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22h
comment Having trouble understanding some stuff about delta functions
Haha, you beat me.
Oct
13
comment What is $V^\mu$ if $\nabla_{\mu} V^{\mu}$=scalar?
I'm not sure what you mean. $V^\mu$ is a vector with components $V^0$, $V^1$, $V^2$, $V^3$.
Oct
11
comment Special Relativity: Finding the Euler Lagrange of a massive particle
I don't understand this. You can certainly choose $\lambda = \tau$ if you want to.
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
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
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.
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.
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.
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.
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.
May
25
comment What force is responsible for anti-gravity?
The triangle lifter you're referring to works by ionizing air and forcing it downward, producing lift. The Mythbusters crew tested it in a vacuum and it didn't work, so it's clearly not anti-gravity. It's just a highly inefficient way of obtaining lift.
May
5
comment QED photon propagator to one-loop order gets different answers
Well that's not really what you asked for. How about this: 152.78.192.50/hepwww/staff/D.Ross/qft/aqft3.pdf ?
Apr
20
comment Transforming an equation to the co-vector version
You're right. I've changed my answer to reflect this.
Apr
20
comment Transforming an equation to the co-vector version
It's true as long as $\frac{d}{d \lambda} g_{\mu \nu} = 0$, which is true for most choices of $\lambda$ when considering a timelike geodesic (which is usually what one's interested in) when there's a timeline Killing vector. In physical scenarios where there's no t-symmetry, i.e. the FLRW metric, one's generally not interested in test particles anyway.
Apr
19
comment How do forces work
Necessary classic: youtube.com/watch?v=wMFPe-DwULM
Apr
11
comment Einstein's theory tells us that gravity is a curve in space and time but how does that causes attraction in mass?
youtube.com/watch?v=DdC0QN6f3G4
Apr
4
comment Why does the index of refraction change the direction of light
The easiest way to answer this is, of course, with Fermat's Principle. But that may not be what you're looking for.
Mar
23
comment Universe Expansion and two tennis balls
Well, since we have no idea what "dark energy" actually is, that's a tough question to answer.
Mar
23
comment Universe Expansion and two tennis balls
You're not reading it wrong, but you're missing Jerry's point. The reason the universe is expanding is because it is filled with dark energy, i.e. there is a nonzero cosmological constant. By "clear the universe of all matter" it was assumed that dark energy should also be removed. Remove the DE and space doesn't expand. Leave it in and it will, albeit at a different rate than we observe because the normal matter in the universe helps to "counteract" the effect of DE.