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General Relativity's Big Year?

If we're lucky, 2015 could be the year we confirm both the virtues and the limits of the theory

YOU might think that physicists would be satisfied by now. They have been testing Einstein's theory of general relativity, which explains what gravity is, ever since he first described it 100 years ago this year. And not once has it been found wanting. But they are still investigating its predictions to the nth decimal place, and this centenary year should see some particularly stringent tests. Perhaps one will uncover the first tiny flaw in this awesome mathematical edifice.

Stranger still is that, though general relativity is celebrated and revered among physicists like no other theory in science, they would doubtless react with joy if it is proved to fail. That's science: You produce a smart idea and then test it to its breaking point. But this determination to expose flaws isn't really about skepticism, far less wanton nihilism. Most physicists are already convinced that general relativity is not the final word on gravity. That's because the theory, which is applied mostly at the scale of stars and galaxies, doesn't mesh with quantum theory, the other cornerstone of modern physics, which describes the ultra-small world of atoms and subatomic particles. It's suspected that underlying both theories is a theory of quantum gravity, from which general relativity and conventional quantum theory emerge as excellent approximations just as Isaac Newton's theory of gravity, posed in the late 17th century, works fine except in some extreme situations.


General relativity was not just the last of Einstein's truly magnificent ideas, but arguably the greatest of them. His 'annus mirabilis' is usually cited as 1905, when, among other things, he kick-started quantum theory and came up with special relativity, describing the distortion of time and space caused by traveling close to the speed of light. General relativity offered a broader picture, embracing motion that changes speed, such as objects accelerating as they fall in a gravitational field. Einstein explained that gravity can be thought of as curvature induced in the very fabric of time and space by the presence of a mass. This, too, distorts time: Clocks run slower in a strong gravitational field than they do in empty space.

Einstein presented his theory of general relativity to the Prussian Academy of Sciences in 1915, though it wasn't officially published until the following year. The theory also predicted that light rays will be bent by strong gravitational fields. In 1919 the British astronomer Arthur Eddington confirmed that idea by making careful observations of the positions of stars whose light passes close to the sun during a total solar eclipse. The discovery assured Einstein as an international celebrity. When he met Charlie Chaplin in 1931, Chaplin is said to have told Einstein that the crowds cheered them both because everyone understood him and no one understood Einstein.

While Newton's theory of gravity is mostly good enough to describe the motions of the solar system, it is around very dense objects like pulsars and black holes that general relativity becomes indispensable. That's also where it might be possible to test the limits of the theory with astronomical investigations. But the highest-profile test of general relativity is the search for gravitational waves. The theory predicts that some astrophysical processes involving very massive bodies, such as supernovae (exploding stars) or pulsars orbited by another star (binary pulsars), should excite ripples in space-time that radiate outwards as waves. The first binary pulsar was discovered in 1974, and we now know the two bodies are getting slowly closer at just the rate expected if they are losing energy by radiating gravitational waves.

The real goal, though, is to see such waves directly from the tiny distortions of space that they induce as they ripple past our planet. Gravitational-wave detectors use lasers bouncing off mirrors in two-kilometre-long arms at right angles, like an L, to measure such minuscule contractions or stretches. Two of the several gravitational-wave detectors currently built have just been upgraded to boost their sensitivity, and both will start searching in 2015.

If we're lucky, then, 2015 could be the year we confirm both the virtues and the limits of general relativity. But neither will do much to alter the esteem with which it is regarded. The Austrian-Swiss physicist Wolfgang Pauli called it"probably the most beautiful of all existing theories." Many physicists (including Einstein himself) believed it not so much because of the experimental tests but because of what they perceived as its elegance and simplicity. Anyone working on quantum gravity knows that it is a very hard act to follow.

(Philip Ball is a science writer and a former editor at Nature. His most recent book is 'Invisible: The Dangerous Allure of the Unseen'.)

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Publication:Qatar Tribune (Doha, Qatar)
Date:Feb 9, 2015
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