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I was reading Feynman lectures on physics (volume 1) when I encountered the following paragraph:

Returning again to quantum mechanics and fundamental physics, we cannot go into details of the quantum-mechanical principles at this time, of course, because these are rather difficult to understand. We shall assume that they are there, and go on to describe what some of the consequences are. One of the consequences is that things which we used to consider as waves also behave like particles, and particles behave like waves; in fact everything behaves the same way. There is no distinction between a wave and a particle. So quantum mechanics unifies the idea of the field and its waves, and the particles, all into one. Now it is true that when the frequency is low, the field aspect of the phenomenon is more evident, or more useful as an approximate description in terms of everyday experiences. But as the frequency increases, the particle aspects of the phenomenon become more evident with the equipment with which we usually make the measurements. In fact, although we mentioned many frequencies, no phenomenon directly involving a frequency has yet been detected above approximately $10^{12}$ cycles per second. We only deduce the higher frequencies from the energy of the particles, by a rule which assumes that the particle-wave idea of quantum mechanics is valid.

Thus we have a new view of electromagnetic interaction. We have a new kind of particle to add to the electron, the proton, and the neutron. That new particle is called a photon.

I can't understand how Feynman concludes the existence of photons. Can anybody please clarify this for me?

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    $\begingroup$ My answer here is relevant, as it shows that photons are an experimental fact physics.stackexchange.com/q/490409 $\endgroup$ – anna v Jul 8 at 15:21
  • $\begingroup$ I don't see how you can explain the photoelectric effect, and Einstein's Nobel Prize-winning paper on it ,f you don't accept that photons have a particle aspect. $\endgroup$ – Michael Walsby Jul 8 at 16:33
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    $\begingroup$ @MichaelWalsby There are many experimental phenomena that require photons/quantum field theory, but ironically the photoelectic effect is not one of them. It was actually Nobel Laureate Willis Lamb and Marlan Scully who first pointed out that all you need is a quantum atom+classical field and the photoelectric effect can be explained as a resonant wave phenomena. You can read the origional paper here $\endgroup$ – Punk_Physicist Jul 8 at 18:26
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Feyman adds this new particle to the colection because of his previous statement: "...things which we used to consider as waves also behave like particles, and particles behave like waves; in fact everything behaves the same way." His point is that everything is quantized, including light and so there must be some minimal amount of light, called a photon.

I would give an advise based on my personal experience while learning physics: don't imagine the photon as a little ball of light. When we hear the term 'photon' for the first time, many tend to imagine it as a little sphere of light because that's how we first imagined the proton or the electron. That mental picture will be an obstacle for learning physics in depth. Neither the proton nor the photon are spheres nor are they localized.

A photon is not a "particle" in the usual sense of the word. Better try to understand the photon from the beggining as the minimal excitation of a field, not a "particle".

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    $\begingroup$ Yes. Another way of looking at it: Feynmann is saying that light is obviously not a wave, and obviously not a little ball of something. So it must be something else, something we call "photon". $\endgroup$ – garyp Jul 9 at 0:57
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I think it can be understood the following way: light as a natural phenomena definitely exists, and some aspects of its behaviour can be described as the behaviour of waves on the water surface (such as diffraction and interference). And before the 20th century only these aspects of the behaviour of light could be observed. But in the 20th century some new aspects of the behaviour of light, which don't occur in "waves on the water surface" model, were found. And from perspective of these aspects it is more convenient to look on the light as a the swarm of the particles. And proposition: "Photon exists" means that some aspects of the behaviour of light are intuitively identical to those of particles (by word "particle" we intuitively mean something small and indivisible).

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Feynman is not concluding the existence of photons merely from the few facts he paints in this paragraph. Rather he concludes the existence of photons from the extensive knowledge he has of quantum electrodynamics, including large amounts of experimental evidence for the accuracy of that theory. Here he is just alerting the reader to some of the basic concepts in any quantum field theory, especially that the distinction between particle and wave vanishes; both are subsumed into a type of entity which can be thought of as a field excitation. If the electromagnetic field exists, and it surely does, then so do its exitations, called photons.

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