
Physics facts not always taught in school - monort
https://backreaction.blogspot.com/2018/07/10-physics-facts-you-should-have.html
======
Xcelerate
I think the coolest fact that I didn't grok until undergrad is that gravity
relates to the stress-energy tensor and is not just due to rest mass alone.
Which means that anything that has energy associated with it (i.e., everything
we've ever observed) affects the gravitational field.

Examples: A compressed spring weighs more than the same spring in an
uncompressed state. A box of light (e.g., a box with perfectly reflective
mirrored walls) weighs more than the same box without light, despite the fact
that photons have zero rest mass. The bulk of a proton's "effective mass" is
due to the kinetic energy of the quarks that comprise it. The joint earth +
moon system weighs less than if you added up each component weighed in
isolation.

The other interesting fact is that the Heisenberg Uncertainty Principle is
commonly misunderstood to mean that you can't simultaneously measure the
position and momentum of a particle. In fact, you can indeed obtain _partial_
information on both properties at the same time, and there's quite a few
papers out there describing how to perform joint measurements of incompatible
observables.

What HUP more accurately entails is that, for a quantum state corresponding to
a specific system, you cannot obtain complete information about the two
properties — no matter what you do. This is because performing a position
measurement destroys your ability to perform a subsequent momentum measurement
(on the same system) and vice versa. One of the postulates of quantum
mechanics is that measuring a system collapses it into an eigenstate of the
observable corresponding to the type of measurement that was performed. Since
the position and momentum operators don't commute, it's not possible to put
the two into a joint eigenbasis (they can't be simultaneously diagonalized).

~~~
sls
Since you obviously like this sort of thing, let me point out that quantum
uncertainty isn't simply a factor of measurement/observer effects[1][2]. (The
only part of what you said that I am addressing by this is the "this is
because" section.)

[1] [https://www.nature.com/news/quantum-uncertainty-not-all-
in-t...](https://www.nature.com/news/quantum-uncertainty-not-all-in-the-
measurement-1.11394) [2]
[https://en.wikipedia.org/wiki/Uncertainty_principle](https://en.wikipedia.org/wiki/Uncertainty_principle)

~~~
Xcelerate
Yeah, you may also find interaction free measurements interesting
([https://en.wikipedia.org/wiki/Interaction-
free_measurement](https://en.wikipedia.org/wiki/Interaction-
free_measurement)).

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wodenokoto
What kind of school? I have a masters degree, and I fail to see where these
facts would fit into to my education.

Quantum mechanics and Feynman diagrams in first year of highschool?

~~~
golergka
Don't they teach basic quantum mechanics in American high schools?

~~~
pytyper2
It is offered but the student must elect to take the course, it is possible to
pass through an American public school system without taking any advanced
topics. Someone who does not enroll in courses of at least college prep level
will arrive at university and be required to take some remedial credits to
catch up with their peers.

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TheCoelacanth
> Wormholes are science fiction

It would be more accurate to say the wormholes are hypothetical than to say
that they are fictional. They are an idea someone came up with of something
that might really exist, we just don't have any evidence of them.

~~~
akvadrako
Wormholes could potentially exist; there is even a well-known non-crackpot
theory called ER=EPR that roughly suggests entangled particles are connected
by wormholes.

What is fiction is wormholes you can send information or people though.

------
dschuetz
The title is misleading. It's an opinion from some blogspot blog without
sources.

" _Aspects of theoretical physics_ not always taught in school" would be more
appropriate.

~~~
robinhouston
I think it’s misleading to characterise Sabine Hossenfelder’s blog as “some
blogspot blog”. She is a serious and respected theoretical physicist, not some
random blogger.

~~~
dschuetz
She's a pop-science writer. She _is_ a random blogger. She has a job as a
physicist. It doesn't make her opinion a fact, just because she's a
theoretical physicist with a job and a blog.

~~~
nabla9
What she says are all kind of basic facts for any physicist, not something new
research that needs to be sourced and skeptically considered.

He just blogged educational insights.

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noobermin
I like the first part, entropy is essentially a measure of degeneracy for a
state (the elementary definition is the log of the number of states multiplied
by a unitful constant). Technically, if you're a supposed super-intelligence,
that is good at remembering detail, every state of a system can be
distinguishable and thus have small entropy. For example, consider a finite
number of legos in a room. A computer could potentially remember where every
lego is placed, while person can't do as well but could distinguish between a
state where the legos are strewn about or built into a castle. So, the person
would lump all the disordered states into just one state (the "mess" state)
and give it a high entropy compared to the number of organized states (castles
or ships made out of the legos).

I guess I always knew this, and this is sort of what we mean when we say "high
entropy" but it's kind of fun to say it out explicitly like this. Most of the
others seem like conflating the strict applicability of a theory vs. it's
practical limits, like that QM doesn't really mean "small" or (equivalently)
high energy, it just means when you're near the Heisenberg uncertainty limit
for the observations you're making, which in most cases means small.

Allow me to add another one: "Special Relativity means when you go faster,
time slows down for you!"

And another (controversial may be): "In Schrodinger's cat, the cat is both
dead and alive at the same time!"

~~~
PopePompus
> For example, consider a finite number of legos in a room.

This works with legos, but not subatomic particles. No matter how smart you
are, you can't distinguish between two states which differ only by having two
electrons swapped.

~~~
noobermin
Fair. This is mainly for macroscopic systems, not fundamental particles or
other species that cannot be distinguished at all, then all you can do is lump
the states together and give it a degeneracy.

------
mehrdadn
> You can fall into a black hole in finite time. It just looks like it takes
> forever.

This one is really interesting! I've definitely never seen anyone emphasize
this before.

~~~
darkmighty
There are variations on this that are more problematic (which I've never seen
solutions to).

1) It looks like it takes forever, to an outside observer, for anything to
reach the event horizon. So if no matter appears to reach event horizons, how
can they (event horizons) form in the first place (i.e. exist in our
universe)?

2) While it takes forever for matter to fall in (again to outside observers),
it takes finite time for black holes to evaporate via Hawking radiation. So
not only do black holes would take forever to form, they seem to have to
extinguish before being able to exist (again in the sense of event horizons).

I'm not a physicist, but my speculation is that Black Holes don't really
exist, they're just a limit of a process that approaches but doesn't converge
to a singularity, which I think is unphysical.

~~~
mehrdadn
I'm also not a physicist but I think I can guess a plausible answer to these?

1\. Because if you don't have a black hole yet then there is no event horizon
to prevent you from falling in and creating one. I think the event horizon
wouldn't form at exactly the same point whose crossing would finally increase
density enough to create it, so this shouldn't be a problem. (Although, again,
since the event horizon doesn't exist yet, I think it might still not be a
problem even in that case?) I would also expect that quantum fluctuations can
also inject matter into a black hole, just as they can remove matter from it.

2\. Again, they wouldn't take forever to form -- see above.

I'm pretty sure black holes have been observed indirectly, so the idea that
they actually don't exist would require a pretty rock solid alternative
explanation!

~~~
pdonis
_> I'm pretty sure black holes have been observed indirectly_

What we have observed indirectly are compact objects that emit no light, but
contain a large enough mass in a small enough volume that they can't be
anything else but a black hole, _if_ we only take classical GR into account.

As I responded in another post just now, it is possible that black holes, in
the sense of objects with actual event horizons (boundaries of regions from
which light will never escape, even in the infinite future), cannot exist when
quantum gravity effects are taken into account. If that is the case (and it is
not clear whether it is--as I said, this is an open area of research), then
the objects we call black holes based on our current observations won't have
actual event horizons--eventually, in the very far future, light will escape
from those compact regions. But they will still have _apparent_ horizons,
i.e., surfaces from which outgoing light is not escaping now (or for a very
long time in the future). And that, in itself, is sufficient to show that the
issues raised by darkmighty are not valid.

------
sunstone
I, for one, thought that conservation of energy was iron clad. It turns out
that "except for the expansion of the universe" proviso, it is. As a religious
adherent of the conservation laws this makes me unhappy. And brings up new
questions. I'll have to sleep on this one.

------
platz
> the idea that any system tries to minimize its energy is just nonsense.

Very interesting, I feel somewhat misled if this is true.

~~~
lostmyoldone
I'll try to answer as best as I can.

While it is true that a system doesn't minimize the energy of the entire
system, as energy IS conserved, except apparently for a miniscule amount due
to the universe expanding, it is also true that locally it often looks like
minimizing energy in some sense.

If you have a vibrating spring on a table, after a while it will have stopped
because it has created motions in the air and in the table, spreading out at
the speed of sound in whatever material it encounters, and some of it also at
the speed of light in the form of thermal radiation.

So, looking only at the spring, it is minimizing it's energy by returning to a
stable quiscent state. In the same way, if we look at a sphere with radius as
"the speed of light times the time from the start of the experiment", all
energy can still be accounted for within that volume.

One system, several perspectives.

I think the minimization model is often used because the statistical model of
entropy can be hard to explain, as it simultaneously allows complex relatively
ordered systems like us humans to exist, while at the same time explains the
almost complete disorder of thermal energy and how it disperses.

Thus, since the energy minimization of the local state can be used as a
relatively truthful proxy, we learn that model first. With the possible bonus
that it also allows one to temporarily avoiding some rather (afaict) gnarly
math until you actually have the tools you need to understand it.

------
dschuetz
To evaluate the validity of those facts, whom can I ask for advice? Because, I
cannot afford years of study in physics to understand and assess the relevance
of those facts to graduate school.

Besides that, when we talk about _facts_ , why is that I cannot find any solid
scientific sources to those claims in that article?

~~~
pdonis
_> To evaluate the validity of those facts, whom can I ask for advice?_

You could ask other physicists. But you would not know whether to believe them
any more than you know whether to believe the author of this article.

 _> when we talk about facts, why is that I cannot find any solid scientific
sources to those claims in that article?_

Where have you looked?

~~~
dschuetz
I looked for footnotes, haven't found any, besides some other blogspot links
and an aeon article.

The title is misleading, there are little to none sources, the whole thing
smells like an opinion and pop-science. I might not be a physicist, but I'm
not lacking competency in reading.

~~~
pdonis
_> I looked for footnotes, haven't found any_

It's a blog post, not a scientific paper, and the statements it makes are very
general, of the sort you will find somewhere in many textbooks, but not in one
specific place that can be referenced. What you're asking for is kind of like
asking for a specific footnote for the definition of a prime number in a blog
post about number theory.

If what you are looking for is a quick way to know whether what the article
says is true without having to take any time to actually study science, there
isn't one. There is no royal road to learning.

------
avmich
The very first item - disorder vs. likelihood. Can somebody explain the
crucial difference?

Why the dough is of higher entropy than the (random) distribution of dough
components?

------
kgwgk
> Which state is more orderly, the broken egg on flour with butter over it, or
> the final dough?

> I’d go for the dough.

Strange.

~~~
avmich
Thanks, you're answering my question. It's not a random distribution - it's
separate substances (butter separate from flour separate from egg), so their
order is actually higher. Then I agree - it's strange the author goes for the
dough.

~~~
stan_rogers
I think that's meant to be an illustration of the naïve tendency to think of
something purposeful - an intended state - as being more ordered than
something broken or scattered. A dough is a thing you made for a reason, a
mess on the floor is just a mess on the floor. Don't take it as a physicist
thinking in physical terms, but as the immediate impression of an ordinary
person in ordinary circumstances - you need to reflect and understand to see
the truth.

~~~
cygx
Not only that: Think about a canvas with some random splatters of paint,
compared to a canvas uniformly coloured with a homogeneous mixture of that
paint. What's more 'orderly'?

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asimpletune
Wait, there’s no conservation of energy?

~~~
nabla9
That's true.

Conservation of energy is not a physical law. It's a consequence of
mathematical theorem called Noether's theorem. All systems with certain
differentiable symmetries have a corresponding conservation laws.

In a system that is not time translation invariant, energy don't have to be
conserved. Expansion of universe breaks the time symmetry.

~~~
sunstone
"Noether's" theorem. Is that a joke or what?

~~~
grkvlt
Nope, named for Emmy Noether.[0]

0\.
[https://en.wikipedia.org/wiki/Noether%27s_theorem](https://en.wikipedia.org/wiki/Noether%27s_theorem)

~~~
sunstone
Wow, and in 1915, I doubt a scientist never had a more fortuitous and timely
name :)

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craftyguy
> If the original title begins with a number or number + gratuitous adjective,
> we'd appreciate it if you'd crop it. E.g. translate "10 Ways To Do X" to
> "How To Do X," and "14 Amazing Ys" to "Ys." Exception: when the number is
> meaningful, e.g. "The 5 Platonic Solids."

[https://news.ycombinator.com/newsguidelines.html](https://news.ycombinator.com/newsguidelines.html)

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chasing
This is interesting, but I'm going to be one of those people for a second...

I hate headlines that assume what I do and don't know. Just call 'em 10
physics facts that most people aren't aware of and are pretty interesting.
Because I actually did know a bunch of those facts from school and I was a
liberal arts major...

~~~
dang
Yes, it's annoying and, as other users pointed out, broke the site guidelines.
There's the magic number thing but also the linkbait 'you'. We've reworded it
to follow them instead.

~~~
dustingetz
please embed the faq on the submit page and the reply page and these problems
will go away and this conversation can stop happening

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throwaway2048
Please keep click-bait title crap off HN.

