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12552
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location Austin, TX
age 34
visits member for 3 years, 8 months
seen 5 hours ago

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.


5h
comment Does spin-0 or spin-2 describe massive or massless particles?
YOu can have both massive and massless particles at both spins. There are beleived to be stability problems with massive spin-2 theories, though.
5h
comment Can the magnetic fields of EM radiation be harnessed or measured?
Note that the magnetic field is generally only negligible because most ordinary objects travel with speeds $\ll c$. For objects moving relativistically, then since the magnetic force $\propto vB$, and since $E/B =c$ tells us that the magnitude of the electric field is greater than that of the magnetic field by a factor of $c$, we see that the forces are then roughly equal.
5h
comment F-mu-nu notation
@SmikGames: the latter one would be a field tensor that takes values over a Yang-Mills group
10h
comment The Alcubierre drive and closed timelike curves
Alcubierre's original published metric is free of CTC. You can get superluminal global travel without CTC, so long as there's no turnaround -- just think of the stars beyond the cosmological horizon. Their proper distance with respect to us is greater than the speed of light, but it just doesn't matter, because there's no way to send a signal back and forth, or for them to "turn around".
1d
comment Why there does not exist any Gravitational Magnetic Field?
@Dvij: you can't predict normal magnetism using classical mechanics only. To see that you need magnetism if you have an electrical force, you have to make an appeal to Lorentz invariance.
1d
comment Charge Distribution in Reissner-Nordström Black Holes
Same thing. The Kerr solution is an electrovac solution (and reissner-nordstrom is a special case of the Kerr solution)
1d
comment In a Big Crunch, would there be more mass than at the Big Bang?
One quibble: some non-asymptotically flat spacetimes do have definiable local energies over things like the cosmological horizon--you just need a null or timelike 3-surface with a null or timelike killing vector, and you can define the energy contained in that surface. This isn't the case for cosmological horizons, though.
2d
comment How can space be euclidean when light bends?
Space can be made aribitrarily close to Minkowski(spatial part Euclidean) by choosing a small enough four-cube of spacetime. So, sufficiently small volumes are, in a sense, exactly Euclidean.
2d
comment Maximum curvature in a black hole
Could you expand this question a little bit? It reads to me like "If a is true, then is it possible that a is valid?"
2d
comment On the coordinate independence of general relativity
In particular, there are several solutions to Einstein's equation in vacuum that are NOT just Minkowski space, because they contain gravitational radiation, for example (other, more exotic contents are possible)
2d
comment Is there a substance that doesn't reflect OR absorb light from the visible light spectrum?
Well, incoming energy can always be transmitted.
2d
comment Lagrangian for FRW metric
are you sure, that's not supposed to be $\left(\frac{d{\vec x}}{dt}\right)^{2}$?
2d
comment On the coordinate independence of general relativity
Flat space is <b>Riemann</b> flat, not Ricci flat. All Riemann flat spacetimes differ from the Minkowski metric by a coordinate change. It's actually true that Einstein's equation will have multiple solutions, IN GENERAL for a given stress-energy tensor. You also need to specify initial conditions and boundary conditions for the metric.
Jul
26
comment $F=ma$ calculation taking relativity into account?
@BenCrowell: and with elementary questions like this, I'd rather provide hints and places to look and things to think about than I would provide a comprehensive answer.
Jul
26
comment $F=ma$ calculation taking relativity into account?
@BenCrowell: force increases the momentum, but the asymptote in the formula for momentum allows the momentum to increase without bound, even while $v < c$
Jul
24
comment number of gravitons launched by a proton
Though I do hesitate to say that the off-shell particles are "there" at all. The feynman diagram above is, after all, ultimately an artifact of perturbation theory, which depends on our approximation scheme, not on physics.
Jul
24
comment Electrostatics coloumb's Law
@SeñorO: the question is poorly phrased, but not incorrect. Are the spheres conducting? How far apart are the surfaces of the two spheres? I will not answer this question, however.
Jul
21
comment The problem in Sredniki's textbook: How do I calculate loop corrections for $\phi\phi\to\phi\phi$ with this Lagrangian?
How can the theory be truly nonrenormalizable if it is related to an exactly solveable one by a change of variables?
Jul
21
comment Frame dragging — is there a “non-tiny” example?
Just for a point of clarity: there is no shell theorem for an axisymmetric spacetime like there is for a spherically symmetric spacetime. We therefore don't expect the spacetime outside of a spinning mass to be exactly the Kerr metric, as the actual geometry would depend on the multipole moment distribution of the matter distribution. Now this wouldn't matter much in most practical cases, since we would be expanding to first order in the angular momentum in any case...
Jul
21
comment Relatvity of Promise
@Iota: it put a limit on programming computers to do nonsense. But GIGO is totally true before you invoke relativity.