# jld

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 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? 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 20 awarded Fanatic May 11 awarded Citizen Patrol