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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.


1d
comment Does metal sublimate?
Mercury is a liquid at room temperature, so it's boiling, not sublimating.
2d
comment Noether's theorem in general relativity
If you're considering an arbitrary geodesic in a fixed spacetime background, then the charges you get out of Nother's theorem will depend on the fixed metric and its symmetries.
Apr
21
comment Black hole with two singularities?
@HarryJohnston: the short answer is because the time of an internal observer and an external observer is different. The long answer would look at HOW the horizon evaporates and would involve drawing a Kruskal diagram of this spacetime, and singularity formation would depend on this answer.
Apr
21
comment Naked Time ( Is there such a thing ?)
And you can come up with observer-dependent notions of time, but these will ultimately reduce down to one of the classical notions that one of those philosophers I cited used.
Apr
21
comment Naked Time ( Is there such a thing ?)
No. it's part of the underlying spacetime geometry. It cannot be meaningfully separated from space in relativistic theories.
Apr
21
comment Naked Time ( Is there such a thing ?)
This question has been treated pretty massively by the ancient Greeks and pretty much any natural philosopher since. I think it would be helpful for you to at least read on the Aristotelian notion of time, which does not explicitly rely on measurement. Hume also had useful things to say about this. I'm sure Kant did too, but it's been so long since i"ve read his impenetrable nonsense, that I don't remember.
Apr
20
comment Black hole with two singularities?
@HarryJohnston: OOOOHH. Yes. That's true. I'm kind of suggesting a bit of a cheat -- break up the spacetime into a foliation of spacelike surfaces, and then call each of these surfaces "an instant". It's an unphysical construction, but it works mathematically.
Apr
20
comment Black hole with two singularities?
@HarryJohnston: there's a topology change in the spacetime-- you go from two black holes to one. Any spacelike foliation will have this state. Similarly, the horizon is a coordinate-independent thing. How long the period lasts and the like may be coordinate-dependent, but a middle period where the horizons have merged, but the singularities have not IS something that can be defined in a foliation-independent way.
Apr
20
comment Black hole with two singularities?
They can, of course, be made precise, given an appropriate foliation of the spacetime.
Apr
17
comment Black hole with two singularities?
And there are other, vague restrictions you have to make for physicality -- the hole could have a nonzero Taub charge or something, but this answer is right enough for the puroposes of the question.
Apr
17
comment Solving Lagrangian equations of motion for two point-bodies with gravitational interaction
It will make deriving the equations a lot easier if you do the substitution first.
Apr
17
comment Solving Lagrangian equations of motion for two point-bodies with gravitational interaction
If you do this, then you'll see that your answers are yes, yes, and yes
Apr
17
comment Solving Lagrangian equations of motion for two point-bodies with gravitational interaction
You'll save yourself a lot of time and energy by replacing ${\vec r_{1}}$ and ${\vec r_{2}}$ with ${\vec r} = {\vec r_{1}} - {\vec r_{2}}$ and ${\vec R} = \frac{1}{m_{1}+m_{2}}\left(m_{1}{\vec r_{1}} + m_{2}{\vec r_{2}}\right)$
Apr
17
comment Was Nikola Tesla right about his ether theory?
I don't doubt that Tesla said something like that. He was probably refuting older theories that tried to assert that the field wasn't real. Also note that the dominant paradigm in chemistry at the time was Wilhelm Ostwald's energeticism, which asserted that there were no atoms and matter was infinitely divisible. In this mindset, electric "matter" was just another fluid. In 1890, this was plausible. By 1930, it was all definitively shown to be completely wrong.
Apr
17
comment Was Nikola Tesla right about his ether theory?
sigh Nicola Tesla did the bulk of his work before Einstein did his. He worked in the old paradigm. For a while after Einstein's paper and the michelson-morely experiment, there was some debate about special relativity. There is no longer any room for debate on the matter. A large chunk of our modern machinery needs special relativity and/or quantum mechanics to work.
Apr
17
comment How exactly does curved space-time describe the force of gravity?
This derivation is actually wrong, because $\frac{delta R}{\delta g^{ab}}$ is only equal to $R_{ab}$ up to boundary terms, which is enough to obtain the field equations, but will give you the wrong Hamiltonian, etc. And you have to take the variation inside of the integral.
Apr
17
comment How exactly does curved space-time describe the force of gravity?
However, I would say that if you're going to go into the technical details of taking the variation of the Hilbert action, you shoudl be honest enough to work through all of the details, and the variation above is fudged to work out right, but doesn't follow the rules of either the standard variation, or the Palantini version -- the $\frac{\delta R}{\delta g^{ab}}$ bit is just sleighted away. Of course, it takes Landau-Lifschitz 15 pages to do it.
Apr
17
comment How exactly does curved space-time describe the force of gravity?
I never got why Wheeler wasn't as famous as Feynman. He had that same magical way of reducing things down to really clear, simple statements that made complicated things seem obvious.
Apr
15
comment What is the significance of angular frequency $\omega$ with regards to wave functions?
Maybe this image can help... echteinfach.tv/formeln/geometrie/animations/…
Apr
12
comment Physical intuition on $\mathbf{v}\otimes \mathbf{w}$
@PyRulez: what I gave is more physical than any of the other answers. and a (1,1) tensor is equivalent to a (0,2) tensor in a metric space, which is every space that the OP has ever seen.