PeterDonis's latest activity

"Compressible" is a poor term. The key thing that a system of bosons can do that a system of fermions can't is to form a Bose-Einstein...

Not a vacuum spacetime, of course. But you might be able to by patching together regions with appropriate stress-energy tensors. I think...

Yes, it does! That was my point in post #15. Maximally extended Schwarzschild spacetime, which is a solution that meets the requirements...

That's correct. There are. One of them is...Schwarzschild spacetime! As I already pointed out in post #15. You are misunderstanding...

Yes, that's true, you can--for example, maximally extended Schwarzschild spacetime, with black hole and white hole singularities. Nor...

I think it is proven by Birkhoff’s theorem. Birkhoff’s theorem shows that the spherical vacuum metric is unique. If the metric you...

This is correct. See the Insights article I referred to, or MTW for more details.

This isn't possible; the "center" is defined as the locus ##r = 0##. Since ##r## is the areal radius, it's impossible for it to be...

No. The areal radius ##r## is an invariant. Picking it as one of the coordinates helps in making the proof of Birkhoff's Theorem look...

Right, which means, to amplify on @PeroK's answer a bit, that the system cannot be in any of these configurations. It must be in a...

Well, exactly! There is no realistic configuration that satisfies the requirements. I'm suprised this example isn't shown more often...

I think it's worth mentioning that this statement isn't true. The "electricity + length-contraction = magnetism" concept works for some...

Yes, that is the case. I think you misunderstood the videos, but you correctly understood the physics. The fact that the wire is...

A PF search should find several threads on that theorem with references.

From what I can see, again on a quick skim, these lectures don't discuss eternal inflation models at all.