Trying to Catch the Elusive Gravity Wave
Is it possible to design an instrument that can detect gravity waves?
According to Albert Einstein’s theories, waves of gravitation should exist. But if they do, they are so faint that scientists have never been able to detect them. They are still trying, though, and they may make it.
How do we know they’re there if we can’t detect them? Einstein worked out the general theory of relativity in 1916 and showed that the presence of matter distorted space, resulting in the gravitational force. Every time matter is redistributed in space, the nature of the distortion changes and this produces a disturbance--a “gravitational wave”--that spreads outward in all directions at the speed of light.
Astronomers are quite certain by now that general relativity is correct, so that these gravitational waves must exist. The Earth must give them off as it turns around the sun, for instance. The Earth loses energy in this way and therefore is spiraling gradually inward toward the sun.
In that case, why don’t we detect these gravitational waves? The answer is that gravity is by far the weakest force we know. The electromagnetic force that holds atoms together is a thousand trillion trillion trillion times as intense as gravitation. The only reason we’re so aware of gravitation is that the Earth is an enormous body and the gravitational pull of its myriads of particles adds up to something noticeable.
Gravitational waves are therefore the weakest and shallowest waves that exist and they simply don’t produce any effects we can detect. The amount of energy Earth loses by gravitational waves is so tiny that even in its billions of years of existence, Earth has spiraled inward toward the sun only a trifling distance.
Naturally, more energetic redistributions of mass will produce stronger gravitational waves. A really massive redistribution such as the collapse of a star into a black hole, or a collision of two stars, might produce gravitational waves just strong enough to detect. If so, a detecting instrument can give us information about the really great catastrophes that may be taking place here and there in the universe--information we might get in no other way.
In the 1960s, a scientist named Joseph Weber at the University of Maryland tried to detect gravitational waves. He used large aluminum cylinders. If a gravitational wave swept over it, a cylinder would compress and expand by a distance of about a ten-millionth the width of an atom. Nevertheless, the strongest gravitational waves might produce a compression just large enough to be detected.
To make sure whatever was detected actually was a gravitational wave, Weber made use of two cylinders, one in Maryland and one in Illinois. A gravitational wave would be so long and shallow it would encompass the entire Earth and should affect both cylinders simultaneously. Weber thought he detected the waves and for a while there was considerable excitement. However, others could not repeat the experiment and the feeling was that while Weber did important work, his instruments just weren’t quite delicate enough for the job.
Scientists won’t give up, however. The desire to have another demonstration of the truth of general relativity, and to be able to detect whispers of great events in the distance, keep them working on new “gravitational telescopes.”
One hopeful design for such a “telescope” is being considered at the University of Glasgow in Scotland by a team under Jim Hough. The instrument would consist of two evacuated tubes (tubes from which all the air has been removed, creating a vacuum) at right angles. In each tube, a beam of laser light would be reflected back and forth a thousand times or so. If the tubes are completely undisturbed, the light-wave will remain in perfect step.
If a gravitational wave sweeps over the tubes, however, one tube would be compressed a very tiny bit more than the other, and that would throw the laser beams out of step. This could then be detected, making it possible not only to spot a gravitational wave, but to estimate its energy content.
Right now, the Glasgow people are working with tubes that are each 10 meters (about 33 feet) long, just to test the workings of laser beams. Things look hopeful, but what they will need eventually, if they are to have a chance of detecting gravitational waves, are tubes about a kilometer (5/8 of a mile) long. Such an instrument would cost about $25 million to produce.
What’s more, to do it properly, there should be four such instruments distributed over the whole world, so that all four would be affected almost simultaneously. That would make it certain it was a gravitational wave and not something else. There would be tiny differences in the time of detection, since it would take the wave about 1/23 of a second, moving at the speed of light, to pass from one end of the Earth to the other. By working with such tiny time discrepancies, it might be possible to locate the direction from which the waves are coming.
