I was listening to a physics professor lecturing on QM and he raised the question "How big is a photon?" and indicated it had arisen during his PhD defense. He than began to discuss the accurately known frequency and wavelength of a laser emitted photon (and thus accurately known momentum) in the context of the uncertainty principle. The product of the uncertainty in position and accurately measured momentum is greater than or equal to (up to a small numerical factor) Planck's constant. He then concluded that in the direction of travel, the position was unknown to within a surprisingly large distance - a meter or so. Or more accurately, that is how I interpreted it. He didn't say position is unknown, he was still talking about "size". Without really saying it, the implication was that the answer to the question was that a photon was a meter or so long in the direction of travel (and the emitting aperture wide in the transverse direction). Does that description of "size" make sense to those here? I would probably have said that the photon is a point particle unless we're doing string theory, and the location was just poorly known along the travel direction. If the photon was truly "big" I'd think it would take 3 nanoseconds to finish arriving at a detector assuming a foot or so per nanosecond speed of light, and that should easily be measurable. Do photons take a finite amount of time to "arrive" at a detector? Thanks.