Linked Questions

13
votes
1answer
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 ...
0
votes
1answer
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 ...
1
vote
0answers
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 ...
37
votes
5answers
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 ...
47
votes
2answers
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 ...
15
votes
2answers
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, ...
8
votes
3answers
823 views

Covariance in gauge theories: why should the Lagrangian be gauge invariant

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)...
8
votes
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 ...
4
votes
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 ...
3
votes
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 ...
1
vote
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 ...
9
votes
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 ...
1
vote
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 ...
1
vote
1answer
330 views

Why is helicity important in quantum field theory?

What makes helicity an important quantity in quantum field theory? I know that one can classify particles by mass and spin. For particles without mass one uses helicity (correct me if this is wrong). ...
0
votes
2answers
420 views

Why Lagrangian of electromagnetism with Lorenz Gauge evolve Klein Gordon equation?

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 ...

15 30 50 per page