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seen Dec 15 '13 at 13:04

Sep
6
comment Why is the $\langle v_{x}^{2} \rangle=\frac{1}{3} \langle v^2 \rangle$?
Generally $\langle A + B \rangle = \langle A \rangle + \langle B \rangle$, this is more or less build into the setup. After all expectation values are computed by evaluating certain sums or integrals.
Jul
27
comment Can physics get rid of the continuum?
@RonMaimon You are right, I am not terribly satisfied with my answer myself. The idea I tried to convey was that numerical analysis has been very successful in simulating classical physics, because they deal with differentiable functions, those functions look boring locally and differential equations can therefore be discretized successfully. The "therefore" sweeps the whole field of numerical analysis under the rug and you are right that there are many subtle problems, energy conservation for example.
Jul
26
comment Can physics get rid of the continuum?
@NickKidman I modified the last sentence, hopefully that is not bad form.
Jul
26
revised Can physics get rid of the continuum?
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Jul
26
answered Can physics get rid of the continuum?
Jul
6
comment Why is ${\partial^i}{\partial_i\phi}$ = ${\partial^i {\phi}}{\partial_i{\phi}}$?
Where did you see that? One thing you can do is partial integration, but then you are missing at least one $\phi$. In deriving the Euler-Lagrange equations, you use partial integration.
Jul
1
comment What is Supersymmetry (SuSy)?
Each of those terms have a rather technical meaning. To explain them usually takes several lectures each, at least. That you ask about them in one question indicates that you have no, or little prior knowledge. I think you should at least indicate why you are interested in those questions.
Jun
27
comment Snooker/billiards cueing
@CaptainGiraffe Sommerfeld treats side queing in his Lectures on Mechanics, if I remember correctly. He certainly treats unver/over cueing.
Jun
27
answered Exercise QFT and CFT
Jun
26
revised What's the distinctions between Yang-Mills theory and QCD?
edited body
Jun
26
answered What's the distinctions between Yang-Mills theory and QCD?
Jun
21
revised Space-time in String Theory
added 94 characters in body
Jun
21
revised Space-time in String Theory
added 232 characters in body
Jun
21
answered Space-time in String Theory
Jun
17
revised Bell states entanglement
added 25 characters in body
Jun
17
comment Where do I start with Non-Euclidean Geometry?
@RonMaimon: I agree with you that unlike QFT and String theory the fundamental facts about general relativity are explained well in some text books, but that does not mean that there are no difficult questions left. To just give one example: Although one knows special solutions to the Einstein equations, there are no general existence theorems. Even a priori estimates are extremely hard.
Jun
16
answered Bell states entanglement
Jun
9
asked Helium Fusion and the Origin of Elements
May
30
answered Crash course on algebraic geometry with view to applications in physics
May
27
comment Derivation of the supergravity action in 11D
@LuboŇ°Motl : Is the procedure of beginning from the free spectrum and adding interactions trial and error, or does it follow some algorithm? I imagine that you begin by postulating some supersymmetry transformation between the fields, compute and find what terms do not cancel, add those and so on. At some point you find that the remainder can be written as a total derivative and you are done. I think that is more or less what was done in the original paper. Given that there are many of those miraculous actions I feel that there should be a systematic way.