation to offer about why an sin(x) and sin(2x) are orthogonal.

------
vijay_nair
• Magnetism. There are plenty of videos out there calling it the result of a
relativistic charge imbalance. But I've never been able to use this point-of-
view to practical use cases like understanding how permanent magnets work or
how increasing the number of windings in inductors boosts the magnetic field
strength. There were more situations I tried to put this POV into use but I
can't remember them off the top of my head.

• Qualia. What is this subjective experience that I know as consciousness?
I've gone through Wiki, SEP and a fair number of books on philosophy and a few
on neuroscience but I still don't understand what it is that I experience as
the color "red" when in reality it's just a bunch of electric fields
(photons). Why can't I get the same experience — i.e., color — when I look at
UV or IR photons? These too are the very same electric fields as the red,
blue, green I see all the time.

• Photographic composition. I'm a designer. I know them. I use them. But only
empirically. I just do not understand them at a neuroscientific level. Why
does rule-of-thirds feel pleasing? Is the golden ration bullshit? My gut says
yes but I'm unable to come up with a watertight rebuttal. Why do anamorphic
ultra-widescreen shoots feel so dramatic/cinematic? Yet to see an online
exposition on the fundamental reasons underlying the experience. Any questions
to artists are deflected with the standard "It's art, not science" reply.

• Wave-Particle duality. "It's a probability wave that determines when a
particle will pop into existence out of nothingness." okay, where exactly does
this particle come from? If enough energy accumulates in a region of empty
space, a particle pops into existence? What is this "energy"? What is it made
of? What even is an electron, really? I've followed quite a few rabbit holes
and come out none the wiser for it.

• Convolution. It's disappointing how little I understand it given how wide
its applications are. Convolution of two gaussians is a gaussian? Convolution
in time domain is multiplication in frequency domain and vice-versa? How do
these come out of the definition which is "convolution is sliding a flipped
kernel over a signal"?

~~~
aeternum
The issue with wave-particle duality is that most of us think about it
backwards.

The universe is actually made of quantized fields. Both particles and waves
are imprecise models/approximations. There's no such thing as a particle,
instead there are just excitations of this field which we cannot measure with
complete accuracy.

~~~
Lichtso
> Which we cannot measure with complete accuracy

I very much dislike this phrasing, because it suggests that it is just us that
are not capable of building an apparatus to enable us to do so.

Imagine a gear transmission or a lever: You can transform distance into force
and vice versa. It is up to your choosing if you want to go with more speed or
more force by changing the point along the lever, where your transmission
happens. It is not possible to build a transmission, which gives you the most
distance and the most force simultaneously. In this system of transmission,
one is the other, just a different perspective.

And it is the same with the location and impulse of a quantum. You can choose
to have more information in the shape of location or more in the shape of
impulse by changing your measurement (like the point along the lever). But you
can't have both, because there is only a constant amount of information which
is represented in a combination of location and impulse.

Actually, the uncertainty part of heisenberg uncertainty principle is a purely
mathematical limitation (called Gabor limit) and only the Planck constant
makes it physical.

Gabor limit: a • b >= 1/(4•PI)

Heisenberg's uncertainty principle: a • b >= h/(4•PI)

So the Planck constant is kind of the maximal sampling resolution of the
fields / signals in our universe.

~~~
aeternum
Good point

------
eranation
Lot's of quantum related phenomenon here, but what keeps bothering me is that
while I get it that light is both a wave and a particle, but I have no clue
what that means. I mean, a wave of sound, is made from air particles, a wave
of ripples in a pond is made of movement of water molecules. In the double
slit experiment, it's explained that the single photon has to be "interfering
with itself", so I don't get it if by being a "wave" it means that the single
photon is basically a bunch of "magic" photon ghosts that behave like a wave,
but once it is measured or any other reason to "collapse" these ghosts
"disappear". I just don't get what the "wave" of light/radio wave is. Is it
just an abstract concept of something that behaves like a wave but not the
same as sound waves / ripples since we simply don't know? Or is it just a wave
of these "not yet collapsed" probabilities of the photons locations that are
interfering with each other right until we ask them to choose a location, then
they just collapse magically into a single "real" photon. Another thing I
don't get is in the double slit experiment, a LOT of the measure before,
measure after, etc, are told to be thought experiments, but it's also claimed
that someone managed to actually replicate them. Why isn't there a video
showing it? I obviously believe they happened, and understand why more or less
(e.g. in the one photon at a time experiment, it's spooky that over time you
get the same pattern that indicates interference as if you shoot many) but the
more spooky result is that thought experiment, that if you measure which slit
the photon actually traveled through, you'll see 2 slits on the screen vs the
famous pattern. e.g. you'll cause the wave to collapse back to particles. So
any video of reproducing of that thought experiment or explanation why it's so
hard to reproduce, will be super helpful.

~~~
SAI_Peregrinus
> while I get it that light is both a wave and a particle, but I have no clue
> what that means.

Because it's wrong. It's a quantum of the electromagnetic field. It's
_neither_ a wave _nor_ a particle. It just happens to have some properties of
both.

~~~
AlanSE
I've done plenty of layman reading into QFT, and I'm truly good with this. All
particles are really waves. There are no particles. There's self-interaction,
and all other kinds of weird stuff. (it's important to qualify that stable
orbits and molecules and stuff exist)

But for the duality, there's something bigger that the responses always seem
to blow past. Is wave-like nature for explaining behavior (wavy double-slit
intensity pattern), or is it something to have a mathematical mapping to
measured probabilities?

Quantum stories always seem so backwards. The root phenomenon is some sort of
irreducible probability. But then the mechanical part (inference in double-
slit) goes a totally different direction. Instead of just turning the
situation into a probability of one-or-the-other slit, it STAYS as a wave.

Okay, now you have a new hole in the story. If the photon refuses to choose
just 1 slit to go through, why does it choose 1 spot on the photo paper to
land on?

Why do we not still have to consider interference in outcomes after the photon
makes its mark on the paper? Why does there appear to be like a limit on
entanglement, such that it goes away beyond a certain scale? Why are quantum
computers hard?

~~~
aeternum
> If the photon refuses to choose just 1 slit to go through, why does it
> choose 1 spot on the photo paper to land on?

The photon (as a field excitation) goes through both slits, but is quantized
so only has enough energy to trigger a mark at 1 spot on the photo paper.

> Why do we not still have to consider interference in outcomes after the
> photon makes its mark on the paper?

If we want to be completely accurate, we should. However so many interactions
happen so quickly that the law of large numbers quickly takes over and
obfuscates the quantum reality. Technical term for this is decoherence.

> Why are quantum computers hard?

Exactly because of this decoherence. It is very difficult to keep the qubit
state isolated from the environment throughout the computation.

------
tomp
Flight. Apparently "air flows faster on the top side of the wing, lowering the
pressure" is an incomplete explanation; I even heard we don't completely
understand why it works (?!?).

~~~
geocrasher
I've been informed that I'm part of the problem. Comment removed, sorry for
the trouble, folks!

~~~
na85
>The top of a wing is curved, making it longer than the bottom of the wing.
This means that air takes longer to go over it, meaning it has to spread out
further to go the same distance as the air under the wing. As a result, the
air going over the top of the wing is less dense, (aka lower pressure). The
wing tries to equalize the pressure by moving in the direction of the low
pressure, which is Up. We call this Lift.

100% completely false.

Imagine you have two particles of air, and they are immediately adjacent to
each other. Suppose now that one goes above the wing, and one goes underneath.
In your example, the particle going upward goes further in the same amount of
time.

But ask yourself this: Why do the particles of air have to arrive at the same
time? What mechanism from physics requires that they meet up again at the far
end of the wing?

Then ask yourself this: If what you described is true, then how do aircraft
fly upside down?

~~~
Biganon
For years I thought I was crazy or stupid, whenever I heard this story of "air
has to go faster" I was like "but how does air know?? It's not like it has a
Google maps plan telling it it needs to reach the other end of the wing at a
precise time!"

By chance, in the last few years I've started reading more and more comments
debunking this absurd explanation. Not that I understand perfectly now, but at
least I know I'm not crazy.

~~~
na85
In fact that "air has to go faster" silliness only works if you completely
neglect air friction entirely, because then the air can be said to part around
the wing like butter around a hot knife.

But of course anyone who's seen snow billowing off the back of a car knows
that air doesn't just close up behind the object like a ziplock bag: it's
messy and turbulent and gets all over your windows while you're tailgating.

------
Dutchie85
Why does time slow down/go faster with movement compared to another object.

The well known example that if you travel into space you'd gain let's say 5
years and people on earth 25 in the same time or so.

I just don't get it and I can't find any logic explanation.

For instance: Two twins who came to live exactly at the same moment in the
year 2000 and both die on their 75th birthday at the same time. One travels
into space, the other stays on earth. Earth-brother dies on earthyear
2075,space-brother dies in earthyear 3050 or so...

I know its Einstein's point but that just doesn't instantly make it correct to
me.

~~~
opinion-is-bad
All things in the universe have four measures of velocity. Three of these are
easily observable to us in terms of x, y, and z movement. To understand time
dilution we need to realize that we are also physically traveling through
time.

The total velocity of our <x, y, z, t> vector will always be equal to the
speed of light constant, c. You can think of something that has no physical
movements as moving forward in time at the speed of light. As x, y, or z
increases the magnitude of t will decrease so that the speed of light constant
is always achieved.

Why this link has to hold is more complex and I cannot explain it well, but
hopefully this gives some insight into time slowing as velocity increases.

~~~
thr0w__4w4y
> The total velocity of our <x, y, z, t> vector will always be equal to the
> speed of light constant, c. You can think of something that has no physical
> movements as moving forward in time at the speed of light. As x, y, or z
> increases the magnitude of t will decrease so that the speed of light
> constant is always achieved.

I've gone decades without hearing it explained that clearly and simply. Thank
you (sincerely).

------
Crazyontap
What happens when you actually fall inside a black hole and what is the
singularity.

I never really understood what happened really when the guy fell inside it in
Interstellar and how come he started seeing all those photos. I just accepted
it as Hollywood bs.

I know my question is based on a movie but would still like to know what will
someone witness (assuming of course they somehow live)

~~~
hartator
Singularity in AI?

~~~
guerrilla
No,
[https://en.m.wikipedia.org/wiki/Gravitational_singularity](https://en.m.wikipedia.org/wiki/Gravitational_singularity)

------
sloaken
Avogadro constant - I can accept the number, if someone can show me how, over
a hundred years ago, without having every really seen a molecule, this number
could be derived. Obviously no one ever sat there and counted them but I find
it hard to believe, they could have decided this without being able to see it.

~~~
Balgair
Here's how to estimate the size of an oil molecule just using water and an oil
drop: [https://spark.iop.org/estimating-size-molecule-using-oil-
fil...](https://spark.iop.org/estimating-size-molecule-using-oil-film)

(They use some other stuff, but you get the idea)

You can back out Avogadro constant starting with this experiment.

~~~
sloaken
Thank you. I do get the idea. And now I can go back and finish High School
chemistry. I appreciate you help in understanding this.

------
mynegation
Why tardigrades are so hardy, how their biology is so different?

How immune system and medications work.

Why some plastics are recyclable and others are not.

~~~
ramraj07
I've been mulling making a youtube channel with ten minute videos on
immunology; what would be a good starter video that might interest someone
like you? I thought I'll do something about antibodies as drugs!

~~~
mynegation
Thank you and please please do it and post a link here or send me an email (in
profile)! For me the most interesting is the recognition/pattern matching
aspect: how antibodies find what to attack and what to leave alone.

~~~
ramraj07
Most definitely one of the hardest questions to answer :) I'll take it up as a
challenge!

------
davidmanheim
Non-interactive zero knowledge proofs.

ZK proofs have a number of good explainers, mostly using graph colorings. Non-
interactive versions, however, require quite a bit more than that explanation
allows - and despite asking experts, I still haven't found a good, basic
explanation.

~~~
itcrowd
Maybe the blog of Prof. Matthew Green of JHU is of use. Specifically, the two-
part series about zero-knowledge proofs. Part II discusses non-interactive ZK
proofs. Part I is really required to grasp the extension to non-interactive
ZKP's, so you may need to read that first.
[https://blog.cryptographyengineering.com/2017/01/21/zero-
kno...](https://blog.cryptographyengineering.com/2017/01/21/zero-knowledge-
proofs-an-illustrated-primer-part-2/)

------
yters
A non handwavy explanation of how does evolution result in such complicated
and carefully orchestrated mechanisms, way beyond our engineering capability.
The computer analogy of genetic algorithms certainly doesn't explain much,
they are not very effective, and if they were, we should be able to generate
marvels of engineering with GAs and current computational power.

~~~
sloaken
A whole LOT of time, a whole lot of space.

So estimate 1 minor good random mutation per 10,000 population. Assume 1 major
mutation per 1000 years, per 10,000 population.

~~~
yters
That's essentially the same kind of explanation as 'god did it'.

~~~
cambalache
I dont know if you are being serious, but if you are, Dawkins books are pretty
good (The blind watchmaker,or the Ancestors tale).

~~~
yters
I will have to check them out, but I remember being unimpressed with the table
of contents. Looks like it will be more handwaving.

~~~
cambalache
Ah OK, you are not being serious, fair enough. Magic god did it all.

~~~
yters
I am being serious. I just purchased Dawkin's book "Blind Watchmaker". My
point is no explanation should amount to "magic X did it" whether X is god,
evolution, the earth spirit, aliens, etc. All the above are bad explanations.

It is not scientific to substitute one bad explanation for another. The
scientific approach is to say we don't know, and then look for a good
explanation.

------
sam
Mach's principle. Why is there a "preferred" rotational frame of reference in
the universe? Or as stated in this Wikipedia article,

"You are standing in a field looking at the stars. Your arms are resting
freely at your side, and you see that the distant stars are not moving. Now
start spinning. The stars are whirling around you and your arms are pulled
away from your body. Why should your arms be pulled away when the stars are
whirling? Why should they be dangling freely when the stars don't move?"

[https://en.wikipedia.org/wiki/Mach%27s_principle](https://en.wikipedia.org/wiki/Mach%27s_principle)

~~~
contravariant
The unsatisfying mathematical answer is that it is impossible to have a
uniform distribution of rotational speeds, therefore there must be a preferred
one.

It's the same reason the universe has an average speed (unlike what you might
expect from special relativity), although it is unclear if this is true for
the _entire_ universe or just the portion we can see. We can measure how fast
we're moving w.r.t the cosmic microwave background radiation though (it is
red-/blue-shifted in a particular direction).

~~~
panic
This is an interesting argument! Wouldn't it also work for positions, though?
That is, either the universe is finite, or, since there can't be a uniform
distribution over an infinite space of positions, there must be some preferred
"center" of the universe?

~~~
contravariant
You'd think, but of course we know that not to be the case. It's hard to
pinpoint the exact reason though. Sure we know time and space are rather
special, but its hard to say exactly why.

In the end though I reckon the most obvious reason is that speed is a property
that directly corresponds to energy, therefore for each region of space to
have a well defined energy (which is required for e.g. general relativity)
every region of space needs to have a well defined distribution of speeds.

I suppose this does leave open a small loophole, as you can easily correlate
speed with position in order to get a distribution that is uniform in both
(but correlated). But this goes against our assumption that the universe is
uniform everywhere (which might turn out to be false, but so far it's holding
up well).

------
bunya017
Asynchronous programming

With the addition of async to django core, I felt its time to finally learn
the concept. I first took interest in async early last year when I re-read a
medium post on Japronto; an async python web framework that claims to be
faster than Go and Node.

Since then, I've been on the lookout for introductory posts about async but
all I see is snippets from the docs with little or no modifications and a lame
(or maybe I'm too dumb) attempt at explaining it.

I picked up multi threaded programming few weeks ago and I understand (correct
me if I'm wrong) it does have similarities with asynchronous programming, but
I just don't see where async fits in the puzzle.

~~~
pixelmonkey
The first couple of paragraphs of the documentation for asyncore, the module
in Python's standard library that implemented the machinery for async IO all
the way back in 2000, has a great description of what async programming is all
about. Here it is:

[https://python.readthedocs.io/en/latest/library/asyncore.htm...](https://python.readthedocs.io/en/latest/library/asyncore.html)

'There are only two ways to have a program on a single processor do “more than
one thing at a time.” Multi-threaded programming is the simplest and most
popular way to do it, but there is another very different technique, that lets
you have nearly all the advantages of multi-threading, without actually using
multiple threads. It’s really only practical if your program is largely I/O
bound. If your program is processor bound, then pre-emptive scheduled threads
are probably what you really need. Network servers are rarely processor bound,
however.'

'If your operating system supports the select() system call in its I/O library
(and nearly all do), then you can use it to juggle multiple communication
channels at once; doing other work while your I/O is taking place in the
“background.” ...'

------
henearkr
[Sorry I have misinterpreted the subject, I thought it could be some
phenomenon still not well understood by science.]

Placebo. There are biological bases of it (I don't believe in soul). Find
these bases, study them, make a model of them. Then use proxy variables to
measure it instead of trying to eliminate it statistically. Predict it in
studies to avoid the need of placebo groups (and possibly of double blind
methodology). Also, after it is completely measurable and its mechanisms are
understood, if (very hypothetical) it has a really substantial effect, just
use it to help treat patients.

------
sarthakjshetty
What actually happens to the money that gets put into the stock market? I
mean, I understand that there are people holding stocks and then people offer
to buy/sell them at a given price; but, how does the company plan to benefit
if the stock just keeps moving back and forth multiple third parties? Doesn't
a company go public to raise money? If so, how does the company benefit from
the daily changes in the stock and when someone buys stock in the company?
Does the company have a fixed number of stock that they trade as well on the
market?

~~~
drran
Company raises money from market by giving away part of the company.

If company shares are cheaper than other source of money, then company can buy
back them, or other potential owner can buy them. It also sends strong signal
to owners and investors about future of the company.

If company shares are costing more than other sources of money, then company
can sell more of them to return costly money back or to expand business.

For example, bank rent rate is 10% per year. Company worth is $1 million.
Company created 1 million of shares. 1 share is $1 of company worth. Company
pure profit is 10% per year. Company needs $1 million in circulation to
operate.

If company will rent money from bank, it will have 10%-10%=0% of profit. If
company will sell 100% of shares, it will have 10%-0%=10% of profit, but this
profit will go to someone else. If company will sell 50% of shares and will
rent $0.5 million from bank, it will have 10%-5%=5% of profit, 2.5% will go to
company, 2.5% will go to other owners. By reinvesting this profit, company can
reduce bank money or expand company to worth more, increasing company profit
share.

------
airstrike
The one-electron universe is always a personal favorite. Though more a far-
fetched theory than a proper "scientific phenomena", I'd be eager to learn
more about it in layman's terms

[https://en.wikipedia.org/wiki/One-
electron_universe](https://en.wikipedia.org/wiki/One-electron_universe)

[https://www.youtube.com/watch?v=9dqtW9MslFk](https://www.youtube.com/watch?v=9dqtW9MslFk)

------
vvoyer
vertical alignment in CSS

~~~
JJseiko
this made me laugh!

------
npr11
Automatic differentiation. It's useful to so much computational work, but most
people only get a cursory introduction to the topic (a rough intro to the
minimum they need to know), whereas really understanding it seems to open up a
lot of research.

~~~
OkayPhysicist
Oh man, I read a super cool article about that about a year ago. It provided
an algorithm for automatic differentiation using an imaginary number such that
it times itself equaled 0, but wasn't equal to zero itself. I'll try to find
the link.

I don't know if this was it, but an explanation nonetheless
[https://medium.com/@omaraflak/automatic-
differentiation-4d26...](https://medium.com/@omaraflak/automatic-
differentiation-4d26d03b7508)

------
yfiapo
I'm sure there are great explanations out there but I haven't had time to read
up on them, but a few of the space things that always bother me when I watch
pop-sci space tv:

\- "as soon as iron starts to be produced in the core of a star it instantly
collapses" \- I get that fusing iron costs energy rather than produces it and
this causes a collapse.. but can it really be that quick? There are other
fusion reactions that are still producing energy, right?

\- dark matter / energy - I understand we have observations that indicate
there is some type of matter we can't see but it feels a lot like saying
"magic" or "the ether".

\- how different size stars form - if there is a critical mass where a star
"ignites" and after igniting starts pushing away from itself with the energy
being produced, how do we get stars of such varying masses? Like, why didn't
this 100x solar mass star start fusing and pushing the gases away before they
were caught in its gravity? Do the more massive stars ignite on the same
schedule but continue to suck in additional matter anyway, gravity overcoming
the solar wind?

~~~
c1ccccc1
Not an expert, so take these with a grain of salt:

The core of the star is the hottest and most dense part. Greater heat and
density make it easier for fusion reactions to run. If suddenly the core is
made mostly of iron, then the amount of energy it produces rapidly drops. Even
if there are nice, easily fusible hydrogen atoms farther out from the core,
they will not be fusing at a very high rate, because the temperature and
pressure is lower where they are. Also, the more easily fusible atoms
remaining outside the core can't diffuse into the core fast enough to refuel
it. The only possible outcome is collapse.

In some sense "dark matter" and "dark energy" are just placeholder words for
"whatever thing is causing all this weird stuff to happen". This is actually
very analogous to how "the ether" was a placeholder term for "whatever thing
that radio waves are waves in". (Now we refer to it as "the electromagnetic
field". The "ether" terminology was associated with some incorrect
assumptions, such as a privileged reference frame, which is why people
sometimes say it was an incorrect hypothesis. But the electromagnetic field is
certainly real, it just didn't turn out to work like some people thought it
did.) Scientists have observed so far the dark matter seems to behave pretty
much like ordinary matter, except that it just happens to ignore the
electromagnetic and strong nuclear forces. Not only does it hold galaxies
together, but its gravity also bends the paths of light rays, just as we
expect of anything massive. So calling it "matter" isn't too much of a
stretch. It's still very mysterious, though.

Radiation pressure actually does limit the mass of stars, to something on the
order of 100 to 200 solar masses, see this stack exchange question:
[https://astronomy.stackexchange.com/questions/328/is-
there-a...](https://astronomy.stackexchange.com/questions/328/is-there-a-
theoretical-maximum-size-limit-for-a-star) That doesn't stop smaller clouds of
gas from collapsing to form smaller stars, though.

~~~
yfiapo
Thank you for even trying to answer my rambles! :-)

I think my contention with the iron is the tipping point and how quickly it
goes. Pop-sci tv makes it seem like you fused a single iron atom and bam.
Maybe it is you fused an iron atom and it is like a day, a year or a thousand
years and that adds up; still bam in terms of cosmic timelines but it is not
what I hear when I listen and hear "instant collapse".

Thank you for the thoughts on dark matter and energy as well, and the link on
radiation pressure, I will read it.

------
neilk
When I hear explanations like “space is expanding like the surface of a
balloon” it’s always confusing. Because a surface is an object, separate from
anything on it, but space is the thing we’re all embedded in, so we’re like
drawings on the balloon.

If space is expanding why aren’t the radii of fundamental particles and their
orbits and molecules also expanding? And if that were the case we couldn’t
notice space expanding.

~~~
knzhou
> If space is expanding why aren’t the radii of fundamental particles and
> their orbits and molecules also expanding? And if that we’re the case we
> couldn’t notice space expanding.

> Does space only expand somewhere else? Only between me and the Andromeda
> galaxy, and not _within_ me and the Andromeda galaxy? How would it know to
> do that?

If you start with expanding space in general relativity, and then carefully
take the limit where you get back to Newtonian gravity, then it just
corresponds to a classical force, specifically a very tiny force that weakly
pulls everything apart, growing with distance.

This doesn't expand small objects, because they're rigid. It's the same reason
that I can't make my laptop get bigger by gently pulling on the ends. On the
other hand, it would pull apart two independent, noninteracting objects (such
as the Milky Way and Andromeda).

~~~
cygx
On top of that, FLRW spacetime is a large-scale approximation: More realistic
models should probably follow the 'swiss-cheese' approach, where local
conditions can look rather different.

------
gkolli
Antenna Design/RF Theory. I would love a simple text/YouTube series to learn
this stuff...would definitely be good learning for the summer :)

~~~
kccqzy
I second this. I once worked with an assistant professor to do some of that,
and in the end a lot of what we did boiled down to trial and error: do a
design, run simulations in some software, and if it doesn't look good try
again. It makes me sad that I didn't really encounter anyone capable of
explaining things beyond trial-and-error.

------
abetusk
* Lie algebras and Lie groups - I still don't understand this, what they're used for or how to use them in any practical sense.

* Galois Theory - I have a basic understanding of abstract algebra but for some reason Galois theory confounds me, especially as it relates to the inability of radical solutions to fifth and higher degree polynomials

* "State-of-the-art" Quantum Entanglement experiments and their purported success in closing all loopholes

* Babai's proof on graph isomorphism being (almost/effectively) in P - specifically how it might relate to other areas of group actions etc.

* Low density parity checks and other algorithms for reaching the Shannon entropy limit for communication over noisy channels

* Hash functions and their success as one-way(ish)/trapdoor(ish) functions - is SHA-2 believed to be secure because a lot of people threw stuff at the wall to see what stuck or is there a theoretical backpinning that allows people to design these hashes with some degree of certainty that they are irreversible?

~~~
aesthesia
Probably the best place to get started with Lie groups is to wrap your head
around SO(n), which has both a nice geometric interpretation as rotations of
n-dimensional space as well as a concrete representation as the space of
orthogonal (determinant 1) nxn matrices. With a little matrix calculus you can
work out what the tangent directions are at the identity matrix: they’re
precisely the skew-symmetric matrices. This is the Lie algebra so(n). Where
the Lie group consists of rotations, the Lie algebra consists of directions in
which you can rotate something, or really velocities of rotation. This is why
classical angular momentum is actually an element of a Lie algebra.

~~~
bawana
I’m sorry but you used so many big words so close together, I could find no
purchase with the things I know. Perhaps you could tell me what was the
problem that Lie groups are supposed to solve. I read something about an
infinitely symmetric transformation- the rotation of a circle is a Lie group.
But then what are all the letters about- the A,B,C etc all the way up to E 8.
And why did it take 77 hours on the Sage supercomputer to solve E8.

------
jhylands
Light, a mixture of why a thing is the colour that it is? As well as why
reflection would be angle of incidence is angle of exit.

My current understanding of colour is that the colour of an object is defined
by the ability of the electrons in the compound jump different energy levels.
I don't know if that in itself is enough to result in all the colour we see.

My current understanding of reflection is that because of wavyness of light
when lots of light gets absorbed (to my understanding a single photon exciting
a single electron to jump some amount) and reemited (the electron falling back
down) together the light ends up forming that angle pattern. Under than
understanding single photons don't bounce in the same way rays of light do?

I don't know how correct either of those understandings are, but my
understanding has been put together from so many places and I've never heard
any source explain either like that so I don't trust they are correct.

~~~
rom16384
You are mixing a lot of different things:

* The reflection angle laws are due to the laws of conservation, see [https://en.wikipedia.org/wiki/Snell%27s_law](https://en.wikipedia.org/wiki/Snell%27s_law)

* For a pure colour, the colour is simply the energy of the photons. Atoms have discrete stable electron orbits, and electrons moving between these levels will absorb or emit discrete levels of energy in the form of photons, which is why we have spectral lines. Reality is more complicated because part of the energy may be converted to vibrations of the atom itself (phonons).

* Another factor is the perception of colour. In physics to characterize light one measures its spectra, the intensity of the light versus its wavelength (wavelength = speed of light in vacuum / frequency). The perception of colour of these distributions isn't always always what one would expect.

------
Rerarom
The connection between elementary particles and group representations. I get
the math, I just don't see in what way it corresponds to reality (i.e. in
addition to being a bijection).

------
ben_w
Wormhole time machines.

The idea is, you can transform a normal [0] wormhole that isn’t a time machine
into one which is by:

1) accelerating one end to high speed relative to the other

2) keeping on end in a lower gravitational potential than the other

Why are either of these considered meaningful statements, never mind
_correct_?

In the case of 2 in particular, isn’t GR supposed to require smooth values? So
any time dilation effect would be almost identical on a pair of points +δ and
-δ from the throat? Making it similar to the case of a gravitational potential
without a wormhole?

And in the case of 1, the more I think about it, the less I understand the
concept. What is being moved? An imaginary clock that would’ve been in the
part of the wormhole at the far end? The apparent speed as measured going
through the throat will be zero regardless of the apparent speed of the same
as measured when going the long way around.

[0] yes, I know

~~~
c1ccccc1
I'll answer 2. You are correct that clocks just to either side of the throat
will stay synchronized. But it's important to remember that time dilation
isn't some kind of absolute effect. The easiest way to talk about it is in
terms of clocks following paths.

Imagine you have two spaceships in London. At the fire of a starting gun, they
both take off, and fly around, each one taking a different path. Each ship has
a clock that records the time elapsed since it took off. Eventually, they both
return to the same landing pad in New York, landing at the same time. Thanks
to time dilation, the readings on the clocks of the two ships might be
different, even though they started and landed at the same places and times.
Imagine we start both ships from a space station in deep space. One ship
doesn't leave the station at all, it just stays in its docking bay, with its
clock ticking along. The other flies down to the surface of Earth, sits there
for a few years, and then flies back up to the space station. Thanks to the
gravitational field of Earth, the ship that stayed home has more time elapsed
on its clock than the ship that went to Earth.

Now suppose that each ship is carrying one end of a wormhole. The clocks on
either end of the wormhole must stay synchronized. Someone sitting in the
middle of the wormhole would be able to see the inside of one ship by looking
to their left, and the inside of other ship by looking to their right. The
clocks start out synchronized. As you point out, no matter how the ships move
about, this does not change as the ships fly around. Anyone standing in the
middle of the wormhole always sees that the clocks on the wall of each ship
are in sync.

So: Entering the wormhole from ship A when ship A's clock reads X means you
exit at ship B, at the time when ship B's clock reads X. And vice-versa for
going from B to A. Now thanks to time dilation, ship B might arrive back at
the station when its clock reads X, while ship A, which stayed behind, has a
clock reading of (X + 20 minutes). If you are the station master, you can go
into each ship to look at the clocks, and you will find that ship A's clock is
always 20 minutes ahead of ship B's. But suppose that instead of walking
between the ships through the station, you use the handy wormhole that
connects them directly.

Suppose you enter ship B when its clock reads Y, and walk through the
wormhole. You exit at ship A when its clock also reads Y. Then you step out of
ship A, and walk through the station to ship B. It's clock reads (Y - 20
minutes), since according to people on the station, ship A's clock is still 20
minutes ahead of ship B's. When you originally entered ship B, its clock read
Y. It now reads (Y - 20 minutes). Time travel. By retracing your path in the
opposite direction, you can also travel 20 minutes into the future.

~~~
ben_w
That helps, thanks.

It doesn’t fully solve my confusion, but I suspect I need to study more before
I can even ask the right next question. :)

------
ars
How can metric expansion of space redshift light? Doesn't that violate
conservation of momentum, since the redder light has lower momentum?

And adding on to that: Will light inside a box redshift? If I weigh the box
(i.e. weigh the light inside the box), then wait a bit for the light to
redshift, then weigh the box again?

~~~
antognini
You are correct! Cosmological redshift does violate conservation of momentum
(and energy as well [1]). But conservation of energy and momentum does not
actually apply if spacetime itself is globally changing.

The underlying reason for this is that Noether's theorem tells us that every
physical symmetry implies a conservation law for some physical quantity.
Conservation of energy and momentum comes from the fact that the physical laws
are the same throughout time and space. However, cosmological expansion
violates that assumption, so there is no reason that energy and momentum
should still be conserved. [2]

[1]: One side note here is that relativistically, energy and momentum are not
really separate physical quantities, but instead two components of the same
underlying physical quantity. Unfortuantely, this quantity does not really
have a good name (despite Taylor & Wheeler's attempt to call it "momenergy").
It ends up being called the momentum 4-vector, but the temporal component of
this 4-vector is energy.

[2]: This is only true _globally_. Locally, the laws are approximately the
same from one moment to the next, so conservation of energy and momentum hold
for small distances and short times.

------
Buttons840
Flight. How can a plane fly when it's thrust to weight ratio is less than one?
It's like, if you can produce 10 pounds of thrust, who would look at that and
say "ah ha, we can use this to keep a 100 pound machine miles in the air
indefinitely"?

I understand flight from a mathematical point of view. I've actually read a
few books on the subject, and I could explain how flight works to someone.
However, I'm still fishing for an explanation that "feels" more satisfying
though. Per the question, I still want it explained better.

EDIT: There's already a thread about flight. I asked the same question there,
but phrased a bit differently:
[https://news.ycombinator.com/item?id=22993460](https://news.ycombinator.com/item?id=22993460)

~~~
punnerud
The 10 pounds trust is used to overcome the airodynamic friction, when the
plane is in high speed. Because the "way" around the wing is longer on one
side compared to the other this create a lower pressure on one side, and
higher on the other. This pressure both lift and push the plane up. If there
was zero friction you would hold the plane up with zero thrust ;)

I build this at school, using the same principle:
[https://no.wikipedia.org/wiki/Sivilingeni%C3%B8r#/media/Fil:...](https://no.wikipedia.org/wiki/Sivilingeni%C3%B8r#/media/Fil:Wind_turbine_from_student_project_at_NTNU.jpg)

~~~
aeternum
The problem with this description is that you don't need a fancy shaped wing
to fly, a flat board will work. Aerofoils provide better efficiency but are
not required.

~~~
punnerud
Don’t agree, unless you only did read the first 1/4 of my comment?

~~~
aeternum
I did, but if I use an extremely thin wing with zero aerofoil (a wing made out
of credit-cards or flat balsa wood for example), one "way" around the wing is
not longer. Why would pressure be different on the top vs. bottom of this flat
wing?

------
redlock
I wish I could get a clear explanation of why faster than light travel breaks
causality. I have seen the math but intuitively I am having trouble grasping
it especially when you have only two reference frames (most explanations will
use three reference frames to show the violation)

~~~
OkayPhysicist
The simplest answer is that events that happen further apart in space than in
time can be observed in different orders depending on the frame of reference.
If an effect can happen before its cause, causality is violated.

The three-reference frame example is the easiest, because you can start with a
frame where two events, A and B, happen simulateously. A reference frame (say,
a spaceship), flying along a line in the A to B direction will observe A
happen, then B. A ship flying the opposite direction will experience the
opposite, B then A.

So whose observations were correct? All them are perfectly valid. The problem
is if we allow A to cause B, in which case the B then A frame has the effect
happen before the cause.

~~~
redlock
Isn’t cause and effect mediated by forces that travel at the speed of light?
Simultaneous events cannot be in a causal relationship, no?

------
throwphoton
Absorption spectra baffle me and I've never seen an explanation that helps me
understand.

It seems conceptually simple, except the requirement that the energy of an
photon _exactly match_ some required energy in order to be absorbed seems
really unlikely, since photon energy not a discrete quantity, and varies
according to doppler effects and other things.

It seems like the vast majority of photons would just fly through the universe
without interacting with anything, unless there are other ways for photons to
interact with matter besides being absorbed. (If there are other ways, they
are seemingly never mentioned as a potential alternative fate for the photon).

~~~
galimaufry
> It seems like the vast majority of photons would just fly through the
> universe without interacting with anything, unless there are other ways for
> photons to interact with matter besides being absorbed.

Maybe someone who actually knows will chime in, but afaik:

\- Most light doesn't have a fixed frequency. If it did, it would have a fixed
momentum, but then you would have no idea where it is! Instead it is some
superposition of many frequencies. That could be part of the story.

\- Light could be stopped by something other than absorption into an
individual atom. Metals don't have discrete spectra.

------
qqqqquinnnnn
Another frustrating one - what is heredity? If it's possible to inherit
something due to a shift in behavior (i.e. it's a cultural change that leads
to a biochemical change), how does that connect neatly to mendalian
inheritance?

~~~
epmaybe
Are you talking epigenetics? It's relatively closely related to mendelian
inheritance. At a high level view, if genes are present in your genome that
aren't "on" in a previous generation, but due to changes in behavior become
more active, then in a future generation that gene could be expressed more.
It's not that the genes have changed their code, they are just flipped on or
off.

~~~
qqqqquinnnnn
No, more things that are modifications without any genetic basis - like
myopia. Doesn't seem to have any genetic/epigenetic basis. It's purely a
physical use case, where the eye matures improperly if it's only exposed to
near-field work.

~~~
epmaybe
ah, I see what you're saying. Luckily, I've looked into this specific topic!

Myopia is definitely hereditary, especially the pathologic variants that can
lead to retinal tears and the like.

That being said, there is the process of refractive development that occurs
early on in life. The eye develops at a frighteningly fast pace, and you
achieve near-adult globe size after about 18 months. The genes that drive this
refractive development could be hereditary, if that is what you're trying to
figure out.

Now, we can make a claim that this adaptation during infancy could eventually
affect our genome, but I have not delved into the epigenetic literature to
determine if that has been borne out or not.

~~~
qqqqquinnnnn
90% saturation in a few generations suggests that it is something other than
purely hereditary mutations accumulating: [https://www.nature.com/news/the-
myopia-boom-1.17120](https://www.nature.com/news/the-myopia-boom-1.17120)

~~~
epmaybe
I know, I'm not saying that these are hereditary mutations. I'm saying that
genes in all of us are involved in refractive development, and thus _have_ to
be inherited if mutations exist. I'm saying that our environment of myopic
environments is feedback that forces our eyes to develop to support better
focus at those distances.

------
crazypython
Quantum Mechanics, and why we need interpretations of it.

~~~
qqqqquinnnnn
Like.... why are sub-atomic phenomena important in general?

~~~
Cheyana
To add to all this, the double-slit experiment. What exactly does it mean that
light moves as a wave or a particle?

~~~
Rury
>What exactly does it mean that light moves as a wave or a particle?

One idea is known as the Copenhagen interpretation.

It basically says that the wave-like effects we associate with matter is
merely a wave of probabilities. Or in terms of the double-slit experiment and
in other words, light behaves like a particle, but the wave-like effects you
see is just the result of probabilities where the particles end up. Dark areas
are areas of low probability, and lighter areas high probability.

One might imagine the light particles streaming through the slit end up having
slight variation in trajectory from one particle to another (for various
reasons such as interference with other particles), which results in areas
where most particles end up and others where few end up... representing a
wave.

------
phkahler
Wave function collapse. As far as I can tell there is no discernable
difference between a particle whose wave function collapsed (perhaps via
measurement with another entangled one) and one that hasn't.

~~~
rkagerer
This is super naive but I just think of it as you (or whatever other particle
is interacting with the one of interest) have now become part of its system.
Like a causal event cone reaching you. Nothing about the particle changed,
just your relationship with it (hence locking down the uncertain parameters).

I'm sure you or another physicist could point out the flaws in my mental
model.

------
unexaminedlife
Sort of trying to piggy-back on this thread in hopes someone with knowledge
will be able to share it.

I've been having conversations about viruses recently and in those
conversations / thought experiments I keep coming back to a point someone made
to me.

Someone this person knows, with extensive medical expertise, explained that
the "membrane" of the cell contains a ridiculously large number of unique
types of proteins.

Understanding, in vague terms, how viruses penetrate cells the question I pose
is "is this true because each of those proteins has a unique and distinct
function in the cell membrane? Or is it more a matter of scale and utility?"
In other words, does the observation simply indicate that our bodies are not
as perfect as we'd like to think they are and the body's process for creating
/ repairing cells is more a utilitarian function where the "rules" of cell
construction are extremely flexible such that these molecules are constructed
in various ways where our cells are using materials available to them at the
time?

If this is the case it starts to make a lot of sense to me at a molecular
level why certain people tend to be more susceptible to contracting certain
diseases. Could a lot of it really just come down to diet, along with probably
a hint (or more) of DNA's interaction with those proteins we're providing to
our bodies? And to what extent does each of those play a role? DNA and the
proteins.

------
scld
Time dilation with regards to the "no absolute rest frame" in physics.

The famous twin thought experiment where one gets in a spaceship, accelerates
away from the planet, turns around, and comes back.

The twin that stayed on earth is old and the traveling twin is young still.

On one hand, I know that time will "pass differently" for each twin....but why
is it the twin in the spaceship that ages less? Why isn't it true that the
entire universe accelerated away from the spaceship and then returned, leaving
the entire earth young?

~~~
strgcmc
You're in luck my friend, the Wikipedia article does a rather nice job of
describing just this "paradox":
[https://en.wikipedia.org/wiki/Twin_paradox](https://en.wikipedia.org/wiki/Twin_paradox)

> In physics, the twin paradox is a thought experiment in special relativity
> involving identical twins, one of whom makes a journey into space in a high-
> speed rocket and returns home to find that the twin who remained on Earth
> has aged more. This result appears puzzling because each twin sees the other
> twin as moving, and so, according to an incorrect[1][2] and naive[3][4]
> application of time dilation and the principle of relativity, each should
> paradoxically find the other to have aged less. However, this scenario can
> be resolved within the standard framework of special relativity: the
> travelling twin's trajectory involves two different inertial frames, one for
> the outbound journey and one for the inbound journey, and so there is no
> symmetry between the spacetime paths of the twins. Therefore, the twin
> paradox is not a paradox in the sense of a logical contradiction.

There's multiple explanations included to resolve the "paradox" from different
lines of argument; I particularly like this one:
[https://en.wikipedia.org/wiki/Twin_paradox#A_non_space-
time_...](https://en.wikipedia.org/wiki/Twin_paradox#A_non_space-
time_approach)

~~~
scld
Hahah 2 separate sources each for why I'm incorrect and naive. Thanks!

------
mihaaly
The rising cost and decreasing productivity of scientific research.

------
bhk
When two particles get closer, their mutual gravitational attraction
increases. As the distance approaches zero, the force approaches infinity. In
the limit of d -> 0, the _energy released_ -> infinity. Obviously at some
scale the notion of a point mass breaks down, but even quantum theory would be
problematic if we think of a wave function as describing a _probability_
distribution, wouldn't it? What's the "official" story on this?

~~~
abernard1
What follows is very hand-wavy, and the renormalization sibling post may touch
on it.

An answer is that the d->0 approaches infinity presumes a nice, continuous
analytic function. If d->epsilon, you can't get to that singularity.

There was an equivalent problem in the E/M space with "The Ultraviolet
Catastrophe" [1], which turned out to go away if you assumed quantization.

I'm not going to claim this is a perfect analog to the gravity problem, only
that a lot of physics doesn't quite work right when you assume continuity.
(The Dirac delta is a humorous exception that proves the rule here, in that
doing the mathematically weird thing actually is closer to how physics works,
and it required "distribution theory" as a discipline to prove it sound.)

[1]
[https://en.wikipedia.org/wiki/Ultraviolet_catastrophe](https://en.wikipedia.org/wiki/Ultraviolet_catastrophe)

~~~
knzhou
This is definitely related, though not the whole story. Quantization did get
rid of some infinities, but as GP kind of states, it also introduced others.
My comment focuses on what we do for those.

~~~
abernard1
> In the limit of d -> 0, the energy released -> infinity.

I believe the poster's general premise to be false. While renormalization may
be useful in resolving infinities in general, I don't think it's necessary for
this one.

You can't commute the dp*dx of a Hamiltonian to be zero in a quantized world,
so if gravity has quantum properties, you don't need to worry about what
happens when d -> 0\. There is no "0" distance.

~~~
knzhou
I guess, but it depends on how one parses the poster's setup. You're correct
that for particles interacting under a 1/r^2 force, the energy turns out
finite in quantum mechanics. My comment was referring to the fact that once
you quantize the field that gives rise to that force, the infinities return,
but for a different reason.

------
ncmncm
I like that for a full decade, people discussed measurements at reputable labs
indicating that certain radioactive species decayed at rates that varied >0.1%
depending on the season, and explored possible neutrino flux influences.

The measurements were finally shown to be effects of the immediate environment
on the measurement apparatus.

That detectors used in labs may vary with time by >0.1%, unknown to their
users, seems pretty important. How did everybody involved not know?

~~~
MichailP
Because people live in a busy world, where knowledge is not transferred with
enough love and integrity. And also people are afraid to say "I don't know"
and what little they know they tend not to share.

To make things more specific, those labs had uncertainty budget with something
like 20 terms for the things they measured. Each of those terms had associated
probability distribution etc. They had uncertainty budgets for all the methods
they did etc., and some of those where probably dated, done by someone else,
etc. etc. Who checks that? Is the check rigorous enough? Are some assumptions
made that don't hold to scrutiny?

So it is actually very easy for error to creep in, I would say actually very
likely.

------
Waterluvian
Why does my bread last for a while and then go bad? Or is it constantly going
bad and I just haven't hit critical mass of mold for me to notice?

~~~
Balgair
Correct. Mold will grow where there is food for it to grow, like bread. It
will divide and grow exponentially, up to a point. That fact of exponents
where they seem slow at first and then really rocket away? That's why mold
seems to just appear overnight.

~~~
Waterluvian
So it might be a fair statement to say that when I eat fresh bread there's
already a molecular amount of mold already on it?

~~~
carapace
There's a molecular amount of mold already on _everything._ Mold spores are
ubiquitous.

------
inopinatus
Why bicycles stay upright.

For every authoritative-sounding, in-depth explanation, there is an equally
plausible, yet conflicting and contradictory alternative.

~~~
CamperBob2
Two conditions are necessary for a bicycle to remain upright (at least with
most riders.)

One is gyroscopic forces. Ever picked up a spinning hard drive? Notice that it
feels strangely hard to turn in some directions? Same idea.

The other is the feedback loop consisting of the bicycle and its rider.

If the bike is stationary, it's hard to keep it upright because you have no
assistance from gyroscopic forces. At low speeds, you have some assistance but
not enough. At higher speeds, the bike wants to maintain its current
orientation, and it's easy to feed in the slight corrective forces needed to
keep it that way. Hop off the bike and it will keep going until something
causes it to veer off course.

You can throw a ton of math at it, as in the paper mentioned elsewhere in the
thread, but at the end of the day, gyroscopic forces and negative feedback are
all that's necessary. The Schwab paper appears to show that the gyroscopic
forces aren't necessary, but no bicycle in the real world is ever going to
work that way except in rare corner cases, e.g., if you're one of those riders
who can stay upright at a standstill.

~~~
inopinatus
That is indeed one of the authoritative explanations, and in a few seconds of
ad-hoc search we may find at least two refutations from authoritative sources.

For example, gyroscopic-forces-as-stabiliser don't need the Schwab paper's
"ton of math" to be undermined. A simple counter-rotating wheel was used
empirically at Cambridge to show as much, alongside notes that gyroscopic
forces _are_ relevant to the dynamics of a loaded bicycle, but misconstrued;
far from assisting to hold it upright whilst ridden, they induce _instability_
at the beginning of a change in direction, and more so at speed, a phenomenon
(counter-steering) familiar to cyclists and relied upon by motorcyclists.

Then there's a simpler observation that can be made: people have to learn to
ride a bicycle. The fact it stays upright when rolling unloaded, but not when
loaded, is indicative of how _small_ the gyroscopic effect is, not how
significant it is. Ergo, that argument would suggest, it is tiny shifts body
position that contribute all of the stability.

And then others, proposing further explanations, etc etc ad nauseum.

I have come around to the view that in fact they don't stay upright, and they
are almost always falling over, but in a many-branched configuration of the
universe our observer effect sends us preferentially down the vanishingly
unlikely path where they didn't, and there are an uncountably many
alternatives of Me that have nothing but knee scars to show for it.

~~~
CamperBob2
Sorry, I don't buy anything beyond what I stated. Those papers are exercises
in violating Occam's Razor. Your quantum model makes more sense. :)

When you learn to ride a bike, you're simply training the feedback mechanism.
Much like a PID controller, your brain has to keep track of the amount of
error and null it out with proportional and integral terms (at least). Once
those constants are dialed in, it's "just like riding a bike" \-- they're
yours for life.

Then there's the matter of learning which way to lean so that the gyroscopic
instability inherent in turning doesn't send you into the nearest ditch...

~~~
inopinatus
Occam’s razor is for hypotheticals. You’re rejecting an actual physical
demonstration, by a reader of engineering dynamics and vibration at Cambridge.

The feedback mechanism you’ve described is likely correct, and also a complete
furphy, since the central nervous system is not part of the bicycle.

All of which is par for the course and rather confirms the point, viz. that
people will happily hold forth on any explanation they care to latch on to,
secure in the knowledge that the total absence of consensus makes it
impossible to say, definitively, “that is wrong”

~~~
itcrowd
Great discussion, I agree with inopinatus who, if I am not mistaken, supports
my position that the science is not quite out on this topic yet. Many
explanations exist, none seem to be all-encompassing.

That being said, it doesn't answer the question posed in this forum: "What
scientific phenomenon do you wish someone would explain better?"

Instead, it answers the question: "What scientific phenomenon do you wish
someone could explain?"

~~~
CamperBob2
I guess my question on this specific topic would be, "What aspects of bicycle
operation are _not_ adequately explained by gyroscopic action under the
control of a human rider?" That would give us a good basis for inquiry into
other possible principles.

Sure, a bike could work some other way, but my point is, it doesn't need to.
Anyone who has ever picked up a hard drive should understand how a bicycle
remains upright. What else is there to know? It's not like an airplane wing,
where the "obvious" conventional wisdom is inadequate, misleading, or
incomplete.

~~~
inopinatus
Well, apparently it is.

------
pjungwir
Entropy. Sometimes you read that it's a measure of randomness; sometimes,
information. Aren't randomness and information opposites?

~~~
itcrowd
Let me give an analogy and then a solution to your paradox.

Temperature. Sometimes you read that it's a measure of warmth; sometimes cold.
Aren't hot and cold opposites?

Yes, hot and cold are opposites, but in a way they give the same kind of
information. That's also true for information and randomness. Specifically,
little randomness means more (certain) information.

------
asfarley
Why are clouds flat on the bottom?

~~~
lllr_finger
Because that's (roughly) the Lifted Condensation Level:
[https://en.wikipedia.org/wiki/Lifted_condensation_level](https://en.wikipedia.org/wiki/Lifted_condensation_level)

Water vapor around the LCL starts condensing and turning from a gas into
liquid cloud droplets. This process happens considerably faster once it begins
for a variety of reasons, so once you can have cloud droplets, you get a ton
of cloud droplets - not a gradual transition from water vapor to cloud. It's
almost like a light switch.

Most air masses are relatively homogenous anyways, so unless there are
underlying processes causing things like undulatus asperatus, it will
certainly appear very, very flat over a large area.

------
pgt
Gravity wells. I only realised in my 20s that the only reason satellites can
orbit the Earth without crashing into the ground is by going sideways really,
really fast. So as they inch closer to the ground, they also travel parallel
to the ground fast enough so that they stay approximately the same height from
the ground.

~~~
freeone3000
This is Newton's Cannonball. Honestly, I've found the best way to learn more
about orbital mechanics is with a simulator - Kerbal Space Program is a fun
version.

~~~
DavidSJ
What other subjects would simulation enlighten people on?

~~~
CamperBob2
Turing equivalence comes to mind.

------
CJefferson
Human Biology, particularly the "bigger scale" things like joints & ligaments.

I have arthrtic knees, and I'd like a better understanding of how joints work,
and where the various clicks, pops and swellings come from.

It's easy to find really simple things, but harder to understand "how things
go wrong".

~~~
petargyurov
This won't help with everything you asked for but there is neat piece of
software called Complete Anatomy that helped me with this sort of thing.

It renders an interactive model of the human body and you can toggle different
layers, from bones to nerves, to various layers of muscles and ligaments. It
also contains animations of treatment exercises/stretches, surgeries and
highly detailed models of various biological components.

It helped me understand my injury and why certain exercises help. It's paid
software but it comes with a free trial.

------
CamperBob2
I've always had trouble following Searle's Chinese Room argument as it applies
to the nature and identification of intelligent action. I've never understood
how the Chinese Room shows what its adherents say it shows, so that would be
one topic I'd like to see more perspectives on.

~~~
ShamelessC
"Searle understands nothing of Chinese, and yet, by following the program for
manipulating symbols and numerals just as a computer does, he sends
appropriate strings of Chinese characters back out under the door, and this
leads those outside to mistakenly suppose there is a Chinese speaker in the
room. "

I have trouble with this too. I think it's actually incorrect, or at least
misleading. I think what it's _trying_ to say is that even if an entity can
perform a complex task doesn't mean it can understand a complex task.

I think the more important result of this argument is that certain complex
tasks can be "pre-baked" into rulesets _by an existing intelligence_. To me
this just means that intelligent entities can sort of copy parts of their
intelligence into other entities which are not intelligent i.e. computer
programming.

I think with this argument they're trying to say "a series of sufficiently
complex if statements isn't necessarily intelligent" by choosing something we
know computers are good at - string manipulations and applying it to something
we consider intelligence - language translation.

The argument holds that the computer is obviously not intelligent because it's
just a function that takes a character and outputs another character.

But it needs to be a convincing translation, right? The computer would then be
able to spit out not just accurate translations but also properly converted
cultural idioms and new combinations of words where one didn't exist in the
other language. That requires context of surrounding characters, memory of
common language use, statistical analysis and creativity.

One implication that arises from this argument is actually about humans. How
do we know that we aren't all just incredibly detailed rulesets ourselves
without any actual understanding?

Well, first off - we technically can't prove it for anyone other than
ourselves. More pragmatically, it's obvious that we, unlike the computer
translator, can probe ourselves and be probed by others on whether or not we
understand the subject. It's not like we're a bunch of Boltzmann's Brains that
just happened into existence. We evolved intelligence in order to survive, not
to "trick" other intelligent beings into thinking we're more intelligent than
we are. There's no need for that. There's no one smarter around that we need
to "trick".

------
jbanfill
I have wondered if heat and photons impart a partial charge on atoms and
molecules which causes several phenomena. Faster Brownian motion due to the
increased repelling action of stronger charges which cause pressure/volume
changes in gasses.

Also are these charges responsible for some weather effects such as the jet
stream. In a tornado is the negative charges on the dry side of the dry line
interacting with the moist air on the wet side really just a local intense
acceleration of the dry air trying to "get to" the oppositely charged moist
air?

Are the rotation of low and high pressure systems basically due to the same
condition? Is lightning also just basically a flood situation of the charges?

~~~
OkayPhysicist
Charge can be pretty easily shown to be a conserved, quantized value. So
chargeless particals imaprting a partial charge would be a big no-no.

~~~
jbanfill
I didn't mean it was equivalent to the charge of an electron. What I was
wondering is how do heat and light impart more kinetic energy to physical
objects? Light may be the quantity of a photon but heat seems to be more of an
analog property. And what are some ramifications of these energized particles
in small spaces like enclosed gasses or large unenclosed spaces with effects
like the weather.

------
plurinshael
Spin aka intrinsic angular momentum

~~~
OkayPhysicist
Basocally, we can think of momentum in two parts: the momentum of an object
orbitting another, and the momentum of the object spinning on an axis.

At the subatomic level, we observe that electrons have some extra angular
momentum, beyond what we'd expect from their "orbits". We call that spin,
because it's intrinsic, like the spinning of a macroscale object.

------
somezero
What's the point of defining summation methods for divergent series? Do they
have any connections to any other area of mathematics or physics? Is analytic
continuation of complex analysis relevant to these things? How about p-adics?

~~~
abetusk
This is not an answer as this is something I've only just started wondering
myself, but, if I understand correctly, perturbation theory [1] uses divergent
series (and _wants_ divergent series in certain conditions over convergent
series(?)) in it's methodology.

I've started but haven't finished the physics lectures by Card Bender on
mathematical physics, where he features perturbation theory prominently [2].

If someone could chime in on this, I would also be appreciative. Also if
someone has better resources to learn about perturbation theory, I would also
be appreciative.

[1]
[https://en.wikipedia.org/wiki/Perturbation_theory](https://en.wikipedia.org/wiki/Perturbation_theory)

[2] [https://youtu.be/LYNOGk3ZjFM](https://youtu.be/LYNOGk3ZjFM)

------
spodek
Consciousness.

How is it possible that the thread is up 5 hours and ctrl-f consciousness
returns nothing?

~~~
JabavuAdams
Because no one knows what it is or even agrees on a definition. Consciousness
is a pre-scientific concept at this point.

There are attempts to rigorously define it. I'm currently reading this paper,
but not really convinced:
[https://journals.plos.org/ploscompbiol/article?id=10.1371/jo...](https://journals.plos.org/ploscompbiol/article?id=10.1371/journal.pcbi.1003588)

------
yamrzou
Evolution.

How does large scale randomness result in such complicated and intelligent
systems, while after decades of research and all the computing power we have
today, we still struggle to model and reproduce the intelligence of an insect.

~~~
dreamcompiler
"The Selfish Gene" was the book that finally made this click for me.

~~~
yamrzou
Unfortunately, The Selfish Gene leaves it an open question of how the brain
developed its capacity to represent and model the world, which is the very
notion of intelligence.

From the book:

> The evolution of the capacity to simulate seems to have culminated in
> subjective consciousness. Why this should have happened is, to me, the most
> profound mystery facing modern biology. There is no reason to suppose that
> electronic computers are conscious when they simulate, although we have to
> admit that in the future they may become so. Perhaps consciousness arises
> when the brain's simulation of the world becomes so complete that it must
> include a model of itself.

------
8bitsrule
Low, medium, and high-frequency radio waves (below 30MHz, wavelengths longer
than 10 meters) are a form of EM radiation. Which means that an antenna is
'transmitting' low-energy, long-wavelength photons. (Which gets us into the
whole wavicle question.) But the signal from a radio station appears to be
continuous.

So: how to picture this? Is the signal made of discrete 'photons' overlapping,
or combined somehow? Or is it that the 'wave-like' aspect of these photons is
so predominant at these frequencies? (I've grappled with this one for a long
time.)

~~~
vlasev
A radio station in the 1MHz range is transmitting in the 10s of kW range. The
energy of a single photon is tremendously small. So if you divide out the
large transmitting power by the energy of a single photon, you'll see that an
unimaginable number of photons is being released, giving the impression of a
continuous process.

More detail:

Say you have an AM station that transmits at frequency f = 650kHz and uses
power P = 50kW.

The equation for a photon's energy is E = hf, where E is the energy, h is
Planck's constant and f is the frequency. Here h = 6.63 _10^(-34) Js and f =
6.5_ 10^5 Hz. Thus the photon's energy is E = 4.31 _10^(-28) J. This is very
tiny number.

The number of photons per second is n = P/E = 1.16_10^32.

Let's try to visualize this. Avogadro's number is 6.022*10^23 for each mol of
something, so if we divide it out from n, we see that there are almost 200
million mol of photons being released every second!

Water is 18g/mol, which takes up about 16 cm^3. 200 million moles of water is
about a million gallons. If a photon was like a water molecule, a "water AM
station" would be releasing about a million gallons of water per second.

~~~
8bitsrule
Thanks, Vlasev, helpful ... easy to scale down to my hometown's measly 2,000
gallon AM station. I guess I'll need to go back to the radio physics math and
puzzle it out more (directional arrays, groundwaves, and the like.) The idea
of a photon with such a miniscule energy yet a wavelength of a thousand meters
is a tad non-intuitive to me.

------
VygmraMGVl
I had a very mathy explanation of Spinodal Decomposition in my graduate work.
I wonder if there's a more intuitive explanation than just "that's how the
energy landscape works".

~~~
elcritch
Hmmm, you might look up how dendrites form. They’re roughly analogous in how
the thermodynamics are favorable to forming more complex structures, IIRC. But
it’s easier/more intuit to see how dendrites form. Dendrite formation is also
a huge problem in many fields, like electronics manufacturing (eg tin
whiskers).

------
GnarfGnarf
Quantum entanglement. How do they know it's happening?

~~~
monktastic1
Have you studied Bell's Inequalities at all? There are various YT videos:

[https://www.youtube.com/watch?v=zcqZHYo7ONs](https://www.youtube.com/watch?v=zcqZHYo7ONs)

[https://www.youtube.com/watch?v=f72whGQ31Wg](https://www.youtube.com/watch?v=f72whGQ31Wg)

[https://www.youtube.com/watch?v=qd-
tKr0LJTM](https://www.youtube.com/watch?v=qd-tKr0LJTM)

------
enriquto
The periodic table.

Can the properties of the elements be computed from the first principles of
particle physics, or do you need to observe the atoms in real life to figure
them out? For example, some isotopes are stable and others have a finite half-
life. Can you know beforehand or you have to observe the decay? Can you
compute exactly the mass of each atom without measuring it? Can you know
compute its electronegativity? Etc.

------
partyboat1586
Basic probability. How come when I flip a coin it is just as likely to come up
heads or tails even if it has just been heads 5 times in a row?

I understand the physical properties of the coin make it so it is an
independent event but if I were to run the experiment multiple times the
number of times it would be heads after 5 heads would not be an even
probability, it would be unlikely since 6 heads in a row is a rare event.

~~~
Rerarom
Yeah, but 5 heads followed by 1 tail is also a rare event. Basically, after
drawing 5 heads you have a choice between two rare events which are equally
probable. The same after drawing anything else, not just 5 heads.

------
elgfare
The mathematics of training a neural network. I understand how they work once
trained, but that you can train them almost seems too good to be true.

~~~
supernova87a
Also, for me, how all these advances in ML/computing are alleged to be on the
horizon, when I hear that A/D/C(NN)s are actually so slow in learning. How can
something that has to be trained with 1M of <xyz> be smart? What's the next
thing?

~~~
OkayPhysicist
You and I probably took 4 or 5 years to recognize the alphabet. Cut the
machines some slack.

------
billfruit
Lagrangian and Hamiltonian mechanics. That is usually presented in a very
mysterious manner. Also I think most engineers do not get taught these.

------
wsieroci
I didn't see good explanation of Einstein's twin paradox. Have you seen one? I
have seen only clear explanation based on fact that observer will feel
acceleration when he will come back and this is what breaks symmetry, but it
appears that explanation based on acceleration are wrong, because it seems
that acceleration is not necessary to explain this paradox.

------
tjpnz
Double slit experiment. There's a lot out there detailing what happens but
relatively few explanations go into _why_ observing the state of a photon
results in it "choosing" one slit over the other. I suspect many come away
with the impression that it's magic despite there being a reasonably simple
explanation for what's going on.

~~~
aryc19
This video explains the double slit experiment using probabilities and it's
the only one that made sense to me without bringing in quantum mystery:
[https://www.youtube.com/watch?v=iyN27R7UDnI&feature=share](https://www.youtube.com/watch?v=iyN27R7UDnI&feature=share)

It also goes on to explain the delayed choice quantum eraser experiment but I
don't think that's quite convincing.

------
jccalhoun
Electricity. What is it? Is it related to electrons? I thought it was? If the
plug in my house is AC then why is one wire "hot" and one "neutral?" (on plugs
in North America at least) if there is a ground wire then why are plugs
polarized? Why do some things come with a three prong plug and others don't?

------
egberts1
Plasma ball, and how to create one. I’ve seen two of those with my own eyes:
One in nature after a lightening strike to a lone maple tree in a pasture and
one accidentally man-made via 50,000 VDC. And none of the literature shows how
how such a plasma ball can travel in a fairly straight line at steady level
from the ground.

------
yigithan
Morphological development of organisms. Mainly, how do cells form the shape of
the organism and how do they specialize to different type of tissues? How do
cells know where they are? What is the mechanism that puts the constraints on
the shape of the body? I can understand mitosis but the rest from there is
magic.

------
dandare
The cosmic inflation. Yes, the universe is smooth but how is that enough to
justify this unimaginable expansion? There seem to be no proof or possible
experiment yet the scientific community apparently accepted this theory with
its abrupt start and abrupt end, only secondary to the big bang itself in its
scale and energy.

------
mech422
BlackHoles - where does the 'extra' mass/gravity come from?

I've been watching a lot of documentaries lately, and I can't figure out how a
star that _radiates_ light, collapses and suddenly light can't escape? Doesn't
that mean the blackhole has more mass/gravity then the star that created it?

~~~
emblaegh
What teachers tell you in school is that as you get close to a uniform sphere
the gravitational force increases with the square of the distance to the
center of such sphere.

What they don't tell is that once inside the sphere the force decreases
linearly, because the planetary mass ahead of you is partially balanced by the
mass behind you.

With this you can see that the gravitational pull of a planet/start/spheroid
is largest at its surface. So, if something happens to make a start shrink by
some factor, the gravity pull at its surface is increased by the square of
this factor, even if the mass of the star remains the same.

Actually I believe starts eject a lot of mass when they become black holes,
and this is just a Newtonian argument for a phenomenon intrinsically
relativistic, but I hope you get the gist of it.

~~~
mech422
Yeah - a lot of mass gets 'blown away' which really made it even stranger that
gravity increased. While I found your explanation illuminating, I still don't
see how you can end up with 'more' gravity then you started with? Mass
determines gravity right ? not volume ?

------
dandare
How a single hormone can regulate so many different things. Looking at
Wikipedia, a single hormone like progesterone which is apparently one molecule
regulates a plethora of effects. How is this implemented? As a coder, if I had
to use one variable to simultaneously regulate 10 different things I would go
crazy.

------
bjourne
Fermat's theorem: a^n + b^n = c^n For n > 2, why are there no integers a, b,
and c that satisfy the equality?

~~~
ken
One of my advisors in college said "The proof takes about 10 years of graduate
mathematics to understand".

~~~
wesammikhail
Is there really no intuitive way to communicate the answer to that question
without needing 10 years of grad math? I find that to be somewhat hard to
believe

~~~
itcrowd
No, there is no intuitive way to communicate it. The theorem has taken >350
years to prove, which makes it clear that the proof is not some intuition that
was somehow missed by hundreds of people for centuries.

Fermat's Last Theorem (book) by Simon Singh is the source to check out if
you're interested in the details of how it eluded mathematicians and a general
idea of how the problem was solved, without getting too technical. It's a
great story well told.

~~~
wesammikhail
Thanks for the recommendation good sir!

------
shadowprofile77
the fundamentals of computer science. Unlike many who comment here regularly,
I am not a programmer or developer and though it might seem silly, the way in
which a bunch of code written in a programming language of any sort translates
to a computer physically or electrically causing things to "happen" because of
it has always been a bit fuzzy. I know that electronic systems and machines
translate all instructions to binary code but still, from there how does the
rest happen? such as an OS on my laptops working the way it does, or more
specialized: an unmanned spacecraft autonomously navigating its way through
the solar system and doing complex physical tasks. Anyone have a suggestion
for a good starting point on learning through these fundamentals and on
upwards?

~~~
grisha
“Code: The Hidden Language of Computer Hardware and Software” by Charles
Petzold should be what you are looking for. Author describes computing from
the very ground up, and in clean, approachable manner.
[https://en.wikipedia.org/wiki/Code:_The_Hidden_Language_of_C...](https://en.wikipedia.org/wiki/Code:_The_Hidden_Language_of_Computer_Hardware_and_Software)

------
autonomousErwin
Protein Folding

~~~
bobosha
Isn't protein folding considered one of the biggest problems ever?

~~~
qqqqquinnnnn
sure is.. but somehow biology manages it with few issues. It seems like it has
a lot to do with the presence of chaperone proteins that babysit the protein
as it's coming off the ribosome, preventing pieces of it from sticking
together that shouldn't touch, etc.

[https://en.wikipedia.org/wiki/Protein#/media/File:Chaperonin...](https://en.wikipedia.org/wiki/Protein#/media/File:Chaperonin_1AON.png)

I'm working on making a model of this chaperone complex relative to a folded
protein to get a sense of how it might be interacting with the amino acid
chain before it becomes globular

------
LVB
Genetics. I still have no intuition for how combining my wife’s and my DNA has
resulted in children with traits from both of us. My brain always tries to
imagine interleaving two binaries and hoping for a resulting program that
works a bit like the two sources, which it of course wouldn’t.

~~~
vikramkr
Don't think if it as a monolith. It's more like thousands of microservices and
thousands of different binaries all interacting in a whole bunch of different
and convoluted ways. Sexual reproduction is jumbling together two different
people's thousands of microservices and seeing what happens. Or, each program
is relying one library, imagine just randomly mixing around different
libraries. Genes and proteins are discrete parts that can be swapped out and
in.

------
andrewflnr
Hawking Radiation. I know just enough to know that the story of "anti-particle
of virtual pairs happens to always fall in the hole" is a Lie Told To
Children, but the explanations seem to go straight from there to rather dense
math and I've never wrapped my head around it.

~~~
guerrilla
I hope this helps:
[https://www.youtube.com/watch?v=qPKj0YnKANw](https://www.youtube.com/watch?v=qPKj0YnKANw)

~~~
andrewflnr
Actually... yes, which just makes it worse that something about that
presenter's style really rubs me the wrong way. Do they have their content in
text format somewhere? Anyway, thanks!

~~~
guerrilla
I'm sure somone's made transcripts but I have't come across them yet

------
rsync
Water pressure (like PSI) and how it relates to water flow (like GPM).

It's difficult to relate the two together and even after hearing every
heuristic and every cutesy analogy, I still can't quite wrap my head around
what happens to one when I increase the other (and so on).

------
dennis_jeeves
Interesting that you mentioned exosome, and no one else did. That in itself
should give a hint on why people cannot do a better job at explaining stuff.
It's complicated, , not well understood, extremely messy and does not fit well
in a textbook.

------
superbaconman
I'd love to see shows like 'Nature' and 'Planet Earth' but focused exclusively
on single celled organisms; I find that whole world very interesting. Journey
to the Microcosmos on youtube is the closest thing I've found.

~~~
willhoyle
Cells at work!
[https://en.m.wikipedia.org/wiki/Cells_at_Work](https://en.m.wikipedia.org/wiki/Cells_at_Work)!

It’s an animated series that takes place inside the human body. I’ve been
meaning to watch it myself. It’s supposed to be pretty accurate.

------
BiteCode_dev
Relativity.

I just can't grok it.

I can't understand how time would flow differently depending of your speed.

I don't get why C is a constant no matter the referential, for any other
object the speed is relative to your referential. I just don't see how those 2
are compatible.

------
dreamcompiler
Frame dragging. How does space know a body is rotating if the body is
"smooth"?

~~~
ffhhj
"Know" in which way, what kind of interaction? What about magnetic field due
to poles rotation?

------
adipasquale
The Coandă effect [1] and how it applies to plane wings and sailboats.

[1]
[https://en.wikipedia.org/wiki/Coand%C4%83_effect](https://en.wikipedia.org/wiki/Coand%C4%83_effect)

------
ur-whale
Why does quantum mechanics need complex numbers ?

Every time I read an introductory QM book / article, the complex numbers just
come out of nowhere and no one bothers to explain how that makes any kind of
physical sense.

~~~
OkayPhysicist
Imaginary numbers, pretty much everywhere in physics, represent an extra
dimension. e^ix results in a unit vector spinning around.

~~~
ur-whale
Thanks for your answer, but that doesn't really cut it for me. For example, if
complex numbers are so "natural" at modeling rotations, why don't they show up
e.g. in Maxwell's equations ?

------
0xfaded
The unification of electo-magnetism. I know the classical equation well, but
never formally studied physics and have never been able to get my head around
Maxwell's equations.

------
stevebmark
The LIGO detector. I've never heard a logical explanation for it. If your
explanation is that gravitational waves stretch and squash spacetime so light
takes different amount of time to bounce from an emitter back to a measurer,
then you don't have even a slight understanding of how it works. If your
explanation doesn't involve higher spatial dimensions (not time) then you
don't understand it. If you haven't even _considered_ higher spatial
dimensions when explaining LIGO then you shouldn't even try for the incorrect
explanation above because you don't have any of the pieces of the puzzle.

~~~
knzhou
> If your explanation is that gravitational waves stretch and squash spacetime
> so light takes different amount of time to bounce from an emitter back to a
> measurer, then you don't have even a slight understanding of how it works.

Well, I'll bite. I'm a physicist and I understand LIGO. What's your
alternative explanation?

~~~
stevebmark
I have no idea how it works, I don't have an explanation, I just know the one
spouted by people doesn't make sense

~~~
knzhou
Well, if you're uncomfortable with LIGO, you're in good company. Once a whole
decade went by in physics where people couldn't agree if it would work in
principle or not. And it is true that a lot of common explanations are bad
(e.g. "it's just a ruler" is not complete by itself because "why doesn't the
light get stretched too?"). Nonetheless, today we have a variety of
independent explanations.

Maybe you'll find this paper helpful:
[https://aapt.scitation.org/doi/10.1119/1.18578](https://aapt.scitation.org/doi/10.1119/1.18578)

~~~
rrss
Would you say it's accurate to say LIGO is a big interferometer measuring
changes in lengths of the arms due to gravitational waves, with some extra
analysis to figure why it still works even though the light waves get
stretched? Or is OP correct that such an explanation means I "don't have even
a slight understanding of how it works" and a correct explanation must
"involve higher spatial dimensions (not time)?"

~~~
knzhou
I'd say that's a perfectly fine explanation. And in fact the light _does_ get
stretched. For sufficiently low frequencies, that's fine because there's
enough time for _new_ (unstretched) light to enter and exit the apparatus. At
higher frequencies this causes the sensitivity of LIGO to fall.

Part of the popular confusion around how LIGO works is the freedom in
coordinates: there are different, perfectly good definitions of space and time
you can use, and the explanation sounds different in each one. So people can
get them mixed up. For example, my previous paragraph makes sense in
"transverse traceless gauge", but not in others.

I'm not sure what GP was referring to with "higher spatial dimensions".

------
JanisL
I'd really love an explanation of what specific impulse is. I've looked it up
a few times but the units seconds^-1 confuses me, what does this represent?

~~~
TheFlyingFish
I think the unit is just one of those weird dimensional equivalences that pop
up from time to time. E.g. fuel efficiency in cars is measured as distance /
volume (of fuel consumed), so it's dimensionally equivalent to area^-2. But we
don't use this because "1 mile per gallon" makes a lot more sense than "42.5
cm^-2".

------
nojvek
Quantum computers. I still don’t get it. What are they useful for, what are
it’s computing bounds and why it’s a big deal. When will the future actually
come ?

------
est
How to design compact antenna without those math voodoo.

------
pvaldes
We aren't short of interesting problems.

To pick a couple fo them randomly, understanding amyotrophic lateral sclerosis
or alzheimer would be terrific starts.

------
kf6nux
Superconductive flux pinning. Specifically, I'm curious what behavior it'd
exhibit if used as a core for superconducting inductors.

------
hartator
Quantum Physics. Still can’t wrap my mind around multiverse being a simpler
explanation then the future holds data we won’t have in the now.

~~~
ivan_ah
If you remember your linear algebra basics, you could easily pick up QM ---
the parts that deal with finite size systems. Here is an excerpt from my book
I posted a while back in another thread, and refreshed today since you asked:
[https://minireference.com/static/excerpts/noBSLA_quantum_cha...](https://minireference.com/static/excerpts/noBSLA_quantum_chapter.pdf)

As for the multiverse, I don't know enough to talk about it. I just know it's
one of the possible interpretations of quantum mechanics. Note that the
various interpretations are generally considered more philosophy than science,
and have no (or very little) practical implications. I would suggest ignoring
all analogies and not looking too deeply for interpretations, and instead
focus on basic concepts like "What is a quantum state?" and "How do I compute
measurement outcomes?" which are super well understood and the same under all
interpretations.

You can think of the various interpretations of QM as different software
methodologies, scrum, agile, waterfall, etc. just stuff people like talk about
endlessly, but ultimately irrelevant to the code that will run in the end.

------
karatestomp
The Oberth Effect. I’ve seen a bunch of awful attempts at the “why” and some
“well it’s just in the math, so there” but nothing satisfying.

~~~
knzhou
I answered this here:
[https://physics.stackexchange.com/questions/287101/where-
doe...](https://physics.stackexchange.com/questions/287101/where-does-the-
extra-kinetic-energy-of-the-rocket-come-from/287102#287102)

The basic answer is that the extra energy that goes to the rocket comes from
harvesting the kinetic energy that _the fuel itself_ had by virtue of being in
the moving rocket.

------
jppope
Why is it that the smartest people in various fields constantly need reference
materials, but medical doctors never look anything up?

~~~
Balgair
They look things up _constantly_ , you just don't see it because it's
typically very boring and not a good use of time for you or the MD. They get
symptoms from you and then will go off to research. Also, a broken arm or
strep isn't very difficult to figure out. ~80% of visitations are fairly
simple.

The site most MDs use is here:
[https://www.uptodate.com/home](https://www.uptodate.com/home)

~~~
jppope
thanks for the reference. I'll taking a peek.

------
kosmodrom
First principle reasoning in real life examples.

------
Rerarom
I think we should have this thread more often.

------
diehunde
\- How computers translate bits into electrical signals

\- What does it mean the universe is expanding

\- Bayesian statistics

\- How information is stored in magnetic tapes

~~~
OkayPhysicist
First one's easy. You've got a bunch of electronically controlled switches
(transistors), and you encode a higher voltage as 1, and something near ground
as 0.

You can build logic gates out of 2 or 3 transistors, and combine those logic
gates into more complex gates until you've got a computer.

But how does a transistor work? Basically, you've got semiconducting materials
of two types (Phosphorus or Boron doped silicon), one of which wants a few
more electrons to be a conductor, and the other wants a few less electrons to
be a conductor. If you stick the two types next to each other, the electron-
wanting (N) one snatches up the electrons from the electron-offering (P) one,
and you get an Electric field going from the P to N. Now, that alone makes a
diode. Already cool, nonlinear electronics. But what if we go N-P-N? Now we've
got two electric fields, going opposite directions. With three leads, you can
adjust the strength of those electric fields with one, creating a variable
resistor, a transistor.

------
wiz21c
Electricity.

Electricity is always explained by its effects, but never by its actual
nature. I'd like better explanation :-)

------
dandare
How did Mendeleev and his peers knew something was the final indivisible
element and not just another molecule?

~~~
OkayPhysicist
It was pretty wild wild west, really.

The idea that some some compounds didn't just contain some fire was still
common when the first list of elements was put together. A big leap forward
was realizing air had two principle components, burn-y air and not burn-y air.

They figured out water wasn't an element when the burn-y air and some mystery
gas burned to make it.

Basically, everything was maybe an element until they either broke it into
pieces, or made it out of other stuff.

------
elgfare
The twin paradox. All explanations seem to be just "something something one
twin has to accelerate"

~~~
gliese1337
The amount of time you experience is directly related to the length of your
worldline--the path you trace out in 4D spacetime as you move through space
and simultaneously move forward in time.

It should be easy to see that a non-accelerating object (or person) will trace
out a straight worldline. If you ever change your velocity, though, either
through smooth acceleration or instantaneous rotation of your velocity vector,
you will trace out some non-straight curve in spacetime. If you leave your
friend behind and then, at some later time, meet back up again, if you did not
undergo exactly the same amount of acceleration throughout your journeys
(i.e., because one of you stayed behind and hardly accelerated at all, tracing
out a boring straight line path), then you will have different world-line
lengths (different "path integrals") between the starting and ending points,
and thus will have experienced different amounts of subjective time.

Now, in a Euclidean spacetime, the traveling twin would end up older, because
a straight line is the shortest distance between two points. But our spacetime
is not Euclidean--it is a Minkowski space, in which acceleration is equivalent
to a hyperbolic rather than Euclidean rotation of your velocity vector, so it
turns out that straight line is actually the _longest_ distance between any
two points, and the twin who leaves and comes back will have a shorter
worldline, and thus will have aged less.

------
skanga
Can anyone explain disc loading and solidity in a propeller and if there are
any equations governing them?

~~~
na85
Disc Loading is just the force on the actuator disc (such as a rotor, in which
case the force is the weight of the vehicle the rotor holds aloft) divided by
the swept area (pi times r squared).

It's used when discussing propulsive efficiency, as it's a proxy measurement
for how much "work" each blade is doing. Because propeller/rotor blades are
just high-aspect wings, if you have high disc loading your blades are at a
high lift coefficient which means they'll be incurring lots of lift-induced
drag which increases your power requirements.

Solidity in the same context refers to the amount of volume within an actuator
disk that's occupied by actual solid material. If you have a 4-bladed rotor
and you move to a 5-bladed rotor, all else equal, you've increased your
solidity.

There are many many equations, and as most things in fluid mechanics you can
get as deep into the weeds as you want. As a starting point, have a look at
the wiki article for Blade Momentum Theory[0]

[0] -
[https://en.wikipedia.org/wiki/Momentum_theory](https://en.wikipedia.org/wiki/Momentum_theory)

~~~
skanga
Thanks na85

Let's take a contrived example. I have a 4 engine high wing airplane with 2
bladed 55 inch props on each engine with 100kg force (1 kg-f is equal to 9.8
N) thrust per engine.

Now, I need to make that a low wing airplane so need to change to 22 inch
blades with the same thrust and I don't want to change engines. So I want to
add more prop blades. How many more blades do I need to add?

~~~
na85
Couple of points:

1) 100kg is a measure of mass, not force. Thrust is a force, not a mass. But
let's say you have 100N of thrust per engine.

2) Why would changing from a high-wing to a low wing monoplane require you to
add prop blades?

~~~
skanga
Sorry, edited question

~~~
na85
So, your question is a little off, because propeller thrust changes
dramatically with airspeed. Propeller aircraft are typically referred to as
being "power producers", i.e. we talk about power when we analyze props, not
about thrust, because the mechanics and mathematics behind propeller thrust
are annoying and complex.

But to get to the heart of your question: You want to reduce the prop diameter
(ground clearance perhaps? The engineer in me asks why you don't just make the
landing gear taller) and not change the engine. You don't actually have to add
blades (maybe). You could also just make the existing blades fatter, or change
the airfoil, or increase the propeller RPM. Lots of ways to attack that.

But, playing along that adding blades is the only way:

1) Take your existing high-wing plane and calculate power required for all
phases of flight: Take-off, climb, cruise, etc.

2) Take your new low-wing plane with its smaller prop diameter and work
backwards to ensure you can actually meet the power requirements to stay aloft
through your envelope. Adding blades reduces efficiency because the blades'
wakes interfere with each other so you'll have to dig into some experimental
data based on the prop of your choice. Much depends on blade pitch and
washout.

Very likely you will need to increase the RPM (if your engine can deliver
enough power) or change engines to a more powerful model because your props
are now less efficient.

Such is the nature of aircraft design - almost nothing can be changed without
affecting something else.

------
dvdkhlng
Wave-particle duality (of e.g. light)

~~~
gus_massa
The usual misunderstanding is that light is sometimes a wave and sometimes a
particle, but that is wrong. Light is always a weird thing, that you have
never seen before in macroscopic objects, and you need to use some math to
describe.

In some experiments the weird mathematical thing can be _approximated_ as an
almost classical particle. That approximation simplifies the calculation a
lot, and sometime you can get some result intuitively. But it is never true,
it is only a very good approximation.

In some experiments the weird mathematical thing can be _approximated_ as an
almost classical wave. That approximation simplifies the calculation a lot,
and sometime you can get some result intuitively. But it is never true, it is
only a very good approximation.

Try to read again everything you have read about the subject, but every time
the text says "here light is a wave/particle" use a red marker to rewrite that
sentence as "here light can be approximated as a wave/particle".

------
crosser
Non-linear optic explanation from quantum standpoint (classical explanation is
quite clear).

------
rockmeamedee
Kalman filters. The explanations always start easy and then get too confusing.

------
danesparza
Einstein's (summarized) quote, "Time is a persistent illusion"

------
machawinka
\- Hegel. I haven't found any resource that can explain it clearly.

~~~
efficax
I’d suggest Kojeve’s Introduction to the Reading of Hegel (a classic
expository work on the Phenomenology of Spirit)

------
gjvnq
How depression, autism and gender dysphoria work on a molecular level.

~~~
smnthermes
As for autism, it's related with a kind of general reduction of gene
expression:
[https://www.researchgate.net/publication/328734204_A_theory_...](https://www.researchgate.net/publication/328734204_A_theory_of_evolution-
biased_reduction_of_gene-
expression_manifesting_as_autism_the_antiinnatia_theory_Improvedupdated_presentation_of_A_theory_of_general_impairment_of_gene-
expression_manifesting_as_)

------
invisibIe
All the credible theories of why there are 3 generations of particles.

------
rambojazz
How EM radiations propagate through the air and what is an EM field.

------
AmericanOP
Why is there no evidence of black matter annihilation in astronomy?

~~~
knzhou
You mean dark matter annihilation? There are some hints: an excess of gamma
rays detected by Fermi-LAT, an excess of X-rays detected by Chandra, excess
ionization detected by EDGES, and excess positrons detected by AMS-02.
However, all of these anomalies are contentious and could well be explained by
systematic effects or unknown astrophysical sources.

If none of these hold up, it could be that dark matter just doesn't
annihilate. That's not weird or anything -- I mean, your room is full of
matter right now, and it's not annihilating itself either.

------
nightfly
Eddy currents. Why does a magnet move slowly down a copper tube?

~~~
OkayPhysicist
A changing magnetic field induces a current in a material, which creates an
opposing magnetic field which slows the moving magnet.

~~~
carapace
The kinetic energy turns into heat due to resistance to the induced current?

------
keithyjohnson
Agile, an empirical survey of whatever the hell it actually is.

~~~
Davidbrcz
Agile is a short-lived V cycle repeated for ever.

Legacy V-cycles (needs - spec - code - test - integration - product) were such
that everything was written down and planned in advance for months/years. So,
if the customer had made an error, his/her needs had changed... you were
basically screwed.

Agile advocates for Short V cycles while getting often user feedback. But __it
's __a V cycle.

\- PO speaks to the customer = get needs \- PO writes tickets, UX design
something = specification \- And then it follows the classical cycle :
develop, test, integrate, deliver.

What's remain around agile (ceremonies & co) feel much more like bullshit to
me, and people follow it religiously without understanding the core idea of
agile as they think V cycle is an insult.

------
billfruit
Wavelets: Usually given explanations are hard to understand.

------
Razengan
Amperes, Voltage, Watts, without using a liquid analogy.

------
wegymoo
ionization. The resources I can find on it are mostly conspiracy theory types
or the same basics explained over and over again.

------
econcon
Axial flux motors

------
nabla9
Gauge theory.

------
ykevinator
Quantum stuff

------
daenz
Why a flat earth is impossible.

~~~
cygx
Gravity. Gather enough stuff, and you end up with balls (unless you spin
things really fast).

------
yanovskishai
How airplane wing really works.

~~~
nnd
I found this article useful: [https://www.scientificamerican.com/article/no-
one-can-explai...](https://www.scientificamerican.com/article/no-one-can-
explain-why-planes-stay-in-the-air/)

~~~
yanovskishai
thanks!

------
geocrasher
Reactance.

~~~
itcrowd
If you have a specific question about electrical reactance, I can help. If not
electrical, I probably can't help since I don't know what you are referring
to.

------
Madblenderuser
why does blender at times become slower for no fucking apparent reason

------
zander312
Why the earth is a sphere.

~~~
OkayPhysicist
Systems tend towards low-energy states (because small perturbations keep
things from getting stuck at local minima or saddle points. The lowest
potential energy configuration for a bunch of particles pulled together by
gravity is a sphere, because every point is as close to to the middle as
possible.

------
unixhero
Human counsciousness.

------
tprice7
1\. Carbon dating. Sure, I get that carbon decays over time and this changes
the proportion of isotopes. But why does this give you any information? That
carbon didn’t come into existence just to be in that bone, it was made in the
sun billions of years before that, so why does the age of the carbon tell us
anything about organic matter? The key fact, which I think is not emphasized
enough, is that the ratio of isotopes in atmospheric carbon is kept at a
constant equilibrium by cosmic rays. So you can use carbon dating to tell
roughly when the carbon was pulled out of the atmosphere. Without this
additional fact, the concept of carbon dating makes absolutely no sense.

2\. The tides. The explanation I was given is roughly something like “the
tides happen because the moon’s gravity pulls the water toward it, so you have
high tide facing the moon. There’s also a high tide on the opposite side of
the earth, for subtle reasons that are too complicated for you to understand
right now and I don’t have time to get into that.”

The first problem with this explanation is this: gravitational acceleration
affects everything equally right? So it’s not just pulling on the water, it’s
also pulling on the earth. So why does the water pull away from the earth?
Shouldn’t everything be accelerating at the same rate and staying in the same
relative positions?

The second problem is that, when viewed correctly, the explanation for why
there is a high tide on the opposite side of the earth as the moon is equally
simple to why there is a high tide on the same side as the moon.

The resolution to both these problem is this: tides aren’t actually caused by
the pull of the moon’s gravity per se, but are actually caused by the
difference in the strength of the pull of the moon’s gravity between near and
far sides of the earth, since the strength of the moon’s gravitational pull
decreases with distance from the moon. The pull on the near water is stronger
than the average pull on the earth, which again is stronger than the pull on
the far water. So everything becomes stretched out along the earth-moon axis.

3\. This one isn’t so much a problem with the explanation itself, more about
how it’s framed. I remember hearing about why the sky is blue, and wondering,
“ok, more blue light bounces off it than other colours. But isn’t that
essentially the same reason why any other blue thing is blue? Why are we
making such a big fuss about the sky in particular? ” A much superior
motivating question is “why is the sky blue during midday, but red at sunrise
/ sunset”? I was relieved when I saw this XKCD that I’m not the only one who
felt this way:

[https://xkcd.com/1818/](https://xkcd.com/1818/)

~~~
itcrowd
Carbon dating can't say anything about dinosaurs (in the colloquial
definition) because dinosaurs went extinct >>50k years ago and CD doesn't work
after such long time spans.

~~~
tprice7
Ok fair enough. I admit I regret this comment and think I posted it a bit
hastily, I tried to delete it a few minutes ago but it appears to be too late
now. I edited out the part about dinosaurs though.

------
zarkov99
Coroutines in C++.

~~~
ncmncm
Coroutines are most easily understood as a way to write a state machine in a
way that looks like a function. I.e., it's just a notational trick to make one
function do different things according to when it's called.

To see it, imagine you have a struct with a data member for each local
variable of your function, and replace your function with a member function
that has no local variables, but uses "this" to get at what was local data.

Add one more data member, a number that is set differently right before each
place the function returns.

Finally, insert some code at the start of the function that, according to the
number, jumps to just after the last return statement executed.

Then, each time you call the function, what happens depends on what happened
last time.

There are more details, but that is the gist.

You can write that yourself in C++98, with the body of the function inside a
switch statement. Getting it past code review would be the real challenge.

~~~
zarkov99
I think I get coroutines in theory. It's the C++ specific complexity that is
the hardest barrier. Thanks for the explanation though

~~~
ncmncm
Yes, a lot of extra junk is needed to make them actually work, but the extra
junk goes a long way toward obscuring what you actually need to know.

Ultimately, though, you are right that you have to understand it all, once,
even if you can't remember it all a month later. The explanations I find
online are not good at presenting just the details you need when you need
them, and building up to the full picture.

------
foreigner
The Coriolis Force

~~~
cygx
Drag a marker with a constant radial velocity (from your perspective) across a
spinning disk. It'll trace out a curved line, so from the perspective of an
observer on the disk, an acceleration must have been present.

The full 3d Coriolis force is more complicated than that (eg accounting for
the Eötvös effect): The spinning disk example only gets you to the -2vω term
(where v denotes radial velocity and ω angular velocity).

------
kbradero
celullar replication and virus code insertion

------
aaronblohowiak
Orbital mechanics

~~~
ghostpepper
Have you heard of / played the game Kerbal Space Program?

I believe even folks at NASA have even said it helped cement their
mathematical knowledge with a better intuitive understanding.

------
druml
Escape velocity

~~~
sloaken
My dad was an aero space engineer, and I always had trouble with this. It is
the speed you need to be going at, to not be pulled back by gravity. As a kid,
I always thought, well big deal, if I am in a plane and just keep going up,
eventually I will be out. The key I always missed was the velocity at that
point was with no added power. So when the engine cuts off, what is your
velocity? Is it enough to escape? Or do you need more power. Of course there
is a good chance I still do not understand it.

------
curiousgal
Measure theory.

~~~
ivan_ah
This is a very good playlist:
[https://www.youtube.com/playlist?list=PLBh2i93oe2qvMVqAzsX1K...](https://www.youtube.com/playlist?list=PLBh2i93oe2qvMVqAzsX1Kuv6-4fjazZ8j)

Starts from basic concepts and builds up a nice overview.

------
barrenko
Conciousness.

------
fegu
Yoneda lemma

~~~
aesthesia
As with most things in category theory, the way to really understand the
Yoneda lemma is to sit down and prove it yourself. Break the statement down
into the underlying definitions, draw lots of diagrams, and convince yourself
that it’s true.

The other thing you can do is think about what it means for particular types
of categories. For a posetal category, it says that an element of a poset is
uniquely determined by the set of all elements that come before it in the
ordering. For a group, it says that every element is uniquely determined by
its action on the group. (This is basically Cayley’s theorem.) See this MSE
post for more intuition: [https://math.stackexchange.com/questions/37165/can-
someone-e...](https://math.stackexchange.com/questions/37165/can-someone-
explain-the-yoneda-lemma-to-an-applied-mathematician)

------
2Trips
Women!

------
chrisMyzel
Math!

------
buboard
electromagnetic waves

------
myth2018
Haskell's Monads

------
dimitristi
coriolis force

------
RocketSyntax
string theory

------
Madblenderuser
why does blender become slower at times for no apparent reason

------
sloaken
Reminds me of a story:

A guy was walking along the beach and found a lamp. Of course he rubs the
lamp, and sure enough a genie appears.

Genie: master of the lamp I can grant you a wish, you may wish for anything.

Guy: Wait, isn't it supposed to be 3 wishes?

Genie: One or nothing, and do not wish for more wishes.

Guy thinks for a while ....

You know I have pretty much everything I need. But I have always wanted to
travel to Hawaii. But I get sea sick and am afraid to fly.

Genie: Very well I will take you there.

Guy: No no, if you take me there, I wont be able to come back. And what about
next year? Since I only get one wish, I want a bridge built to Hawaii.

Genie: That does not make any sense. Please make a different wish. One that
does not involve so much construction.

Guy: hmmmm you know I know, can you explain women?

Genie: So do you want the bridge to be a suspension bridge or truss? and how
many lanes ....

