# QED: the strange theory of light and matter. Light is made of particles ?

by StudentJoseph
Tags: light, matter, strange, theory
P: 1,943
 Quote by ZapperZ I still don't see how in these arguments, you could say that the wavefunction representing a single photon is a wavepacket.
The wave function representing a single photon during the time interval [t_1,t_2] is a wave packet if and only if at each time in this interval the amplitude is virtually zero outside a tiny region (which moves with the speed of light, and grows with time). Many such solutions of the free Maxwell equations exist.
 Quote by ZapperZ It is also is confusing because you're bringing up different arguments under different situations that don't quite apply to this case, which is similar to your previous post as well.
If you can pin down the source of the confusion a bit more precisely, I'll be glad to learn how to express myself more clearly. This is the main purpose I have in being here on PF.
 Quote by ZapperZ Can you show me an experiment that has a single photon that is actually a superposition of several frequencies?
On slide 14-28 of my lecture at http://www.mat.univie.ac.at/~neum/ms/lightslides.pdf , I discuss one of the experiments from the literature producing single photons on demand. The photons produced are quite localized - they need to be it to be useful for signalling information. They are realistically described (as most real photons) by density matrices rather than wave functions, but if you consider the rank one approximation, you get ordinary wave packets.

Since the Fourier decomposition of every wave packet is composed of a wide range of frequencies, you get the required superposition.
P: 612
 Quote by A. Neumaier _All_ waves that correspond to a solution of the Maxwell equation in vacuum are associated with a single photon if only a single photon had been around before the spectacle began. Only the intuitive particle view of a photon cannot tolerate this and breaks down.
I think I see your point and could agree with phrase "associated with a single photon". But I'd very much hesitate to say "comprise a single photon" which you seem to be asserting. I think it's a matter of definition to say what a photon is comprised of and strict definitions are lacking in the field as far as I can gather.

You may be entitled to develop a definition based on your understanding and conception, but others might do so with different qualifications. I'm wondering if it might not necessarily be a step forward to always dispense with a semi-classical perspective. One of the potential problems with your line of thinking as I understand it so far is in dealing with both transverse and longitudinal solutions of the Maxwell equations in dispersive media. Those are well known and studied in the field of Plasma Physics. Naturally the longitudinal solutions are not self-propagating but do have trajectories that diverge significantly from the transverse solutions. I'd tend to call those "effects of a photon's transit" rather than the photon itself as the energy perturbations are no longer localized.

Thanks, by the way, for the lecture notes. I'll try to read them over the next few days.