Retelling the story of gravity
(appeared in Oct 2017)

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Gravity is perhaps the first sensation that a new-born becomes aware of, says S.Ananthanarayanan.

After a tranquil nine months of suspension in the placental fluid, the new-born experiences the sensation of up and down. But it took many centuries for humans to formulate gravity as not just a feeling but a force that acted between things that had mass. A first triumph of classical physics was to recognise that it was this same mass, which gave rise to gravity, that gave an object its inertia and was the basis of the laws of motion.

The Nobel prize for physics for 2017, to Rainer Weiss, emeritus physicist at MIT and Barry Barish and Kip Thorne, from Caltech, is for their role in the LIGO experiment, which has detected waves of gravity, very faint emissions from cataclysmic events 1.3 billion light years away. While the prize has gone to three persons who played major roles in the actual detection in 2015, the work itself was the culmination of many who participated in the quest set off by Einstein’s General Theory of Relativity a century earlier.

Albert Einstein refined the understanding of the laws of motion with a revolutionary re-interpretation of the nature of space and time. While this led to the equivalence of mass and energy, Einstein went on to relate the acceleration that objects feel when they attract each other to a curvature of the space itself in the vicinity of massive objects. The body attracted was thus reacting to the geometry of its local space, rather than an object at a distance.

Einstein’s first formulation, published in 1905, was about the motion of objects without considering the effect of gravity. The work was hence called the Special Theory of Relativity. The theory brings out that when speeds are comparable to that of light, speeds do not add up in the way that we are familiar with, but always to a little less, and the total can never equal the speed of light. A well-known observation at the time was that although the earth is hurtling through space at the speed of 30 km a second, the speed of light stayed the same in all directions. Einstein shows that this is because the measurement of lengths and time are observer- dependent, whose effects become appreciable when the observer is moving fast enough.

This strange effect of a thing not going sufficiently faster although pushed by a strong force dovetails with the apparent mass of a moving object being more than the mass of a stationary one. The work done by the push then gets accounted for in the greater energy of a heavier body in motion. This idea, of energy and mass being the same thing but from different viewpoints, gets formalised into the energy content of space and time and a different geometry of space when a massive body is present.

This view of gravity was published as the General Theory of Relativity, in a celebrated paper of 1915, which used the ideas to reconcile a nagging problem of the time, of anomalies in the orbit the planet Mercury, as observed and as worked out by the conventional laws of motion. The revolutionary ideas of Relativity, packaged with a solution to a flesh and blood problem that had puzzled astronomers, took the scientific world by storm. Despite the paper being written in German, and during World War I, an expedition to verify the theory was organised just after the war ended.

That massive objects would bend the fabric of space implied that straight lines should curve towards the object when they went past. Specifically, starlight shining past a large object should get bent around the star, and deviate. A total solar eclipse had been forecast for May 1919 and this was an opportunity to test the idea. If space were bent near the sun, then stars that were normally hidden should become visible at the moment of totality! English astronomers Sir Arthur Eddington and Sir Frank Dyson led expeditions to South Africa and Brazil to photograph the star field during the eclipse and sure enough, images of stars that should have been hidden sprang into view!

Gravity waves

The properties of electric and magnetic forces, which grew, like gravity, weaker by the square of the distance, had been elegantly described by the laws of electromagnetism. There were electrical forces from isolated charges, magnetic effects of moving charges and currents of charges set in motion by changing magnetic fields. And in consequence, acceleration of a charge or a magnet gave rise to a series of reciprocal effects, in the form of an electromagnetic wave, which carried energy away from the accelerated system.

As positive and negative gravitational charges were not known, the forces of gravity could not be described in the same way. For all that, the mathematics with which Einstein had treated the energy associated with a gravitational field allowed for energy in rapid acceleration of masses to propagate in the form of waves. Electric and magnetic forces are comparatively strong forces and the forces between the smallest electric charges can very strongly bind the lightweight particles found inside the atom. The forces can similarly be harnessed to generate power to light cities or run heavy machinery. In contrast, gravitational forces are millions of times weaker. While, it is true that gravitational forces keep the planets in orbit around the sun and the stars, the force of gravity between objects of everyday use is infinitesimal. The strength of gravity waves, which would manifest as some miniscule and instantaneous variation in the dimensions of space, is hence unimaginably feeble. The equations of Einstein did say that such waves could exist, and would travel with the speed of light, but nobody imagined that they would ever be found!

This was till the discoveryof exceedingly dense objects in the cosmos, like pulsars and black holes. The force of gravity at the surface of a black hole bends space in such a way that light itself cannot escape. The first evidence for the existence of gravity waves was found in 1978, when a pair of neutron stars orbiting each other was found to be losing energy and spiralling in. The rate of slowing was exactly what it should be if they were radiating gravity waves.

The drive to detect gravity waves has been with the experiment, LIGO (laser Interferometer Gravitational-wave Observatory), a pair of L-shaped, 4-kilometre-long light paths, one in Louisiana and one in Washington. A laser beam is shone, with the help of beam splitter, down each of the L-shaped arms of the arrangement, and reflected back and forth, 280 times, to increase the effective length, before the beams are brought together. As the arms are of equal length, the beams should travel for the same time and there should be no interference, or creation of dark and light fringes when the beams are received in a telescope.

If a gravity wave were to pass through the earth, however, one of the arms of the L is going to get just a little shorter, while the other gets longer, for just an instant. When this happens, the light paths will not be equal and the arrangement will detect the difference in the times of arrival of the laser beams.

As the path difference that a gravity wave will cause is miniscule indeed, however, we can imagine that an event can easily be swamped by all kinds of false signals, or ‘noise’. LIGO, which has been in development now for 30 years, incorporates all kinds of noise suppressing or compensating devices, apart from the light path being in high vacuum. The arrangement can now detect changes of one part in a thousand billion billion (1021). And two arrangements, separated by 3,000 kilometres, are used so that only events that are reported in both are counted, to eliminate false signals in either.

A third detector, VIRGO, has now been added in Italy and two more are being added, one in Japan and one in India.

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