Does quantum theory describe light as a wave?

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Summary:

Trying to sort out some missunderstandings about photons.

Main Question or Discussion Point

Hello!
I recently had a discussion with a person who's well-read on quantum physics and I was suprised by his claim that "light is in no sense regarded as a wave" in quantum mechanics.

His support for this claim was that there are no wave crest or wave trough, there is nothing moving. What matters is distance. What we call wavelength is the periodicity of the wavefunction of the photon. Half a wavelength is the distance for which negative self-interference occurs.

Futhermore, photons do not behave as waves in the sense that they follow the rule of superposition. What we observe is self-interference. As an example, it would be theoretically impossible to build an interferometer from two different lasers even if they had exactly the same wavelength, this is because the only interference we observe is self-interference, i.e. the interference of one photon with itself.

My questions for this thread is:
1. Is light, in any way, regarded as a wave in quantum theory?
2. Do you agree with the claims above?


Lastly, sorry for my english, it's not my mother tongue.
 

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  • #2
anuttarasammyak
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An interesting topic. I would like to know the answers too.

Your friend says based on individuality of photons. I am afraid whether QFT allows such individual trace of photons.
 
  • #3
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1. Is light, in any way, regarded as a wave in quantum theory?
2. Do you agree with the claims above?
Your friend seems quite well informed and I agree with all his statements.

Light has wave-light behavior in some ways and particle-like behavior in other ways, but it is in no way either of those things.

And your English seems fine. I had not noticed and found myself surprised at the apology.
 
  • #4
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In classical electrodynamics light is described by radiating solutions of the Maxwell equations, these are functions of time and space which fall off with distance no faster than ##1/r## and thus carry nonzero energy to all of space. You can call these waves if that makes you happy.

In quantum electrodynamics light is described by the state of the electromangetic field; there are single or multi-photon fields but these do not correspond to classical radiation solutions. In fact one must use coherent states which have an indefinite particle count but give the correct “average behavior” of the classical solution. Whether these are “waves” or “particles” is just a question of language.
 
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  • #5
Cthugha
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Futhermore, photons do not behave as waves in the sense that they follow the rule of superposition. What we observe is self-interference. As an example, it would be theoretically impossible to build an interferometer from two different lasers even if they had exactly the same wavelength, this is because the only interference we observe is self-interference, i.e. the interference of one photon with itself.
Let me just cite Roy Glauber, who received the Nobel Prize for his contributions to quantum optics. From Nucl.Phys. A774, 3-13 (2006), also available on the ArXiv here :

" When you read the first chapter of Dirac’s famous textbook in quantum mechanics [8], however, you are confronted with a very clear statement that rings in everyone’s memory. Dirac is talking about the intensity fringes in the Michelson interferometer, and he says,

Every photon then interferes only with itself. Interference between two different photons never occurs.

Now that simple statement, which has been treated as scripture, is absolute nonsense. "

There is nothing to add to that. The assumption that one only observes self-interference is known to be wrong. It was shown to be wrong already 50 years ago or so. Just look up the Hong-Ou-Mandel effect for a simple case of two-photon interference.
 
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  • #6
vanhees71
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Concerning the claims in the OP, I'd state the precise opposite. Quantum theory, and if you are dealing with photons, it's necessarily relativistic quantum field theory, light (or better electromagnetic fields) are never regarded as particles. I would define a particle to be an entity which can be localized, and a photon cannot be localized (along its propagation direction). Formally that is reflected in the fact that it is mathematically impossible to define a position operator for a photon.

Photons are certain states of the electromagnetic quantum field, the socalled one-photon Fock states.

Also single photons in many ways behave like classical electromagnetic waves, but you have to reinterpret the meaning of the "intensity" of the wave as a probability distribution for detecting a photon with a detector located at a given place. E.g., if you conduct a double-slit experiment with sufficiently coherent single-photon
states, at a detector like a photoplate or more modern a CCD cam for each photon sent through the slits you detect only a single spot. You cannot predict at which precise pixel of the cam each photon will be registered, but if you run many such single photons through the slits, you'll get an interference pattern as predicted for classical em. waves. In this sense the single photons behave like waves rather than particles, but they make only a single spot on your detector, and the latter notion has brought up the idea that photons may have particle-like features.

Of course, Glauber is right in saying that there's not only single-photon interference but I don't think that this is the point in answering the question whether photons are particles or waves (fields). One must simply say that when it comes to the quantum realm all classical pictures are inadequate and a photon is simply a photon and no classical particle nor a classical wave but a certain state of the electromagnetic quantum field.

The same holds for massive particles. They are all described as single-particle Fock states of quantized fields. The difference to a photon, which is a massless quantum, is that for massive quanta you can define a position observable and thus they are localizable in some sense restricted by the position-momentum Heisenberg uncertainty relation. So here a particle interpretation is possible to some extent but also of course not completely adequate.

I'd not waste to much time about thinking about "quantum weirdness" like "wave-particle duality", which is a remnant of the socalled "old quantum theory", which was not a consistent theory and has been used for only 25 years in the struggle of the physicists to make sense of what was observed in atomic physics and particularly the interaction between electromagnetic fields and matter. All this has been resolved with the discovery of "modern quantum theory" (including quantum field theory) in 1925/26. There are no more classical particles nor classical fields but only quantum fields, which describe the behavior that looked like "wave-particle duality", i.e., observations that tended to be better understandable as a continuum theory (field theory, "waves") or as a point-particle theory ("particles"). Modern quantum theory provides a conistent theory describing what's observed in terms of a single concept, which I'd simply call quanta rather than "waves" or "particles".
 
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  • #7
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Wow, thanks for great answers!
Some of which was new and some was supporting my idea of QM. :smile:

I agree with the statement that "all classical pictures are inadequate", as stated by vanhees71. However I would not formulate it as:
"Light is in no sense a wave."
I would formulate is as "light has some wave-like properties and some particle-like properties".
But trying to dress QM in the terminology of classical physics is always up for problems.
 
  • #8
vanhees71
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To the contrary, I'd formulate "Light is in no sense a particle". Though there's no way to describe light in all details as a classical field nor as classical particles (or a continuum-mechanical system), it's usually closer to a classical electromagnetic wave field than anything like particles or a fluid stream.
 
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