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Jul
22
comment Massless particles and the speed of light - New? Theories of existence
Your assumptions about "matter based mass" do not make much sense, since in theory one could have a composite massive particle made entirely of massless particles, if those massless particles are not exactly unidirectional. Glueballs are an example of one such hypothetical particle. There is also no such thing as "pure energy"--energy is a property that things have. As for the rest, I'm sorry, but I can't make much sense of it at all.
Jul
18
comment Finding the metric tensor from the Einstein field equation?
Spherical symmetry alone determines the Schwarzschild ansatz. Kerr was working from Petrov classification (on non-Type-I vacuum spacetimes) and proved that all Killing vectors are one of two types. The only unsubstantiated guess was a convenient form for an asym. timelike Killing vector; otherwise, the discovery was rather rigorous. Notably, there was no claim that this was solution was unique until a decade later, proven by Robison. I'm not saying this comment-answer is wrong (as typical presentations go), but relativists aren't nearly as morally loose as it might appear at first glance.
Jul
13
revised Computing the Christoffel symbols with the geodesic equation
added 35 characters in body
Jul
12
comment Computing the Christoffel symbols with the geodesic equation
The main trouble is that your first equation is actually incorrect, because of product rule and differentiation of $\nu$.
Jul
12
answered Computing the Christoffel symbols with the geodesic equation
Jul
12
comment Is the apparent lack of (Ricci) curvature in the Schwarzschild metric due to a choice of coordinates?
"Shouldn't there always be curvature in the presence of mass... ?" -- There is no curvature inside a spherically symmetric shell: spacetime is flat there. So it's not true that the presence of mass necessarily guarantees that spacetime is everywhere non-flat.
Jul
5
reviewed Approve suggested edit on mathematician or physicists
Jul
3
answered Is time subjective?
Jul
1
comment Can a curvature in time (and not space) cause acceleration?
I feel that Schwarzschild geometry isn't a good example, because it is static, and hence there's a distinguished timelike direction, in this case Schwarzschild time. But the point certainly holds for most spacetimes.
Jun
30
reviewed Approve suggested edit on What to do when you are living in very cold environment?
Jun
28
answered Is it ok to have two events $A$ and $B$ so that for one person $A$ occurs before $B$ but for another $B$ preceds $A$
Jun
28
comment Is it possible to derive Lorentz transformation equation without Einstein's postulates?
@V.Moretti that simply replaces the speed of light postulate with the postulate that the permittivity and permeability being the same in all inertial frames. Since free-space EM can be equivalently described with constants $(c_0,Z_0)$ as with constants $(\epsilon_0,\mu_0)$, we're effectively trading invariance of lightspeed with invariance of lightspeed and invariance of impedance. That's not an improvement, and arguably, a step back, since it postulates more things.
Jun
28
comment Is it possible to derive Lorentz transformation equation without Einstein's postulates?
One of the minus signs on the Euclidean "boost" matrix shouldn't be there. Also, what about Galilean boost? (One can do $e^{\epsilon\lambda} = \mathrm{cosp}\,\lambda + \epsilon\,\mathrm{sinp}\,\lambda$ with $\epsilon^2 = 0$ to give $dt' + \epsilon\,dx' = e^{\epsilon v}(dt + \epsilon\,dx)$, but it's probably not worth to stretch the trig analogy too far.)
Jun
27
reviewed Approve suggested edit on How do centripetal forces and gravity work for objects in a rotating cylinder?
Jun
27
reviewed Approve suggested edit on How badly could someone be injured by concentrated sunlight?
Jun
26
revised Slowing of time under gravity
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Jun
26
reviewed Approve suggested edit on Does Earth really have two high-tide bulges on opposite sides?
Jun
26
answered Slowing of time under gravity
Jun
23
revised Why doesn't time change in the non-relativistic limit of Lorentz transformations?
added 58 characters in body
Jun
23
comment Why doesn't time change in the non-relativistic limit of Lorentz transformations?
@brightmagus: I probably should have emphasized more clearly in the answer that I'm taking a tangent-line approximation--I did suggest it, but it was vaguely worded and likely confusing. With that clarification in mind, I disagree that it's mathematically invalid. It's a bit like saying than the transformation $\theta' = \theta$ being a t-l approx to $\theta' = \sin\theta$ at small $\theta$. In the end, they're still two different transformations, so they are not required to be identical to be mathematically valid. But perhaps we should just stop quibbling about wording at this point. (: