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11h
comment What is the electric field at the midpoint of the hypotenuse of a triangle?
Could it be that the book's answer is the component of the E-field along each axis?
23h
comment Same photon or different photon?
Perhaps the real take-home here is that the "photons" of QED diagrams (and, hence, of the Standard Model) are not really the same things as the "photons" we first meet in atomic-scale quantum mechanics. For example, QED photons don't have frequency as an intrinsic property, but QM photons do. The QM photon is an ensemble phenomenon made up of many interfering histories involving possible QED photons.
Apr
28
answered Can you use infra-red goggles (or similar principle) to see through mist and fog?
Apr
24
comment Why is 1 kg of cotton heavier than 1 kg of stone?
If you measure out a quantity of either cotton or stone by weight, what you'll usually measure is its actual mass minus its buoyancy in air anyway. So, if you're using the same weight, one will not be heavier than the other -- by definition, since otherwise they wouldn't both measure as 1 kg in the first place!
Apr
11
comment Can lasers lift objects?
The basic fallacy here is you're computing "how much energy does it take to lift a 100 g weight one meter?" and then jump, without any argument or reason, to the conclusion that this amount of energy packaged up as laser light in particular would constitute a workable lifting apparatus. That's like saying that just because the price of a Big Mac is \$4, one ought to be able to construct a machine that produces a Big Mac when supplied with \$4 worth of crude oil.
Mar
30
comment Are black holes very dense matter or empty?
@CeesTimmerman: It would probably be accurate to say that a collapsing star converges towards how an eternal Schwarzschild black hole would look like. The longer time has passed for the outside observer since the collapse began, the more like a Schwarzschild black hole will the remains look like, but it will never reach exactly that shape.
Feb
25
comment Signal travels with speed greater than light breaks causality
@Beman: It should not be necessary for you to translate between two frames with a greater mutual velocity than $c$ -- just assume that the signal can be made to leave one event and be received at another end such that the ratio between (difference in space coordinates) and (difference in time coordinates) is greater than $c$. You don't need to set up a frame where the signal doesn't move.
Feb
25
comment Can radioactivity be slowed through time dilation?
@EricLippert: Oh noes! Now the sun is radioactive. Humanity is doooomed! :-(
Feb
25
awarded  Organizer
Feb
25
revised Can radioactivity be slowed through time dilation?
retag: there seems to be nothing GR here
Feb
25
suggested approved edit on Can radioactivity be slowed through time dilation?
Jan
25
comment Weightlessness in Satellites
@MartinJames: If the passenger is inside the satellite, the gravitational pulls from different parts of it will tend to cancel out even if he is not at its exact center. (Famously, they cancel out completely everywhere inside a uniform spherical shell). And even if they don't cancel out exactly due to irregularities of shape or density, the sum will not tend to pull him towards the center-of-mass.
Jan
21
revised How many neutrinos would it take to accelerate a 75kg human?
A few additional engineering numbers
Jan
21
revised How many neutrinos would it take to accelerate a 75kg human?
deleted 8 characters in body
Jan
21
revised How many neutrinos would it take to accelerate a 75kg human?
deleted 18 characters in body
Jan
21
revised How many neutrinos would it take to accelerate a 75kg human?
oops, the 12 came from nowhere
Jan
21
comment How many neutrinos would it take to accelerate a 75kg human?
Yes, I know $N_A$ is supposed to be dimensionless, but remembering it as "baryons per gram" seems to be easier for back-of-the-envelope calculations.
Jan
21
answered How many neutrinos would it take to accelerate a 75kg human?
Jan
21
comment How many neutrinos would it take to accelerate a 75kg human?
Relevant what-if, though it doesn't provide sufficient hard numbers to answer this one.
Jan
21
awarded  Guru