
Bohr–van Leeuwen theorem – magnetism in solids is a quantum mechanical effect - apsec112
https://en.wikipedia.org/wiki/Bohr%E2%80%93van_Leeuwen_theorem
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xxpor
Neither can even basic chemistry. Intro to chemistry is sort of just a giant
hint at quantum mechanics. When you get into electron shell configurations,
inevitably there will be a student who will ask why electrons arrange
themselves like this. The teacher will say "well, it's complicated". But it's
just basic quantum mechanics. I wish they would have explained the concepts at
least. Obviously the math is complicated but the ideas really aren't too bad.

~~~
casperc
I’d love to read more. Can you or someone else provide a relevant link?

~~~
Synaesthesia
Well classically, atoms wouldn’t even be stable. Their electrons would radiate
energy when orbiting and therefore spiral into the nucleus. So without quantum
physics you don’t have fundamental properties of atoms like energy levels
which explains their chemical properties.

~~~
shakow
> Their electrons would radiate energy when orbiting

What phenomenon would make them lose energy?

~~~
jbay808
Under the assumption that the electron is a point charge moving in a circular
path, it radiates electromagnetic energy as it moves, in the form of field
ripples.

This gif is for a 1d oscillation rather than an orbit, but it does a good job
of conveying the idea.

[https://thumbs.gfycat.com/ExhaustedAlarmedDeinonychus-
size_r...](https://thumbs.gfycat.com/ExhaustedAlarmedDeinonychus-
size_restricted.gif)

Interestingly enough, if the electron were a continuous ring (like Saturn's
rings) rather than a point charge, it could orbit without losing energy,
generating only a static magnetic field. I don't know why this model rarely
comes up in explorations of classical physics models of the atom.

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

Richard Feynman explaining magnets is one of the best layman explainers I've
seen (in between the ranting). I honestly didn't understand why magnets worked
and found this on a late night youtube binge.

(But don't search youtube for anything related to magnets, you end up amongst
the perpetual motion nuts)

Edit: Just to be clear with this video (it had been a while since I last
watched it), he doesn't 'explain' magnets as much as he hints at the concepts
involved. I already knew about the spins being lined up etc but it was the
"you can't put your hand through the chair" comment that gave me the eureka
moment.

~~~
axaxs
I don't know, maybe I'm the odd man out, but I've been linked to this video
tens of times and always hated it. I find Feynman comes across extremely
condescending and wastes time explaining that the interviewer is too dumb to
understand in a very roundabout way. One thing I've come to really respect in
my CS career are people not only able but willing to distill knowledge down
into an approachable way. I've come across so many people with the same
attitude as seen in the video, especially in the DNS world for some reason,
maybe that's why it strikes a nerve.

~~~
kybernetikos
I agree with this. I found it very off putting and not what I'd expected from
someone lauded as an amazing teacher.

Having said that, there is a lot to think about in his irritation. For
example: the mysteries of magnets are no more mysterious than lots of other
things that we take for granted and treat as entirely unmysterious. To
priviledge magnets with an almost magical respect when we haven't spotted that
gravity is at least as mysterious is a failure of imagination.

There's also the question 'what kind of answer would you accept'? When we can
only express bafflement about a phenomenon, we're still in the prescientific
myths and magic world. You can't really get to the scientific world until you
can formulate some kind of question. The passage from Hitchikers about the
Ultimate Question touches on something pretty profound.

Still, I think an honest expression of bafflement is an excellent beginning
(or at least it's where I often find myself), and I think there may have been
more sympathetic ways Feynmann could have expressed his thoughts on the topic.

~~~
acqq
I understood that differently:

[https://news.ycombinator.com/item?id=22816968](https://news.ycombinator.com/item?id=22816968)

Hint, the questions were:

"Now, what is it, the _feeling_ between those two magnets?"

" _There 's something there_, isn't there?"

" _what 's going on_ between these two bits of metal"

" _What does that mean, or why are they doing that, or how are they doing
that?_ "

I don't know what phenomena you study, but I assure you that "magnets" aren't
the only topic for which the "feeling" and "what does that mean" and "why"
questions would warrant the same answer.

And where answering "how" would be cheating if the answer would use some false
analogy.

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HPsquared
Magnetism is a side effect of special relativity: moving charges have length
contraction (depending on frame of reference) which makes them appear
more/less dense and therefore an attractive/repulsive force.

Veritasium on YouTube made a much better explanation:
[https://youtu.be/1TKSfAkWWN0](https://youtu.be/1TKSfAkWWN0)

~~~
dnautics
Yeah but that doesn't explain permanent magnets, which is the subject of this
theorem.

~~~
kjeetgill
This video was a collaboration between Veritasium and Minute Physics.

They did a second one on that subject on the other channel:
[https://m.youtube.com/watch?v=hFAOXdXZ5TM](https://m.youtube.com/watch?v=hFAOXdXZ5TM)

~~~
dnautics
Indeed, that is one of my favorites; I should have pointed that out in my
comment.

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Invictus0
Why is it called the most deflationary finding of all time? What does
deflationary mean in this context?

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ganzuul
For me
[https://en.m.wikipedia.org/wiki/Magnetic_refrigeration](https://en.m.wikipedia.org/wiki/Magnetic_refrigeration)
really opened my eyes to post-classical physics. Not only is it a great way to
understand entropy, but soon we may have quiet and efficient refrigerators in
every home thanks to this research!

I wish there was a similar intuitive application for quantum plasma and
superfluidity too, but the existence of high-temperature superconductivity
seems to indicate there is none as of yet.

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ptrincr
There is an interesting answer here with regards to how the Pauli Exclusion
principle is related to magnetism.

[https://physics.stackexchange.com/a/246439](https://physics.stackexchange.com/a/246439)

As I understand it, with my limited knowledge, deeper down it involves the
exchange of virtual photons between electrons. I can see how this explains
repulsion.

Though how this leads to attraction and how this looks as a Feynman diagram
I'm not sure.

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effie
> *"and means that classical physics cannot account for diamagnetism"

[assuming classical statistical physics assumptions, such as nothing special
happens at the boundary]. The whole result is due to unrealistic boundary
condition (for magnetic materials) that the Boltzmann isotropic distribution
of momenta is valid at the boundary. Diamagnetism is present in classical
physics, for example, electron(s) circling in magnetic field create magnetic
moment opposed to the magnetic field.

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amelius
The reason why electrons don't fall into the nucleus due to Coulomb attraction
is QM. So from there, you can say that a lot of things are actually a QM
effect.

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Blackthorn
I'm always fascinated how discoveries like this can often be simultaneously
discovered only a few years apart, like what happened here.

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ajawee
During my classes, I always see waves going up and down to represent sound or
magenetic waves. I really cannot visualize in real life. What exactly the ups
and down in real time?

~~~
twic
For sound, at least, the waves are not up and down, but longitudinal. Perhaps
most easily illustrated using a slinky:

[https://www.youtube.com/watch?v=GIkeGBXqWW0](https://www.youtube.com/watch?v=GIkeGBXqWW0)

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

For electromagnetic waves, as i understand it, there isn't actually movement
up and down in space; there are oscillations in the electric and magnetic
fields. Which do somehow have a spatial direction, which is why light can be
polarised. I don't have an intuition for it at all!

~~~
jcims
The challenge with the longitudinal example, which is of course technically
correct, is that it’s hard to map it to the spherical shells that most of
these phenomena radiate into.

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Koshkin
Also, it is a widely known fact that classical electrodynamics is not
compatible with classical mechanics.

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peter_d_sherman
>"The significance of this discovery is that classical physics does not allow
for such things as paramagnetism, diamagnetism and ferromagnetism and thus
quantum physics are needed to explain the magnetic events.[5]"

Well, that might be true and all, but I like the following simpler
explanation:

1) There is an extra dimension in space.

2) This extra dimension in space can be occupied by magnetic and other fields.
They are unseen, but present when a magnetic object is present, much like
radio or other electromagnetic waves are unseen, but can be present.

3) These fields, when present, exert a force on the magnetic objects they
occupy, and can be used to exert force on other magnetic opjects, without the
two objects ever physically touching.

Which also means that:

4) Magnetic fields are somehow intricately related to the materials they
occupy, that is, the matter, at either the molecular or atomic levels...

If this is true, then one weird model for magnetism might exist as the
"displacement of space"...

For example, you take a stone, you put it in some water, it displaces a little
bit of that water.

But not enough to really see anything...

So now the $64,000 question:

What happens when you have a fixed
ditch/gutter/trench/trough/channel/container of water, and you start to cram
in stones, a whole lot of stones, such that each stone starts to displace more
and more water?

?

Oh, and lets suppose that it's open on one side... and in that side is a much
thinner open trench, which loops around to the other side, the closed side of
the container?

Well, put enough stones (atoms, molecules) there, and they'll displace the
water such that it flows into the small looped external open trench...

So, is a magenetic field -- actually the _displacement of space_?

I don't know, but there might be a case to be made for it...

Also, other observation: IF a magnetic field is in fact the displacement of
space -- then this would mean that the magnetic field lines... represent a
different kind of space than regular space... in other words, _there are at
least two kinds of space_ , _regular space_ and _magnetic field line space_.

Then of course the next question is, how are these two types of space the
same, and how are they different?

How do you make one from the other (without using a magnet!), and vice-versa?

But of course, all of the above is highly speculative...

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yters
Magnets are teleological, whereas classical mechanics is entirely efficient
and material causes.

