May 13, 2002
A Philosopher's Stone
Could superconductors transmute electromagnetic radiation into gravitational waves?
By George Musser
Raymond Chiao remembers the day, during his childhood in Shanghai, when his brother built a crystal radio set and invited him to try it. "When I put the earphones on, I heard voices," he says. "That experience had something to do with my going into physics." Chiao has since become well known for his work in quantum optics at the University of California at Berkeley. Now he is preparing an experiment that, if it works (a not insubstantial if), would be the biggest invention since radio.
Chiao argues that a superconductor could transform radio waves, light or any other form of electromagnetic radiation into gravitational radiation, and vice versa, with near perfect efficiency. Such a feat sounds as amazing as transmuting lead into gold--and about as plausible. "It is fair to say that if Ray observes something with this experiment, he will win the Nobel Prize," says superconductivity expert John M. Goodkind of the University of California at San Diego. "It is probably also fair to say that the chances of his observing something may be close to zero."
Chiao presented his hypothesis at a March symposium celebrating the 90th birthday of Princeton University physicist John Archibald Wheeler (the paper is available at arXiv.org/abs/gr-qc/0204012).
His analysis, like most discussions of gravitational radiation, proceeds by analogy with electromagnetic radiation. Just as changes in an electric or magnetic field trigger electromagnetic waves, changes in a gravitational field trigger gravitational waves. The analogy is actually quite tight. To a first approximation, Einstein's equations for gravitation are a clone of Maxwell's equations for electromagnetism. Mass plays the role of electric charge, the only difference being that its value must be positive (at least in classical physics). Masses attract other masses via a "gravitoelectric" field. Moving masses exert forces on moving masses via a "gravitomagnetic" field. Gravitational radiation entwines gravitoelectric and gravitomagnetic fields.
Over the years a number of physicists have suggested that if a superconductor can block magnetic fields--giving rise to the famous Meissner effect, which is responsible for magnetic levitation over a superconductor--then it might block gravitomagnetic fields, too. When Chiao adds the gravitomagnetic field to the standard quantum equations for superconductivity, he confirms not only the gravitational Meissner-like effect but also a coupling between the two breeds of magnetic field. An ordinary magnetic field sets electrons in motion near the surface of a superconductor. Those electrons carry mass, and so their motion generates a gravitomagnetic field.
Thus, an incoming electromagnetic wave will be reflected partly as a gravitational wave, and vice versa. The same should occur in any electrical conductor, but in a superconductor the electrons all move in unison, greatly amplifying the effect. In fact, Chiao ventures that the incoming energy will be divided evenly between the two types of radiation.
"His mathematical arguments seem to be correct," remarks Bryce DeWitt of the University of Texas at Austin, a pioneer of quantum gravity. But DeWitt, Goodkind and the half a dozen other leading lights of physics interviewed for this article have assorted ideas about where Chiao might have gone astray--pointing out, for instance, that he makes various simplifications and leaps of faith. And you have to wonder why this coupling, if it is really so strong, hasn't been noticed before.
By the time the theory is vetted, though, Chiao will probably have conducted his experiment and settled the question. Working with Berkeley electronics specialist Walter Fitelson, he plans to beam specially polarized microwaves onto one slab of superconductor and use a second slab to look for rebounding gravitational waves. The setup, which uses off-the-shelf parts, is not much more complicated than a crystal radio.
If it works, you could probably come up with 30 ideas for applications in as many seconds, from new gravitational-wave detectors for astronomy to graviton antennas for telecommunications, which could send signals through the solid earth. Chiao's idea is a reminder that for all the attention paid to cutting-edge research such as string theory, radical new physics may lie within the interstices of conventional theories.
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