Anyone know how “long” a photon wave packet envelope is? I get the impression from Purcell that it might be only about 1 wavelength long. But then I think I saw on this forum someone mention that it could be “thousands” of wavelengths long. How would or could one determine that? I am thinking, for example, of the Kennedy-Thorndike experiment, where the two arms of the interferometer were deliberately and significantly different – maybe by about 10 inches, as that was the “limit of coherence”. Would that be a clue to how long the envelope might be? If they were experimenting with optical wavelengths, then the “thousands” of wavelengths would be correct. Any thoughts are appreciated.