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Daedalus

Gravity waving to us

Physicists are still exasperated by the elusiveness of gravitational waves. Laser interferometers and resonating metal bars have so far failed to detect them. Einstein showed that a gravitational field can be detected by its bending of light. A distant galaxy in the same line of sight as a nearer one can form an ‘Einstein ring’ image around it; the light is deviated in passing through the nearer galaxy's gravitational field. If an Einstein ring is ever found around an empty patch of sky, a clump of dark matter will have been discovered. Similarly, says Daedalus, the light from a distant star or galaxy should be deviated in phase with any gravitational waves through which that light passes. An observer would see the object apparently vibrating back-and-forth in the sky, in time with the gravitational waves.

The effect would be extremely small, and is probably below the resolution of the best modern telescopes. But Daedalus is undismayed. He points out that phase-sensitive detection can extract a periodic signal from vastly greater amplitudes of noise, provided the frequency and phase of the signal are known. Several types of astronomical object, such as spinning pulsars and tightly orbiting binary neutron stars, should radiate gravitational waves at their own rotational frequency. So he wants to train a big telescope on such an object, and study the image of a distant star or galaxy close to it in the sky. The image should be vibrating ever so slightly in time with the rotation frequency of the gravitational source. Demodulate the position-signal of that star with a phase-sensitive detector locked to the rotating source, and the amplitude and direction of the vibration should slowly emerge out of the optical noise.

This elegant scheme only works with gravitational waves of known period. But once the method has been well worked out, it should be possible to select the image of a distant galaxy, and to try decoding its position-fluctuation with a whole range of likely frequencies and phases. The occasional positive result would indicate the presence nearby of some gravitational source with that frequency and phase. The real prize would be to detect several such objects, all vibrating in the same phase as some invisible central source of gravity waves. This would reveal dark matter in the form of two extinct neutron stars, dead and dark but still locked in each other's gravitational embrace, and still waltzing endlessly round each other.

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Jones, D. Gravity waving to us. Nature 393, 24 (1998). https://doi.org/10.1038/29895

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