Question

Who hasn't heard about the double-slit experiment? It figures in any book of quantum physics. But there is something no one can explain to me : I understand why the light cannot be described only as a wave, but I do not understand why it cannot be explained only in terms of a particle, having some trajectory, following other laws of phyisics we may not know. Usually, every book in which I have tried to look for an answer considers that if it cannot be described as a classical trajectory (which is clearly the case in this experiment), then it is not a particle, that is, a material point. Could it not be a particle following new laws?

Answer 1

I point out that Feynman, in his 1979 Douglass Robb Memorial Lecture, part I (actually entitled "Photons - Corpuscles of Light") on QED, video freely available at http://vega.org.uk/video/programme/45, asked this very question. He answered definitively that Newton was correct and that light was indeed a particle which, however, propagates according to the usual q.m. laws of adding complex amplitudes, thus reconciling its particle nature with observed interference phenomena. Furthermore see his closely related answers on this question at http://www.youtube.com/watch?v=_7OEzyEfzgg&feature=related.

The determining evidence ruling out waves includes 1. the photoelectric effect since waves can not cause ejection of electrons with the same frequency energy relationship, and the non relationship between the E field amplitude and energy 2. Compton scattering (not mentioned in his lecture) not explainable under a wave theory 3. An argument he alluded to relating to the equation for scattering multiple particles not corresponding any more to Maxwell's equations (which I don't quite follow). With the adoption of the probability amplitude propagation theory of QED, apparently no evidence is against the particle hypothesis. (I myself would also like to see a QED derivation of the old Einstein entropy density of black body radiation in a closed cavity, however, i.e. the one that looks like 1/2 particle and 1/2 wave)

Note related to a comment below:I previously (mis)attributed these arguments to his 1964 Messenger Lectures at Cornell, available online (only for PC's?) here and later published as "The Character of Physical Law", but the correct ref. is as above.

PS. I myself do not feel that the question is confused at all, and actually voted it up, but it is the sort of question that often gets many confused answers. I'm not too sure how we should handle this)