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location Austin, TX
age 35
visits member for 4 years, 8 months
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I am a Ph.D. general relativist working as a software engineer. I like to still go and do physics as a hobby, and to keep up my skill and knowledge.


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awarded  spacetime
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comment Why do electrons, according to my textbook, exist forever?
Technically, the neutrinos continually decay into each other: en.wikipedia.org/wiki/Neutrino_oscillation
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comment How close would you have to be to the merger of two black holes, for the effects of gravitational waves to be detected without instruments?
OK, I think I have a lazy, quick way of order of magnitude estimating this thing. I'll put it up tonight or tomorrow.
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comment Is momentum an invariant?
Like, the quantity you're talking about is explicitly frame-dependent, as your notation makes manifest. It's not an invariant.
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comment Is momentum an invariant?
@user12262: the frame velocity is 100% not Lorentz covariant. It is explicitly present in the equation. Therefore, a boost will change the frame velocity, and change your momentum.
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comment Is momentum an invariant?
@user12262: well, your formula has explicit $v$'s in it, so how invariant do you expect it to be?
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comment How close would you have to be to the merger of two black holes, for the effects of gravitational waves to be detected without instruments?
Are you detecting the plunge phase, the inspiral, or what?
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comment Why can I put my hand through sand but not a table?
Well, to answer your last question, what happens when you push a hammer through the table as hard as you can?
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awarded  Good Answer
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awarded  Nice Answer
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comment Why we don't see time dilation in stars orbiting black hole?
@BrandonEnright: be aware that there is expected to be a large amount of non-luminous matter around these objects, so perehelion precession will also be governed by this superious matter.
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revised Why we don't see time dilation in stars orbiting black hole?
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comment Why we don't see time dilation in stars orbiting black hole?
@eli.rodriguez: also note what I said in my answer when talking about accretion disks. Largely, they are still in orbits until they do the final plunge, but the final plunge is very fast, so the bulk of the luminosity of the disk will come from the plasma orbiting at a radius greater than $6M$ for a Schwarzschild hole.
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revised Why we don't see time dilation in stars orbiting black hole?
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answered Why we don't see time dilation in stars orbiting black hole?
Jul
21
comment When is Einstein summation implied by Lorentz indices?
I also can't think of a case where anyone would take the sum $\sum_{\mu}\sum_{\nu}A^{\mu \nu}$. Almost, universally, they are taking the trace of the matrix, which is labeled ${\rm Tr}(A^{\mu \nu})$ or they are contracting it on the metric, $g_{\mu \nu}A^{\mu \nu}$. The latter thing would usually just be called $A$
Jul
21
comment What would the consequences be if it was experimentally shown that antimatter bends spacetime opposite from matter?
at that point, you're no longer talking about the standard model. In the standard model, the photon is its own antiparticle. And yes, that's the thought experiment.
Jul
21
comment What would the consequences be if it was experimentally shown that antimatter bends spacetime opposite from matter?
@AsphirDom: it's pretty well established what antimatter is in particle physics.
Jul
21
comment What would the consequences be if it was experimentally shown that antimatter bends spacetime opposite from matter?
We've measured the response of antimatter to an external gravity field. It falls down at exactly the same rate as ordinary matter.
Jul
21
comment What would the consequences be if it was experimentally shown that antimatter bends spacetime opposite from matter?
I disagree with this pretty strongly. If antimatter gravitates negatively, it is basically negative energy in Einstein's equation. Unless there's a quantum proof tht bulk antimatter is impossible (and I don't think that there is, CP not being an exact symmetry aside), you've now found a way to violate the central assumption necessary to derive most of the consistency arguments in GR. Also, there's a simple thought experiment to show that if antimatter antigravitates, then you can build a perpetual motion machine.