Saturday, February 27, 2016

There Are No Pea-Shooters For Photons

I wrote an article several years ago titled "There Are No Peashooters For Photons". The link will take you to the where the article is posted. In the article, I take apart the various arguments which claim to debunk the wave theory of light in favor of the particle theory. You know the arguments: the black-body spectrum, the photo-electric effect, the Compton effect and and so forth. None of these arguments, which were so persuasive in the early days of quantum mechanics, remained valid after the discovery of the Schroedinger equation in 1926. All those arguments depended on the difficulties of having a wave (light) interacting with a particle (electron). None of those arguments remained valid once we understood that the electron was also a wave. Yet  by then the phton picture, with its magical "quantum leaps", was so well entrenched that the dominant physicists refused to abandon it. Instead, they reinvented the old quantum leap in the guise of Born's "collapse of the wave function", and particles ruled. Waves were out.

Even Schroedinger was cowed into acceptance of the new Copenhagen orthodoxy, and he did not seriously attempt to challenge it again until the 1950's when he was no longer taken seriously by people at the "cutting edge". But over the years, this or that group (notably the Jaynes group in the 1960's) has worked out the alternative formulation of quantum mechanics in terms of waves rather than photons; and by now it is recognized by most people at the highest level that at the very least, the photo-electric effect has a straightforward interpretation in terms of the wave theory of light. But even people who should know better continue to claim to be troubled by the apparent discrepancy between the power density of the e-m wave and the physical cross-section of the atom. People who are trouble by this should read up a little more on classical antenna theory, especial my blogpost on the Crystal Radio. But that's another story.

Even the people who accept the wave explanation of the photo-electric effect are quick to stipulate that the Compton effect is impossible to explain by waves. Schroedinger's own beautiful explanation, published in 1927, languishes in such obscurity that when I independently rediscovered it on my own ten years ago, I thought I would be a shoo-in for the Nobel Prize. The Compton effect remains a huge debating point in favor of the particle theory of light, but in fact it is no more than a debating point.

Feynmann weighs in on the wave-versus particle argument by citing the photo-multiplier tube as incontrovertible experimental evidence of the particle nature of light: you can actually hear individual photons being counted by the detector. But Feynmann is simply wrong. There is a 1200-volt power supply hooked up to the tube, and there is no way you can argue from logic that it must be a particle rather than a wave which triggers the process whereby that 1200-volt supply generates a pulse of current. I explain the wave mechanism in my article on Quantum Siphoning. The article actually deals with the mechanism for generating flecks of silver on a photographic plate, but the principle is the same.

But the proponents of the particle theory have one final impenetrable refuge, which they have relied upon since probably the mid-1980's or perhaps a little earlier. There are now experiments, so they say, where you can actually shoot a single photon at a beam splitter and measure that it goes either one way or the other; it is never split in two as you would expect if light were a wave.

This is a very weighty argument, and I talked about this in my article, "There Are No Pea-Shooters For Photons". It turns out that the experiment is a little more complicated than shooting a single photon at a target. You have to use something called parametric down-conversion, which creates entangled pairs, and by measuring one member of the pair, you can be sure that you have shot the other member at your desired target. As they tell us, "it comes to exactly the same thing as shooting single photons at a target."

Or does it? In my original article, I expressed my doubts. Hence the title of the article. But that was just me. Would any serious physicists back me up?

Maybe they have now. I came upon this post in the Physics Stack Exchange site the other day. It seems that some guys have analyzed the statistics of parametric down-conversion, and it is after all not identical to what you would get from a true single-photon source. The original Physical Review article by Bashansky, Vurgaftman, Pipino and Reintes is here, behind a paywall. From the abstract, it looks pretty significant to me.

Almost all of the cosmic paradoxes of Quantum Mechanics originate with the hypothetical, "suppose you shoot a single photon at...". But what if you can't? That would represent a huge paradigm shift in the way we have to look at the microscopic universe.

I've been saying it for years, and it turns out not that maybe I was right. Bashansky et al have my back on this: there really are no pea-shooters for photons.

At least not yet.


David Reishi said...

Just read your post and your original paper. I very much enjoyed both. But I still don't get what the basic argument is (or was) when it comes to Compton scattering demonstrating particle-like behavior. Is it merely because the re-emitted "photon" of a new wavelength is always found to travel in a single, definable direction? "A big wave-blob couldn't do that!" Is that about the gist of it?

Marty Green said...

Thanks, David. But remember, it was Schroedinger's argument for wave-on-wave Compton scattering, not mine. (That's why I didn't get the Nobel Prize, right?) I analyze only the very simplest case, a center-of mass system with a light wave and an electron wave interacting head-on. In that situation, the superposition of the incident and reflected electron waves creates a perfect diffraction grating (or an array of charged sheets) which provides an impenetrable barrier to light of the same wavelenght. Since wavelength is momentum, that's the Compton effect.

I don't analyze the further complications because the historical argument is that the electron is a tiny ping-pong ball which simply cannot interact with the wave in any manner whatsoever. Once you identify the basic flaw in that argument, I figure "the reset is commentary".

George Herold said...

Hi Marty, I seem to have found your physics blog a bit late. I did want to tell you that if you measure the cross section for an atom to absorb a "photon" resonant with some energy level in the atom you find the answer is not the area of the electron orbit, but closer to the wavelength squared of the "photon". (With factors of 2 and pi here and there.)

Marty Green said...

Thanks, George. Yes, that's the calculation I do in my article on the Crystal Radio, which is linked to above. It's a very classical calculation and doesn't make any sense if you treat light as a particle. And yet everywhere you go, the so-called paradox of the photo-electrric effect starts of with the premise that to eject an electron the "photon" must concentrate its power in an area the size of an atomic cross-section.