Why do we say that photons are particles?

Question

This question may appear stupid but I really do have to understand. Maybe it's just semantic and nothing else.

Why do we say that photons are (elementary) particles?

They are pure radiation, since they are massless, aren't they? So they cannot be treated as point like particles, and I think that it's a nonsense to think about them as particle according to the definition of a particle.

Once I also read about he hypothetical mass of a photon which (if existed) should be smaller than $10^{-54}$ kg more or less. This however may be useless to know.

Answer 1

The standard model of particle physics has a quantum mechanical Lagrangian where the elementary particles of the table enter as point "particles" with the mass and the quantum numbers in the table. This is a very successful physics model fitting an enormous amount of data from resonances organized in the eightfold way , to LEP data with the culmination of the Higgs discovery at LHC .

"Particles" with quotation marks because they are not classical billiard balls, they are quantum mechanical entities manifested in experiments microscopically with probability distributions.

The reason one calls photons and gluons and gravitons and Z and W particles is because of the validation of the standard model.

The above is the current state of particle physics, experiment and the theory that describes them and can predict new behaviors.

The photon emerged as a particle, at the time not separated from a classical particle because of the photoelectric effect. It was a proof that light was composed by quanta and these were named photons, to finally be called quantum mechanical entities, "particles". With quantum field theory the emergence of the classical electromagnetic radiation from the photon field is shown in this blog post of @Motl.

Answer 2

There is something called the Compton effect, where an electron and a photon interact with each other, and the scattering happens with a large, billiard-ball style change of momentum, rather than the "soft" sort of interaction you would expect from a fluid or continuous field.

Answer 3

All the elementary particles are described as excitations of quantum fields. What you think of as a particle is actually a much stranger object. It is an excitation in an operator field that spans all of spacetime.

While this may seem an odd theory it works exceedingly well and it makes some previously mysterious aspects of particle behaviour very clear. For example particles can be created by adding energy to the quantum field and destroyed by removing energy from the quantum field, which neatly explains how matter can be converted to energy and vice versa. It also neatly explains the wave particle duality. Particles are neither particles nor waves - the energy in a quantum field can behave in wave like and particle like ways under different circumstances.

The point of all this is that photons are described by quantum field theory in exactly the same way as all the other particles, so there is no reason to regard them as any different to the other particles. Photons are massless gauge vector bosons, and their behaviour is somewhat different to massive fermions like electrons, but these differences are all well described by quantum field theory.

Answer 4

The observed selection rules of atomic light emission allow for an emitted photon only when one quantum of angular momentum change occurs in the electron complement of the emitting atom. So, by conservation of angular momentum, we must have a departing particle containing that angular momentum: this is a boson, called a photon. The equations of motion of a photon are those of continuous electromagnetism, but the interaction with matter shows that photons have particle properties.