WASHINGTON — Until recently the coldest spot in the universe has been the empty void of deep space. But space is balmy compared to the ultra-frigid temperatures being pursued by laboratories in New York and Florida, where dueling refrigerators are approaching the ultimate in cold: a state called absolute zero.
For scientists who pursue record cold temperatures, absolute zero is a place of beauty and mystery. In this super-cold clime, strange and new phenomena begin to unfold: Conductors become super-conducting (losing all resistance to the flow of electricity); fluids become superfluid (losing all viscosity); and all matter attempts to reach a state of perfect order.
"We have theories. We have laws. But we really don't know what will happen until we cool things down," said Robert Richardson of Cornell University. "Low-temperature physics is largely the physics of the unknown."
Near the ultimate limits of cold, the research refrigerators that fill entire buildings are so sensitive that even stray radio waves, or earthquakes on the other side of the world, can create vibrations that jiggle the super-cold atoms, which--like rubbing two sticks together--will send temperatures in the experiment soaring.
Absolute zero is 459.67 degrees below zero Fahrenheit. Scientists measure such extremes not in degrees, but in a scale that uses units called Kelvins, or K, which are named for William Thomson Kelvin, the 19th-Century British physicist who proposed the scale. Absolute zero is 0 K.
Until scientists learned to manipulate cold in their laboratories, the lowest known temperature in the universe was 3 K, occurring in the void of space between the stars. Early in this century, scientists managed to liquefy helium gas by subjecting it to repeated cycles of cooling in special refrigerators. Once liquefied, helium stays colder than 4 K and can be stored in the laboratory equivalent of a thermos bottle. Several years later, in 1911, scientists found that certain materials suddenly become superconductors at these low temperatures.
In the last few years, laboratories in Britain, Japan and Finland--using ever more sophisticated refrigerators and better insulators--have reached temperatures of a few millionths of a Kelvin. In special settings, individual atomic nuclei have been cooled to a few billionths of a Kelvin.
Scientists at new laboratories funded by the National Science Foundation at Cornell University and the University of Florida are now trying to match, and perhaps exceed, these record lows. They hope to learn how matter organizes itself without the disturbing effects of heat and why materials become super-conducting or superfluid. Researchers are trying to cool down silicon, helium and thin films of metals such as silver and platinum.
In all materials, in solids as well as gases and liquids, the atoms are constantly in motion, vibrating and colliding with each other, creating thermal energy. The wilder the motion, the greater the heat.
"Before the laws of quantum mechanics were revealed, absolute zero was thought of as the temperature when all motion would come to a stop," when atoms, and their subatomic components such as electrons and protons, would stop their frenetic vibrations and collisions, "and would collapse as if into a black hole," said Dwight Adams, a low-temperature physicist at the University of Florida at Gainesville.
This is now known to be untrue. At absolute zero, atoms would still vibrate. Electrons would still swirl in a cloud around the nucleus. Indeed, helium would even remain in a liquid state at absolute zero, unless it was placed under enough pressure to force it into a solid.
But the atomic order at 0 K would be perfect. In a solid, each atom, though quietly wiggling, would occupy a precise location in the material. In a liquid or gas, even though the atoms may move from spot to spot, they would move in unison. They would all be in perfect order, or in what physicists call the same quantum state.
To reach this state, all thermal energy must be extracted from the system. Atomic motion must be slowed to a crawl. To get to this cold, scientists must first employ a special "dilution refrigerator," which is basically an elaborate version of an ordinary kitchen fridge, but with a lot more pumps, valves, heat exchangers, condensers and tubes, which are filled with liquid helium instead of Freon.
Liquid helium, already under 4 K, can cool materials down to a few thousandths of a Kelvin. But to go further requires super-conducting magnets. These magnets are immersed in the liquid helium and surround a rod of copper. The low temperatures and the high magnetic field cause all the nuclei in the copper atoms to align like tiny compass needles that all point in the same direction. The researchers then reduce the magnetic field, which causes the copper nuclei to resume their random orientations. This return to random order absorbs the last rays of heat from the environment.