
Ask HN: What is a science fact that blew your mind when you learned it? - itronitron
I&#x27;ll go first. When I read that it takes a photon over 100,000 years to exit the Sun as visible light, I was completely astounded. Curious what other insights from science people have learned that were completely unexpected to them.
======
Stratoscope
Electrons in a copper wire do not travel at the speed of electricity. Not even
close.

Electricity travels at nearly the speed of light.

Electrons themselves travel like molasses:

"In the case of a 12 gauge copper wire carrying 10 amperes of current (typical
of home wiring), the individual electrons only move about 0.02 cm per sec or
1.2 inches per minute (in science this is called the drift velocity of the
electrons.). If this is the situation in nature, why do the lights come on so
quickly [when you flip the switch]? At this speed it would take the electrons
hours to get to the lights."

This completely caught me by surprise, but it makes sense once it's pointed
out. Imagine a pipe filled with solid balls that just fit in it, with little
friction. If you push a ball in one end, a ball pops out the other almost
immediately. But not the same ball! Even if you keep pushing balls in, that
first one you pushed will take a while to get the other end.

Update: as rrobukef notes in a reply, this would be the case for direct
current (DC). With the usual household alternating current (AC), the electrons
barely move at all!

[https://www.uu.edu/dept/physics/scienceguys/2001Nov.cfm](https://www.uu.edu/dept/physics/scienceguys/2001Nov.cfm)

[https://wtamu.edu/~cbaird/sq/2014/02/19/what-is-the-speed-
of...](https://wtamu.edu/~cbaird/sq/2014/02/19/what-is-the-speed-of-
electricity/)

~~~
rrobukef
For AC, they move back as well. Thus electrons move less than 4µm from their
original place (excluding Brownian movement, I assume)

~~~
quickthrower2
Another mindblower: you can chuck a capacitor at the other end and you’ve got
a circuit with one wire!

Hence you can get shocks from one live wire.

------
aazaa
Science doesn't "prove" hypotheses - it _disproves_ them.

For this to work, a hypothesis must be falsifiable. Most pseudoscience (and
religion) makes non-falsifiable claims, meaning they are incompatible with
scientific discourse.

This simple observation is a powerful tool in any bullshit-detection kit.

Once produced, a scientific hypothesis of any merit will be attacked
vigorously with experiments until enough parties are convinced that disproof
is sufficiently unlikely. The process isn't always pleasant for those making
the falsifiable claims.

Sadly, this is not how science is taught in most schools. There, students are
given the "truth" and, on a good day, asked to verify it experimentally. We
are now living with the terrible consequences of generations of youth who
think science is about "proving" the truth.

~~~
dschuessler
You're correct in the sense that this is what science is per Popperian
definition. However, I would be careful to assume that many scientists are
actually so busy refuting hypotheses. The Popperian ideal and the reality in
science are two different things.

To cite a paper I currently read:

> "As Chalmers (1999) remarks, when we turn to history of science, the idea
> that falsificationism is actively undertaken borders on laughable (see
> especially Feyerabend, 1975). Not only do scientists exhibit extreme
> reluctance to falsify their work (Woodward and Goodstein, 1996), but if
> falsification had been followed as a scientific method, then many of the
> theories generated by some of the best minds in science would never have
> progressed beyond their earliest explications (see Chalmers, 1999: ch. 7;
> Feyerabend, 1975; Kuhn, 1970c: 234; 1970d: 13; Von Dietz, 2001: 22)."
> ([https://journals.sagepub.com/doi/10.1177/1350508409104504](https://journals.sagepub.com/doi/10.1177/1350508409104504))

I haven't read all the sources. But it's worth mentioning that your point is
controversial.

~~~
DataDaoDe
I would like to add to your remarks that science actually says something along
the lines of "Grant me these assumptions/axioms/miracles(i.e. space-time,
causation, etc.)" and then I can give you a theory of gravity, statistics,
etc. This I think is hinted at in Gödel's core insight from his incompleteness
theorem.

------
Xcelerate
A compressed spring weighs more than the same spring uncompressed. The moon
and earth weigh less together than if you weighed them separately and added
the values together. Most of the weight of solid objects is due to the high
speeds and binding energies of the elementary particles within them rather
than the rest mass of the constituent particles. The number of particles that
exist is relative to how fast you are accelerating. Everything that has energy
(which is everything we know of) affects the gravitational field; this means
that even photons are "attracted" to each other.

A non-relativistic quantum state will return arbitrarily close to its initial
state an infinite number of times. There is such a thing as interaction-free
measurements: you can take photos of things without ever letting light hit a
detector and you can tell whether a bomb is "active" without actually
interacting with the detonator.

Energy is just a number that is calculated as a function of the state of a
closed system — that this number is a constant results from the time
transitional invariance of the laws of physics. Similarly, conservation of
momentum is due to the spatial invariance of the laws of physics, and
conservation of angular momentum is due to rotational invariance. Also,
conservation of energy does not hold under general relativity.

~~~
thaumasiotes
> The number of particles that exist is relative to how fast you are moving.

I thought the existence of particles was relative to how fast you're
accelerating? If it's relative to how fast you're moving, wouldn't that imply
that absolute velocity exists, i.e. that given two objects with known relative
velocity to each other, we could potentially establish which one was "really"
moving and which one was "really" still?

~~~
Xcelerate
Should have said how fast you are accelerating. Edited now. See
[http://www.scholarpedia.org/article/Unruh_effect](http://www.scholarpedia.org/article/Unruh_effect)

------
cbsks
I think "mind blown" is an accurate description when I first saw the Hubble
Deep Field image:

[https://hubblesite.org/contents/articles/hubble-deep-
fields](https://hubblesite.org/contents/articles/hubble-deep-fields)

I already knew that "the universe is incomprehensibly large", but seeing how
many entire galaxies there were in a random dark patch of sky was eye opening
to me.

~~~
perk
Don't know exactly why, but thinking about the vastness of the universe always
cheers me up. Thanks for sharing :)

~~~
take_a_breath
Relatedly, my dad always told me that thinking about the vastness of the
universe "weirded him out."

My hypothesis is that he has narcissistic tendencies so pondering the vastness
of the universe only highlights his own insignificance. I've noticed similar
reaction from people who visit a large city for the first time and get a sense
of how small they are in the grand scheme.

~~~
tfandango
Reminds me of the Total Perspective Vortex from Hitchhikers Guide to the
Galaxy, which exposed an individual's insignificance in the universe.

------
ffpip
Not science, but Mathematics - That there are more ways to arrange a deck of
52 cards than seconds that have elapsed since the big bang took place.

Thousands of similar questions on reddit for anyone interested-
[https://duckduckgo.com/?t=ffab&q=science+fact+site%3Areddit....](https://duckduckgo.com/?t=ffab&q=science+fact+site%3Areddit.com%2Fr%2Faskreddit)

~~~
akmarinov
Also chances are that no one has shuffled a deck in the same order as someone
else has, ever in history.

~~~
edanm
This one gets me too. It's _so_ counterintuitive to me.

(Caveat: this is assuming people shuffle well, which of course most people
don't, therefore in fact people probably _have_ shuffled to the same state).

------
lazyjones
A 2016 poll of 1,500 scientists reported that 70% of them had failed to
reproduce at least one other scientist's experiment (50% had failed to
reproduce one of their own experiments).

~~~
RhysU
I would love to see this number for computational scientists.

~~~
aewens
While anecdotal, its easy to run into issues like this[0] where scientist with
little CS background assume certain behaviors in code that don't actually
exist.

[0] [https://www.vice.com/en_us/article/zmjwda/a-code-glitch-
may-...](https://www.vice.com/en_us/article/zmjwda/a-code-glitch-may-have-
caused-errors-in-more-than-100-published-studies)

------
Liquidity
When i could "see" my visual blindspot. [https://lasikofnv.com/try-
these-3-fun-tests-to-find-your-vis...](https://lasikofnv.com/try-these-3-fun-
tests-to-find-your-visual-blind-
spot/#:~:text=Position%20your%20head%20to%20look,with%20the%20surrounding%20yellow%20color).

~~~
sloaken
OMG me too. I was having an eye exam. The technician had a black field with a
bunch of lines. Holding a rod with a white dot, and told to let them know when
it disappeared.

I laughed, "Well that will only happen if you put it behind your back."

They laughed, "Everyone has an astigmatism where the nerves enter the eye. We
are looking for that or any other issues."

I was so shocked when the dot disappeared.

Also happy to hear it was the normal blind spot.

~~~
m463
There is one driving your car too. :)

~~~
moistly
Not if your mirrors are set correctly. Lean your head to the side window, set
the mirror so that it just misses the tail end of your car. Position your head
over the centre console, set the mirror. There is now approximately no blind
spot. You can fine-tune by careful observation on a multi-lane highway as
people pass/you pass others. Done correctly, there is no blind spot.

~~~
shaftway
There are always blind spots. You covered ___a_ __blind spot, but not all of
them. Some of them are surprising.

[https://www.wthr.com/article/news/investigations/13-investig...](https://www.wthr.com/article/news/investigations/13-investigates/13-investigates-
millions-vehicles-have-unexpected-dangerous-front-blind-
zone/531-9521c471-3bc1-4b55-b860-3363f0954b3b)

------
RaoulP
It never fails to amuse me that any pressure or force that we intuitively feel
is "pulled" by a vacuum, is only caused by air pressure on the other sides of
an object.

When you see something hanging from a suction cup in your kitchen or bathroom,
it's fun to imagine that the air around you is hammering the suction cup
enough to keep it stuck there. With quite some force you'll notice, if you try
to pull it straight out! But let some air in through a small gap, and it will
help even things out.

A lack of air by itself does nothing. It's just about the net forces.

~~~
Corrado
Wow, I never thought about that before. So, if you could draw a vacuum in your
whole kitchen or bathroom, would the suction cup fall down?

~~~
s_gourichon
Yes.

------
teh_klev
Ok, so maybe not a "fact" but the hypothesis of the False Vacuum:

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

This quote blew my mind:

 _" The possibility that we are living in a false vacuum has never been a
cheering one to contemplate. Vacuum decay is the ultimate ecological
catastrophe; in the new vacuum there are new constants of nature; after vacuum
decay, not only is life as we know it impossible, so is chemistry as we know
it. However, one could always draw stoic comfort from the possibility that
perhaps in the course of time the new vacuum would sustain, if not life as we
know it, at least some structures capable of knowing joy. This possibility has
now been eliminated."_

I immediately went out and bought Greg Egan's "Schild's Ladder" after find out
he'd used this as a plot device.

~~~
cercatrova
You should also read the Three Body Problem trilogy, specifically the last
book, although I won't spoil it. It uses a plot device very similar to a false
vacuum.

------
ManuelKiessling
Not a fact per se, but: the sheer amount of living things that came before me,
in the sense of my direct ancestors and their direct ancestors etc. etc – not
only my human ancestors, but the whole of my ancestors all the way back to the
origins of life itself – and I would not be here today if even one of them
would have died before reproducing... that manages to blow my mind every
single time I think about it.

~~~
Geee
Try counting the total number of your ancestors: you have 2 parents, they have
4 parents in total etc., i.e. 2^N where N is the number of generations. Going
just 40 generations back (about 1000 years), you had 2^40 = 1,099,511,627,776
ancestors.

~~~
AnimalMuppet
But not necessarily 2^40 _unique_ ancestors. That is, the same person may be
your ancestor by more than one route through the subsequent generations.

------
akmarinov
The sun orbits the center of the galaxy every 250 million years, so when the
dinosaurs were around, they were on the other side of the galaxy, compared to
where we are now.

------
tashmahalic
Gravity travels at the speed of light (the speed of causality). If the sun
suddenly disappeared, the earth would continue orbiting around where it used
to be, for about 8 minutes.

Nothing can travel faster than light through space, in a vacuum. However, if
you pick two points in space that are far enough apart (e.g. at opposite sides
of the observable universe), these points will be moving apart faster than
light, because space itself is expanding.

The expansion of space isn’t coming from a single point outward, like an
explosion. It’s expanding by the same amount at every point in the universe.
People analogize this in lower dimensions to stretching fabric or blowing up a
balloon.

~~~
masked_titan
How do we know that the space is expanding if there is nothing to compare it
to?

~~~
tashmahalic
The farther away a galaxy is, the more red-shifted the light from that galaxy
is, when it reaches us.

Also, really far-away galaxies are receding from us faster than light.

And, the existence of the cosmological event horizon seems to support this
too, I think. Things beyond that horizon will emit light in our direction, but
that light will never reach us. In fact, without this horizon, I believe that
the night sky would be much brighter.

I find it kind of sad that, as time passes, more of the universe becomes
unobservable. Eventually the only stars visible from our POV will be those in
our own galaxy (and galaxies that ours has merged with, in the interim).

More info at
[https://en.wikipedia.org/wiki/Expansion_of_the_universe](https://en.wikipedia.org/wiki/Expansion_of_the_universe)

------
xlm1717
The selective attention test, aka the "invisible gorilla" experiment, and the
finding that 50% of people didn't see the gorilla.

------
rootbear
I was pretty amazed by the Casimir Effect when I first learned of it.

    
    
      https://en.wikipedia.org/wiki/Casimir_effect

~~~
m463
I think I need to get an advanced degree just to be able to read that
wikipedia entry.

------
tito
A gallon of gasoline creates 20 pounds of carbon dioxide.

So my trip to the grocery store could be adding as much carbon dioxide to the
air as the groceries I pick up!

~~~
m463
That is fascinating. A gallon of gasoline weighs 6 pounds, so it probably must
be using 14 pounds of oxygen.

------
loandigger
The dual slit experiment. Run the experiment with no one watching it, it
produces result A. Run it again and watch it, it produces result B. Watching
the experiment changes the result. WTF????

~~~
drran
It's false information. It's debunked multiple times. "Watching" means
"placing or turning on an additional detector", which ruins interference
pattern. To develop intuition, look at double slit experiment made at macro
scale using walking droplet.

~~~
JohnDeHope
I'd really appreciate a few links to this. I am on a youtube binge lately
watching quantum dynamics lectures for lay people. The constant talk about
"watching" and "observers" drives me crazy. I refuse to believe anything has
anything to do with whether or not anybody is "watching" anything. Bouncing
photons off of it? Sure I can see how that would affect all sorts of things.
But if the bouncing photons alter what's going on (and it seems they cause as
much quantum decoherence as anything else might) then that's not the same
thing as just "watching". That word implies to me "there are ambient photons
or some other field or background that does not alter the experiment, and I am
merely observing that background to get an idea what's going on within the
experiment". If the experiment cannot be conducted without interference within
an ambient background of photons flying around, or within some other medium we
passively non-destructively-to-the-experiment observe, then we cannot
currently be said to be "observing" anything at all with respect to that
experiment. Let's just say that, can't we? If we can't not alter the relevant
circumstances of what's going on, then let's just admit that, without using
faulty and improper language to "describe" what's happening. Can somebody help
me out with this?

~~~
Viliam1234
You make a good point. The problem is interacting with the particles, e.g. the
photons bouncing off them. Don't poke the particle -- it only interferes with
itself. Poke the particle -- now it is entangled with your measurement device,
that means zillions of other particles, which changes the interference
patterns.

But it is more complicated that this. Consider
[https://en.wikipedia.org/wiki/Elitzur%E2%80%93Vaidman_bomb_t...](https://en.wikipedia.org/wiki/Elitzur%E2%80%93Vaidman_bomb_tester)
\-- if you only poke the particle in a "parallel universe", that also changes
its pattern in this one.

At the end, quantum physics says there are multiple "versions" of how the
particles move, and those versions interfere with each other. By interacting
with each other, particles become entangled, which means that the "versions"
of their states are no longer calculated independently, but together.

There are disagreements of what exactly this means: some people believe that
reality only has multiple "versions" on a microscopic level, but when the
entangled configuration becomes large enough (how large? no one knows), the
parallel computation collapses, one of these "versions" is randomly selected
to become the actual reality and the remaining ones disappear. Other people
believe that multiple "versions" is the whole story; that observing the
outcome means that you (being composed of particles that follow the laws of
physics) also become entangled with the particles in the experiment, and now
there are multiple "versions" of you, each observing a different outcome.

So, I'd say you got it half-right. Yes, it is about "observation =
interaction"; and "observer" is just a shortcut for "the thing that poked the
experimental particle, optionally also a display connected to that thing,
optionally also a person observing the display". (That is, you could also have
a completely impersonal "observer", e.g. a machine that measures the particle
but no one is looking at its display.) But quantum physics is a different
thing than mere classical physics where you correct for photons being actual
things that hit the measured particle. It means there actually are multiple
outcomes, which then interfere with each other, at least on the microscopic
level.

------
JohnDeHope
I absolutely cannot psychologically deal with the Monty Haul paradox.
[https://en.wikipedia.org/wiki/Monty_Hall_problem](https://en.wikipedia.org/wiki/Monty_Hall_problem)
I've coded it multiple times in multiple programming languages as a cathartic
exercise. Despite seeing the results obviously printed out in a console in
front of me, I just can't handle it. I lose SAN just thinking about it.

~~~
cbruns
The explanation that makes the most sense to me, from wiki:

Yet another insight is that your chance of winning by switching doors is
directly related to your chance of choosing the winning door in the first
place: if you choose the correct door on your first try, then switching loses;
if you choose a wrong door on your first try, then switching wins; your chance
of choosing the correct door on your first try is 1/3, and the chance of
choosing a wrong door is 2/3.

~~~
JohnDeHope
The bit that gets me... If he opens one door, which is a goat, so there is 1
goat and 1 prize remaining... then the chance should be 50% staying put or
switching, right? That's the part that gets me. The whole bit about 3 doors
and opening one of them and "do you want to switch" is all a ruse, it seems to
me. At the end of the day there are two doors, one with a goat and one with a
prize, and however you choose between 2 doors, you will win or lose 50% of the
time. Right? Aargh the SAN loss...

~~~
emiliobool
It would only be 50-50 if you could only pick from 2 options, but your initial
choice is between 3 options.

------
kpwags
That when we look up at distant stars, nebulas, & galaxies and the like...we
are looking back in time. What we see could have been gone for decades,
centuries, or longer.

~~~
perardi
“longer” is an understatement. The Andromeda galaxy, which is viewable by the
naked eye, is 2.5 million lightyears away. And that’s the _closest_ galaxy to
us.

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

~~~
pyfgcrl123
One of the closest, anyway. Yeah, most distant objects observed are over 10
billion years "in the past".

------
atmosx
Not a science fact per se: Until recently I haven't realised the amount of
damage inflicted by religion or in the name of religion if you will, to our
species. Take Heliocentrism[^1] for example, it was alluded in the 5th century
BC that the Earth revolves around the sun and that stars are other "suns".
Then religion came along. Galileo nearly died for re-iterating a theory that
was accepted by part of the _scientific community_ nearly 2000 years before
him.

In similar fashion, years ago while studying the circulatory system for the
anatomy class I came across a wikipedia article. This particular part of the
circulatory system was documented in detail by Egyptians in 200 BC. Knowledge
came from the mummification process. The next breakthrough in this area was
made in the 19th century.

Looks like our species could have a colony in Mars by now, if science were
allowed to breakthrough linearly.

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

~~~
DevX101
I'm not personally religious but I'd be very careful before assuming religion,
an institution that's been with humans for thousands of years, was a net
negative.

I think humans probably have some innate desire for institutions with
religious-like properties. And as we've become more secular over the past 60
years, there's been an increase in people that seem to be making politics the
replacement, which may end up being more destructive than theistic religion
ever was.

~~~
atmosx
> I think humans probably have some innate desire for institutions with
> religious-like properties.

Hm, but we grow in a religious setting, the larger part of the population. So,
is it an _innate_ desire or a designed reality for population control?

------
hbcondo714
Growing up, I was taught that the main reason why it's bad to drink is because
it kills brain cells and brain cells don't grow back. Apparently that is not
the case anymore:

[https://www.scientificamerican.com/article/the-adult-
brain-d...](https://www.scientificamerican.com/article/the-adult-brain-does-
grow-new-neurons-after-all-study-says/)

~~~
adsjhdashkj
What level of drinking would cause this, though?

~~~
berbec
Come over to my house and I'll show you...

That's why haven't had a drink for the last 5 years

~~~
adsjhdashkj
Well, i just meant i had not heard this - so i was curious if a glass of wine
a night causes damage.

------
dmckeon
Only 10% of the cells in a human body are human, but that 90% is only 2-3% of
human body weight. [https://www.nih.gov/news-events/news-releases/nih-human-
micr...](https://www.nih.gov/news-events/news-releases/nih-human-microbiome-
project-defines-normal-bacterial-makeup-body)

~~~
sloaken
So if your body was a democracy, you would lose.

------
proactivesvcs
How prion diseases work and spread. They're basically infectious lego. You may
not thank me if you read about them...

~~~
m463
I read somewhere they survive even 1000°C cleaning of stuff.

Maybe dental implements.

~~~
tandr
I get what you are saying, but it just occurred to me "survive" might not be
fitting for these ... "things"? They are not quite "alive", are they?

------
raint
The physics & biology behind eyesight.

Our eyes collect light that is "left over"(not reflected) from other surfaces.

Can't explain why but I had always had a sense that objects somehow emanated
their own "image". Learning that colors manifest themselves because _every
other wavelength was absorbed_ was fascinating.

~~~
anchpop
Why do you say not reflected? It seems like the light that enters our eyes
often has been reflected?

~~~
pushECX
I'm thinking the parent commenter meant to say "not absorbed"?

------
gadders
Obvious when you think about it, but I didn't realise for ages that all the
planets orbit in the same plane.

~~~
sloaken
Is the obvious reason because the sun is spinning? Or some other not so
obvious reason I do not know of?

~~~
gadders
Well this may not be correct, but I'm assuming the plane they spin in is the
same as the equator of the Sun and in the same direction as the Sun's
rotation. That would make sense if they were made of "stuff" that was thrown
out from the centre and clumped together.

------
sieste
That there is such a thing as deterministic chaos (aka the butterfly effect),
ie. infinitesimal perturbations can quickly produce macroscopic changes, and
all the philosophical implications that result from it.

That despite the apparent complexity of the weather many atmospheric phenomena
can be explained from first principles with pen and paper calculations.

That the existence of elementary particles can be derived from simple symmetry
considerations (That one blows my mind every time.)

That when we look out into the universe we see elements roughly in the same
proportions as they appear on earth. (We are all made out of star dust!)

~~~
Chris2048
Isn't that just probability distributions?

------
znpy
we can see our nose all the time, but our brain filters it out.

if you close one eye and keep the other open you'll suddenly see one side of
your nose.

~~~
krapp
We also have a blind spot in each eye where our optic nerve passes through our
retina, which the brain constantly edits out.

~~~
akmarinov
Also while turning your head you don’t really see, but your brain pieces
things together and plays an image of what it thinks you should see.

------
scott31
Average density of the universe is about 1 proton mass per cubic meter

~~~
pyfgcrl123
That's including dark matter, I think? With just the matter we can see, it's
not even that.

------
mjevans
For me it's material science misconceptions, corrections to what a kid / the
over-simplified public conception are. As I'm not a material science expert,
if I'm wrong or slightly incorrect please explain what actually happens.

"diamonds are forever"

It's appealing to believe that a crystalline structure, a pretty one too,
might exist until actively changed.

Diamonds are not forever. What 'blew my mind' is that diamonds are the result
of compression and very slowly decompressing. That slowly over time the outer
layer leaves that state and turns to dust.

Glass is a 'liquid'

I'm much less sure about glass, I'm not even sure that science is sure about
glass. Apparently one process for making glass in the old days involved
something blowing and spinning discs of it to produce nearly flat segments.
When installed the artisan making the window would place the thicker end down
for stability or some other reason. I'm not positive if glass is a liquid or
not, but the reason many people might think it's a liquid is that intentional
selection bias when fitting the panes of glass.

The whole concept of glass possibly being a liquid though changed the way I
perceive solid, liquid, and gas states. Those labels better reflect much more
temporally localized potential change and interaction than they do to uniquely
describe matter.

BTW, if there is an expert, is glass actually a liquid or a solid?

~~~
pauldelany
May be wrong, but I think it correct to say glass is a 'fluid' (it flows -
very slowly), gases and liquids are generally fluids. But glass is not a
liquid.

------
juancn
Information has mass (and thus energy). As in a full hard drive weighs more
than an empty one.
[https://aip.scitation.org/doi/10.1063/1.5123794](https://aip.scitation.org/doi/10.1063/1.5123794)

------
Corrado
The gold in my wedding ring was formed inside a star. I kinda sorta knew all
matter was formed inside stars but it never really clicked until I thought
about something as simple as a lump of gold and where those atoms really
originated.

~~~
AndrewOMartin
Related, most of the atoms in a new born baby are 13.7bn years old.

~~~
riffraff
as Moby would say, we are all made of stars. It was a bit mindblowing to me to
realize this was true and not just some hippie-ish slogan.

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

------
mistersquid
In Physics 2 (Light, Electricity, and Electromagnetism), I could not derive
the correct answer for calculating the flux of an infinite sheet.

TA: The flux is S for an infinite sheet.

Me: How? We have S for this side plus S for the other which yields 2S!

TA: For an infinite sheet, there is no other side.

Me: O_O

~~~
jabirali
To me that sounds mostly like a philosophical distinction. In most physics
calculations, “infinite” just means “finite but so large that we can neglect
edge effects”.

------
shansense
That the Amazon soil is actually very poor in nutrients and the dust from the
Sahara desert is actually what keeps the Amazon rainforest alive.

Long ago the South American and African continents were one. To think of it,
there is still a major connection!

------
yongjik
BTW, "it takes a photon over 100,000 years to exit the Sun" is misleading - it
sounds like there's a single photon that's generated deep inside the Sun's
core and winds its way through like a cicada until it emerges at the surface.

In reality, any photon traveling inside the Sun will almost instantly collide
with some other particle, which may emit zero, one, or more photons as a
result. When we say "100,000 years", I believe we're summing up the total of
these photons' expected lifetimes, basically following the flux of energy
rather than individual photons.

------
thedevindevops
The 100-step rule in neuroscience, that states that no primary brain operation
(e.g., face recognition) can take more than 100 neuron firing “steps.”
(Feldman & Ballard, 1982)

~~~
lazyjones
This is a bit misleading since there are way more steps / neuron firings
involved, the rule just says that these operations happen in less than 500ms
and 1 firing per 5ms is the maximum throughput of a single neuron. These brain
operations aren't implemented as a sequential chain of neuron activations
though.

------
macando
Creating something out of nothing: The Banach - Tarski Paradox

[https://youtu.be/s86-Z-CbaHA](https://youtu.be/s86-Z-CbaHA)

~~~
Caylio
That's a consequence of the axiom of choice, which has historically been a
highly controversial axiom.

------
pyfgcrl123
That had interactions between protons and neutrons been but a tiny bit weaker
or stronger than they are, the universe would probably contain a lot less
carbon, nitrogen and oxygen ([https://en.wikipedia.org/wiki/Triple-
alpha_process#Resonance...](https://en.wikipedia.org/wiki/Triple-
alpha_process#Resonances)), perhaps precluding existence of life as we know
it.

------
pier25
Gravity is not really a force or objects "attracting" themselves but a
consequence of the curvature of spacetime.

At least that's my pedestrian interpretation :)

~~~
drran
What releases energy then, when objects are accelerated towards each other?

------
BrandoElFollito
I usually ask people to imagine they are putting a rope tight around the
earth, following the equator (idealized sphere).

Then they add 1 meter to the rope (it is now longer by a meter) and spread it
around the earth uniformly (it hovers above the earth, everywhere at the same
distance from the earth)

Estimate that hovering distance.

Try yourself to estimate it, then make the calculation.

------
bjourne
Exactly anything you can think of can be represented as a segment of pi's
infinite decimal expansion.

~~~
sigjuice
[https://github.com/philipl/pifs](https://github.com/philipl/pifs)

------
meiraleal
A man can fast for more than a year without die
[https://en.wikipedia.org/wiki/Angus_Barbieri%27s_fast](https://en.wikipedia.org/wiki/Angus_Barbieri%27s_fast)

------
CrazedGeek
The life cycle of the lancet river fluke:
[https://www.damninteresting.com/a-fluke-of-
nature/](https://www.damninteresting.com/a-fluke-of-nature/)

------
fsflover
Many statements here:

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

~~~
blablabla123
> which states that entropy, or a loss of the energy available to do work,
> must rise over time.[2] Stars will eventually exhaust their supply of
> hydrogen fuel and burn out.

Since entropy is a statistical quantity, the second law of thermodynamics is a
statistical statement. And because modern physics essentially deals with
Hamiltonian systems, it can be proven that a time exists when entropy goes
down again. The intuitive proof is really nice. Since energy is conserved, one
can imagine the global state (position, momentum) as position in a park
covered in snow. So someone walking through it will eventually walk over his
own footsteps. Not precisely but it's arbitrarily close, the longer one waits.

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

I really like that theorem

------
merpnderp
That 'normal' matter only makes up 5% of the universe. That some omnipresent
dark energy field makes up the majority (63%) of the universe.

------
rimunroe
From Introduction to Elementary Particles by David Griffiths: a neutrino of
moderate energy could easily penetrate a thousand lightyears of lead.

------
arkanciscan
That mitochondria have their own DNA and used to be separate organisms. Or
when I heard that trees feed each other and fungi through their roots.

------
yuningalexliu
You always weigh less in the morning compared to the previous night because
you have exhaled carbon atoms from your body!

~~~
buwka
Isn't most of reason you weigh less in the morning than at night due to the
amount of water weight you have lost while sleeping.

------
RichardCA
That something trivial like seeing speckle patterns in my eyes when I'm tired
is proof of the quantum nature of light.

~~~
AnimalMuppet
Interesting. How does that work?

~~~
RichardCA
I'm sure I'll do a poor job articulating it. It's similar to bokeh that
photographers are familiar with. You look at a light source and the light
seems to bounce through the pupil and back out again. If there's a drop of
liquid on your eyelashes you'll see a bokeh ring with interference ripples
around the edge.

I just remember as a child I thought it was strange, then in high school
physics I saw how laser light was always grainy, and even things like the
solidity of matter can be thought of as optical illusion (caused by the
wavelengths of visible light casting a well-defined shadow).

Then later I got into photography and realized what you can do with even a
little knowledge about light and optics. I still have a sense of wonder about
it even though I'm an average shooter at best.

Hope that all made sense.

~~~
RichardCA
This is what I'm taking about, when I was a kid before I got fitted with
glasses, I remember the world looking like this...

[https://photo.stackexchange.com/questions/99407/what-is-
the-...](https://photo.stackexchange.com/questions/99407/what-is-the-cause-of-
inclusions-in-my-bokeh)

~~~
AnimalMuppet
OK, but that's proof of the _wave_ nature of light, not quite of the _quantum_
nature.

~~~
RichardCA
Let's try to work the problem in a way that avoids pedantry, just for a
moment.

This is one of my favorite PBS clips from the 80's in which Richard Feynman
explains how strange the nature of light and by extension all of
electromagnetic radiation.

[https://www.youtube.com/watch?v=FjHJ7FmV0M4](https://www.youtube.com/watch?v=FjHJ7FmV0M4)

If you listen carefully, you notice that he never talks about wave/particle
duality (because he wasn't giving a classroom lecture).

What he does talk about is an electric field that waves slosh around in, and
that something "elaborate and complicated" is going on at a deeper level.

That's what I'm on about, if you were pressed to explain it so that a bright
child would get it, how would you do that?

~~~
AnimalMuppet
For _this_ problem/phenomenon, I'd probably just talk about waves and
interference. I might mumble something about something elaborate and
complicated going on at a deeper level, or I might ignore that and just talk
about waves.

------
m463
Trees form out of thin air.

------
quickthrower2
Black hole singularity is infinitely dense and zero size

------
giantg2
Hot water freezes faster than cold water (sometimes).

------
cvhashim
This question feels AskReddity

------
cameronfraser
camels have three eyelids

------
Chris2048
look at the time-lines of the universe and some of the entries, especially the
last few, are mind blowing (
[https://en.wikipedia.org/wiki/Timeline_of_the_far_future](https://en.wikipedia.org/wiki/Timeline_of_the_far_future)
)

e.g.

"Due to the gradual slowing down of Earth's rotation, a day on Earth will be
one hour longer than it is today"

"From its present position, the Solar System completes one full orbit of the
Galactic Center"

"All the continents on Earth may fuse into a supercontinent (Pangaea Ultima,
Novopangaea, or Amasia)"

"Tidal acceleration moves the Moon far enough from Earth that total solar
eclipses are no longer possible."

"the Andromeda Galaxy will have collided with the Milky Way, which will
thereafter merge to form a galaxy dubbed "Milkomeda" ... There is also a small
chance of the Solar System being ejected. The planets of the Solar System will
almost certainly not be disturbed by these events"

"time until stellar close encounters detach all planets in star systems
(including the Solar System) from their orbits"

"time until those stars not ejected from galaxies (1–10%) fall into their
galaxies' central supermassive black holes. By this point, with binary stars
having fallen into each other, and planets into their stars"

"estimated time for rigid objects, from free-floating rocks in space to
planets, to rearrange their atoms and molecules via quantum tunneling. On this
timescale, any discrete body of matter "behaves like a liquid" and becomes a
smooth sphere due to diffusion and gravity"

"... they [Positrons] find a distant electron to pair with and the two enter
into a highly excited state of positronium, with a radius larger than the
current universe. Over the next 10^141 years they will gradually spiral
inwards until they finally annihilate"

"time until a supermassive black hole with a mass of 20 trillion solar masses
decays by Hawking radiation ... marks the end of the Black Hole Era. Beyond
this time, if protons do decay, the Universe enters the Dark Era, in which all
physical objects have decayed to subatomic particles"

"... time for all nucleons in the observable universe to decay ..."

"... estimated time until all baryonic matter in stellar-mass objects has
either fused together [into iron-56] ... or decayed from a higher mass element
into iron-56 to form an iron star"

"Estimated time for a Boltzmann brain to appear in the vacuum via a
spontaneous entropy decrease"

"estimate for the time until all iron stars collapse into black holes ...
which then (on these timescales) instantaneously evaporate into subatomic
particles ... Beyond this point, it is almost certain that Universe will
contain no more baryonic matter and will be an almost pure vacuum until it
reaches its final energy state ..."

"Because the total number of ways in which all the subatomic particles in the
observable universe can be combined is <big number> a number which, when
multiplied by <big number>, disappears into the rounding error, this is also
the time required for a quantum-tunnelled and quantum fluctuation-generated
Big Bang to produce a new universe identical to our own ..."

