# If photons travel at c, how do they spin?

1. Mar 22, 2006

### kmarinas86

"If photons travel at c, how do they spin?"

I thought the time dilation would mean that the photon would be in a frozen like state. Is this spin real? How does a point particle spin? How does a wave spin? I'm not sure whether this spin is actually a spin in the literal sense. Also, if it is not a literal spin, then is the angular momentum of a photon ±h/2pi not a literal angular momentum?

2. Mar 22, 2006

### Staff: Mentor

Fundamental partlcles don't "spin" in the classical sense, because they're pointlike as far as we know, but they do have intrinsic angular momentum.

The intrinsic angular momentum of fundamental particles contributes to the total angular momentum of an object and can be observed macroscopically under the right circumstances.

Have you ever done or seen the demonstration where someone sits on a turntable while holding a spinning wheel (usually a bicycle wheel)? If the person and turntable are initially stationary, and then the person flips the spinning wheel, the person and turntable start to rotate so as to keep the total angular momentum constant.

Replace the person and turntable with an a chunk of iron or other magnetizable material, and the spinning wheel with the electrons in the iron that produce its magnetization when their intrinsic angular momenta are oriented in the same direction. Start with the iron magnetized in one direction, suspend it so it's free to rotate, and make it appear to be stationary. Then reverse the magnetization, which flips the intrinsic angular momenta of the electrons and the iron starts to rotate so as to keep the total angular momentum constant. This is the Einstein - de Haas effect.

And I think it has been demonstrated that when an object absorbs circularly polarized light (which consists of photons whose intrinsic angular momenta are all oriented in the same direction), it acts as if something exerted a torque on it.

3. Mar 23, 2006

### dextercioby

We can't really measure the value of the photon's spin, i.e. the eigenvalues of the spin operator, because, in order to do so, we'd have to place ourselves in the rest frame of the particle (as to insure zero eigenvalues for the orbital angular momentum), but the photon doesn't have something like that, since it's a massless particle. We can measure however the projection of the spin vector upon the photon linear momentum vector and we find that this is quantized as well. It can take the values +/- \hbar. We call the operator

$$\hat{\lambda} \sim \hat{\vec{S}} \cdot \hat{\vec{P}}$$

the helicity operator...

Daniel.

4. Mar 23, 2006

### LnGrrrR

Not to hijack the thread, but to piggyback...I've heard of instances where three particles were entangled....how do they measure that? When talking about 'spin' on two entangled particles, are they just talking in terms of percentages, instead of direction or something that is a strict +/-? For instance, is it that two entangled photons show one spin at 50% and the other spin at the opposite 50%, and three entangled photons show 33% spin among them?

(Just having a hard time visualizing it. Any help is appreciated.)