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location Austin, TX
age 35
visits member for 4 years, 8 months
seen 19 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.


1d
comment Wavelength vs Wavenumber etiquette
Wavenumber is just a convenience -- it's a lot easier to write $e^{ikx}$ than it is to write $e^{2\pi i x\frac\lambda}$. And if you're writing the first thing, you need a name with which to refer to $k$. They are 100% the same thing, though.
1d
comment Why is SiO2 solid while CO2 is a gas?
Considering that the sublimation point of dry ice is only like $-70{}^{\circ}$ C, a factor of 2.5 of the mass is actually probably plenty to explain this.
2d
comment How are standing waves a result of constructive and destructive interferences?
This is a weird question to me. I'd more say that standing waves are examples of "normal" waves, and other waveforms result from constructive and destructive interference of standing waves.
Jul
29
comment Density of dark matter along the galaxy
@RussellMcMahon: of course, modified gravity may still be right, but you're asking way more of Occam with most realistic modified gravity schemes (seriously, look up the TeVeS Lagrangian) than you are by just saying "let's extend the standard model to include a stable 20 TeV [or whatever] WIMP plus a one-parameter cosmological constant". YMMV, of course, but non-radiative matter isn't a wild assumption.
Jul
27
comment Why do electrons, according to my textbook, exist forever?
Technically, the neutrinos continually decay into each other: en.wikipedia.org/wiki/Neutrino_oscillation
Jul
27
comment How close would you have to be to the merger of two black holes, for the effects of gravitational waves to be detected without instruments?
OK, I think I have a lazy, quick way of order of magnitude estimating this thing. I'll put it up tonight or tomorrow.
Jul
27
comment Is momentum an invariant?
Like, the quantity you're talking about is explicitly frame-dependent, as your notation makes manifest. It's not an invariant.
Jul
27
comment Is momentum an invariant?
@user12262: the frame velocity is 100% not Lorentz covariant. It is explicitly present in the equation. Therefore, a boost will change the frame velocity, and change your momentum.
Jul
27
comment Is momentum an invariant?
@user12262: well, your formula has explicit $v$'s in it, so how invariant do you expect it to be?
Jul
27
comment How close would you have to be to the merger of two black holes, for the effects of gravitational waves to be detected without instruments?
Are you detecting the plunge phase, the inspiral, or what?
Jul
27
comment Why can I put my hand through sand but not a table?
Well, to answer your last question, what happens when you push a hammer through the table as hard as you can?
Jul
27
comment Why we don't see time dilation in stars orbiting black hole?
@BrandonEnright: be aware that there is expected to be a large amount of non-luminous matter around these objects, so perehelion precession will also be governed by this superious matter.
Jul
27
comment Why we don't see time dilation in stars orbiting black hole?
@eli.rodriguez: also note what I said in my answer when talking about accretion disks. Largely, they are still in orbits until they do the final plunge, but the final plunge is very fast, so the bulk of the luminosity of the disk will come from the plasma orbiting at a radius greater than $6M$ for a Schwarzschild hole.
Jul
21
comment When is Einstein summation implied by Lorentz indices?
I also can't think of a case where anyone would take the sum $\sum_{\mu}\sum_{\nu}A^{\mu \nu}$. Almost, universally, they are taking the trace of the matrix, which is labeled ${\rm Tr}(A^{\mu \nu})$ or they are contracting it on the metric, $g_{\mu \nu}A^{\mu \nu}$. The latter thing would usually just be called $A$
Jul
21
comment What would the consequences be if it was experimentally shown that antimatter bends spacetime opposite from matter?
at that point, you're no longer talking about the standard model. In the standard model, the photon is its own antiparticle. And yes, that's the thought experiment.
Jul
21
comment What would the consequences be if it was experimentally shown that antimatter bends spacetime opposite from matter?
@AsphirDom: it's pretty well established what antimatter is in particle physics.
Jul
21
comment What would the consequences be if it was experimentally shown that antimatter bends spacetime opposite from matter?
We've measured the response of antimatter to an external gravity field. It falls down at exactly the same rate as ordinary matter.
Jul
21
comment What would the consequences be if it was experimentally shown that antimatter bends spacetime opposite from matter?
I disagree with this pretty strongly. If antimatter gravitates negatively, it is basically negative energy in Einstein's equation. Unless there's a quantum proof tht bulk antimatter is impossible (and I don't think that there is, CP not being an exact symmetry aside), you've now found a way to violate the central assumption necessary to derive most of the consistency arguments in GR. Also, there's a simple thought experiment to show that if antimatter antigravitates, then you can build a perpetual motion machine.
Jul
20
comment Curvature of spacetime and the equivalence principle
@Javier, if nothing else, $R^{abcd}R_{abcd}$ is not zero in nonzero gravitational fields.
Jul
20
comment Curvature of spacetime and the equivalence principle
@MaddeAnerson: it does not. They are two seperate statements. However, any theory of gravity that encodes gravity as space curvature automatically satisfies the EEP. So, the opposite direction of your causality completely works.