Aug. 30, 1999 Stanford physicists have put a modern twist on Galileo's classic 16th-century experiment of dropping objects from the Leaning Tower of Pisa.
Instead of dropping balls of wood and iron to prove that gravity acts equally on objects made of different materials, as Galileo did, the Stanford researchers directly compared the force of gravity acting on individual atoms to the force it exerts on an object like a baseball that contains billions of atoms. Their conclusion: The force of gravity is virtually identical at the atomic and the everyday, "macrosocopic" levels.
The study is reported in the Aug. 26 issue of the scientific journal Nature . The researchers who performed the experiment are Nobel Laureate Steven Chu, the Theodore and Frances Geballe Professor in the School of Humanities and Sciences; graduate student Keng Yeow Chung; and Achim Peters, a former graduate student now at the University of Konstanz.
To make this comparison, the researchers used a new technique, called atom interferometry, to make the most precise measurement ever made of the acceleration of gravity on individual atoms . They estimate that their measurement is accurate to 3 parts per billion. By directly comparing this result with a measurement of the acceleration of gravity made with a state-of-the-art gravimeter, they were able to show that the gravitational force acting on atoms, which are subject to the laws of quantum mechanics, is the same as that acting on familiar objects governed by the classical laws of physics with an uncertainty of only 7 parts per billion.
Although the finding does not come as a big surprise to scientists, it differs significantly from results of a series of experiments conducted by scientists at the University of Missouri-Columbia and elsewhere who attempted to measure the force of gravity acting on subatomic particles called neutrons. Using a technique called neutron interferometry, they found a difference of a few percent between the gravitational force acting on neutrons and that acting on larger objects.
The new results strengthen the likelihood that the neutron measurements are incorrect. "Since the basic physics principles used in neutron interferometry and atom interferometry are the same, our experiments show that there must be some aspect of neutron interferometry that isn't properly understood," says Chu. As important as the result itself is the fact that this experiment represents the coming of age of atom interferometry, a powerful new method for making extremely precise measurements. It "demonstrates that this type of atom interferometer can be used to make absolute measurements comparable with the most sensitive measurement tools in physics," the scientists write in Nature. Previously, atom interferometers had demonstrated high levels of sensitivity, but the highest accuracy that had been claimed was an uncertainty level of a few parts per thousand. The current work represents a million-fold increase in absolute accuracy over previous atom interferometers.
Researchers have shown that atom interferometers can act as extremely precise gyroscopes and accelerometers. In 1997, for example, Chu and his students reported that they could measure the acceleration due to gravity with a precision of 100 parts per trillion. The current experiment demonstrates that their apparatus also can act as an ultra-sensitive absolute gravity meter. Similar instruments, called gravity gradiometers, are currently used in oil exploration.
Atom interferometers exploit many of the same basic principles as optical interferometers, instruments that have been employed for more than a century to make precise measurements of distances and other physical quantities. An optical interferometer divides light into two or more beams that take different paths and then reunites them. When brought back together, the waves add or subtract with each other, forming a pattern of light and dark bands called interference fringes. The position and spacing of the fringes allows scientists to measure the difference in the distances of the light paths with great precision.
An atom interferometer does much the same thing, but it uses atoms instead of photons. Making use of the fundamental wave/particle duality that characterizes quantum mechanical phenomena, the instrument splits atoms into two waves separated in space. When the two parts recombine, they interfere with each other, forming a pattern analogous to optical interference fringes. To compare the gravitational force acting on atoms and larger objects, Chu arranged to have a state-of-the-art gravimeter operated by the National Oceanic and Atmospheric Administration run next to the atom interferometer. The gravimeter is an optical interferometer set up so the light beam on one arm is reflected off a freely falling glass cube. By measuring the speed at which the interference pattern moves as the cube falls, the instrument can measure the force of gravity acting on the cube to within 2 parts per billion.
The first atom interferometer was built in 1991 by researchers at the University of Konstantz, the Massachusetts Institute of Technology, the Physikalisch-Technische Bundesanstalt and Stanford.
Three months later, Chu and Mark Kasevich, now on the faculty at Yale University, created a new type of atom interferometer that replaced the diffraction gratings used in the first instrument with laser-cooled atoms and optical pulses. The Chu interferometer uses lasers to chill a cluster of cesium atoms to within a few millionths of a degree above absolute zero. At room temperature, atoms zoom about at supersonic speeds. When cooled down, they move at only a centimeter per second-- making it much easier to measure their position and speed. Then the slow-moving atoms are gently lofted into a free-falling arc. Laser pulses cause them to split apart and then recombine. Reading the interference fringe patterns that are produced provides an accurate measurement of the atoms' velocity in free fall.
Because the wavelengths of particles are much shorter than those of photons, the atom interferometer has the potential of making much more precise measurements than its optical counterparts. "We've only been at it since 1991, yet we can already equal the precision and accuracy of the best of the competing techniques," says Chu. "And I think we can improve the precision another three or four orders of magnitude and the accuracy by one to two orders of magnitude." The research was supported by the National Science Foundation and the Air Force Office of Scientific Research. Chung was partially supported by the National University of Singapore.
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