Recent content by guillefix

Thanks, and another question Thanks for that! I've got another question though. In the same document, a bit later, he says that one can also derive the Schrödinger Lagrangian by taking the non-relativistic limit of the (complex?) scalar field Lagrangian. And for that he uses the condition...

In pages 41-42 of these notes: http://www.damtp.cam.ac.uk/user/tong/qft/two.pdf , it is said that |\vec{p}|\ll m implies |\ddot{\tilde{\phi}}|\ll m|\dot{\tilde{\phi}}| Why is this so?

That was the key I was missing, probably missed due to writting it so abstractly :P. Thanks!

Again, from the Peskin and Schroeder's book, I can't quite see how this computation goes: See file attached The thing I don't get is how the term with (\partial^{2}+m^{2})\langle 0| [\phi(x),\phi(y)] | 0 \rangle vanishes, and also why they only get a \langle 0 | [\pi(x),\phi(y)] | 0 \rangle...

Hello, I'm looking at the following computation from the Peskin and Schroeder's book: See file attached In the second page, the second term that's being integrated, I don't understand why it has a negative i in the exponential, that'll keep the energy term the same, but will swap the sign...

Thank you for the help, and yeah it was quite lengthy/mistake-prone. I think I found my fundamental mistake. It was in not changing the sign of the i inside the exponential of the Fourier transform of Pi and Phi for the a_dagger. Incidentally, following your definition, I actually arrive to the...

Sorry, yeah it's the Klein-Gordon field (in 3+1 dimensions). The expressions I got from the quantum harmonic oscillator, and the fact that the KG field can be considered as one of these per momentum. I haven't verified it in any literature actually, but it made sense, and gave me the right...

Oh true, that's it! Thank you very much!

Sorry for reopening a closed thread. But I have exactly the same doubt as this guy: https://www.physicsforums.com/showthread.php?t=346730 And the answer doesn't actually answer his question. I do get delta(p+p'), but they just help me in getting a_{p}a_{-p} and a_{p}^{\dagger}a_{-p}^{\dagger}...

Hello, I'm having trouble calculating this commutator, at the moment I've got: \left[a_{p},a_{q}^{\dagger}\right]=\left[\frac{i}{\sqrt{2\omega_{p}}}\Pi(p)+\sqrt{\frac{w_p}{2}}\Phi(p),\frac{-i}{\sqrt{2\omega_{p}}}\Pi(p)+\sqrt{\frac{w_p}{2}}\Phi(p)\right]=i\left[\Pi(p),\Phi(q)\right]=i\int...

Yeah, sorry, it's actually quite simple, but should have made it clearer. I decided instead of rotating the bottom surface to displace it a bit, which is maybe more practical?. Anyway, in the picture, the space between the first surface and either of the bottom ones is glass say. The yellow beam...

Probably this is an easy question, given that Newton's rings are probably one of the most common and famous light phenomena. In any case, I was wondering that given that say you have the second interface at a position in which the reflectivity on the first is 0, but then you are allowed to...

Just read in here: http://hep.ph.liv.ac.uk/~hutchcroft/Phys258/CN6EMWaves.pdf that I was right yeah. Walter Lewin's lecture was a bit missleading.

Hello, The energy density of an electromagnetic wave is ε_{0}E^{2}. To calculate the energy flux, at least in the derivation's I've seen, people just multiply by the speed of the wave, i.e., c. But doesn't this assume that the energy density is constant at all points?; but E changes...

Yes that's what I meant :P True, I should have posted more detail. The model I've used is quite simple, with just coupled inductors and resistors. The schematic is attached. The equations I used can also be found in hyperphysics here (with another schematic acutally)...