Yesterday’s historical physics poll was about precision measurements. Who were those people, and why are they worth knowing about?
As usual, we’ll do these in reverse order of popularity…
First up is Ole Rømer, a Danish astronomer who is no stranger to this blog, having been profiled as part of the Top Eleven series back in the early days of ScienceBlogs. Rømer’s big accomplishment was the first really good measurement of the speed of light, which he did by timing the eclipses of Jupiter’s moon Io. These are seen to occur slightly sooner when Earth and Jupiter are on the same side of the Sun than when they’re on opposite sides, and the time difference combined with the orbital radii give you the speed of light.
Next is Willis Lamb, who only died last year. Lamb was an American physicist who did extremely precise microwave spectroscopy of hydrogen atoms, and discovered that the lowest excited states of hydrogen are in a slightly different place than predicted by theory. As hydrogen is the simplest atom, and the only one for which we can do an exact quantum-mechanical solution, this Lamb shift was kind of a big deal, and attempts to explain it led directly to quantum electro-dynamics (QED), which is just the coolest thing going.
Robert Millikan, third no matter which direction you count in, is responsible for not one but two impressive measurements. The first was the “Millikan oil drop” experiment, which demonstrated that electric charge is quantized by measuring the voltage needed to levitate small charged droplets of oil. This led to the first measurement of the electron charge.
Millikan followed this up by taking on the photoelectric effect, because he thought that Einstein’s explanation of the effect in terms of quantized light was lacking “any sort of a satisfactory theoretical foundation.” Ironically, his exacting measurements served only to confirm Einstein’s model, and provided the first really good measurement of Planck’s constant.
In third place is Henry Cavendish, whose measurement of the gravitational constant G was another of the Top Eleven. Gravity is a ridiculously weak force, and Cavendish invented the torsion pendulum technique to measure it in 1797. Astonishingly, the same basic method is still used today for the most precise measurements of G.
Finally, the winner of the poll is Albert Michelson, who is especially beloved of this blog, both because he was born in Poland (well, Prussia at the time, but modern Poland), and because his partner in his most famous experiment was Williams alumnus Edward Morley. The Michelson-Morley experiment was also the winner of the Top Eleven voting back in the day, and is probbaly the most famous failed experiment in history. Michelson and Morley set out to measure the effect of the Earth’s travel through the “luminiferous aether” then believed to be the medium through which light propagated. They failed utterly, because light does not require such a medium, but in the process invented the ridiculously precise measurement technique of interferometry, variants of which are now used to measure everything from potential gravitational waves to the position of manufacturing equipment– from LIGO to Zygo, as it were.
And that’s who’s who in the history of precision measurement.