3k views

### Why does photon have only two possible eigenvalues of helicity? [duplicate]

Photon is a spin-1 particle. Were it massive, its spin projected along some direction would be either 1, -1, or 0. But photons can only be in an eigenstate of $S_z$ with eigenvalue $\pm 1$ (z as the ...
888 views

### What is the spin of photon [duplicate]

I am reading a book. It says that spin $s$ describes the symmetry property of a particle. Basically, the number of different wavefunctions which are transformed into linear combination of one another ...
253 views

### If a photon is a boson and has spin 1, shouldn't it have 3 spin orientations since spin 1 is a triplet? [duplicate]

I've gotten used to the fact that a spin can be described by its total spin and its $z$-component. And I've learned that a particle (really, anything) with spin 1 forms a triplet with three possible ...
20k views

### Difference between spin and polarization of a photon

I understand how one associates the spin of a quantum particle, e.g. of a photon, with intrinsic angular momentum. And in electromagnetism I have always understood the polarization of an EM wave as ...
8k views

### How do I construct the $SU(2)$ representation of the Lorentz Group using $SU(2)\times SU(2)\sim SO(3,1)$ ?

This question is based on problem II.3.1 in Anthony Zee's book Quantum Field Theory in a Nutshell Show, by explicit calculation, that $(1/2,1/2)$ is the Lorentz Vector. I see that the ...
3k views

### When an atom emits a photon are all directions equally likely?

When an atom has an electron in an excited energy level and it transitions to a lower level it emits a photon. What direction is it likely to emit the photon in? Are all directions equally likely, ...
823 views

I am following a course about gauge theories in QFT and I have some questions about the physical meaning of what we are doing. This is what I understood: When we write a Lagrangian $\mathcal{L}(\phi)... 1answer 2k views ### What is a gauge theory? Please note that I just read about 20 forum discussions, none of which answered my question. This question is related to my earlier question Is spacetime symmetry a gauge symmetry?. I am looking for ... 1answer 2k views ### Why helicity is proportional to the spin of particle and has two values? How can it be shown without using the little group formalism? Let's have the Wigner's classification for the irreducible represetation of the Poincare group. For the massless case the eigenvalues of ... 1answer 1k views ### The definition of a$\pi$polarized photon? I am looking at the definition of$\sigma^\pm$and$\pi$polarized photons (in the context of atomic transitions), however I have seem to come across two (both seen in numerous sources surrounding the ... 0answers 2k views ### Degrees of freedom of the photon in$d=n$It is well known that in ordinary$4$dimension, the photon has on shell only two physical degrees of freedom. Physically this means its elicity is either$\lambda=+1$or$\lambda=-1$but cannot ... 1answer 340 views ### Is the third spin vector of a photon always suppressed? I like to tell people interested in light polarization that the photon is a vector boson for which the third spin axis, the one in the direction of travel, is suppressed due to photons being massless ... 2answers 587 views ### Transverse polarizations of a massless spin 1 particle Physical polarization vectors are transverse,$p\cdot{\epsilon}=0$, where$p$is the momentum of a photon and$\epsilon\$ is a polarization vector. Physical polarization vectors are unchanged under a ...
Simply Lagrangian without a source for Maxwell equation is $$L = -\frac{1}{4}F^{\mu\nu}F_{\mu\nu}$$ Also Lorenz Gauge condition is $$\partial_{\mu}A^{\mu}=0$$ and if so I can briefly add this ...