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bio website math.mit.edu/~shor
location Cambridge, MA
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visits member for 4 years, 5 months
seen 5 hours ago

Apr
9
comment Why does red light travel faster than blue light?
@Jon: Right. (Bangs head.) I meant refraction.
Apr
3
comment Do quasiparticles have antiparticles?
Some quasiparticles have antiparticles, and some quasiparticles act as their own antiparticles, just like ordinary particles. Charged quasiparticles will have antiparticles with opposite charge; uncharged quasiparticles may or may not be their own antiparticles.
Apr
3
comment Why does red light travel faster than blue light?
@Jon: I have never seen a prism produce a rainbow with a caustic in it; rainbows produced by diffraction are always neatly ordered from red to violet. So while red light may not always travel faster through materials than blue light, it seems to me that materials where it doesn't must be incredibly rare.
Mar
25
comment Doesn't the Schwarzschild metric combined with Hawking radiation imply that nothing ever gets past the event horizon of a black hole?
According to the standard theory, an outside stationary observer perceives that there is Planck-scale black body radiation just outside the event horizon. If I understand it correctly, the AMPS paper conjectures that the particle sees Planck-scale radiation inside the event horizon.
Mar
24
comment Can I blow soap bubbles (of air) inside a vacuum chamber?
Sorry. What I meant is that if the inner pressure is low but positive, and the outer pressure is 0, then I don't see why a soap bubble shouldn't be stable.
Mar
24
comment Can I blow soap bubbles (of air) inside a vacuum chamber?
You're right: the air pressure inside the bubble is larger than that outside. But couldn't a bubble exist in vacuum if the inner pressure was very low?
Mar
24
comment Can I blow soap bubbles (of air) inside a vacuum chamber?
There's surface tension: the same thing that keeps bubbles in equilibrium at atmospheric pressure (the pressure on the inside of a bubble is the same as the outside. So if you have very low internal pressure inside the bubble, and vacuum outside the bubble, I don't see why this shouldn't work.
Mar
24
comment Doesn't the Schwarzschild metric combined with Hawking radiation imply that nothing ever gets past the event horizon of a black hole?
No, I'm referring to Hawking radiation, or Unruh radiation. One way of viewing the paradox is: if the particle never interacts with the high-temperature soup from its point of view, quantum mechanics says the information cannot be transferred to the soup.
Mar
24
comment Doesn't the Schwarzschild metric combined with Hawking radiation imply that nothing ever gets past the event horizon of a black hole?
From the view of the outside observer, the freely falling particle hits a Planck-temperature soup of particles and is destroyed. From the view of the freely falling particle, it passes the event horizon without incident. It is very difficult to reconcile these two views, and neither of these is the scenario indicated by your answer.
Mar
8
awarded  Nice Answer
Mar
6
comment Why does a quantum cloner imply superluminal communication
That's not cloning. What you're doing with the EPR state is an allowed operation in quantum mechanics. Cloning is an operation on a pure state that takes $|0\rangle \rightarrow |0\rangle|0\rangle$, $|1\rangle \rightarrow |1\rangle|1\rangle$, $|+\rangle \rightarrow |+\rangle|+\rangle$, $|-\rangle \rightarrow |-\rangle|-\rangle$.
Jan
20
comment When an unitary operator is a quantum gate?
Using just the Hadamard and Toffoli, you can never get any complex entries in the unitary matrices, so it's not true that any gate set can be constructed out of them.
Jan
18
comment Coherence of spin state
This is just the size of the off-diagonal term in the density matrix $\Psi$.
Jan
8
reviewed Close Light Lab Misunderstanding. Please help
Jan
8
reviewed Close Gravitational potential energy of a system
Jan
8
comment Thought Experiment - Poking a stick across a Black Hole's Event Horizon
@rob: that's only if you don't mind falling in shortly after the stick.
Jan
7
awarded  Custodian
Jan
7
comment Thought Experiment - Poking a stick across a Black Hole's Event Horizon
How strong is the pole? Can you make a pole strong enough that it won't be ripped apart by gravity when you try this experiment?
Jan
7
reviewed Close How is heat and temperature related? How will a thermometer react in the following conditions?
Jan
2
awarded  Nice Answer