Foucault's Pendulum

(This column was first published in the March 7, 2004 issue of The Buffalo Sunday News.)

In a back hall of the Buffalo Museum of Science is an unusual display. A heavy metal pointer hangs at the end of a long wire stretching down from several floors above. Once it has been set in motion, the pointer slowly and hypnotically swings back and forth.

The display is a replica of Foucault's pendulum.

Leon Foucault was a 19th century French scientist who, because he did not hold an advanced degree and in particular was not facile with advanced mathematics, was considered an amateur outsider by the scientific establishment of his time. He also edited a journal that explained science to the public, another role too often discredited by mainstream scientists.

You might think of him as an early version of the hero of the motion picture, "Good Will Hunting".

But Foucault was in no way a second rate scientist. His carefully designed experiments significantly improved estimates of the speed of light. He invented the gyroscope and adapted it for the telescope-aiming mechanisms astronomers still use. He also initiated the use of silvered telescope mirrors.

Far more important, the pendulum he built and finally set in motion on January 6, 1851 (the wire broke three days earlier) finally proved that the earth rotates. Remarkably, this long accepted belief had not been proved until then.

How does his pendulum establish this? Foucault, the excellent science popularizer, explained it this way. Suppose you swing a pendulum over a table and you rotate the table slowly. The pendulum will stay in line as the table turns. But if you sit on the table as it turns: the pendulum itself will appear to rotate. The pendulum, Foucault said, is "fixed in absolute space while, like the table, we and the planet rotate under it." The pendulum appears to us to turn slowly as it swings back and forth but it is really we who are rotating around the pendulum.

If you see in that explanation the kind of relativity that would later prove important to Einstein, you are on the mark.

Even before Foucault, most astronomers believed that the earth rotated. A principle called Occam's razor - the simplest explanation is usually the best - strongly supported the idea. If you stand outside at night and watch the stars, you soon notice that they "move" in a circle around the North Star. Camera time exposures show this even better. Now either all those stars move in that similar pattern or the earth rotates: Occam would have us accept the latter.

Until Foucault performed his experiment, however, no one had been able to demonstrate this.

Foucault even came up with a simple formula that tells how long it takes the pendulum to rotate 360∞. The time in hours is equal to 24 divided by the sine of the latitude where the pendulum is located. The latitude of the Buffalo Museum of Science is 42∞54' North so it takes the museum pendulum about 35 1/4 hours to rotate. It turns just over 10∞ per hour. The original in Paris at 48∞50' North took just under 32 hours. With or without math, you should be able to determine how long it would take at the North Pole.

Clearly Foucault brought to his experiment deep insight. Unlike an engineer who would employ a series of trials he supported his theory by a single experiment.

Much of the information for this column is derived from a delightful book by Amir Aczel entitled Pendulum: Leon Foucault and the Triumph of Science (Simon & Schuster). I highly recommend reading it for more about this hero of science.-- Gerry Rising