Photon spin - experimental evidence

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Discussion Overview

The discussion revolves around the concept of photon spin and the experimental evidence for photons carrying angular momentum, particularly in the context of a rotating disk when exposed to a beam of polarized photons. The scope includes theoretical implications, experimental observations, and the relationship between classical and quantum mechanics.

Discussion Character

  • Exploratory
  • Debate/contested
  • Technical explanation

Main Points Raised

  • Some participants assert that a beam of photons with like polarization directed at a disk can cause it to rotate, suggesting this as evidence of photons having angular momentum.
  • It is noted that this effect occurs if the photons are absorbed or reflected by the disk.
  • Questions arise about the source of such a beam of photons and the distinction between classical angular momentum carried by electromagnetic waves and the spin of individual photons.
  • One participant argues that classical electromagnetic waves carry angular momentum independently of quantum mechanics, challenging the necessity of quantum explanations for this phenomenon.
  • Another participant agrees that the transfer of angular momentum from the electromagnetic wave to an object does not necessarily indicate quantum mechanics but emphasizes the belief in the existence of photons as relevant to the discussion.
  • A further point is made regarding the need for a specific discrete amount of momentum transfer to demonstrate photon angular momentum, suggesting that a large number of photons complicates the ability to isolate this effect from other influences.

Areas of Agreement / Disagreement

Participants express differing views on the implications of angular momentum transfer from electromagnetic waves and the necessity of quantum mechanics to explain photon behavior. There is no consensus on whether the rotation of the disk serves as definitive evidence of photon angular momentum.

Contextual Notes

The discussion highlights limitations in distinguishing between classical and quantum angular momentum, as well as the challenges in isolating the effects of individual photons from a large ensemble.

Usaf Moji
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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?
 
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It is true if the photons are absorbed or reflected.
 
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?
 
turin said:
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.
 
clem said:
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.
 
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.
 
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.
 

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