Discovering Gravitational Waves (Kindle Single) by John Gribbin

Author:John Gribbin [Gribbin, John]
Language: eng
Format: epub
Published: 2017-05-21T00:00:00+00:00

Raising the Bar

The first person to attempt to detect gravitational waves here on Earth was an American physicist, Joseph Weber, working at the University of Maryland. Weber’s background was in electronic engineering, but in the 1950s he became intrigued by the general theory of relativity and its predictions, in particular the possible existence of gravitational waves. He decided to build a detector to search for this radiation, starting out from the standpoint of an engineer. His philosophy, as he later told science writer Marcia Bartusiak, was to ‘build something, make it work, and see if you find anything’. (Bartusiak’s book is the best source of information about Weber’s work, based on an interview with him. I am grateful to her for letting me re-use some of her material here.) It was an ambitious idea, because the predicted effects of gravitational waves here on Earth are so small. As a wave passes by, it first squeezes and then stretches space, which makes objects squeeze and stretch accordingly. As Bartusiak has spelled out, an event, such as the collision of two black holes, which produces an intense burst of radiation, might briefly stretch a nearby object to twice its original length; a human being two metres tall would be stretched to four metres, with unpleasant results. But if the same event occurred as far away from us as the centre of the Milky Way, an object one metre long here on Earth would be stretched by a billionth of a billionth (10-18) of a metre, very roughly a hundred-millionth of the size of an atom. In engineering terminology, the strain would be 10-18 metres per metre. Such a strain would change the distance between the Earth and the Sun by the size of a bacterium. Yet Weber published a paper in 1960 setting out how it might be possible to measure such effects, then set out to build an experiment to do just that.

The key to Weber’s idea was that although a gravitational wave passing through a solid metal cylinder would stretch and squeeze the cylinder only by a tiny amount, if the cylinder were just the right size this would set it ringing, briefly, like a bell struck with a hammer. This ringing would be more pronounced if the length of the bar matched the wavelength of the gravitational radiation, so in effect it would be tuned to respond to certain waves. This is like the way a string on a guitar leaning against a wall will resonate when a note that matches the fundamental note of the string is played on another instrument nearby. Weber’s electronic expertise came in to play with the design of the instruments to measure very small vibrations of the bar – a ring of detectors around the waist of the cylinder to convert the vibrations into electric signals that could be recorded and analysed.

Weber’s first detector was constructed with the assistance of Robert Forward, a graduate student who went on to become a top physicist, and also a leading science fiction writer.

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