In the ongoing string theory comment thread (which, by the way, I’m really happy to see), “Who” steps off first to ask an interesting question:
One way to give operational meaning to a theory being predictive in the sense of being empirically testable is to ask
What future experimental result would cause you to reject the theory?
I think what worries a lot of people about string thinking is that it seems so amorphous that it might be able to accomodate any future experimental measurement. In fact I am not aware of any string theorist’s answer to this basic question.
It’s an interesting question, and not just for string theory.
The book I’ll be using for my “Quantum Optics” class in the spring term spends a good deal of time on the history of experimental attempts to prove the existence of photons. It turns out to be a really difficult thing to do, because people can keep finding theoretical loopholes. When you start looking itno the history, the Qwiki page for “Photon” has the definition about right:
A little word that can cause a lot of problems…
The simplest experiment you can do– setting up a detector or a camera that records individual photons– is a complete non-starter. You see discrete spots, or hear discrete “clicks” on your detector, but there’s nothing about that that proves you’ve got photons.
Historically, a lot of textbooks point to Einstein’s theory of the photoelectric effect (which is officially what got him the Nobel) as the demonstration that photons exist. The photon theory works very nicely in this case, but even after Einstein’s model was experimentally confirmed, there were people who really didn’t buy it. In fact, you can develop a semi-classical model (treating the atoms as quantum objects with discrete states, but the light as a continuous classical wave) that reproduces all the features of the photoelectric effect. The Compton effect was actually more convincing, historically, but there’s a dodge around that one, too (I don’t recall what it is, though, and the book is at work).
There are a number of other experiments that have been claimed as proof of the photon nature of light (Hanbury-Brown and Twiss, for example), but all of them turn out to have semi-classical explanations. The experiment that finally settled the question for just about everybody was the observation of photon anti-bunching. In 1977, a full 72 years after Einstein’s paper on the photoelectric effect.
(More after the cut.)
(The anti-bunching experiment is a pretty cool one. What you do is set up a photon detector looking at a light source, and measure the probability of detecting a second photon some interval dt after detecting the first photon. It turns out that classical sources of light either give you a flat distribution as a function of dt (a completely random distribution), or a peak at dt=0 (photon bunching).
(If you use the right kind of light source, though, you can find that the probability goes to zero as dt goes to zero. That is, you never detect a second photon immediately after the first one. This is impossible to reproduce with a classical model of light– you need photons to do it.
(The simplest example of such a light source is the light scattered from a single atom. The atom emits light only when it drops down from an excited state to the ground state, so after you detect the first photon, you have to wait for the atom to be excited again, and drop back down, and that takes some time. There’s no way for the atom to emit two photons right on top of one another (for most atomic transitions, anyway), so you get anti-bunching, which is a purely quantum effect.)
Now, of course, most people in physics had accepted the existence of photons long before 1977. The anti-bunching experiment was just the last nail in the coffin– a piece of experimental evidence that there was absolutely no way to dance around. It’s not like it took seventy years to get anybody to accept photons– just the die-hards.
But if you’re going to say “It’s been twenty years, when are you going to give up string theory?” remember that it took seventy years to get people to give up classical light…