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location Austin, TX
age 34
visits member for 3 years, 5 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.


1d
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
comment Metric for infinite straight cosmic string
Where is your fourth spatial coordinate?
2d
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.
2d
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.
2d
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.
2d
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
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.
Apr
8
comment Is the sun's solar radiance spectrum matching up with water's absorption spectrum just coincidence?
There's no a priori reason to believe that intelligent life needs light -- you could imagine creatures evolving using sonar or some sort of chemical sniffing to see and manipulate the world around them.
Apr
7
comment What arguments are in favour of an atomic structure to space-time?
I don't see any a priori reason to believe that space has a finite volume. And it is actually false that the hilbert space for a finite volume is finite. Even something as simple as the infinite square well doesn't work if you assume a finite dimensional Hilbert space -- your eigenbasis no longer spans the function space.
Apr
4
comment Is sun revolving around certain mass in space with reference to some other galaxy?
@dipendra: to be pedantic, the sun revolves around the center of mass of the galaxy, which is governed more by the other stars in the galaxy than by the SMBH at the glactic core. It is not a completely analogous case to that of the sun -- the sun accounts for more than 99% of the mass of the solar system, while the SMBH at the galactic core is something like 1/500 of the mass of the galaxy.
Apr
4
comment Maxwell-Faraday Equation and Electric Fields
@DumpsterDoofus: but in a nice DC circuit like that, there are no time-dependent magnetic effects. It's just ordinary friction. There is a perfectly well defined potential at every point in the circuit.
Apr
3
comment Maxwell-Faraday Equation and Electric Fields
@DumpsterDoofus: it is of course, easy enough to explain that away by saying that the heating is caused by friction between the electrons and the metal ring, just like you get heating when you run a DC current through a light bulb.
Apr
2
comment Acceleration of the Expansion of Universe
Note that if this is true, then the cause of the acceleration is NOT dark energy, or at least not a simple dark energy like a cosmological constant.
Apr
2
comment Poincaré symmetry and linearized gravity
As Prahar points out, this is certainly not the case if you choose a non-trivial background for your linearized theory.
Mar
31
comment Does inflation predict a multiverse?
If the graviton doens't exist, then either General relativity is somehow not quantum (in which case, you have to explain how to couple a classical theory to a quantum one withough causality issues) or is wrong.
Mar
30
comment Is it foolish to distinguish between covariant and contravariant vectors?
@RobinEkman: and? The distinction between covectors and vectors has to be tied to a manifold. If you go one level of abstraction down, you don't have the structure to distinguish between vectors and covectors, after all.
Mar
29
comment Null Geodesics in Einstein Universe
@user3421870: ds is not zero for the null case - null geodesics trace out "affine parameter". Think of it as "how many perpendicular null geodesics you cross as you trace out the null curve". Now, I WILL be zero, but that's no problem.
Mar
29
comment Null Geodesics in Einstein Universe
you had it mostly right :)