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I understand what spacial and temporal coherence mean when we speak waves, but when we start speaking photons I can't wrap my head around the concept. So what does it mean for photons to have temporal and spatial coherence?

Jack
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  • Perhaps you are thinking that photons are like particles, not waves? A photon is sort of like both. How can a red light photon be different from a blue light photon? – mmesser314 Dec 14 '23 at 02:49
  • Roughly temporal=same/similar phase, spatial = same/similar direction. – PhysicsDave Dec 14 '23 at 04:08
  • @PhysicsDave what is the phase of a photon? – Jack Dec 14 '23 at 07:00
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    The best model of a photon is as a wave in the EM field!! Maxwell's wave equation as shown many times on this website (the sinusoidal E and M fields perpendicular to each other in a 3D diagram) has generally proven itself in many experiments. The photon is said to be a particle in the "Standard Model" which is kind of baloney as the model only gives 2 choices, everything is either a field or an excitation of the field (excitation = particle) .... – PhysicsDave Dec 14 '23 at 14:21
  • In a laser photons mostly all get in phase ... i.e they begin their existence starting at the same point in space and in amplitude and in the same direction of propagation .... highly temporarily coherent and spatially coherent (called stimulated emission). Some lasers are more coherent than others, a cheaper laser diode has a small semi-conductor crystal and in action actually has multiple breakdown sites at the same time ... and these are short bursts .... so in a second there are really 1000s or more lasers ... so its somewhat less temporarily coherent. – PhysicsDave Dec 14 '23 at 14:31
  • Also for a laser depending on the exit aperture and diffraction some lasers are more spatially coherent than others. – PhysicsDave Dec 14 '23 at 14:32

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