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 Nov 24 awarded Scholar Nov 24 accepted Spectrum of Free Strings Oct 29 answered Is it possible to recover Classical Mechanics from Schrödinger's equation? Oct 10 answered Differences between the gravitational constants $G$ and $g$? 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? deleted 35 characters in body 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.