Quite a while back, Clifford Johnson at Cosmic Variance had a post seeking nominations for “The Greatest Physics Paper Ever.” Back after a long hiatus, he’s now holding a vote among five finalists: Isaac Newton’s Principia Mathematica, Albert Einstein’s General Relativity, Emmy Noether’s paper on symmetry and conservation laws, Dirac’s theory of the electron, and the Einstein, Podolsky, and Rosen paper on quantum non-locality.
(Newton’s Principia Mathematica had a comfortable lead when I last checked, so if you’re a partisan of one of the other candidates, go over there and vote…)
Of course, as some have noted, these are all theory papers. This is a natural result of the overabundance of theory types in blogdom, but it still just doesn’t seem right. For most of the history of science, after all, the development of theory has been driven by experimental results, not the other way around.
So here’s an attempt to restore the balance. I’m soliciting nominations in comments for the best experimental work in physics. What experiment do you think was the greatest, that is, what is the one experiment that you think did the most to change the field of physics (hopefully for the better)?
Nominate your favorite experiment in comments. If I get enough comments (which is not by any means a sure thing, as this ScienceBlogs thing may be too new to have the necessary audience), I’ll compile the results, and we’ll put this question to a vote as well.
I’ll put a few suggestions below the fold, but here’s your chance, experimental types. Why should the theorists get to have all the fun?
An obvious starting point for this sort of discussion would be the “Most Beautiful Experiments” list generated a few years back. I posted about this at the time, but it’s recently resurfaced with spiffy animated graphics (a site so nice, Cosmic Variance linked it twice.) There are some problems with the list (like the fact that the Leaning Tower of Pisa story is probably apocryphal), but it’s not a bad place to start.
Of the experiments on the list, the best are probably Young’s double slit and Rutherford’s discovery of the nucleus. Young’s experiment provides clear and unambiguous evidence of wave-like behavior in light, with nothing more than a couple of slits cut in a card. It scores big points for elegance, as well as for directly contradicting Newton, a bold move in 1800.
Rutherford’s experiment is more complicated, but like Young’s, it completely smashed the prior theory. My favorite part of the story is that the discovery supposedly came about because Rutherford had hired a new student, and set him a silly make-work task to get him used to the apparatus. What he found required a complete re-thinking of existing models of the atom, and produced one of the great quotes in experimental history:
It was 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.
(That’s Rutherford describing the discovery.)
Among experiments not on the “Most Beautiful” list, I’d nominate the Michelson-Morley experiment, and not just because Morley was a Williams grad. The real role of the experiment in launching relativity is somewhat unclear, but it’s certainly a fabulous example of the ingenuity of experimentalists. Michelson set out to make a ridiculously difficult measurement, and in the process invented a technique that’s still in use (and that I’ll be teaching in lab tomorrow morning) for precision measurements.
Another good candidate (and I’ll stop here) would be a Bell Inequality experiment, but here we hit the major problem with sorting out great experiments. Many of the best experiments in physics only put limits on particular quantities, limits which are constantly being questioned and refined. In the case of Bell’s Inequality, all the experiments to date have left loopholes that allow some possibility for local-variable theories (albeit increasingly baroque local-variable theories) to survive. There hasn’t really been a definitive Bell Inequality experiment.
Still, the experiments by Alain Aspect and co-workers are awfully impressive, and most physicists regard them as fairly convincing proof that quantum mechanics is a non-local theory, and that Einstein, Podolsky, and Rosen were wrong (in an inspired manner, but still wrong). Given that quantum non-locality is one of the absolute strangest things in the entire world, I’d say that an experiment proving it deserves a place among the all-time greats.
So, what’s your nomination for the greatest experiment in physics?