# Spin of the photon

1. Oct 29, 2010

### scope

hi,

if i understand relativity well, in our frame of reference a photon does not change(infinite time dilation). then if we try mathematically to describe its spin as an intrinsic angular momentum, then there should be no observed angular momentum in our frame of reference because this angular momentum would be at the event horizon and time stops at the event horizon in our reference frame. is it correct?

2. Oct 29, 2010

### Staff: Mentor

3. Oct 29, 2010

### pervect

Staff Emeritus
See any of the zillion threads about how a photon doesn't have a "frame of reference" in the usual sense.

This doesn't prevent photons from having an affine parameterization. There's a handful of threads that discuss this, too, though not as many as the first. A photon doesn't experience time as its worldline is null, but you can still order events along it's worldline in a sequence by an "affine parameter", even though it's a null worldline (and usually a null geodesic worldline).

4. Oct 29, 2010

### scope

no, the event horizon from special relativity

5. Oct 29, 2010

### scope

thank you, but i do wonder about the spin of the photon. in OUR reference frame, do we see the photon spinning or not, if we apply mathematics?
by the way how does spin transform under Lorentz transformations?

6. Oct 30, 2010

### Staff: Mentor

SR does not have the usual event horizons.

7. Oct 30, 2010

### bcrowell

Staff Emeritus
A spin of +1 along the direction of motion would look like circularly polarized light, with the polarization rotating in the clockwise direction as viewed along the direction of motion (from behind).

8. Oct 30, 2010

### scope

thank you for your great reply. but is it possible that some velocity of this polarization(spin) reaches superluminal speeds, since if we try to interpret the half-integer spin of the electron for a non-zero radius, then it can lead to velocity at this radius that is faster than light(thats why spin causes a problem of definition). Or there is a clear difference between the photon spin and the electron spin regarding this point?

9. Oct 30, 2010

### bcrowell

Staff Emeritus
You can't visualize it as a rotating material object with some radius r. You can draw a diagram showing circular polarization, with a vector sweeping around in a circle, but that vector is a field vector; it has units of field strength, not units of distance.

A similar confusion can occur when you talk about an ordinary plane-polarized EM wave. If you believed that something was moving through space along a sinusoidal path, then its velocity would be >c.

10. Oct 30, 2010

### scope

ok, so if i understand well, even if spin causes polarization, polarization is not the rotation described by spin, and nothing describes the spin rotation physically?

11. Oct 31, 2010

### lightarrow

Yes, and it's the same for the others particles, not for photons only: there is no (accepted) physical model of spin.

12. Oct 31, 2010

### Naty1

Some good insights into spin here:
http://en.wikipedia.org/wiki/Particle_spin

Spin effects show themselves as in Pauli exclusion and polarization (described already) and as a degree of particle freedom...
The same article notes: "Quantum mechanics states that the component of angular momentum measured along any direction (say along the z-axis) can only take on the values..." so I guess in this sense we do measure spin rather directly.....

Last edited: Oct 31, 2010
13. Nov 1, 2010

### 1MileCrash

I think it's backwards - in a photon's own "reference" (which cannot exist but for simplicities sake) time does not pass. In our frame of reference or from the frame of reference of any massive object photons do take time to reach destinations.