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age 34
visits member for 3 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.


1d
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?
1d
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...
1d
comment Relatvity of Promise
@Iota: it put a limit on programming computers to do nonsense. But GIGO is totally true before you invoke relativity.
1d
comment Why Light and Observers have different laws of physics
It doesn't rule out anything about the physics of light. It just tells us that we cant' look at physics <b>from the light's perspecive</b>. It is still 100% consistent to talk about the influences of light rays, and the interaction of the electromagnetic field with matter. There is just no observer that will be able to look at an electromagnetic wave and say that it is static. This makes sense, because it would otherwise break Maxwell's equations.
1d
comment Why Light and Observers have different laws of physics
What's wrong with just saying that a ray of light is not a valid observer?
Jul
18
comment Finding the metric tensor from the Einstein field equation?
@ACuriousMind: if you were frustrated with the non-mathiness of Schutz, then you should pick up Wald's book. That was definitely the jumping off point for me.
Jul
18
comment Finding the metric tensor from the Einstein field equation?
Kerr's approach began with study of algebraically special spacetimes. Most of these turn out to be things like the Taub spacetime that don't really apply physically, but Kerr was insightful enough to see that he had found a generalization of the Schwarzschild spacetime. So, he did leverage symmetry, but it was a symmetry in the algebriac structure of the Weyl tensor, not of normal physical degrees of freedom.
Jul
18
comment how to measure the age of light?
@Jim: but we do it not by direct measurement, but by measuring the distance first and dividing by c. That was my point.
Jul
18
comment how to measure the age of light?
@Jim: sure, if you know the hubble law. But we generally use cosmological distance and redshift data to fit to the hubble constant, not the other way around (though it can be used to find ranges to newly discovered galaxies). Irrespectively, it's still a distance measurement, at least in my head. That is certainly the case with luminosity distance -- you measure the distance, and then infer the age. Not vice versa. In any case, the fundamental thing is the distance from which the light was emitted, not the travel time.
Jul
18
comment How long does it take to travel 36 light years with tolerable acceleration and deceleration?
In principle, you could have a design that picks up mass along the way, say by scooping up the ISM and using it as fusion fuel. This could reduce the fuel cost some.
Jul
16
comment How to properly construct the electromagnetic tensor in curved space-time?
@Giovanni: yes, but see above about the hodge dual. Cross product doesn't generalize trivially to curved space the way that dot product does. It's all a matter of how you want to present the components. I would suggest starting with ordinary cartesian space in three coordinates and seeing what's necessary to reproduce the canned formulae in Griifiths/Jackson.
Jul
16
comment Gravitation as the source of redshift of light beams
@CarlJohanSoderquist: I generally don't draw too big a distinction between "undetectable" and "nonexistent", especially when explaining large, visible effects like Hubble redshift that have detectable causes.
Jul
16
comment Does Kaluza-Klein Theory Require an Additional Scalar Field?
And it should be stated that originally, Kaluza-Klein was an attempt to unify gravity with electromagnetism. They didn't want the dilaton field, so they just assumed that it was nondynamical, and gave it a zero value. Of course, any one value of the metric can be set to one with a choice of coordinates.
Jul
15
comment Why do we say that light travels at a speed?
@DerekRoberts: the rules of relativity say that it's not legal to look at things from a photon's perspective. That insight is one of the things that actually enabled Einstein to formulate the theory.
Jul
14
comment Why isn't resistance proportional to distance squared
A battery isn't a pair of point charges. The basic nature of what a battery is is that it maintains a constant potential difference between the terminals.
Jul
12
comment Solution to Klein-Gordon equation
@pkjag: write down $\phi(x^{\mu}) = A(t)B(x)C(y)D(z)$. Apply the Laplacian, and do the same trick that you do in Griffith's Quantum mechanics to isolate the variables into four ODEs.
Jul
12
comment Solution to Klein-Gordon equation
You'll get a product of four exponentials, with constants $p_{\mu}$ and the constraint $p_{\mu}p^{\mu} = m^{2}$, so it'll amount to the same thing.
Jul
12
comment Solution to Klein-Gordon equation
Well, technically, a general solution would be $\int d^{3}p e^{-it\sqrt{m^{2} + p^{2}}}e^{-i\,p_{i}x^{i}}$, since you can freely linearly superpose plane wave solutions.
Jul
12
comment How fast to travel to end up where i started from?
Well, and the dark energy measurements ruled out the closed universe model, where this question makes sense. Most physicists no longer believe that the universe's topology is such that you come back around to where you came from, at all.
Jul
11
comment Relativity and volume of black hole
THe merger rate is higher, but they aren't GRB candidates, because there is no coupling to the EM field. They are expected to dominate the space of LIGO sources, though.