Recent content by shakespeare86

The answer to my second question may be that under spatial riflession \pi is a pseudo scalar and thus, the first member is a vector too. And so follows the answer to the first question: the expectation value of the vector part of the current vanishes because it's a pseudovector and it's...

I'm reading Zee book on quantum field theory. He wants to explain that pion is the goldstone boson arising from the spontaneous symmetry breaking of the chiral symmetry. So he start with the weak decay \pi^{-} \rightarrow \bar{\nu} + e^{-} and write this equation <0|J^{\mu}_{5}|k>=f k^{\mu}...

Yes you are right: he says that. Infact in this way he is not proving gauge invariance and most of all he can't say that from d j =0 follows gauge invariance, as now i see we all agree :)! He has just proven that if we start with a massive spin one boson, then we can take the limit m->0 safely...

yes i agree. The amplitude for the diagrams is independent of m just thanks to current conservation, that makes the k^mu k^nu / m^2 in the propagator irrilevant. So j is a conserved quantity whatever m>0 we take and it sounds wrong that from this he can deduce gauge invariance that holds just if...

if you give the photon a mass, QED lagrangian is still invariant under U(1) global trasformation, because the mass term does not change! Under a global transformation A -> A! So whatever mass we give to the photon, it seems to me that j is a conserved quantity.

So is Zee wrong? I think if j is conserved, you just can't argue that you have gauge invariance, because j would be conserved also if the photon has a mass. Do you agree?

Hi, excuse the funny title :). In his book on quantum field theory Zee says (pag 245, fouth line) that QED gauge symmetry follows from the conservation of the current j=ψ γ^μ ψ (with the bar on the first spinor). I'm confused because that current is the noether current resulting from the...

Hi. I'm spending little time writing an easy C-code which simulates a gas on a lattice. I'd like to ask if you know any nice (and easy :) ) program which can take in input some files of positions of particles at different times and make a video of them. thanks

Yes, but the main problem, at least mine, was to understand why the emitted radiation is in phase with the stimulating radiation. Actually don't think we could speak about the phase of a photon. Anyway the question is perfectly well defined from a classical point of view.

don't worry ;)

yes, of course there is not a rule that explicitly prohibits simultaneous measurements. And this is the reason why I made my question in the first post. Anyway there are practical difficulties. They are not "mere practical difficulties" since physics is an experimental subject, so maybe, the...

Finding a frame where the measurements are simultaneously taken is not a problem. It's the one fixed with the point where the entangled state is created. Call it O. When S1 and S2 are measured, the second photon is out of the light cone of the first, but experimentalists in these points could...

Sorry my stupidity :( But I don't understand. It's ok that the order of observations does not matter. But I asked what's the correct interpretation of the correlation of spins if the two measurements are done at the same time. What I mean is: if the two measurements aren't done at the...

Hi. I didn't get the answer to the second question. Did you mean it's not really possible to measure S1 and S2 at the very same time?

In a singlet state like |e>= |+> |-> - |-> |+> if we take the two particles far apart and measure the spin of the first particle S1 first, we get the answer +1 or -1 with the same probability. This means that if we perform many measurements of S1 over many equal states |e> we get...