Recent content by Feynlee

Thanks for pointing that out! I think that makes sense. I was thinking about the "contradiction" I brought up in my previous post: I think I figured out what's wrong with it. I was assuming that the operation only applies to the spin but not the motion of the particle. But it seems that the...

Notice that in eq.(111) the operation is on a state that's already moving with momentum k, which can be seen as boosted from k0, when you do another boost from k to \Lambdak, if they are not collinear, then the result will differ from a boost from k0 directly to \Lambdak by a rotation. In other...

I understand the difference between quantum and classical. But even in quantum, we can define the spin direction by asking whether it is the eigenstate of a spin operator in that direction. If the state is an eigenstate of the spin operator in some direction, then I say the spin is pointing in...

It seems to me different people are saying different things. Let me explain my confusion a little bit more. I understand that I can retain the spinor direction if I boost it in the direction of the spin from the rest frame. What I don't understand is how to find out the direction of a spinor...

Since the Dirac spinor can be thought as a spinor directly boosted from the rest frame, if your claim is true, does it mean that when we say "a spin-up Dirac spinor", we don't mean a spin that's pointing in the +z direction if the momentum is not parallel to z direction? Which direction is it...

Say I have a particle in the rest frame with spin up in the z direction. If I boost the system in x direction, does the spin still point in the z direction? Why or why not? In Dirac Spinor, if I choose "spin up", namely setting \phi=\begin{bmatrix}1\\0\end{bmatrix} in the notation shown in...

Thanks for your reply! Is there any reference you can recommend that shows how exactly this works?

It seems to be a dumb question. But I haven't seen anyone making this connection between QED and Classical EM in a complete fashion. The only example I've seen is the connection between two particle scattering amplitude calculation in QED (Peskin's book), and the amplitude of a particle...