How a scientist swung into the truth

Léon Foucault's pendulum proved what scientists knew, but nobody before him could show: the world turns

This intriguing account of the work of a 19th-century French physicist exemplifies a lesson that humanity seems forever reluctant to learn: How the world appears depends on your frame of reference. Someone else with a different reference frame can have a different world view that is equally valid. The trick is to translate between reference frames and find the underlying truth.

That's the trick Léon Foucault pulled off 152 years ago when he presented the first definitive evidence of Earth's rotation. You can see that evidence today in many museums where a pendulum swings ponderously over a sand-covered platform. The track it traces in the sand rotates 360 degrees as the world turns.

Such unequivocal proof of Earth's rotation eluded the most skillful seekers for thousands of years. The spin has no obvious effect on our everyday reference frame: the ever-changing sky above and the seemly motionless ground beneath. It's easy to believe the sky rotates around a stationary orb. When Foucault showed this common-sense perception to be deceptive, a stubbornly held world view changed forever. (Galileo knew the truth earlier but couldn't demonstrate it.)

This triumph of scientific thinking over faith-based misconception is the hero of Amir Aczel's "Pendulum." Though not a biography of Foucault, it does sketch key details of his life. And these make the point that you can't fully understand a scientist's work divorced from its political, religious, and social context.

Foucault was a brilliant engineer, but a self-taught, mathematically naive physicist. Members of the French Academy of Sciences despised him. He might have been sidetracked but for the help of a higher power: Napoleon III became a self-taught amateur scientist during his years in exile and in prison and felt an affinity with the self-taught physicist. As Emperor of France, he saw to it that Foucault had the honors and resources he needed.

Foucault's experiment confronted misguided religious faith. Its physics relies on a different kind of faith. We explain the physics today in terms of a force described by French engineer Gastard-Gustave Coriolis 16 years before Foucault swung his pendulum. This "Coriolis" force acts on the motions of bodies as observed in a rotating reference frame. On Earth, it diverts a moving mass to the right in the northern hemisphere and to the left south of the equator. It determines how air flows around a low-pressure storm system. It gently guides Foucault's swinging pendulum on its daily round.

Physicists explain the pendulum rotation in terms of two distinct reference frames: that is, two sets of coordinates that specify the position of an object in three dimensions (east-west, north-south, up-down). One frame - a so-called inertial frame - has no motion relative to the stars. The second reference frame rotates with Earth.

The mathematical equation that describes the pendulum's motion in the inertial frame is simple: There is no Coriolis force. The pendulum always swings in the same direction. When you do the math to transform that equation so that it describes the pendulum in the rotating reference frame, the Coriolis force appears.

The trouble with this neat story is there is no fundamental reason to believe an inertial frame can actually exist. Philosophers have debated this for centuries. Working scientists just take it on faith.

Foucault knew nothing of Coriolis force math. But like Galileo before him and Einstein after him, he did know that the physics we perceive depends on our reference frame. His genius lay in identifying a crucial experiment that would reveal the influence of Earth's rotating reference frame to everyone's eyes - and in having the engineering skill to pull it off.

That's a humbling thought for this reviewer, whose faith-based geophysical education made him a true believer in the mystical power of a rotating Earth.

Robert C. Cowen writes about science for the Monitor.

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