In the Eye of the Beholder
Physics World features a list of the "Ten Most Beautiful Experiments in Physics" this month, with some interesting commentary about what constitutes "beauty" in this context (Though the author shows a disturbing lack of pop-culture savvy when he cites one particular comment from SlashDot: "One of the contributors described watching small plastic bags circulating in wind pockets, commenting that 'sometimes there's so much beauty in the world, I just can't take it'."). The New York Times also picks up on the story, offering the top ten with additional annotations about the history of the experiments.
The "winner" in the survey was the double-slit interference experiment carried out with electrons, with Young's original double slit experiment with light making the list as well. They're hard to argue with:
The double-slit experiment exemplifies the wave-particle duality of light, as well as quantum physics itself. It demonstrates that light interferes with itself in passing through a pair of slits. It also shows that even single electrons - proceeding one by one - interfere. Richard Feynman is said to have remarked that it contains everything you need to know about quantum mechanics.
There really is something magical about the way that single electron counts pile up in just the right way to make an interference pattern. You can do the same trick with light (given a sufficiently sensitive camera), but it's not as impressive-- everybody already expects light to interfere, and the quantization somehow isn't quite as surprising, either.
For the most part, the Physics World survey respondants went deep into physics history for their choices: only three of the ten were performed in the 20th century, while Galileo alone gets two mentions in the top ten, and one experiment dates to the third century BC (Eratosthenes's measurement of the Earth's circumference based on the lengths of shadows at noon at different latitudes). They were also strongly inclined toward simplicity, with the Milikan oil-drop experiment and Rutherford scattering being the only really complicated ones on the list.
As for my own picks, "beauty" is an awfully difficult thing to judge, though, and it's not a term I tend to apply to experimental physics. This is largely because I'm an experimentalist by trade, and tend to be less impressed with simplicity and elegance than I am with experiments that find extremely clever ways to do astonishingly difficult measurements. Cavendish's torsion pendulum is a simple and elegant demonstration of the inverse-square law of gravitation, but the updated torsion pendulum experiments at the University of Washington absolutely blow me away. The ingenuity they've shown in finding elegant solutions for all the problems that crop up in torsion balance experiments, while keeping the Cavendish concept as the core of the device, is simply amazing.
Similarly, while the double-slit experiment with electrons is perhaps the most elegant demonstration of wave-particle duality you'll find with matter waves, I'm rather fond of the recent fullerene diffraction experiments-- everybody already knows that electrons and atoms behave like waves, but these guys have set out to observe diffraction in the largest systems possible. They've seen clear diffraction with C_60 and C_70-- those soccer-ball-shaped molecules consisting of 60 or 70 carbon atoms-- and are planning to try even larger systems (one rumor said they were planning to try diffraction of viruses).
And as long as we're talking about matter-wave intrference, I think the Davisson-Germer experiment (showing the diffraction of electrons by nickel crystals) gets somewhat slighted. It's not what you'd call conventionally beautiful-- the experiment worked only because they broke part of their apparatus-- but they get extra bonus points for serendipity and sheer determination. They blundered into one of the great demonstrations of the wave nature of matter, and helped confirm the daring (and seeming daft) predictions of the nascent quantum theory. They're sort of the physics equivalent of Alexander Fleming and his moldy Petri dishes.
Rutherford scattering (which did make the list) is another great story. The classic experiment demonstrating the structure of the atom grew out of what was basically a make-work project for a grad student. Rutherford was bombarding thin gold foils with alpha particles, which according to then-current understanding, should've passed clean through the foil. He set a student to checking whether any were actually reflected, expecting a null result, and instead found that lots of particles were scattered backwards-- he famously described it as "quite the most incredible event that ever happened to me in my life. It was almost as incredible as if you had fired a 15-inch shell at a piece of tissue paper and it came back and hit you."
My natural bias toward complicated and audacious experiments leads me to agree in large part with the author of the Physics World piece when he says:
I was a little disturbed, however, by the ease with which many people seemed to think that the experiments that they were proposing had been conceived, or could be carried out and understood. This seemed a function, in part, of the way that these experiments are often taught. Demonstrations can vastly simplify the experimental process through the use of modern equipment constructed with the "right answer" in view. Textbooks and Web simulations - which exist for most of the experiments on the top 10 list - can involve far greater simplifications.
One of the worst victims of this effect is the Michelson-Morley experiment (last one, I promise), which I've mentioned before. It's a wonderful experiment (says the professional Optics Guy...), which lasers have rendered a trivial demo-- I knocked together a table-top Michelson interferometer in about fifteen minutes one afternoon a few weeks ago. Michelson and Morley did it without the benefit of lasers, though-- they used a sodium lamp, and arm lengths of several meters. I hate to think of the amount of time they spent squinting at the damn thing while they tried to align it. And then they floated the whole thing on a vat of mercury and spun it around-- all for a null measurement. It's easy to underrate what was really a tour de force experiment (and, indeed, it limps into the survey midway through the "also ran" list of the original article). Michelson and Morley get slighted because people now tend to think of their experiment as being easier than it really was, and because the theory they disproved (the "luminiferous aether") seems a quaint anachronism.
It's interesting to note that, with one or two exceptions, all the experiments on the list are used as class demonstrations or very simple labs these days. Those really are the things that stick with people from physics classes (which is really the only place you hear much about the great historical experiments). That also probably accounts for the inclination toward very simple experiments, rather than the more complicated and clever ones that impress me for their technique-- you can't do fullerene difraction or the real Michelson-Morley experiment in class, but two-slit interference of electrons is possible.
(And again, I'm always struck by the way that even really simple demos can be so amazingly effective-- I broke out the Liquid Nitrogen demos to introduce laser cooling to our freshman seminar class yesterday, to good effect. But even the senior science majors in the Relativity and Quantum Mechanics class afterwards (which I also taught-- two diferent classes, back to back. I was surprised at just how difficult and draining that turned out to be...) were impressed by just seeing nitrogen dumped out on the table.)
I could ramble on about this stuff for hours, but I'll stop here. It's an interesting article, even if the actual results (as with all such surveys) reveal more about the people who responded than the TRVTH of what the greatest experiments in physics really are. It's fun to look at what's on and what's off the list, in the same way that it's fun to argue about whether the 1927 Yankees could've beaten the 1972 Dolphins in roller hockey, and other such sports absurdities. Basically, it's a good excuse to look back and celebrate some of the most clever experiments in the history of science, and remember just how cool it is that the world works the way it does.
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