Guy Gur-Ari

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bio	website	sheerviscosity.net/blog
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String theory grad student.

	755 reputation	bio	website	sheerviscosity.net/blog	visits	member for	9 months
7 badges		location			seen	Sep 27 '12 at 16:03

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Aug 8	revised	Possibility of making dark energy equivalent with dark matter Corrected title (energy -> matter)
Aug 8	suggested	suggested edit on Possibility of making dark energy equivalent with dark matter
Aug 8	comment	What is the fundamental differences between bound and entangled states Here is a toy example. Let's say we have a massive particle with spin 0, at rest, that decays into two photons. The two photons fly out back-to-back due to momentum conservation. Due to angular momentum conservation, the total angular momentum in the photon axis is zero. So if we measure the helicity of photon 1, the helicity of photon 2 must be opposite. Thus the state after the decay is an entangled state. I am not sure what you mean by the second question, since in this example (and in many others) the entangled particles do not exist before the decay.
Aug 8	comment	Ghosts in Pauli Villars Regularization I think your propagator is missing a $-i$ (from $e^{iS}$), no?
Aug 8	revised	What is the fundamental differences between bound and entangled states edited body
Aug 8	answered	What is the fundamental differences between bound and entangled states
Aug 7	answered	Relating angular and linear kinematics
Aug 7	comment	How do I find work done by friction over a curve represented by a polynomial? @user1220376 The normal force you wrote down is for a body that is at rest. When a body moves on a curve the normal force is different because there is acceleration in the normal direction (as you get in circular motion). Beyond that, see Qmechanic's answer below.
Aug 6	awarded	Citizen Patrol
Aug 6	comment	“Time” by epistemic subdivision of a closed system Okay, thanks for letting me know.
Aug 6	comment	Derivation of the enhancement of U(1)$_L$ x U(1)$_R$ to SU(2)$_L$ x SU(2)$_R$ at the self-dual radius Yes, Polchinski section 8.3 (volume 1).
Aug 5	answered	Why are there two ways to solve for energy of a spring?
Aug 5	comment	“Time” by epistemic subdivision of a closed system I'm sorry but I don't think this is a question in physics.
Aug 5	comment	Does String theory say that spacetime is not fundamental but should be considered an emergent phenomenon? @RonMaimon As for AdS/CFT, I think that just by matching the symmetries it is clear that only one dimension is emergent in the CFT description. The non-commutativity you are referring to happens for example with D-brane coordinates but not with bulk coordinates, so I don't see how it changes the answer.
Aug 5	comment	Does String theory say that spacetime is not fundamental but should be considered an emergent phenomenon? @RonMaimon I agree that the nature of spacetime in string theory is not as simple as it appears in the worldsheet action, because of what you say. But the question was whether spacetime is emergent. In the usual (i.e. modern, textbook) treatment that everyone uses today, spacetime points are there from the start as fundamental variables, and therefore I think that spacetime is clearly not emergent.
Aug 5	comment	Does String theory say that spacetime is not fundamental but should be considered an emergent phenomenon? @NickKidman in the usual formulation, where you start e.g. with the Nambu-Goto action, you also have a spacetime metric as part of the string background.
Aug 4	revised	First Postulate of Special Relativity: What does it mean? added 2 characters in body
Aug 4	answered	Does String theory say that spacetime is not fundamental but should be considered an emergent phenomenon?
Aug 4	answered	First Postulate of Special Relativity: What does it mean?
Aug 4	comment	Relativity - time dilation On more general grounds, as someone on earth you cannot gain a speed advantage by doing a calculation on a moving ship (e.g. a ship orbiting the earth). In our frame a moving computer will always take longer to complete the calculation than one staying on earth. This is even without accounting for the time it takes to transmit the result back to earth.