On June 30,1908, a giant fireball exploded in Siberia's remote Tunguska region, leveling trees for more than 20 miles around and causing atmospheric shock waves that were detected round the world. At the time, scientists thought that a giant meteorite had crashed into the earth. Later, when they failed to find a major crater or clearly identifiable meteorite fragments at the site, they began to question their earlier theory.
Many scientists have since attributed the phenomenon to a comet head that exploded in the air before hitting the earth. Others suggest that a stray clump of antimatter from elsewhere in the universe found its way to earth and completely annihilated itself and an equivalent amount of terrestrial matter, thus releasing an enormous blast of energy.
Now, in perhaps the most imaginative theory offered to date, University of Texas Physicists Albert A. Jackson IV and Michael P. Ryan Jr. have proposed that the 1908 explosion was caused by a "black hole" — a bizarre celestial object scientists believe exist in great numbers throughout the universe.
One of the implications of the theory of general relativity is that when giant stars exhaust their nuclear fuel, they collapse so suddenly under their own tremendous gravity that their remnants are compressed into a sphere only about two miles or so across and weighing trillions of tons per cu. in. The gravitational field of the sphere is so intense that no light can escape from it—thus the name "black hole."
Clearly, such an object would have caused far more cataclysmic damage than the Siberian explosion. But in recent years several scientists have proposed the existence of tiny black holes even smaller than a speck of dust. Some of these may have been formed in the so-called "big bang"—the great explosion that cosmologists believe marked the birth of the universe some 10 to 15 billion years ago. Others could be fragments from collisions between larger black holes.
Jackson and Ryan calculate that if such an object (which would have the mass of a moderate-sized asteroid) intercepted the earth's path at a velocity of about 25,000 m.p.h., it would have set off a shock wave quite similar to the one from the Siberian blast. They re port in Nature that the black hole's passage through the atmosphere would have left a deep blue trail of ionized particles like the streak seen by witnesses near the 1908 blast. Finally, the energy released by the black hole (comparable to that of a ten-megaton hydrogen bomb) could easily have caused the observed damage without leaving material residue or a crater.
Jackson and Ryan offer a concrete check for their fantastic suggestion. Witnesses to the Tunguska blast indicated that whatever caused it streaked toward the earth at an angle of 30° from the horizon. If the object was actually a black hole, it would have easily penetrated the earth in an almost straight line and emerged eight minutes later on the other side, about 1,000 miles east of Nova Scotia, triggering underwater and atmospheric shock waves and drawing off a thin, geyser-like column of water as it flew into space. Jackson and Ryan suggest that their theory may be supported by a search of oceanographic records and ships' logs for any reports of strange doings in the North Atlantic on the day of the 1908 Siberian explosion.