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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
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 ?
May
5
answered QED photon propagator to one-loop order gets different answers
Apr
20
comment Transforming an equation to the co-vector version
You're right. I've changed my answer to reflect this.
Apr
20
revised Transforming an equation to the co-vector version
deleted 51 characters in body
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
20
answered Transforming an equation to the co-vector version
Apr
19
answered proper variation of action term
Apr
19
comment How do forces work
Necessary classic: youtube.com/watch?v=wMFPe-DwULM