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13064
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location Austin, TX
age 34
visits member for 4 years, 1 month
seen 4 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.


12h
comment Would an airtight box filled with air act the same way to 1 bar of water pressure as the same box vacuum-sealed at atmospheric pressure?
@JanHudec: the buoyant force only depends on the volume of displaced water. The net force on the thing is the difference between it's actual weight and the buoyant force. Though I think we're arguing semantics now.
13h
comment Is the gravitational potential of a planet in orbit always equal to minus the squared velocity?
@jibe: yes. THat's fair.
17h
comment Would an airtight box filled with air act the same way to 1 bar of water pressure as the same box vacuum-sealed at atmospheric pressure?
THe buoyant force would be the same, but the weight of the box would be different, so the difference between the weight and the buyant force woudl be different, creating a different net external force on the two boxes.
1d
comment Is quantum gravity, ignoring geometry, the theory of a fictitious force?
I wonder if anyone has tried to do quantum gravity using Bohmian quantum mechanics. This seems to satisfy the spirit of what you're asking. But on that note, I dont' think there's a satisfactory Bohmian quantum field theory, so you have a missing intermediate step.
1d
comment Is quantum gravity, ignoring geometry, the theory of a fictitious force?
@JohnRennie: that's right -- in particular, what do you do with discontinuous changes in wave function after an interaction hamiltonian is applied? How does the gravitational tensor respond to that? You can make sense of it if the gravitatiaonal tensor is a quantum thing that is ALSO superposed, but if it's not, then you're stuck either introducing noncausal effects, or invalidating EPR-type effects when interaction with gravity happens. Both are kind of unsatisfying, so people argue that both sides must be classical, or both sides must be quantum.
1d
comment Why don't we have a theory of everything?
And also there are problems with extracting the standard model out of string theory. It contains enough physics to get the standard model, but taking the limit is non-trivial.
2d
revised What is the most compelling evidence of General Relativity in the presence of matter and energy?
added 286 characters in body
2d
answered What is the most compelling evidence of General Relativity in the presence of matter and energy?
Dec
16
comment Akin to gauge field, why GR's lagrangian is not $R_{abcd}R^{abcd}$? What's the mathematical or physical meaning of $R_{abcd}R^{abcd}$?
But your dynamical variable isn't the riemann tensor, it is the metric tensor. And $R$ contains a $g^{ab}\partial^{2}g_{ab}$ term, which can be integrated by parts to a $\partial^{c}g^{ab}\partial_{c}g_{ab}$ term. This is the kinetic term that you want in an action.
Dec
15
comment Are there limits to human/devices perception?
Something like this will eventually come down to the sensitivity of the device. Once you get to "measure the state of the universe", though, you're going to run into problemes of the device belonging to the universe, and it being ambiguous which identical particle is here and which identical particle is there.
Dec
15
comment How does a Wavefunction collapse?
$<p^{2}>$ will be different for the two states, irrespectively, and this will couple to a magnetic field differently.
Dec
15
comment How does a Wavefunction collapse?
@MartyGreen: I'm not invoking the fine structure constant. I'm invoking fine structure splitting of the energy levels, which gives the states different energies. Or is the spectroscopy of the 1930s too advanced to talk about?
Dec
14
awarded  Necromancer
Dec
12
comment Discontinuous momentum in relativity
Well, the first issue here is that you'll have to figure out what "an instant of time" means. The relativity of simultaniety means that your stoppage can only be simultaneous in one reference frame. The light travel thing, you can account for just by waiting, and then filling in the past history on your spacetime diagram.
Dec
11
answered Finding $M_{ij}$ from $J_{i} = -\frac{1}{2}\epsilon_{ijk}M_{jk}$ of the Lorentz group
Dec
10
comment How does a Wavefunction collapse?
@MartyGreen: they have the same spin but different orbital energy, which will translate to different orbital angular momenta. And also, once you factor in fine structure, they dont' have the same spin.
Dec
9
comment How does a Wavefunction collapse?
You can measure the energy eigenstate of a hydrogen atom by measuring the magnetic moment of the atom, which will depend on the angular momentum of the electronic state, the energy of the electron, and the alignment of the electron's spin.
Dec
9
comment What are the principle factors that limit $\beta$ decay to elements with atomic number, Z 100 or less?
You're going to have to get a technical explanation for the exact number. The raw reason is that if you convert a neutron to a proton, you're going to increase the charge of the nucleus, and if you do that, you're increasing the electrostatic repulsion of the nucleus, which makes the whole thing more unstable, and is therefore, not energeticaly favorable.
Dec
9
comment For a rocket travelling to a planet, which reference frame measures the dilated time?
@Hypnosifl -- I specify that they should subtract the light travel time.
Dec
8
comment For a rocket travelling to a planet, which reference frame measures the dilated time?
@Hypnosifl: sure, which is why I have to posit the third event of "arrival of rocket is observed on Earth". There are two paths with the same starting point but different endpoints, and these two paths have different proper times.