I spend a lot of time promoting Rhett Allain’s Dot Physics blog, enough that some people probably wonder if I get a cut of his royalties (I don’t). I’m going to take issue with his latest, though, because he’s decided to revive his quixotic campaign against photons, or at least teaching about photons early in the physics curriculum. We went through this back in 2008 and 2009 (though Rhett’s old posts are linkrotted away, so you only get my side of the story…). I’m no more convinced this time around, even though he drags in Willis Lamb and David Norwood for support.
There are basically two pieces to the anti-photon argument, neither of which I find remotely convincing. In fact, I will happily stipulate that both of the central points are correct, and even after that, I don’t find this to be a problem that requires fixing.
The first claim is that photons are redundant, given that you can explain all of the phenomena they’re usually invoked to explain without ever referring to particle-like characteristics of light. Which is true– you can construct a semi-classical model of the photoelectric effect in which electrons inside metals occupy quantized states and are excited out of them by classical electromagnetic waves of the appropriate frequency. That reproduces essentially all of the features of the quantum model proposed by Einstein in 1905 and confirmed by Millikan’s experiments in 1916. Using that instead of the photon model would be a little ahistorical– the semiclassical model was first published by Mandel and Wolf in the 1960′s, and relies on a Fermi Golden Rule calculation assuming the Schrödinger equation which wasn’t invented until the late 1920′s– but you could do it.
But even allowing that it’s true, I don’t see what the point is. To bring it back to classical physics, in the same way that it’s perfectly true that you can describe the photoelectric effect with a semiclassical model, it’s perfectly true that you can describe an elastic collision between two objects by direct integration of Newton’s second law (or the Momentum Principle, in the language of the Matter and Interactions textbook that Rhett and I both use). But there’s absolutely no reason to do that, other than to make a philosophical point– sane people considering elastic collision problems make use of energy in addition to momentum, because it makes the problem simpler.
And that’s the case for photons in talking about the photoelectric effect: it’s much, much easier. Actually working out the details of the semi-classical model of the photoelectric effect is really complicated: you need to know about the Schrödinger equation, make a few approximations, and do an integral involving complex numbers. The photon version requires subtraction.
We invoke photons for the photoelectric effect because it’s much, much simpler. And since a quantized model of light is known to be necessary to explain photon anti-bunching (a point even Rhett concedes), there’s no good reason not to employ it there.
(I’ll note in passing that Norwood makes repeated references to some sort of experiment that supposedly shows a delay in electron emission for low light intensity. I have absolutely no idea what he’s talking about, and he doesn’t provide a citation. The only “delay in photoemission” measurements I’m aware of are attosecond scale delays after excitation with an ultrafast alser pulse, which is not remotely the same thing.)
The second argument against photons basically amounts to the language used to describe light in terms of particles being imprecise in a way that offends some people’s aesthetic sense. And, again, strictly speaking this is perfectly true. Photons are not perfectly described as particles with all the properties of classical particles. A proper description is that photons are quantized excitations of particular modes of the electromagnetic field.
But you know what else isn’t perfectly described as a particle with all the properties of classical particles? An electron. In fact, strictly speaking, electrons also ought to be described as quantized excitations of an “electron field.” There are some differences between the mathematical descriptions of photons and electrons as field excitations, but we’ve known since Dirac’s day that electrons are best described as field quanta.
And yet, you don’t find many physicists willing to argue that we shouldn’t teach electrons as particles. But all the same linguistic ambiguities are present with electrons that are present with photons. Neither is truly a particle or a wave in the classical sense– rather, they’re both a third kind of object for which we lack a convenient single word. There’s necessarily a lot of imprecision in the language used to talk about this; Lamb’s article includes a kind of snotty remark blaming Bohr for this, but I don’t see any great alternatives.
And even after stipulating that talking about photons as particles is imprecise, I fail to see what the problem is. Or, more specifically, I don’t see where this particular bit of imprecision creates a real problem for anything. If this is a pernicious misconception, what is the physical problem that thinking of light in terms of photons keeps you from solving? Where does it lead students astray in a way that gets a clearly wrong answer, as opposed to getting the right answer by aesthetically unappealing means? I haven’t seen a good example yet, though this is at least the third time I’ve read a bunch of anti-photon rants.
So, like the post title says, I see no reason to drop photons. A fully quantized model of light is unquestionably necessary for more advanced experiments, invoking a simplified version of it makes certain classes of intro problems vastly easier to deal with, and the imprecision that the simplified model introduces doesn’t seem to cause any significant problems (particularly for the vast majority of students who will only ever take introductory-level classes). There’s just nothing there that rises to the level of a problem requiring a change in pedagogy.
(I suspect the closest analogue in classical physics is the “work done by friction” business, where a lot of physics education folks vehemently object to the notion of frictional forces doing work, for reasons I have never quite understood. I’ve had it explained to me several times, and all I remember about it is that it turns on a poor choice of what you call the system. This has basically eliminated a whole class of problems from the intro classes– you won’t find problems where students find the stopping distance for a sliding object using energy methods any more– in order to avoid a “misconception” that seems to me to be almost entirely an aesthetic issue.)