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Oct
11
comment Special Relativity: Finding the Euler Lagrange of a massive particle
Why keep the square root? Remember that if $L$ satisfies the E-L equation then $L'=f(L)$ will also satisfy them as long as your parameter $\lambda$ is affine. I suggest using $L'=L^2$ to get rid of the square root - it will make things much easier.
Oct
11
comment Geodesic equation from Euler - Lagrange
You can always change indices which are summed over because they are just dummy indices! And as long as your free indices match up, your equations should make sense.
Oct
11
answered What is the cause of gravitional force of attraction?
Oct
10
comment Why particle number operator $\hat{N}$ is $\hat{a}^\dagger\hat{a}$ rather than $\hat{a}\hat{a}^\dagger$?
I think he means that $a^\dagger a$ kills the vacuum while $a \, a^\dagger$ doesn't.
Sep
23
answered Special relativity; rocket moving towards a mirror
Sep
7
comment Gravity Concept Question
The sun can't just suddenly disappear. What you're asking us to do is use the laws of physics to describe a situation that violates the laws of physics.
Jun
12
answered What is the physical importance of current density?
May
8
comment Chasing a photon
@brightmagus This seems to have spurred an interesting discussion, but to reply to your original comment: I never claimed this was a proof. Velocity addition was originally derived from the assumption that c=const, so it had better work that way. Marco Prins asked whether or not a photons velocity would still be c if one were trying to chase it at high speeds, and what the equation to calculate such a scenario is. I provided him with what he asked for. Of course, you can't "prove" that c=const must be true simply via theory - this is something that must be confirmed by experiment.
May
6
awarded  Nice Answer
May
6
revised Chasing a photon
added 78 characters in body
May
6
answered Chasing a photon
Feb
10
awarded  Yearling
Jan
17
comment Why don't positive charges move?
They can move, but tend not to because they are held in place by other atoms.
Dec
28
answered What does this summation mean in relativity?
Aug
26
comment How can we deduce the relation $m = \frac{m_0}{\sqrt{1-\frac{v^2}{c^2}}}$ between relativistic mass and rest mass in special relativity?
I remember Feynman did something involving analyzing collisions of masses in different reference frames, and concluding that, while $mv$ is not a conserved quantity, $\gamma mv$ is. It's therefore natural to modify either the definition of momentum or mass in order to maintain momentum conservation in relativity.
Aug
15
answered Why is there a factor of $4\pi$ in certain force equations?
Jul
18
awarded  Nice Answer
Jun
5
comment What's the basic premise of General Relativity?
The Doppler factor (1+v^2/2c^2) that follows after "for our purposes" is an approximation that comes from a Taylor expansion of the exact factor. And Taylor expansion comes from calculus.
Jun
4
awarded  Revival
May
27
answered Can one of Newton's Laws of motion be derived from other Newton's Laws of motion?