# spin of the photon

by scope
Tags: photon, spin
 P: 60 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? thank you for your reply!
 Mentor P: 15,618 What event horizon? Are you talking about Hawking radiation?
 Sci Advisor Emeritus P: 7,207 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).
P: 60

## spin of the photon

 Quote by DaleSpam What event horizon? Are you talking about Hawking radiation?
no, the event horizon from special relativity
P: 60
 Quote by pervect 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).
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?

Mentor
P: 15,618
 Quote by scope no, the event horizon from special relativity
SR does not have the usual event horizons.
PF Patron
Emeritus
P: 5,311
 Quote by 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?
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).
P: 60
 Quote by bcrowell 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).
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?
PF Patron
Emeritus
P: 5,311
 Quote by 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?
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.
 P: 60 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?
P: 1,503
 Quote by 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?
Yes, and it's the same for the others particles, not for photons only: there is no (accepted) physical model of spin.
PF Patron
P: 5,509
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 spin of an elementary particle is a truly intrinsic physical property, akin to the particle's electric charge and rest mass.
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.....
P: 1,068
 Quote by scope hi, if i understand relativity well, in our frame of reference a photon does not change(infinite time dilation).
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.

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