Photon spin - experimental evidence

[Usaf Moji]

I read somewhere that if a beam of photons all of like polarization are directed towards the surface of a disk (the disk being capable of rotation), the disk will rotate. This rotation is supposed to be experimental evidence confirming that photons have angular momentum.

Is this true?

 

[Meir Achuz]

It is true if the photons are absorbed or reflected.

 

[turin]

Where can you get such a beam of photons? Doesn't the EM wave classically carry angular momentum? How can you distinguish the classical angular momentum from the spin of individual photons?

 

[Meir Achuz]

Where can you get such a beam of photons? Doesn't the EM wave classically carry angular momentum? How can you distinguish the classical angular momentum from the spin of individual photons?

The "classical wave" is just a huge number of photons.

 

[turin]

The "classical wave" is just a huge number of photons.

No. You are talking about the correspondence between classical and quantum. What I'm saying is that, even if you don't assume a quantum for the electromagnetic wave (a photon), there is still angular momentum carried by the wave; this does not require quantum mechanics. Since classical E&M preceeds QM, there is no reason to believe that a transfrer of angular momentum from the EM wave to an object is evidence for QM; it is already there in classical E&M.

 

[Meir Achuz]

You are right in that "there is no reason to believe that a transfer of angular momentum from the EM wave to an object is evidence for QM".
But if we believe that photons exist (Don't we?), then "This rotation is ... experimental evidence confirming that photons have angular momentum.", which is what was asked.

 

[turin]

I suppose I am splitting hairs, here. My point is that a transfer of momentum is insufficient to demonstrate photon angular momentum; the transfer of momentum must be a specific discrete amount in order to demonstrate photon angular momentum. So, if single photons hit the object periodically, then you could see this effect, but if you have "a huge number of photons", then you have no way of separating this tiny effect from other tiny effects, say, the beam hitting at a slight angle and slightly off axis.