Precision Measurement Smackdown Explained

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.

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If you include Ole Romer for measuring the speed of light then you also have to inlude Jean-Dominique Cassini on whose exact measurements of the orbits of the Jupiter moons Romer's work was based.

I really hate to write this in response to a "precision measurement" article, and I hate it all the more because I really enjoy reading this blog.

But...

I think you measured wrong.

You say above, "In third place is Henry Cavendish", but according to my measure Henry Cavendish is in 2nd place (being between Michelson at 1st and Millikan at 3rd).

I recused myself from this poll. I did my graduate work with a Michelson interferometer, and I once accidentally drove a fellow graduate student's face into the Millikan oil drop experiment he was doing. So I can't be objective.

The Navy Museum at the Washington Naval Yard in DC has one of Michelson's interferometers, or at least it did when I went through it 22 years ago. Michelson was a Navy officer. An episode of Bonanza depicts Ben Cartwright helping him get into the Naval Academy, which is semi-realistic because he was from the Virginia City, Nevada area. Win a Nobel prize and everyone claims a piece of you.

By Bob Hawkins (not verified) on 31 Jul 2009 #permalink

You say above, "In third place is Henry Cavendish", but according to my measure Henry Cavendish is in 2nd place (being between Michelson at 1st and Millikan at 3rd).

That's a typo caused by writing very fast, late at night.

Michelson was at Clark University for a short time before heading to the University of Chicago (which was professored by a bunch of former Clark profs because Jonas Clark was a cheap guy).

You could play with some of his old stuff in the physics building.

Not so with Robert Goddard's old stuff. That office was a shrine.

The best thing about Michelson is his practical experimental sense: How does one isolate and stabilise an interferometer without the benefit of electronics? Mount everything to a big slab of granite and float it in a tank of mercury, of course!

my vote for Michelson not just because of the beauty of his experimental design but also because his result directly overturned the leading theories of the day. No ether !!! the other experimenters on the list had more evolutionary impact on their contemporaries. Otherwise, like any good caltech alum, I'd have voted for Millikin.