Forever_a_Newcomer
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 Apr 3 comment A simple example of symmetry setting the properties of a Physical System Angular momentum is a good one, but it would be nice to find a more "down to Earth" example (sorry for the pun). Does a spinning top needs to be invariant under any rotation? Or does less symmetric top works? (say invariant under 90 degree rotations - with a square cross section). This looks promissing, I'm trying to understand it: en.wikipedia.org/wiki/Rattleback Oct 7 comment How can perturbativity survive renormalization? But, being more specific: when you do perturbation you have to expand $Exp(- i S_{I}- i S_{CT})$ (S are the action of the interaction and of the counterterms), and pick the first terms. I understand doing that for $S_{I}$ but not $S_{CT}$. It seems that this step in the calculation is invalid. Of course the infinities will cancel but that seems to me like "holding" a limit - it seems like the order in which you take limits ($\lambda$ small vs. huge regulator) make a difference in the result Oct 7 comment How can perturbativity survive renormalization? The question is more specific than "explain me renormalization". Also, if the infinity is just "pushed to the next order", how can you explain truncating the series? Sep 3 comment Ward Takahashi identities from Z invariance Just be sure: so I'm right about this commutation business being all wrong? Also: is the link I posted to the book itself (on Google books) working? Sep 2 comment What´s the importance of the normalization of the Kinetic term? Thanks, I'll have to check that keeping $\hbar$ around. I included some comments in the question. Sep 2 comment What´s the importance of the normalization of the Kinetic term? Included my take on this on the question. Please let me know if I got it wrong. Aug 30 comment Ward Takahashi identities from Z invariance I understand what you are saying, that's what you do when writing supersymmetric models in terms of 2-spinors. But these are usual 4-component Dirac spinors and their sources. I never seen their scalar product defined in any other way than just the sum of the product of their components. In fact this seems to indicate just that. Aug 30 comment What´s the importance of the normalization of the Kinetic term? Thanks for point out the mistakes - I corrected them in the question. As for your answer: does it have to do with the $\langle p | \phi | 0 \rangle$ that appears when you are building the LSZ formula? I think I got it but I want to be sure before including my conclusion in the question itself. Being more specific: I guess there will be a $\sqrt{2} \langle p | \phi | 0 \rangle$ in the case of ${\cal L}_2$ but I can't see how it appears without doing field redefinitions. Aug 24 comment Filming light in motion? Good point, not a native English speaker, and automatic orthography correction didn't help me there :D It would be nice to have laser beans though... Jul 30 comment Surface terms for field path integrals? So the point here is that the "surface terms" vanish when I integrate with ${\cal L} \rightarrow (1+i \epsilon){\cal L}$, right? Jul 19 comment Why is the (free) neutron lifetime so long? @Terry - maybe you did (forget me) but I didn´t forget the question ;) Great answer by the way! My thanks to everyone involved Jul 8 comment Why is the (free) neutron lifetime so long? I not completely satisfied with the answer. It seems to me that dr-bdo-adams and @terry-bollinger are explaining this based on the off-shellness of the W. But still the diference is too big, the neutron lifetime is $10^9$ times bigger than that of the Muon, but the "liberated energy" is only $10^2$ smaller. Is this going like $\left(\frac{E_L}{M_W}\right)^4$? ($E_L$ is the liberated energy). Can someone give me pointers of why we have such a high power in the dependence? Jun 22 comment How to detect ice in thermostat Nice idea, but he would have to figure out a way to avoid ice formation in the tube itself, which could be hard depending on the setup. Jun 22 comment Energy efficiency of antimatter producion Also, even if you could get antimatter stored in considerable quantities, with efficiency close to 1 (not bigger). What would you do with it? Eletron-positron pairs anihilate into 500 keV gamma rays, which is very bad for propulsion. Photons this hard can´t just be reflected for push. You would have to find materials that would absorb and thermalize them, and then eject these. I don't think this is much better than what we already do, at much higher costs. Jun 21 comment Imaginary pertubation to a Hamiltonian: how is it the same as rotation to imaginary time? (...) he uses exactly this substitution to get $Z[J]$. Of course, the potential for a free scalar field is proportional to $q^2$, but wouldn't adding the whole $i\epsilon H$ mess up the momentum integral you need to get from the hamiltonian to the lagrangean? The bothering point here is that this $i \epsilon$ factor will be the one (on page 185) dictating the path of integration to get the propagator. Jun 21 comment Imaginary pertubation to a Hamiltonian: how is it the same as rotation to imaginary time? Yes, I noticed it would be far simpler to add $i \epsilon H$ but, when the author goes on to quantizing a scalar field (page 182) Jun 19 comment What is the relation between (physicists) functional derivatives and Fréchet derivatives That´s exactly my problem with this definition (Dirac delta subtleties apart). It seems your are defining the partial derivative in the specific direction of $\delta_y$, but this very definition is used everywhere as the partial derivative in any direction (are they all the same?). Another way of expressing my problem: is the first equality below valid? Why? (I tried to follow your notation, $f_y$ means f calculated at point $y$): $$\frac{\delta F}{\delta (f_y)}[f] = \frac{\delta F}{\delta (\delta_y)}[f] = \lim_{\epsilon \to 0} \frac{1}{\epsilon} ( F[f + \epsilon \delta_y] - F[f]).$$