91av

This is a classic article from 91av’s archive, republished as part of our 50th anniversary celebrations

A GROUP of researchers at the University of Maryland who pioneered work on the detection of gravitational waves has been regularly observing “events” since the beginning of this year. The leader of the group, Professor Joseph Weber, cautiously avoids making the claim that the events are in fact pulses of gravity, although he cannot relate them to any terrestrial occurrence.

According to Einstein, masses that are accelerating should radiate energy in the form of gravitational waves, just as an oscillating electric current radiates electromagnetic radio waves. The energy radiated by gravity is very small, but Professor Weber has devised a detection scheme and has given hope that experiments on the laboratory scale might see something.

The strong sources of gravitational waves that are likely candidates for detection are rotating double neutron stars and supernovae. Moreover, if the “big-bang” theory of the beginning of the universe is correct, some of the gravitational radiation that should have been created then might still be about and might be detectable. A double neutron star, or a star in the process of becoming a supernova with consequent gravitational collapse, might also emit strongly.

Called a high-frequency detector, Weber’s system should pick up oscillations near 1660 hertz in the curved geometry of space-time that Einstein used to describe the gravitational field. The detector is a 1-ton block of aluminium suspended on wires in a vacuum chamber and elaborately isolated from laboratory noise. The lowest natural compressional mode of the block is also 1660 hertz and it would be set into vibration by gravitational waves since, as the wave passes the block, its length would momentarily change. Quartz strain gauges detect any vibration.

Weber’s group has recorded events as output pulses from the strain gauges five times as high as the average thermal “noise” level. The events were recorded at essentially the same time in each of two gravitational wave detectors positioned more than a mile apart. But these same events failed to trigger a highly sensitive seismometer, gravimeters and tilt meters set up to rule out signals such as earthquakes, local Earth movements or unusually intense sounds. Although the number of recordings is small, the researchers have eliminated the possibility that they occurred by chance. In any case, the signals closely resemble the expected extraterrestrial signals.

No astronomical observations of supernovae have been reported that coincide with the events, however, and this has led Professor Weber to be cautious about his claims. Further work should give concrete evidence one way or the other.

This article was originally published in 91av on 16 November 1967

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