General relativity's big year?

For the record, my op-ed in the International New York Times.

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You might think that physicists would be satisfied by now. They have been testing Einstein’s theory of general relativity (GR), which explains what gravity is, ever since he first described it one hundred 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, although GR 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 breaking point. But this determination to expose flaws isn’t really about skepticism, far less wanton nihilism. Most physicists are already convinced that GR 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 GR and conventional quantum theory emerge as excellent approximations just as Isaac Newton’s theory of gravity, posed in the late seventeenth century, works fine except in some extreme situations.

The hope is, then, that if we can find some dark corner of the universe where GR fails, perhaps because the gravitational fields it describes are so enormously strong, we might glimpse what extra ingredient is needed – one that might point the way to a theory of quantum gravity.

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 travelling 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. That’s one prediction that has now been thoroughly confirmed by the use of extremely accurate clocks on space satellites, and in fact GPS systems have to adjust their clocks to allow for it.

Einstein presented his theory of GR to the Prussian Academy of Sciences in 1915, although 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.

General relativity predicts that some burnt-out stars will collapse under their own gravity. They might become incredibly dense objects called neutron stars only a few miles across, from which a teaspoon of matter would weigh around 10 billion tons. Or they might collapse without limit into a “singularity”: a black hole, from whose immense gravitational field not even light can escape, since the surrounding space is so bent that light just turns back on itself. Many neutron stars have now been seen by astronomers: some, called pulsars, rotate and send out beams of intense radio waves from their magnetic poles, lighthouse beams that flash on an off with precise regularity when seen from afar. Black holes can only be seen indirectly from the X-rays and other radiation emitted by the hot gas that surrounds and is sucked into them. But astrophysicists are certain that they exist.

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 GR becomes indispensible. That’s also where it might be possible to test the limits of GR with astronomical investigations. Last year, astronomers at the National Radio Astronomy Observatory in Charlottesville, Virginia, discovered the first pulsar orbited by two other shrunken stars, called white dwarfs. This situation, with two bodies moving in the gravitational field of a third, should allow one of the central pillars of GR, called the strong equivalence principle, to be put to the test by making very detailed measurements of the effects of the white dwarfs on the pulsar’s metronome flashes as they circulate. The team hopes to carry out that study this year.

But the highest-profile test of GR 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 kilometres-long arms at right angles, like an L, to measure such minuscule contractions or stretches. Two of the several gravitational-wave detectors currently built – the American LIGO, with two observatories in Louisiana and Washington, and the European VIRGO in Italy – have just been upgraded to boost their sensitivity, and both will start searching in 2015. The European Space Agency is also launching a pilot mission for a space-based detector, called LISA Pathfinder, this September.

If we’re lucky, then, 2015 could be the year we confirm both the virtues and the limits of GR. 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 teats 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.