Wave Power

Surveying the field of space, time and the ultimate nature of matter at the widest level.  Published by Cambridge University Press 1979.

Surveying the field of space, time and the ultimate nature of matter at the widest level. Published by Cambridge University Press 1979.

Gravitational waves, as predicted in 1915 by Einstein’s theory of general relativity, exist.  The evidence is there.  The merging of two black holes around 1.4 billion light years away caused ripples in spacetime which were detected in September 2015 by the Laser Interferometer Gravitational-Wave Observatory (LIGO).  In a previous webpost – Cosmic Ripples – I wondered whether it would ever happen, but it has, and we can use our imagination to think ahead to what it all means.

In her recent 2016 book Black Hole Blues and Other Songs From Outer Space, Janna Levin, professor of physics and astronomy at Barnard College of Columbia University, takes us through the story of the process which led to this momentous achievement.  Indeed, creating the most sensitive scientific instrument in history was quite an achievement in itself.  One of the main objectives of LIGO was to open “an observational window on the universe that differs fundamentally from that provided by electromagnetic or particle astronomy”.  That’s what it has done by detecting an event which had “the power in gravitational waves a hundred billion trillion times the luminosity of the sun.”

A comprehensive assessment of the subject as it stood in 1979, a century after Einstein’s birth, was General Relativity: An Einstein Centenary Survey edited by Stephen Hawking and Werner Israel.  It included an analysis by David Douglass and Vladimir Braginsky of sources of gravitational radiation as then understood, ranging from continuous sources such as rotating binary stars to sudden bursts from events such as supernova explosions.  Assessing experiments as then proposed, they felt that the prospects for unambiguous detection of gravitational waves in the near future was good, though it was actually over thirty years before the technique of laser interferometry got the right result.

The authors suggested that, once achieved, an entirely new field of gravitational wave astronomy would be opened up.  As Professor  Levin expresses it in her book, we are “listening directly to the messengers of a fundamental law of nature”.  And one of the significant outcomes of the LIGO discovery is indeed that we may be closer to identifying the graviton, the currently hypothetical quantum particle that carries the force of gravity between two objects that have mass.  Gravitons occur in some versions of string theory, so we may, in due course, see further clarification of the models of the elementary particles in fundamental physics.

But the implications might go considerably further than academic research alone.  In his contribution to the 1979 survey, Quantum gravity: the new synthesis, Bryce DeWitt of the University of Texas at Austin explains how general relativity not only gives dynamical properties to empty space, but also “ascribes to it energy, momentum and angular momentum”.  And, in principle, since gravitational waves are ripples on the curvature of spacetime, they could be used as a propellant for a spaceship.  Observing that this could be seen “as getting something for nothing – achieving acceleration simply by ejecting one hard vacuum into another”, he adds that this is not as absurd as it sounds.

This has further implications for how the human race may eventually control the forces of nature in the service of long-distance space flight.  In his far-sighted paper Beamed Propulsion by Gravitational Waves (JBIS, November 2011), Koichi Mori of Nagoya University in Japan investigated the potential for a transmitted beam of gravitational waves to be used to considerably reduce the flight time of a spacecraft, when compared with other means of propulsion, by deforming spacetime around the vessel, including even the theoretical possibility of faster-than-light travel.

Indeed, once we have control of the graviton, the universe could be opened up.  Never let it be said that any potential source of energy will go untapped.

Richard Hayes, Assistant Editor (Odyssey)

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