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If it is said that photons do not experience time or distance because they travel at light speed, then how can photons get out of phase with one another due to different path lengths? How can phase evolve over time or distance?

I thought the reason physicists know that neutrinos do not travel at light speed and have mass is that they oscillate from one to the other, thus evolving over time. Why does this argument not apply to photon phase?

RC_23
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This is roughly the connection between light and photons:

spinangluramom

Left and right circular polarization and their associate angular momenta

Photons have no phases in space, as they are just described by their energy and the orientation in spin, while classical light as above can have phases.

A single photon can be described by its quantum mechanical wave function, as in the simple case above, on the right. A multitude of photons generates classical light, and it is the combined quantum mechanical wavefunctions that can be coherent. It is the phases between the wavefunctions of different photons that can determine whether light can carry the information of an image or not. These phases are not in space, but control the probability of a photon being measured at an (x,y,z,t).

anna v
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  • Wow it looks like I have some reading to do on this topic, the relationship between photons, their wavefunction, and classical light. Do you happen to have any recommendations for further reading, that covers the concept without being overly mathematical? – RC_23 Oct 01 '21 at 04:52
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    That's what I love about this site – I'm always learning something new! – RC_23 Oct 01 '21 at 04:52
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    One has to study quantum mechanics with all its mathematical tools, and then quantum field theory, there is no simple way I'm sorry to say. these answers of mine are relevant and have links https://physics.stackexchange.com/questions/60603/seeing-colors-photons-vs-waves/125911#125911 , https://physics.stackexchange.com/questions/90646/what-is-the-relation-between-electromagnetic-wave-and-photon/90649#90649 – anna v Oct 01 '21 at 06:02
  • Really nice answer. "Photons have no phases in space, as they are just described by their energy and the orientation in spin, while classical light as above can have phases." This is what I do not fully understand, can you please elaborate on this? A single photon cannot have phase, but a lot of them (building up classical light) do. Now what I do not understand is, where is the limit? Two photons, can have a phase? Or does it need more photons? – Árpád Szendrei Oct 01 '21 at 22:07
  • @ÁrpádSzendrei sure. Think of the pi0, that decays to two photons. The wavefunctions phases have to be fixed so that the invariant mass and all the quantum numbers combined of the two photons give the pio identity. – anna v Oct 02 '21 at 03:30