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PM 2Ring

12:50 AM

The speed of light is 1 in natural units. And I suppose 1 is a mathematical construct...

11 hours later…

user129412

11:22 AM

Yes, sorry. I was thinking about a superconducting film on, say, a silicon substrate. Me and some colleagues were discussing the thermal conductivity of superconductors at mK temperatures, and it was stated that they do not conduct heat. But I think that's wrong; there is virtually no heat carried by electrons, but it is still carried by phonons, right?

Or maybe phrased slightly differently, if we put a purely superconducting link between a 4K stage and a mK stage, it is not a perfect thermal isolation, right? It's just a rather poor conductor

3 hours later…

Slereah

2:30 PM

@RyanUnger I mean basically yeah

1 hour later…

G Warner

3:50 PM

This is all just for fun. I'm no math/physics genius. I saw a nerdy physics related T-shirt in a movie. There is a statement, followed by a mathematical equation. I could see "Here, let me explain.." but the equation was not fully visible. Is there an equation or an incomplete equation that formulates how to try to explain the unexplainable? I'm imagining an over complex way to expalin something that is atually easy to understand.

Charlie

Without seeing at least part of the equation in question it really could be anything

G Warner

Like 2+2=4, but a complex formula that would do the same thing?

I will try to get one.

Charlie

You could make an equation like $2+2=4$ arbitrarily complicated by just doing random operations that leave it valid

G Warner

okay thanks.. m.media-amazon.com/images/I/…

Charlie

That's sort of different, it's a force diagram for an object rolling down a hill, the pun being the obvious one :p

Without a Compass

4:29 PM

I don't fully understand the top answer to this question: physics.stackexchange.com/questions/433678/…

I don't quite get how you can bound the integral of d^3x J(x)G(x-y)J(y) without explicitly knowing the matter field J

5 hours later…

Bohemian relativist

9:40 PM

is it easy to prove that the Pauli-Lubanski vector transforms like a 4-vector under Lorentz transformation, that is, $[W^\mu, M_{\tau\lambda}]=i\delta^\mu{}\tau W_\lambda-i\delta^\mu{}_\lambda W_\tau$? I feel it looks like not easy.

the Pauli-Lubanski vector $W^\mu=\frac{1}{2}\epsilon^{\mu\nu\rho\sigma}P_\nu M_{\rho\sigma}$

ACuriousMind

@Bohemianrelativist if you write it like that, what is there to prove? Everything on the r.h.s. is a proper tensor, so their contractions are proper tensors, too. The whole thing has one free index, so it's a 1-tensor = vector.

Bohemian relativist

10:02 PM

@ACuriousMind I am not quite know what you mean. which one do you mean is a proper tensor? $W_\lambda, W_\tau$?

ACuriousMind

The $\epsilon$, $P$, $M$ are proper tensors, so $W$ is, too

it has only one index, so it's a vector

but if you wish you can just do it by a brute force calculation: You know the commutation relations of $P$ and $M$ with $M$, so just substitute the definition of $W$ in the $[W,M]$ and plug away

the calculation might be tedious, but that's different from being difficult :P

Bohemian relativist

@ACuriousMind I just tried to prove by using $[M_{\mu\nu}, P_\rho]=...$ and $[M_{\mu\nu}, M_{\rho\sigma}]=...$, but it looks not easy to get such a simple result. Or maybe it's because the weather is too hot so that my brain cannot function well.

Slereah

10:28 PM