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seen Jul 23 '13 at 10:51

String theory grad student.


Sep
10
comment Is the Higgs a quantum field or a particle?
I agree that we cannot really claim at this point that the Higgs is elementary.
Aug
10
comment Why is Mendel Sachs's work not taken seriously? Or is it?
Quantity of work is a poor measure of the work's value.
Aug
10
comment Constraining two-point functions of boundary operators on the disk
I am certainly assuming unitarity. The fact that correlators of descendants are determined in terms of primaries is of course true, which is why we don't usually care about such correlators. Perhaps this is enough for your purposes, but note that this alone does not imply that the pairs of correlators you asked about are at all related to each other: each could be related to a different correlator of primaries. My suggestion is that they are in fact related.
Aug
9
comment Constraining two-point functions of boundary operators on the disk
Yes, I see what you mean now. I added an idea for the descendants.
Aug
9
comment Change in appearance of liquid drop due to gravity
@CrazyBuddy What confuses me at least is that you call them 'horizontal' lines -- I never saw a raindrop that looks like that. That is why I thought you meant this flattening. Do you mean vertical lines?
Aug
9
comment Change in appearance of liquid drop due to gravity
Their shape depends on their size. Small drops are spherical, larger ones look sort of like pancakes (this is what I mean by 'flat', perhaps this was not clear), yet larger ones look like parachutes. I don't know at which size (if any) they become elongated, but the shape is due to the flow of air and not gravity. I will try to clarify...
Aug
8
comment What is the fundamental differences between bound and entangled states
Sorry, but I do not understand what you are asking. Specifically the phrases "the particle and the energy are bound to the process of entanglement" and "energy is determined to stay bound ..." make no sense to me. The photons do not exist before the decay. They are created when the particle decays.
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
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
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
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
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.
Aug
4
comment Relativity - time dilation
You have to be careful about simultaneity, which is not a frame-independent concept. In your original question, you defined simultaneous events in the frame of observer A (whether you intended to or not). As long as you stick with a frame you're fine. But now you launch the second chip in one frame, and want to discuss what it looks like in the first chip's frame, so you have to be careful. For example, in the first chip's frame the new launch does not happen when he is $10^{-6}$ months old, but after that! You have to work it out carefully.
Aug
4
comment How is $\frac{dQ}{T}$ measure of randomness of system?
A microscopic description (like statistical mechanics) considers the underlying degrees of freedom, and should explain how the observed 'macroscopic' phenomena emerge from the detailed model. If you derive the ideal gas law by postulating tiny atoms bouncing around -- that is a microscopic description. Of course a microscopic model at one level may become phenomenological once you go to more detailed observations, so this distinction depends on one's point of view. People who study optics would not call particle physicists 'phenomenologists' -- but strings theorists would.
Aug
4
comment How is $\frac{dQ}{T}$ measure of randomness of system?
I don't know about a 'formal' definition, but a phenomenological description is a description that is at the level of the observations. We observe that a system has certain properties that we can measure, like temperature, pressure, and so on. And we observe certain relations between these properties. When we combine these into a (hopefully simple) model -- that is a phenomenological description. For example, the ideal gas law and Ohm's law are both phenomenological models.