Recent content by Rick89

anyway, some physicists are not so sure about the status of fields, I mean if you measure them. It is a part that leaves doubts. In fact for example the Dirac field is such that you can only measure quantities involving even multiples of it, and it isn't clear at all why. (It is required because...

hi, well you include the negative frequency terms exactly to make sure the sum is overall hermitian, it is to constrain it to be such. You're right you wouldn't need both terms, you are in practice including each term twice, this allows you to specify that the operator that goes with +p and...

Hi, honestly I think the mode expansion of psi (i.e. writing it in terms of a and a-dagger) is just the definition of a, no hidden physics (the SHO is why this expansion is useful...because you can then write the Hamiltonian so that you see a and a-dagger are ladder operators). Anyway it is a...

yes, thanx, but that's what I was doing. As I said the delta function is there, it comes from the integral in d^4x of the exp(-i p.x) since there are no other terms with x (the two exponentials of the two psi's cancel) it gives delta(p), imposing the constraints (conservation laws) you're...

Hi, thanks for your reply, I'm not sure about this (I'm not doing that course yet, I'm only reading it for myself) , check what I say please. The delta fn there comes from the spacetime integral of the exponentials. In my case,once acted with a already as in the notes, we commute the c past the...

if you look at that page (it's the number written, not the page of the pdf I meant) you will see that in meson decay he's doing that. just what about the term I was mentioning? Thanx

to haushofer: I know that, it's true. But does it matter for what I asked? to Dickfore: I am not sure I understand exactly where your formula comes from, but I have a feeling my thing is in a step in evaluating it. It's quite simple to see where my matrix element comes from, I'm considering...

I know what you're saying, but then why is the matrix element different from zero, there must be an error, where? Thanx

anyone with more detail? I still don't understand how to treat such terms.. Thanx

maybe I see what you mean, is it because of amputated diagrams and all that story?

well, I suppose the vacuum of the free theory, without interaction would do...is it important? what I'm not understanding is probably trivial, just I don't see why we compute decay of a meson using b-dagger*c*dagger*a (that is destroy a meson and create a pair of nucleon-antinucleon) but cannot...

Hi, I am having this problem learning interacting fields. In scalar Yukawa theory, two scalar fields (one real call meson phi, one complex call nucleon psi) (interaction term psi-dagger*psi*phi) why can't we write at first order in the interaction an amplitude from a single meson state to vac...

sorry, that was not what I meant. I meant: what is the trick to calculate commutators involving gradient and laplacian operators (for example [pi,del squared phi])? Do I treat them as operators? How do I deal with the fact that they only act on the operator immediately to the right? Is this the...

sorry, I'm not sure I see what u mean. I understand how to pass from the first to the second expression of the Hamiltonian used by "integrating by parts" the grad term. But my problem is how to handle the commutators. Any more suggestion? I still can't go on, I'm probably missing something easy...

Hi, could someone give me a hand with the two long commutators on page 25 of Peskin and Schroeder? I'm not sure how to deal with the gradient in the first and the laplacian in the second. Thanx a lot