# elfmotat

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 Sep10 comment Dependence of sign on operators in wave equation? The form of an operator depends on what basis you're working in. For example, in position space the momentum operator $\propto \partial /\partial x$, while the position operator is just x. But in momentum space, position is $\propto \partial /\partial p$ while momentum is just p. In general you should be alright no matter what basis you're using as long as your operators satisfy the commutation relationships. Sep7 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. Jun12 answered What is the physical importance of current density? May8 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. May6 awarded Nice Answer May6 revised Chasing a photon added 78 characters in body May6 answered Chasing a photon Feb10 awarded Yearling Jan17 comment Why don't positive charges move? They can move, but tend not to because they are held in place by other atoms. Dec28 answered What does this summation mean in relativity? Aug26 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. Aug15 answered Why is there a factor of $4\pi$ in certain force equations? Jul18 awarded Nice Answer Jun5 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. Jun4 awarded Revival May27 answered Can one of Newton's Laws of motion be derived from other Newton's Laws of motion? May25 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. May20 awarded Fanatic May11 awarded Citizen Patrol May5 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 ?