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FOOTNOTE:*The most familiar example of a phase change is the transformation, at 32 degrees F, of water from a fluid into crystalline ice.
Iowa's Finnemore compares the movement of the electrons in a superconductor to a crowd moving across a football field. "If they act as individual particles," he explains, "they will bump into each other and scatter. That's the equivalent of electrical resistance. But suppose someone starts counting cadence, and everyone locks arms and marches in step. Then even if one person falls into a chuckhole, he won't fall because his neighbors hold him up." Thus in a superconductor electrons move unhindered.
While the BCS theory works well near absolute zero, some physicists think it will have to be modified or even scrapped as an explanation for the behavior of higher-temperature superconductors. According to Bardeen, his theory can explain superconductivity up to around 40 K. But at 90 K, he says, "I think it's highly unlikely. We no doubt are going to need a new mechanism." In fact, says Schrieffer, "superconductivity may turn out to have as many causes as the common cold."
Confusion at the level of theory has put no damper on the orgy of speculation about potential applications. Some ideas involve upgrading existing superconducting technology; others push marginal technology into the realm of the profitable; still others raise the prospect of entirely new uses of the phenomenon.
Giant particle accelerators are one target for possible upgrading. Currently the most powerful such devices use conventional superconducting electromagnets. If high-temperature superconducting magnets can be developed, millions of dollars could annually be saved in electrical and liquid-helium bills.
Electromagnets are also crucial to fusion energy, which depends on fusing atoms (the same process that powers the sun), rather than splitting them. Key to one promising fusion process, which is under development in several countries, is a "bottle" composed not of any material substance but of powerful magnetic fields, generated at great expense by conventional electromagnets. Such fields are the only envelopes that can contain and squeeze atoms together at the hundred-million-degree temperature required to initiate fusion. But superconducting magnets, especially warm-temperature ones, could produce more intense fields at less expense and thus could "help make fusion power possible and practical," says Harold Furth, director of Princeton University's Plasma Physics Laboratory.
In medicine, superconducting magnets are at the heart of magnetic resonance- imaging machines. The magnets' powerful fields first align the atoms of the body. Then a pulse of radio waves knocks them momentarily out of alignment. When the atoms return to their previous attitudes, they emit radiation that produces detailed images of the body's soft tissues. MRI machines in use today are enormous (6 ft. by 8 ft. by 10 ft.), largely because of the more than $100,000 worth of bulky insulation required to preserve the liquid helium coolant, which costs an additional $30,000 annually. The improved economics of the new superconductors, says Walter Robb, of General Electric's Research and Development Center, should eventually enable medical institutions to install many more MRI machines, which are invaluable for diagnosing disorders like brain tumors.