These questions are provoked by this New Scientist article Vacuum has friction after all. It essentially says that an object spinning in pure vacuum will eventually slow down due to friction with the virtual photons that continually pop in and out of existence even in a vacuum.
1/ This is said to happen because
… so imparting a greater force against the object’s spin than with that spin, thus slowing it down. But my understanding was that the quality called “spin” in atomic physics was just a label for a property that had no real analog in the world of classical physics. The relevant Wikipedia article seems to say this is not so, and elementary particles’ spin is real spin - kinda. That is, it has some of the properties of classical-world spin including angular momentum (or something that acts like it, anyway):
Can anyone clarify this for me; in what sense is a photon’s “spin” actually spin? How does that allow it to transfer angular momentum to a large scale object?
2/ As I understand it these virtual photons occur in pairs, that mutually annihilate after a brief life. But what happens when one of the pair has interacted with something (in this case, lost some of its “spin”) and the other has not? I know this is the basis of Hawking Radiation, but does it take a black hole to cause the pair to behave differently than each other, and interfere with the particles disappearing again?
3a/ The article also says:
But in the situation under consideration we are in a vacuum, unpopulated by any particles other than the virtual ones. Isn’t it so that temperature is solely a measure of particle energy; if so how can there be a higher or lower temperature in a vacuum? On the other hand if we are not in a perfect vacuum it’s unremarkable that the object would experience friction and slow down.
3b/ This thought also triggers another: does the soup of virtual photons have a temperature, absent any real particles?