The SETI@home receiver at Arecibo scans the skies, searching for a signal from an alien civilization. So far no signal has been found, but SETI@home scientists are not losing hope. Searching for a signal is bound to be a long-term process, says chief scientist Dan Werthimer, and its success can only be evaluated on the time scale of generations. In the meantime, Werthimer and his colleagues are making sure that the mountains of data gathered in the search do not go to waste, and are used in shorter-term scientific projects. Most recently, astronomers have begun sifting through SETI@home's data looking for signs of evaporating black holes.
"Black holes" are those astronomical objects, predicted by Einstein's theory of relativity, whose mass and density are so great that their gravity allows nothing to escape their surface - not even light itself. As a result they are like black bottomless pits, which swallow anything in their vicinity.
Until quite recently it was believed that black holes had an unlimited lifespan. "Once a black hole, always a black hole" summed up the understanding of these objects' longevity. This view changed when Stephen Hawking, the famed quadriplegic astro-physicist from Cambridge, showed that black holes do indeed expire. According to Hawking, at some point in their lifetime black holes begin to shrink, until in the end they practically evaporate in a great burst of energy.
More recently, scientists following up on Hawking's insights have concluded that the universe was once populated with a much larger number of black holes than exist today, but those have gradually been evaporating. According to their estimates, a large number of black holes are becoming extinct even now, at the present age of the universe.
There is one problem with this theory, however: no one has ever detected the great burst that marks the passing of a black hole. In theory these events should be detectable - an energy eruption of this magnitude would send out strong electromagnetic signals at all frequencies. If we were to listen to radio transmissions from space, we should be able to hear the dying gasps of black holes.
As it turns out, we are listening, or at least the SETI@home receiver is. Perched above the giant Arecibo dish, it is systematically surveying a large portion of the sky, listening to the signals coming from space. If black holes are expiring out there, SETI@home should be able to hear and record them.
Finding the signals in the haystack of radio data collected by the receiver, however, is no easy matter. Scientists need to look for an extremely short radio burst, perhaps as brief as one millionth of a second. The signal, furthermore, would be of a very wide frequency band. It is, in a way, the exact opposite of the signal SETI@home was designed to look for. An alien transmission would in all likelihood be sustained in time or repetitive, and narrow in bandwidth; in contrast, the last gasp of a black hole would be a single extremely brief burst, with a very wide bandwidth. Because of this, astronomers cannot use the analysis performed by the millions of SETI@home users worldwide. They will have to start over, processing the masses of data collected so far, and analyze it themselves. Instead of the SETI@home network they will be using their own computers, programmed to look for the signature emission of an expiring black hole.
Scientists hope to learn many things from these signals. First and foremost, if they are detected, it would provide striking confirmation for the astronomical theories of Hawking and his successors. In addition, the signals could provide invaluable clues as to the location of black holes. As it crosses the vast distances of space, an electromagnetic pulse is distorted and stretched over time. Because of this, the exact characteristics of a signal from space can be used to estimate the distance it has traveled. Analyzing the signal would give scientists a good idea of how close we are to black holes, and how prevalent they are in our galactic neighborhood.
If no signals from black holes are found in SETI@homes's enormous database, this result would be just as valuable to scientists. It would mean that the theory's prediction was not confirmed, and the theory must be reconsidered. In that case scientists would go back to their drawing boards to see where they went wrong, and come up with a new explanation that better fits the observations.
In this manner, SETI@home contributes to the advancement of science in more ways than one. If the search for an alien civilization is proving to be a long-term project, perhaps the search for dying echoes of black holes will provide more immediate gratification.