Are photons and electrons "elastic" particles?

Question

My teacher told me that Photons are elastic particles (allegedly it was a postulate of Max Plancks's Quantum theory too) and that during the photoelectric effect due to "elastic collision" the transfer of energy occurs simultaneously which results in the ejection of electrons without a time delay. I now know that there is in fact a time delay, something my high textbook also missed out on. In addition, I also figured out that the collision cannot be elastic as the sum of the Kinetic Energy is not a constant like it must be for elastic collision, with work energy being the culprit.

But that gets me back to the original question. Are photons elastic particles? What about electrons? Can elasticity be defined for particles like photons and electrons? A recent research appears to think so.

When one snooker ball hits another, both spring away from each other in an elastic manner. In the case of two photons a similar process -- the elastic collision -- has never been observed. Physicists have now shown, however, that such a process does not only occur, but even could soon be registered in heavy-ion collisions at the LHC accelerator.

But what I came to the conclusion in the first paragraph and what was found by researchers don't seem to be compatible. Is there a reason? My best guess would be that the photons appear elastic only during photon-photon interactions but not during photon-electron interactions.

Answer 1

A free electron colliding with a free photon always (as far as experiments have seen) results in a perfectly elastic collision.

A photon colliding with an electron that is bound in a complex system like an atom does not result in an elastic collision. The difference is that the photon is not interacting with just the electron, but with the whole atom system.

Inelastic collisions happen when some of the initial kinetic energy is transferred into other forms of energy during the collision, leaving you with less final kinetic energy.

Apparently free electrons and photons do not have any internal structure or degrees of freedom that can hold energy. Hence, a collision between the two must always conserve kinetic energy.

But an atom made up of an electron and a nucleus does have internal structure; some of the kinetic energy can go into changing the potential energy between the electron and the nucleus. Hence, a collision between a photon and an atom might be inelastic.

Answer 2

Your teacher was either trying to describe by an analogy which you misunderstood, or was mistaken. Quanta have no internal structure, so the can never interact inelastically. And quantum interactions track and conserve total system energy, which is a trait they share with elastic collisions. However, quanta are not tiny hard spheres, an interaction is not a collision, and inferences based on either supposition are more likely to be false than true.

In particular, far from being representable by tiny hard spheres having a collision, the most likely result of two photons finding themselves in the same place at the same time going in different directions is to pass through each other without any interaction at all. Less likely, they might wink out of existence and be replaced by a massive particle-antiparticle pair, subject to conservation laws. Each is an intersection of two quanta in trajectory that conserves energy, but neither bear any resemblance to an elastic collision.