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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.

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closed as unclear what you're asking by ACuriousMind, CuriousOne, user10851, Norbert Schuch, Daniel Griscom Mar 2 '16 at 17:33

Please clarify your specific problem or add additional details to highlight exactly what you need. As it's currently written, it’s hard to tell exactly what you're asking. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

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    $\begingroup$ "Assuming that a particle is "something" [...] with mass..." no: that's not the definition of particle. See Why are atoms particles? for (a possible) definition of particle, or Rigorous mathematical formalism of particle physics for a more formal definition. $\endgroup$ – AccidentalFourierTransform Mar 1 '16 at 19:25
  • $\begingroup$ Well, maybe my question can be seen without that part! $\endgroup$ – Les Adieux Mar 1 '16 at 19:27
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    $\begingroup$ "So they cannot be treated as point like particles"...why do you say that? The Compton effect, for instance, needs to treat the photon as a particle, it cannot be fully explained in the wave picture at low intensities and frequencies. The blips on a screen on which very low-intensity light falls also look pretty point-like to me. I'm not sure what your question is. We say that photons are particles because that has proven to be useful. $\endgroup$ – ACuriousMind Mar 1 '16 at 19:31
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    $\begingroup$ @ACuriousMind Indeed! One has to think about them as particles because it's conveniente and useful, but in the real sense of the question (for example scattering), it become hard to think about photons as particles, isn't it? Or am I missing something? $\endgroup$ – Les Adieux Mar 1 '16 at 19:34
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    $\begingroup$ Photons aren't particles but quanta. That is what physicists mean when they say "particle" in conjunction with quantum mechanical phenomena. You can, if you like, complain in the English department about our abuses of our own technical terms. :-) $\endgroup$ – CuriousOne Mar 1 '16 at 23:53
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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.

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    $\begingroup$ I think 1 over 137 has a point when he says photons are just radiation providing he means when they interact with matter. Only when they do that they "exist" (or existed) by producing a measurable/observable effect. As such the photon is an abstraction that is made to fit the puzzle and no one knows whether it is real or not - only that the effects are real. $\endgroup$ – Jens Mar 1 '16 at 20:30
  • $\begingroup$ @Jens Your statement is true for anything you observe and decide it exists. Even oneself. Do I exist? Am I real? One works with frameworks. He/she is assuming the classical framework for reality for the definition of particle. We have defined/found a quantum mechanical framework for reality and the vocabulary of reality overlaps and brings confusions. Quantum mechanically since everything has a wavefunction maybe everything then is radiation? Which last is the usual confusion of wave particle duality. Thats why I said "particle". $\endgroup$ – anna v Mar 2 '16 at 5:20
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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.

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  • $\begingroup$ Yes, and what makes me really doubtful is this: treating photons as particles because it's convenient.. I mean it's not simple to think about compton scattering in terms of "billiard balls" when one of them is a pure radiation.. $\endgroup$ – Les Adieux Mar 1 '16 at 19:35
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    $\begingroup$ But photons aren't "pure radiation", because treating them as radiation doesn't match up what we see of their behaviour in the real world. $\endgroup$ – PhillS Mar 1 '16 at 19:39
  • $\begingroup$ @PhillS So we "need" or "have to" treat them as particles.. ok! xD Quite strange but I guess I can understand little by little $\endgroup$ – Les Adieux Mar 1 '16 at 19:40
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    $\begingroup$ @1over137: it's not "treating them as that when it's convenient". It's what the experiment says. There is a consistent framework that tells you when to treat them like particles and when to treat them like waves. It is true that electrons and photons are fundamentally different things, but they have commonality, and physicists use the word "particle" to describe them both. But when they say "particle", they really mean "quantized field that satisfies the rules of quantum field theory," which only somewhat maps to the commonsense notion of what a "particle" is. $\endgroup$ – Jerry Schirmer Mar 1 '16 at 19:40
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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.

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  • $\begingroup$ Good to see that you do note that it is a Duality between classic plane waves and point particles. In fact both are not realistic solutions because of the lack of compact support in their domains. It is intersting that Wavelets (a relatively recent mathematical development) better solve the wave equations. The same maths fun happens for the uncertainty principle (a mathematical relationship for compactly supported square integrable functions - objects with energy;-) $\endgroup$ – Philip Oakley Apr 19 '18 at 21:50
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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.

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