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Slereah

11:32 AM

Big mood

user129412

11:57 AM

Thanks, that is also what I thought! Colleagues of mine said something about superconductors being poor thermal conductors, which I did not really understand because I thought phonons should just propagate through them. But maybe what they meant is that it is hard to thermalize the electrons in the superconducting material.

For the record, I don't think films are any different than bulk superconductors. Of course the boundaries will affect phonon propagation, but that is not specific to superconductivity.

2 hours later…

B. Brekke

1:31 PM

@user129412 Do you mean photons? Phonons are not well-defined perpendicular to the film. Edit: Ah, sorry, you are probably thinking about a heterostructure.

Shashaank

I had a doubt in Reissner Nordstrom solution. It is valid for a static electric field, that is we imagine the collapse of a star which had a static electric field everywhere in the space ( is that right, please comment that too). Now suppose after the black hole has formed, suppose an electron outside the horizon begins accelerating towards it. Now just after the electron starts accelerating towards it, will the Reissner Nordstrom solution be valid, because the charge is accelerating

and will contribute to the stress energy tensor via a radiation term. But the initial Reisner Nordstrom solution was for static field only. So will the solution be still valid or not. Can anyone help

Shashaank

1:47 PM

@Slereah am I right that in such a case Reisnner Nordstrom will not be valid

4 hours later…

Slereah

6:10 PM

The Reissner Nordstrom metric isn't valid for any realistic object

But there are theorems showing that most object of that type will evolve towards the Big Blackhole Solution

If you let all the collapsing stars work out all of their weirdness, eventually the spacetime around them will look like the appropriate black hole of mass M, charge Q and angular momentum J

There's a theorem for that IIRC but I don't remember which

BioPhysicist

@ACuriousMind You have upset someone

Shashaank

@Slereah I understand it isn’t valid for any realistic object. But my concern is in the domain where it is valid. Put in another way I am asking if a charge particle starts accelerating and falling towards the black hole will it constitute a radiation term to the Stress energy tensor appearing in the Einstein eqns or not and thereby changing the soln ( that is the soln won be Reisner Nordstrom).

Slereah

I mean yeah

The same way matter falling in a black hole makes it not a Schwarzschild black hole

Since it's a vacuum solution

Shashaank

6:47 PM

@Slereah yeah. Yeah. We talk about null geodesics without any light actually present to follow them

Slereah

Well you don't need light to talk about that :p

Shashaank

But I had another reason as well. A charged particle falling in the black hole, will it radiate. Seeing from the point of view of Newton it is accelerating but according to GR it’s freely falling. Will it radiate really

@Slereah

Slereah

Paradox of radiation of charged particles in a gravitational field

The paradox of a charge in a gravitational field is an apparent physical paradox in the context of general relativity. A charged particle at rest in a gravitational field, such as on the surface of the Earth, must be supported by a force to prevent it from falling. According to the equivalence principle, it should be indistinguishable from a particle in flat spacetime being accelerated by a force. Maxwell's equations say that an accelerated charge should radiate electromagnetic waves, yet such radiation is not observed for stationary particles in gravitational fields. One of the first to study...

This is what you need

Shashaank

7:11 PM

@Slereah Why are only paradoxes coming to my mind, why not something sensible

Slereah

You will have to ask a psychologist

But GR is complicated

There is a lot of counterintuitive ideas that appear within it

Shashaank

@Slereah :)

@Slereah and still it has stood the test of time. Lucky

Slereah

You have to figure out what things are "real" in GR and what is just a mathematical construct

Shashaank

7:26 PM

@Slereah yeah I do see it. By the way, a quick clarification. Gauss’s law in the form $\nabla . E=\pho/\epsilon_0$ will not hold true for accelerating charges, right

Slereah

There's a GR version of Gauss' law

It does involve the curvature

although of course, black holes are Ricci flat

Shashaank

@Slereah any link. Does it hold for accelerating charges

ACuriousMind

@Shashaank see physics.stackexchange.com/q/150687/50583

if you're doing relativity, "Gauß' law" is not the level at which you should be thinking about electromagnetism - you need to use the "covariant formulation of electromagnetism", where we don't really separate Maxwell's equations into different individual "laws" anymore

Shashaank

@ACuriousMind this looks nice. I want to give a thought to the last paragraph you have written there before getting back. But thanks both of you

@Slereah

2 hours later…