# Existence of Photon

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?

• My answer here is relevant, as it shows that photons are an experimental fact physics.stackexchange.com/q/490409 Jul 8 '19 at 15:21
• 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. Jul 8 '19 at 16:33
• @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 Jul 8 '19 at 18:26