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12/01/92

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RESEARCHER TESTS NOBELIST'S THEORY IN SPACE

STANFORD - When you've worked 17 years on an experiment to fly on the space shuttle, you have to be a little nervous at liftoff.

Stanford physicist and research professor John Lipa can relax a bit now that the shuttle Columbia returned to earth Nov. 1, successfully bringing back data from his experiment that tested a Nobel Prize-winning theory.

"It was exciting to see this type of experiment really working in space and how easy it was to do such things even after so much planning," said Lipa.

Lipa will spend the next 12 months analyzing data that could verify Kenneth Wilson's 1971 theory on the behavior of matter in special types of phase transitions.

A phase transition is the conversion of material from one form to another, such as ice melting to water or water boiling to steam.

Lipa studied the transition of helium from a superfluid to a normal fluid, which occurs at approximately 2 degrees Kelvin (negative 271 degrees Celsius).

The experiment had to take place on the shuttle "because gravity creates pressure differences in the sample that interfere with the accuracy of the results," Lipa said. "We took measurements to within two billionths of a degree on either side of the transition temperature - 100 times better resolution than can be achieved on earth."

Lipa and his team stood watch on 12-hour shifts at the Marshall Space Flight Center in Alabama during the nine days the experiment ran. Shuttle instruments sent transmissions of data, and Lipa could control the experiment from the ground.

"The experiment was designed so it could run on its own," he said. "But after the second day, we changed to manual operations to work around periods of cosmic ray interference."

The project began in 1975 with help from Bill Fairbank, Stanford's late patron saint of low-temperature physics. Lipa had to develop specialized thermometry and thermal control systems that could measure a billionth of a degree and deliver a billionth of a watt of energy.

California Institute of Technology's Jet Propulsion Laboratory provided the "liquid helium facility," the device that houses and runs the experiment.

Simple, first order phase transitions, such as water boiling in an open container, have been understood since the 19th century. But it wasn't until Wilson's theory that the more complicated, second order transitions were explained accurately.

If you boil water without a lid, the water temperature increases proportionally to the amount of heat you add until you reach the transition temperature; there, steam starts to form and the water eventually all converts to steam, Lipa said.

If a lid is placed to prevent the steam from escaping, as in a pressure cooker, a second order transition can occur.

"Now, as you add heat, the water makes a bit of steam, but the pressure increases, causing the water to boil at a higher temperature," Lipa said.

"If you continue heating, the densities of the phases [the water and steam] become similar and suddenly - blink - at a new transition temperature, the densities become the same. All trace of the liquid phase disappears."

Near such a transition, larger and larger amounts of heat must be added to warm the material the same amount, Lipa said.

The difference is in the interaction of molecules, Lipa said. In a first order transition, the water molecules are "stuck" with their nearest neighbors, until the additional energy allows them to pull apart into a gas phase. In a second order transition, there is "an active coupling together of intermolecular forces. The molecules act in concert over longer and longer distances to make the transition occur."

Wilson's theory describes the coupling and predicts the increasing quantities of heat needed near the transition to change the temperature small amounts. The work won the 1982 Nobel Prize in physics.

Lipa chose helium to test the theory, because it conducts heat perfectly in the superfluid phase.

"It is easy to get a thermally uniform sample of helium into the [temperature] region where Wilson's theory dominates the behavior of the material," Lipa said.

While second order transitions occur in the formation of magnetic materials and liquid crystals, "we currently don't have applications that require the precision of Wilson's theory. But perhaps down the road it will become more important from a practical perspective," Lipa said.

"From the theoretical point of view, it is very important to test the limits of Wilson's theory, because it is the only one we have that claims to describe the interactions of large numbers of molecules."

-jns/phasetransition-

This story was written by Peter Chang, a science writing intern at the Stanford News Service.

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