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Recently I have encountered several engineers who do not “believe in” photons. They believe experiments such as the photoelectric effect can be explained with classical EM fields + quantized energy levels in atoms. There is a 1995 paper by Lamb along these lines entitled “Anti-photon”.

What are some easily understood experiments that prove the existence of photons, which I can point to in discussions with anti-photon advocates?

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    $\begingroup$ Uh, so what do they believe in place of photons? $\endgroup$
    – Kyle Kanos
    Nov 24, 2014 at 20:59
  • $\begingroup$ That said, Grangier Roger & Aspect 1986 is probably a good one. $\endgroup$
    – Kyle Kanos
    Nov 24, 2014 at 21:04
  • $\begingroup$ Lamb believes that only waves propagate but the interaction is quantized. "It should be apparent from the title of this article that the author does not like the use of the word "photon", which dates from 1926. In his view, there is no such thing as a photon. Only a comedy of errors and historical accidents led to its popularity among physicists and optical scientists. I admit that the word is short and convenient. Its use is also habit forming. Similarly, one might find it convenient to speak of the "aether" or "vacuum" to stand for empty space, even if no such thing existed." $\endgroup$
    – hyportnex
    Nov 24, 2014 at 21:06
  • $\begingroup$ "...There are very good substitute words for "photon", (e.g., "radiation" or "light"), and for "photonics" (e.g., "optics" or "quantum optics"). Similar objections are possible to use of the word "phonon", which dates from 1932. Objects like electrons, neutrinos of finite rest mass, or helium atoms can, under suitable conditions, be considered to be particles, since their theories then have viable non-relativistic and non-quantum limits. This paper outlines the main features of the quantum theory of radiation and indicates how they can be used to treat problems in quantum optics." $\endgroup$
    – hyportnex
    Nov 24, 2014 at 21:06
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    $\begingroup$ @KyleKanos The flip side is---of course---www-3.unipv.it/fis/tamq/Anti-photon.pdf (the paper that user31748 references) and arxiv.org/abs/1204.4616 $\endgroup$
    – dmckee --- ex-moderator kitten
    Nov 24, 2014 at 21:15

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The issue here, I believe, is not existence of photons, but the fact that people may choose terminology and concepts they find appealing.

The word photon has been coined long ago for an idea that is quite far from the current views on light and the meaning of the word has been evolving many decades.

Its current use in textbooks and papers is quite broad and may be regarded as inconsistent - in one situation photon is a dot on the detector screen, in another it is something that spreads the whole experimental setup, in yet another it is quantum of energy that gets absorbed in a tiny region of space comparable to an atom.

Such liberal use of a word may not appeal to people who like their terms general and clear, which is why they might prefer the term EM field (even in quantum theory) instead.

The possibilities of mathematical modelling of light by continuous fields have evolved to the point where they can account for many experiments that were previously thought to require the idea of particles of light. Photoelectric effect, double slit experiment, black body radiation may be approached from the mathematical standpoint where light is described by continuous fields.

In the end, explanation of an experiment involving light with words and mathematics is just that, and proves nothing about what the light "really consists of".

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    $\begingroup$ This is really the core of (at least one of) Lamb's point. He doesn't argue against QED, but rather that the word in consistently abused in ways that are not consistent with a modern understanding of quantum optics. $\endgroup$
    – dmckee --- ex-moderator kitten
    Nov 24, 2014 at 22:28
  • $\begingroup$ Was the double slit experiment ever thought to require particles? $\endgroup$
    – user253751
    Jul 21, 2016 at 9:49
  • $\begingroup$ @immibis, yes, the double slit experiment with weak light has as a result a screen that shows small spots smaller than distance of interference maxima. There have been people who thought this proves light consists of small particles or localized energy packets. $\endgroup$
    – Ján Lalinský
    Aug 17, 2016 at 19:48
  • $\begingroup$ +1, especially for the last paragraph, which is often overlooked when discussing physics. $\endgroup$
    – Martin Argerami
    Sep 26, 2017 at 4:28
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I would tell them to re-read and understand that paper, and know that few spectroscopists would disagree with it. The point is that far too many people use the word "photon" without knowing what a photon really is or under what context the word can be used. For the vast majority of applications a semi-classial conception of the radiation field is adequate. The author wants to discontinue the use of the word, not negate the real existence of that entity, as defined through a rigorous QED treatment of the radiation field.

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Photon counting statistics cannot always be explained by classical fields. In these experiments, the state of the field is monitored continuously by a photodetector. I believe these represent one of the clearest experimental demonstrations of the quantum nature of the radiation field.

For example, in observing the emission of photons from a single atom, one never detects a second emitted photon immediately after the first. This is due to the fact that after a spontaneous emission event the radiation field is in a Fock state with a well-defined number of photons. This "anti-bunching" effect was observed by Kimble et al. in 1977 and is reported here. It is not possible to explain the experimental intensity distribution as arising from an underlying classical electric field, even if we allow the field to fluctuate stochastically. The quantum theory of light was thus found to be necessary.

Note that these conclusions do not depend on the use of photomultiplier tubes in the detectors or anything else to do with the photoelectric effect. One needs only some device that is capable of measuring the intensity of light with sufficient time resolution.

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  • $\begingroup$ " ...even if we allow the field to fluctuate stochastically" are you talking about SED here? It might be good to mention this if so. $\endgroup$
    – Selene Routley
    Nov 24, 2014 at 22:38
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    $\begingroup$ @WetSavannaAnimalakaRodVance I'm afraid I know nothing about SED. What I mean is the following. Many states of the radiation field can be described by a classical electric and magnetic field distributed according to a positive semi-definite quasi-probability distribution, e.g. the Wigner function. Perhaps such a description is equivalent to some formulation of stochastic electrodynamics. However, Fock states cannot be described in this way: their quasi-probability distributions will be negative or singular. $\endgroup$
    – Mark Mitchison
    Nov 24, 2014 at 22:42
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    $\begingroup$ "It is not possible to explain the experimental intensity distribution as arising from an underlying classical electric field,even if we allow the field to fluctuate stochastically." This is often repeated mantra, but it is demonstrably true, as many no-go claims are, only for certain very restricted view of what classical description is and some obvious way to use it.Anti-bunching is not seen as a phenomenon preventing description by c-number fields with positive Wigner density at all by people at the frontier of classical mode of description-see crisisinphysics.co.uk/optrev.pdf. $\endgroup$
    – Ján Lalinský
    Feb 25, 2015 at 2:09
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    $\begingroup$ What is happening here is people often use a general but outdated theory of classical EM field to convince you that classical EM theory cannot do this and that. They largely ignore the fact people who actually spent lot of time on developing enhancements of that theory did bring many new things to it. Zeldovich made a good point about impossible things in his book on mathematics: it is pointless to stress that something cannot be done [ especially when the rules are not entirely fixed and it is not a mathematical statement]. Give it time and somebody will do just that impossible thing. $\endgroup$
    – Ján Lalinský
    Feb 25, 2015 at 2:16
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    $\begingroup$ @JánLalinský You make a good point. Perhaps it is better to say simply that photon counting rules out some naive or natural interpretations of a fluctuating classical electric field, where one posits a positive probability distribution over a $c$-number field obeying Maxwell's equations. Nevertheless, I do think that this answer is historically pertinent, since if you read the papers of Mandel and the other pioneers of photon counting experiments they often pointed out that their experiments constituted direct evidence of the existence of photons and ruled out popular alternatives at the time. $\endgroup$
    – Mark Mitchison
    Feb 25, 2015 at 12:12
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Photons are observed as radiation with a given spin and other properties. As such, they do exist.

But according to the principles of time dilation and length contraction, from their (hypothetical) proper point of view, they would be reduced to a single momentum. Their proper time would be zero, the distance of their geodesic (and also the spacetime interval) would be reduced to zero. That means that from their (hypothetical) point of view, nothing would be moving - just a momentum transmitted directly without a wave from one electron to another, no particle.

Of course, this is a hypothetical, calculated reality because photons are not observers, and they have no reference frame. But according to the principles of time dilation and length contraction, we can also be sure that our observation does not correspond to reality.

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