A beautiful experiment

What makes a scientific experiment beautiful? A few years ago Physics World asked readers to nominate the “most beautiful experiment in physics.” The results were not terribly surprising. Readers picked Thomas Young’s double-slit experiment demonstrating that light interferes with itself as the most beautiful. My favorite—Rutherford’s experiment discovering the atomic nucleus—came in ninth place.

If beauty is in the eye of the beholder, then I have no reason to complain. However, Physics World also asked readers the reasons behind their selections. Their answers show that readers do not consider this type of beauty purely subjective. Robert P. Crease summarized their reasons as “transformative,” “economy,” and “deep play.”

Those reasons strike me as partly right and partly wrong. What makes an experiment beautiful is that it is cleverly designed, dramatic, and reveals something fascinating about nature. Everything else is fluff. For example, when someone calls an experiment “transformative” it tells us more about modern conceit than the experiment's merit. Like "disruptive" and "path breaking," it's pure cliché.

Thomas Young’s double-slit experiment was beautiful. Unfortunately, it was also somewhat misleading. One purpose of science experiments is to resolve controversies. Thomas Young’s experiment showed the wave nature of light. Other experiments demonstrated the particle nature of light. It wasn’t until much later that a modified double-slit experiment demonstrated wave-particle duality.

I have reservations about some of the other top ten choices. Newton’s “decomposition of sunlight with a prism” illustrated a fundamental concept, but it was hardly very clever. Similarly, the only thing original about Galileo’s “experiment on falling bodies” was his explanation. And Eratosthenes’ “measurement of the Earth’s circumference” relied on a combination of observation and theory—not experimentation.

Rutherford’s experiment showed that while atoms consist mostly of empty space they contain a tiny and relatively massive core he dubbed the “nucleus.” In this experiment, a beam of alpha particles was used to bombard a very thin gold foil. Studying scattering of the alpha particles, Rutherford found most traveled straight through, some were modestly deflected, and one in 8,000 bounced straight back. (He verified that the bounce backs were not just a surface phenomenon.) Rutherford famously stated “It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you.”

Rutherford followed that up with another beautiful experiment. He wanted to prove that alpha particles are helium nuclei. He directed an assistant to make a glass cylinder and place another, slightly smaller glass cylinder inside. The wall of the smaller glass cylinder was extremely thin: 1/100th of a millimeter thick. This allowed alpha particles emanating from inside the inner cylinder to enter the space between the two cylinders (after the air had been removed) but not escape through the outer cylinder’s wall. After collecting alpha particles in the space, he zapped them with electricity and examined them with a spectroscope. They showed the spectrum of helium as expected.

A beautiful experiment doesn’t have to be perfect, but it must significantly increase our knowledge of the natural world in a way that is repeatable and verifiable. With all of the distractions that surround modern science, researchers would do well to revisit beautiful experiments from time to time.