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Q: In what medium do Gravitational waves propagate?

8:42 AM

@JonathanJeffrey you should only delete the question if you think it has no potential use for future visitors - if you think it might be useful in some way, leave it up

3 hours later…

Emilio Pisanty

11:37 AM

I am having a hard time understanding how gravitational waves move. The only reasonable thing that comes to my mind would be the aether but that idea was scrapped.

gravitational-waves aether

duplicate?

11:59 AM

seems more or less equivalent to "in what medium do electromagnetic waves propagate in vacuum"?

It raises the spectre of what curvature means physically, for which I have never found a good answer. We get back to the usual escape clause that physical theories are just descriptions of what is happening and not necessarily actually what is happening.

Philosophers of science seem to enjoy this sort of thing though I can't help feeling it would be a better use of their time to download porn or post cat videos.

@JohnRennie I think the analogy with 2d people living on the surface of a sphere that is their "universe" is very apt - they, too, would wonder what "curvature" is since they cannot see the sphere from outside embedded in some 3d space, and they probably would be as unable to conceive of 2d curvature as a natural geometric property of their universe as we are unable to picture what 3d (or 4d) curvature "really means"

GR is geometry in the mathematical sense, but it is not geometry in a sense that is satisfying to the geometric part of our ability to visualize things

12:32 PM

Reichenbach has a whole chapter on the idea

What it is to live in a torus or something

@ACuriousMind except that if you tell students that they will immediately ask what the universe is embedded in :-)

the answer is yogurt

A friend of mine used to joke that if the universe is closed, what is inside the sphere is Azathoth

So ever since I give that answer

Q: Specific tag to add for my questions

1:02 PM

@JohnRennie Sorry for the double ping, but I’d be curious to know if the electric field pointing along the present position of the moving point charge somehow violates causality?

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Physics Meta

1:40 PM

With this question I would like to know what specific tag I can include in my questions on Physics.SE since most of them are about clarifications or curiosities or doubts that I encounter for my students of an high school of 14 to 18 years old. I have not seen nothing of similar. Thank you very m...

discussion support tags

1 hour later…

2:51 PM

if a bring a magnet closer to another stationary magnet, there comes a point where the second magnet starts moving and clings to the first magnet

but magnetic force = integral (dq(VxB))

why then, did the second magnet feel a force ?

oh hmmm wait, the electrons inside the magnet are moving

3:15 PM

@schn No it doesn't. And that's the surprising thing that Griffiths is referring to. There is no violation of special relativity, and no loss of causality, but the field really does point towards the present position of the particle not where it was a time x/c ago.

@satan29 it's a dipole-dipole force not a Lorentz force.

The dipoles come from the spin angular momentum of unpaired electrons in the ferromagnets.

hmm ok, but in a sense, what I said is also true right?

the electrons inside the magnet/magnetic material are moving

No, the electrons in a ferromagnet aren't moving. How would they be moving?

hmm I was picturing orbiting electrons

Forget you ever heard of the Bohr model!

Electrons do not orbit the nuclei.

yeah, I just realised

@JohnRennie so the proper explanation of this ever so simple question lies in QFT?

3:24 PM

You mean why electrons have a spin magnetic moment?

Or why the electron magnetic moments align to give a macroscopic magnetic moment?

(or both :-)

yeah, I asked this a few days ago and you remarked that spin is understood properly only in QFT

@JohnRennie both , yes :-)

Yes, though no doubt @ACuriousMind will be along any moment to say I have misunderstood things :-)

@JohnRennie wait though.

Jan 27 at 9:35, by ACuriousMind

@123 Ah well, that's now a bit too detailed - you need proper quantum theory to talk about the interactions between the magnetic field and quantum spin. Classically, you should just think of the electron as a tiny current loop (the spin corresponding to the orientation of the loop).

:-)

3:27 PM

Isnt a magnetic dipole simply a current loop?

A current loop is one way to get a magnetic dipole, sure

what are the others

All current loops are magnetic dipoles, but that does not mean all magnetic dipoles are current loops.

oh , I see

@satan29 an electron is not a current loop, but it does have a magnetic dipole.

Likewise any elementary particle with a non-zero spin.

3:29 PM

See my messages after the linked one for a brief description of how the idea of "tiny current loops" can explain the force between magnets, it's called the Ampere model but unfortunately there seem to be very few good descriptions of it on the 'net

I see, Thanks a lot!

3 hours later…

6:06 PM

@JohnRennie Is causality not violated since, if we were to measure the field of the moving point charge, it would still take some time for the field to reach us?

@schn It's hard to explain but it isn't. You'll need to read the derivation in Griffiths to understand it.

6:42 PM

@JohnRennie so when the field is measured at time t, it is the field from the point charge at the position it had at time t? Thanks for the replies.

It is the field of the particle at time $t - r/c$

You only see the retarded potential

But the field is still centered at the current position of the charge...something seems so contradicting.

Think of it like getting mail

You won't get any fresher news than the time it takes for getting the delivery