Over at Dot Physics, Rhett has just completed a two-part post (Part I, Part II) on quantum physics arguing against the use of photons in teaching quantum physics. Part I gives a very nice introduction to quantum physics, which is why I linked it, but Part II goes a little off the rails. There’s not as much physics content, and it ends with a list of phenomena that are able to be described by semi-classical models of light, leading up to a question:
So, if there are no photons, why are they in all the textbooks? That is a great question. I am glad I asked it. I really don’t have a great answer here. Maybe someone wrote a book about photons and a student read it. This student eventually wrote his/her own book and included the photon model of light. This new book was then read by a new student and so on.
The answer to the question is “Kimble, Dagenais, and Mandel.” Specifically, the 1977 experiment they did (Phys. Rev. Lett. 39, 691 (1977)) showing “anti-bunching” of the light emitted by single atoms. This result can only be explained in terms of quantized light, i.e. photons.
(Basically, they saw that when they detected light emitted by a single atom, there was some time delay before they could detect light emitted by that same atom again. In the photon model, this happens because after an excited atom emits a photon and drops down to a lower energy state it must be re-excited and decay again before you can get a second photon out. You can’t explain this with classical waves and quantized detectors, the way you can explain the experiments Rhett lists.)
The inspiration for this whole thing is a rather ranty PDF of a paper by David Norwood of Southeastern Louisiana University, who goes on at some length about the evils of the photon idea. I can’t for the life of me figure out what the problem is supposed to be, though.
Norwood’s argument is, basically, that you don’t need photons to explain most of the simple experiments usually held up as demonstrations of the existence of photons– the photoelectric effect, the Compton effect, and so on. This is perfectly true. All of the experiments we use to introduce photons in modern physics classes can be explained using semi-classical models, where light is treated as a continuous classical electromagnetic wave, and the energy states of atoms are quantized.
Of course, I could equally well say that you don’t need the concept of energy to explain classical mechanics. You can solve absolutely any problem in classical physics using nothing more than Newton’s Laws of Motion. So why do we teach students about energy in introductory physics classes?
Well, we teach them about energy because it’s a central concept in physics, and it’s really difficult to solve interesting problems without it. You can solve any problem in classical physics using nothing more than Newton’s Laws, but it won’t be a pleasant experience.
The same is true with photons. Yes, strictly speaking, you don’t need photons to explain the photoelectric effect, but the semi-classical explanation is hard. It’s basically a Fermi Golden Rule problem, which involves some subtle approximations, and a couple of integrals. This isn’t something you’re going to trot out in a sophomore modern physics class.
And since we know from the Kimble, Dagenais, and Mandel experiment (among others) that photons really do exist, why wouldn’t you use photons as the explanation? The photon explanation is clean, elegant, and can be explained to anyone in about half an hour.
Norwood’s answer seems to be “most students will never deal with a situation where photons are really essential,” to which I reply “So what?” Most students in introductory physics will never deal with a physics problem where energy is really essential, but we teach them about energy all the same, because it’s easier to solve problems using conservation of energy than it is to do numerical integration of Newton’s Second Law.
I might be convinced that photons have no place in the introductory curriculum if Norwood had some evidence of actual harm being done by introducing the comment. If he does, I’m not seeing it– the only thing he gives as an example of a problem is an infelicity of phrasing: the use of “stream of visible photons” instead of “light.” I’ve been known to get bent out of shape over some linguistic matters– ask my former students about my opinion of “utilize”– but this is weak tea.
Note that even Norwood is not arguing that photons don’t exist. You have to look carefully, but in his list of examples, he does grudgingly allow that there are situations that demand a quantum treatment of light. His only gripe is that they’re taught “too soon,” which gets him torqued off for some obscure reason.
So, to return to the original question: Why are photons in all the textbooks? Photons are in all the textbooks because photons are real. The electromagnetic field is quantized, and this description is crucial for certain experiments. It also turns out to be a convenient way to explain a lot of other experiments where it isn’t strictly necessary.