Originally created 08/27/99

Atoms used in gravity study



In a legendary experiment, Galileo dropped stones from the Leaning Tower of Pisa to show that objects of different sizes fall at the same rate under Earth's gravity. More than 400 years later, researchers say they got the same result by dropping atoms.

The findings, published Thursday in the journal Nature, confirm that the rate at which something falls is independent of its mass -- whether it be an atom or chunk of glass used in the modern experiment or a rock dropped from a tilting building in Italy.

"There was no reason to suspect that isolated atoms would fall differently than more massive objects," said Steven Chu, a physicist at Stanford University. "Nevertheless, you want to establish that."

Unlike Galileo's experiment, the modern technology precisely measured the rate at which the atoms accelerated while falling under gravity. The result represents a million-fold increase in accuracy over previous tests, Chu said.

Chu and his colleagues super-cooled cesium atoms to within two-millionths of a degree above absolute zero, the point at which all movement stops. Absolute zero is minus 459 degrees Fahrenheit.

"The atoms are so slow that their motion is predominantly of a gravity free-fall," he said. "They then can be considered objects like a baseball tossed up and down."

The particles' rate of fall was then measured. The researchers ran a similar test in which they dropped a glass prism. The results were the same for both the large and small objects.

The acceleration due to gravity is about 32 feet per second per second, a figure that varies slightly depending on where on Earth it is measured.

The findings of Galileo, Chu and all other physicists may sound odd to anyone who has dropped a tray of food. A slice of bread hits the ground after a glass of wine because of air resistance, not changes in gravitational acceleration.

Chu shared the 1997 Nobel Prize in physics for his laser-cooling invention dubbed "atomic fountains," the same technology used in the atom-dropping experiment. It is also used to improve the precision of atomic clocks.