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


by StudentJoseph
Tags: light, matter, strange, theory
A. Neumaier
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#19
Feb22-11, 01:41 PM
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Quote Quote by ZapperZ View Post
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 Quote by ZapperZ View Post
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 Quote by ZapperZ View Post
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.
PhilDSP
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Feb23-11, 02:58 AM
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Quote Quote by A. Neumaier View Post
_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.
A. Neumaier
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Feb23-11, 04:11 AM
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Quote Quote by PhilDSP View Post
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.
The well-defined thing is the 1-photon state, whereas ''photon'' is a semiclassical notion without a precise definition. When the assumptions for a semiclassical view are not met the notion of a ''photon'' in any other sense than ''1-photon state'' loses its meaning.
Hence in case of disagreement or seeming paradox, one must resort to the common basis from which the semiclassical view arises, which is the 1-photon state.
Quote Quote by PhilDSP View Post
You may be entitled to develop a definition based on your understanding and conception, but others might do so with different qualifications.
And indeed they do, as you can see from the special issue I referred to in the discussion in post #20 of http://www.physicsforums.com/showthread.php?t=474537
Five papers - five different conceptions.
Quote Quote by PhilDSP View Post
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.
I was talking about photons in vacuum, which are always transversal - the kind of photons used in quantum information experiments. Photons in dispersive media are effective photons with different properties, and I am not familiar enough with the literature to give a reasonably authoritative view of how these should be considered. But I suspect it will be similar.


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