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bio website math.mit.edu/~shor
location Cambridge, MA
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visits member for 3 years, 8 months
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Jul
20
comment What force counteracts friction when a block is pulled?
If there were no friction, there would be no forces acting on the gum, and the gum would not move. Thus, if there is only a small amount of friction acting on the gum, the gum won't move much. You don't need a counteracting force to make a body that stays at rest remain (largely) at rest.
Jul
20
comment Non-unqiue basis sets of reduced density matrix in quantum mechanics/decoherence
Presumably he gave the constraint $\langle s_1 | s_2 \rangle = \langle a_1 | a_2 \rangle = 0$, and you just left it out of the question. It's linear algebra. Here, $|s_1\rangle$ and $|s_2 \rangle$ are the eigenvectors of $\rho_S$, and this fact comes from the fact that Hermitian matrices with distinct eigenvalues have a unique set of eigenvectors.
Jun
26
comment Why doesn't a typical beam splitter cause a photon to decohere?
@Slaviks: you're right; probably all that's necessary is that the mirror is localized in space.
Jun
25
awarded  Necromancer
Jun
25
comment Why doesn't a typical beam splitter cause a photon to decohere?
None of the photons are absorbed (or rather, since the apparatus isn't perfect, a very small fraction; much less than 50%).
Jun
25
comment Why doesn't a typical beam splitter cause a photon to decohere?
There's no heat transferred ... the photon has the same energy after it passes through the mirror as before.
Jun
25
revised Why doesn't a typical beam splitter cause a photon to decohere?
added 264 characters in body
Jun
25
comment Why doesn't a typical beam splitter cause a photon to decohere?
If you know how close the phases are, it's straightforward to calculate how much of the interference you lose. But what I'd like to see done is: starting with the thermal state of a mirror, calculate how close these phases are for a given $\Delta p_\gamma$. I think I know how to do it, but it would take me quite a bit of time that I don't want to spare right now.
Jun
25
awarded  Revival
Jun
25
revised Why doesn't a typical beam splitter cause a photon to decohere?
added 15 characters in body
Jun
24
revised Why doesn't a typical beam splitter cause a photon to decohere?
added 99 characters in body
Jun
24
answered Why doesn't a typical beam splitter cause a photon to decohere?
Jun
24
comment Problem in Grandfather paradox
The experiment didn't actually perform time travel, it simulated time travel, which is an entirely different thing.
Jun
23
answered Why is the intensity of Hawking radiation dependent on the size of the black hole it comes from?
Jun
22
comment Why can't a gas be liquified by pressure above its critical temperature?
Does this explain how you can go smoothly from a liquid (below the critical temperature) to a gas (below the critical temperature) without a phase transition by taking a path in the pressure-temperature plane which goes above the critical temperature?
Jun
13
comment A question on lowering the total spin
For a spin-1 particle, the S=1 state has dimension 3, and the S=0 state has dimension 1. You can't connect these with a nice operator.
Jun
10
comment What will happen to compass with north all around it?
You can't have north all around it: south has to leak in to satisfy the divergence-free conditions for the magnetic field. If you do this perfectly, the best you'll get is no magnetic field, and you can probably guess what compasses do in that situation.
Jun
7
comment Is there a theoretical physics masters that accepts mathematics graduates?
@Flint72: I'm not sure you should generalize to "in England, most ..." from the universities of Cambridge, Durham, and Edinburgh. Is this even true at Oxford?
Jun
7
comment Is there a theoretical physics masters that accepts mathematics graduates?
@Hunter: my impression is that the kind of physics courses offered by applied math departments are not the kind somebody would want to tai if they were planning on going into philosophy with an emphasis on physics.
Jun
5
revised Why are the 'color-neutral' gluons confined?
fixed typo and added comment