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Jul
27
reviewed Approve Schwarzschild Solution
Jul
27
comment One-particle scattering: LSZ vs Feynman
I can't explain why $p^2-m^2$ appears in different places, but remember that Feynman diagrams have an implicit $(2\pi)^4\delta(p_{in}-p_{out})$. You're making use of that when you say that $p_{in} = p_{out}$.
Jul
25
comment The Higgs Mechanism-Calculating Fermion Masses
Well, for starters you would have to learn a lot of Quantum Field Theory.
Jul
25
comment How can the orbit of Jupiter's moons be used to calculate the speed of light?
@Ross: Thank you, that makes sense.
Jul
25
comment How can the orbit of Jupiter's moons be used to calculate the speed of light?
Shouldn't the movement of the Earth, instead of its position, be the important factor? In other words, when we move towards Jupiter the eclipses should appear closer (in time) to each other, and farther when we move away from Jupiter, in a sort of Doppler effect; the distance shouldn't matter. But that isn't what the quote is saying.
Jul
24
awarded  Good Question
Jul
24
comment Why do I sink then float in a swimming pool?
@Tetradic: that may be right (I don't know), but it's not immediately obvious to me that 3m can't be completely explained by what I said.
Jul
24
comment Why do I sink then float in a swimming pool?
If you're above water, your weight is stronger than buoyancy and you sink. As you go down the two forces eventually cancel, but because of inertia you keep going down for a little bit. If water didn't cause drag you would oscillate around your equilibrium height.
Jul
20
comment How is it possible that it can get hotter in the car than it is outside?
That law is valid when there are no heat sources. For a car left in the sun, that's not the case.
Jul
19
comment How does the conductor knows which side is outside?
Charges move to the edge if there is an electric field. In your pictures, there should be negative charges at the opposite borders. The direction of the field determines where each kind of charge goes.
Jul
18
comment Co-ordinate rotations
Well, you apply the first one first and the second one second. You've already figured out the order in your post; like I said, you apply the x-axis rotation first and the z-axis rotation second. Remember that you write them backwards because matrices act on the right. Why rotations don't commute, that's a much more profound question.
Jul
18
answered Co-ordinate rotations
Jul
18
comment Where does the $(\ell + x)^2\dot\theta^2$ term come from in the Lagrangian of a spring pendulum?
Because I made a mistake. If you look at how $\theta$ is defined, the vertical coordinate has $\cos\theta$ and the horizontal component has $\sin\theta$.
Jul
18
comment Where does the $(\ell + x)^2\dot\theta^2$ term come from in the Lagrangian of a spring pendulum?
$y = (l+x)\sin\theta$ is the usual vertical $y$ coordinate. We are interested here in the tangential velocity, which has magnitude $(l+x)\dot{\theta}$: radius times angular velocity.
Jul
18
comment Time and gravity relation
Time dilation is unrelated to the fact that gravity decreases, and it can be observed at small heights, where the difference in gravity isn't noticeable.
Jul
18
revised Particle energy in general relativity?
added 360 characters in body
Jul
18
comment Particle energy in general relativity?
@John: As far as I know, yes.
Jul
18
comment Particle energy in general relativity?
@John: well, there should be an $m$ in there, and a minus sign if you're in $(-+++)$ signature. At least to me, that is the only meaningful definition of energy; you could choose to call it something else, but I'm not sure what purpose that would serve.
Jul
18
comment Curvature of spacetime and the equivalence principle
It's worth mentioning that if you're in a vacuum (or in a low density environment such as air), the Ricci scalar is zero (or nearly zero). Of course, there are other scalars that may not be zero.
Jul
18
answered Particle energy in general relativity?