[Physics] the length of a single photon

photons

I don't mean the wavelength, I mean the full length in the direction of propagation. What is the total extent of a single photon in space? Does it even have a length?

Edit to clarify: Let's say a have a polar molecule, e.g. water. A standing EM wave (in this case our photon) will cause the molecule to oscillate (this is how a microwave works). So with a single photon forming the standing wave, how many water molecules can I put in a line, such that the same photon will be affecting them all at the same time? I'm not sure if that even makes sense, but that's what I'm thinking of when I refer to length.

Best Answer

As knzhou says in a comment:

Depends on the photon. It’s like asking what the width of an ordinary particle’s wavefunction is. Sometimes it’s bigger and sometimes it’s smaller.

Some sources, such as lasers, emit photons with very long coherence lengths. For instance, the laser in your CD player probably emits wavetrains with lengths of ~10 cm.

Other sources emit photons with much shorter wavetrains. For example, if you look at thin film interference patterns made by light from a sodium discharge tube, the patterns never have more than ~100 fringes, which is because the sodium atoms emit wavetrains with lengths of ~100 wavelengths. This is determined by the properties of the atom. For example, if the half-life of the transition is ~100 periods of the light wave, then the wavetrain will have a length of $\lesssim100$ wavelengths.