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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.


1h
revised Why should Conservative forces have their curl equal to zero?(intuition)
added 190 characters in body
1h
answered Why should Conservative forces have their curl equal to zero?(intuition)
1h
comment Ricci tensor of Metric black holes with nils and solv geometries of Horizon
Also, what are the $w^{I}$? are they a tetrad basis on the three-space? is $r$ a coordinate on them, or do we expect $r$ to be a function of the $w^{I}$? If so, what of the $dr^{2}$ term above? Are the $q_{I}$ constants? This is a strange-looking metric to me.
2h
comment Is it possible to shoot bullets in space or would the recoil of the gun be too strong?
Would it be possible for the firing mechanism to get enough oxygen to ignite the gunpowder in the bullet?
2h
comment What are the mathematical models for force, acceleration and velocity?
Answering this cleanly will require a bit more than what your'e providing. Are we talking about a Newtonian mechanics in a not-necessarily Euclidean space? Are we allowing for a relativistic dynamics here? Are we doing relativistic dynamics, but assuming that the metric can be decomposed into some sort of $-dt^{2} + f(t)g_{ij}dx^{i}dx^{j}$ ? Is this even a metric space? This sounds like a bunch of technical complaining, but the answer is actually different in all of these cases.
3h
comment What would it take to cause lightning to jump between the Moon and the Earth?
@Jimnosperm--but space isn't a perfect vacuum... Though I wonder if anyone's done studies on the ionization field for such diffuse gas, either.
1d
comment How to solve infinite square well with exponential solution (of oscillatory type)?
Note that by the time you're at your last step, you can substitute $A(e^{ikL} - e^{ikL}) = 2A\sinh(ikL) = 2Ai\sin(kL)$
1d
comment The g-force of common objects hitting the floor
as an extension of what @irishphysics is saying, given a number for the acceleration, such as $2000g$, another approach you can do is to estimate the speed with which the thing hits the floor, and then convert that speed, and your known acceleration, into a time of impact. And then you can convince yourself whether this is reasonable or not.
2d
comment Are there models/simulations of antigravitational antimatter-galaxies?
@lurscher: the force on the dipole would depend on an inner product of the dipole moment and the gravitational field. You could always find an alignment where the net force is zero.
Mar
28
comment Free parameters in the Standard Model
@robertbristow-johnson: Fine, but you've then fixed a scale for everything else and made THEM dimensionless. If you allow the curvature term in the action, it either comes with a dimensionless parameter (which I abstracted to mean G, but fine), or you fix units to make it unity, and that then fixes units for everything else. It's still semantics whether that counts as a "free parameter"
Mar
28
comment Why does $F=ma$? Is there a straightforward reason?
@garyp: a spring scale measures force only because there is a known theoretical construct relating force and acceleration. At heart, force is "amount of push", but how do you define this? You can't, without making some appeal to a deeper law. Your statement about inertial and gravitational mass belies this -- gravitational mass isn't a thing without a gravitational law. There is no a priori reason that there should be such a thing as "gravitational mass"
Mar
28
comment Free parameters in the Standard Model
Well, properly, Newton's constant and the cosmological constant are "free parameters of fundamental physics" and not of the standard model, since the standard model does not deal with gravity. And the particle masses (or at least the combination of yukawa couplings hand the higgs VeV that determine the masses) are most definitely amongst these parameters.
Mar
28
comment Why does $F=ma$? Is there a straightforward reason?
That equation should be the definition of either mass or force.
Mar
27
comment Repelling a weak permanent magnet with an electromagnet
"strong enough" is really a statement about the product of the two magnetic moments, so it's hard to conceptualize one thing being strong enough, but the other one being arbitrarily weak. Put another way, for some fixed nonzero magnetic moment of the permanent magnet, there will always be some current you can push through the electomagnet that will enable levitation.
Mar
26
comment Physics without time
@dmckee: I also think that saying that time is time-reversal symmetric is entirely different from doing without time entirely. A universe that obeys parity symmetry is hardly one in which space does not exist.
Mar
26
awarded  Nice Answer
Mar
26
revised How small does sand have to be to get wet?
deleted 9 characters in body
Mar
25
answered How small does sand have to be to get wet?
Mar
25
comment What would it be like “inside” a star?
I'd be more concerned about radiation than I would be about matter.
Mar
25
comment Schwarzschild: Proof that $\{r<2m\}$ is a black hole
It's a choice of whether the geodesic is future-pointing or past-pointing. In the exterior region, $\dot v > 0$ is equivalent to saying that the geodesic is future-pointing.