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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
Apr
16
answered Interval and proper time
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.
Mar
22
comment Future light cones inside black hole
To add to this, you can't avoid falling toward the singularity any more than you can avoid growing older.