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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.


13h
comment Does GR really allow superluminal movement?
@Jim: what is relative velocity and what is expansion of space is coordinate-dependent.
14h
comment Does GR really allow superluminal movement?
Note that global superluminal travel is allowed for paths that cross apparent horizons. These restrictions are on superluminal travel that allows for global round trips.
14h
comment Does GR really allow superluminal movement?
@Jim: what you say applies to round-trip superluminal travel. But you can certainly have superluminal "relative velocities" invoking only ordinary matter. Consider our relative motion with matter outside of the visible universe, for example.
1d
answered Can weakness of gravity explore new dimensions
2d
answered 'Warp'- type travel; is it possible?
2d
comment Wouldn't angular momentum of a binary star system decrease?
Also, note that the Hulse-Taylor curve is nearly perfect withouta accounting for any quantum effects: astro.cardiff.ac.uk/research/gravity/resources/hulse_taylor.jpg
2d
revised Wouldn't angular momentum of a binary star system decrease?
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Apr
15
comment Metric for infinite straight cosmic string
Where is your fourth spatial coordinate?
Apr
15
comment Wouldn't angular momentum of a binary star system decrease?
Note that this is not definitive, as there are orbit stability effects from general relativity to care about, amongst other things, but this should be enough to show that these orders of magnitude combine badly.
Apr
15
answered Wouldn't angular momentum of a binary star system decrease?
Apr
14
revised What is the notion of a spatial angle in general relativity?
corrected christoffel symbol
Apr
14
comment What is wrong with considering the Atwood machine as a system?
@Shubham: if $m_{2}\neq m_{3}$, the center of mass of the system B moves relative to the pulley, so its net acceleration is $a_{0} + a_{cm}$. It is almost certainly easier to just solve this by doing free-body diagrams for all three masses, rather than having to worry about treating the internal system properly.
Apr
14
comment fusing two air containers with fixed pressures $p_1,p_2$ and temperatures $T_1, T_2$, what end-result $T$ and $p$ will be?
You need a second equation that expresses the conservation of energy.
Apr
14
revised What is the notion of a spatial angle in general relativity?
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Apr
14
revised What is the notion of a spatial angle in general relativity?
added 187 characters in body
Apr
14
comment What is the notion of a spatial angle in general relativity?
And, I should add that the notion of a spatial angle is always definable so long as you have a local notion of time at a point. You just have to take your two vectors, remove the time component from both, take the inner product of the new vectors, and then you have $\cos\theta = \frac{a^{i}b_{i}}{\sqrt{a^{i}a_{i}b^{i}b_{i}}}$. Note that this will depend on the choice of time, but it will have to, because relative spatial angles of vectors are not preserved by Lorentz boosts, much less general coordinate transformations.
Apr
14
answered What is the notion of a spatial angle in general relativity?
Apr
11
comment What arguments are in favour of an atomic structure to space-time?
@Xiao-GangWen: a finite square well still has an infinite-dimensional Hilbert space. Actually, it's making your problem worse, because adding the scattering states makes the dimensionality of the Hilbert space uncountably infinite.
Apr
10
comment If energy can neither be created nor be destroyed,what is the ultimate source of energy?
This doesn't answer why the value of the energy of the universe is $X$ rather than $Y$. If the total energy of the universe is anything other than zero (though I guess you can always add a constant, so this may just be a non-sequitir), I think it's completely valid to ask why it is what it is.
Apr
8
comment Minkowski Metric Signature
Really, the root answer is that you don't get much by adding all of the machinery of complex numbers -- what is the conjugate of a spacetime point, and what does it mean? Should inner products be $v^{*}\cdot v$? Why is the time coordinate the only one that takes a complex number? If you say "there's just a minus sign" in the signature, you avoid these problems.