Theorists twist relativity to eradicate an astronomical anomaly
Black holes breed paradoxes. Although black holes are being "discovered" with numbing regularity these days, many prominent physicists still consider them far from demonstrated. "I don't believe in black holes," declares Philip Morrison, a physicist at the Massachusetts Institute of Technology (and a longtime book reviewer for this magazine).
Such doubters are heartened by two recent theoretical attempts to stamp out black holes--or at least their more far-fetched aspects. Both proposals accept (as Morrison does) that space harbors objects that are extremely massive and dense; astronomers have found ample evidence for such gravitationally powerful beasts. But the theories modify general relativity--Einstein's theory of gravity--in order to make these objects more sensible.
According to conventional relativity, black holes are not just dense but infinitely dense; in their hearts, which are called singularities, space, time and causality itself are mangled beyond recognition. So obscene are black holes that they must be hidden from the rest of the universe by membranelike event horizons, which also strain credulity. Time runs so slowly in an event horizon, relative to the rest of the universe, that anything falling into it is stuck there, seemingly, for eternity. Outside observers thus never actually see anything fall into supposedly voracious black holes.
A proposal advanced by Huseyin Yilmaz, a physicist affiliated with Tufts University and Hamamatsu Photonics in Japan, would eliminate these paradoxes. Since the 1950s Yilmaz has tried to construct a theory that preserves the basic framework of general relativity while doing away with its more anomalous effects. Only recently has Yilmaz produced what he considers to be a consistent version of his idea.
Yilmaz modifies general relativity by assuming that a gravitational field, as a form of energy, also tugs on itself. As one colleague of Yilmaz has put it, "Gravity gravitates." The addition of this factor, for various technical reasons, would prevent large masses from undergoing the catastrophic collapse that spawns singularities and event horizons.
Yilmaz has an enthusiastic supporter and collaborator in Carroll O. Alley of the University of Maryland. The proposal is a "well-defined alternative" to general relativity that "makes a lot of sense," Alley declares. Papers by Yilmaz and Alley have been published in the 1995 "Annals of the New York Academy of Sciences." Alley notes that Einstein himself was appalled at some of the odd predictions of his account of gravity. "We regard this as the completion of Einstein's work," Alley says.
But John W. Moffat of the University of Toronto hardly agrees. According to his analysis, the Yilmaz theory might eliminate event horizons but not singularities. That would leave the singularities "naked," Moffat adds in horror. "This is a disaster." Of course, Moffat may be biased: he prefers his own modified version of general relativity. Together with a graduate student, Neil J. Cornish, Moffat describes his work in this month's "Journal of Mathematical Physics." In their take on relativity, space-time may be twisted as well as bent in response to the presence of a large mass; the addition of this extra "torsion" parameter ousts event horizons and singularities.
Moffat claims his hypothesis also whisks away one of the most troubling problems of modern theoretical physics. Various theorists, notably Stephen W. Hawking, have argued that Einsteinian black holes destroy information and thereby violate basic notions of cause and effect. But if there are no Einsteinian black holes, Moffat contends, there is no information loss.
Both Yilmaz and Moffat are trying to find ways--short of visiting black holes--to test their hypotheses. Ironically, Yilmaz's work helped to inspire an experiment that might falsify his proposal. In the late 1950s a physicist named George Pugh, after hearing Yilmaz present an early version of his model, proposed a complex experiment--involving a gyroscope in a weightless environment--that would rigorously test general relativity. Pugh's plan, combined with others, evolved into the $500-million Gravity Probe B Relativity Mission. The experiment is scheduled to be launched into space in 1999.
Most theorists think Einstein's version of relativity is far more likely to survive such a test than are the alternatives. The Yilmaz suggestion is "junk," "garbage," "not even a real theory," sneers Clifford M. Will of Washington University. The Moffat account, while more deserving of respect, also suffers from technical problems, Will says. But then, he confesses he does not believe there is really a need for such theories, because he does not consider singularities and event horizons to be so odd: "These other theories are much more exotic."
Morrison, needless to say, disagrees. The proposals by Yilmaz and Moffat could come to naught, he says, but they are nonetheless "examples of what you need to have" to make astrophysics sensible again.--John Horgan
SCIENTIFIC AMERICAN July 1995 Volume 273 Number 1 Page 16
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