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(Image: Advanced LIGO)

Update: Since this article was originally published, gravitational waves have been detected in a major breakthrough by the LIGO experiment.

EINSTEIN was a prolific predictor of new phenomena, often relying on ingenious thought experiments to turn reality on its head. Almost as often, he had difficulty coming to terms with the results.

Gravitational waves are a case in point. And although Einstein eventually accepted that these oscillations in space-time could exist, they remain the only major prediction of general relativity still to be verified. The latest and best detector dedicated to finding them has just come online. If nothing’s there, something is deeply awry. “It would be absolutely mystifying,” says of the Max Planck Institute for Gravitational Physics in Potsdam, Germany. “We cannot not see them.”

Matter causes space-time to curve, says general relativity. When a large mass accelerates, that curvature should change. The result is ripples in space-time that spread out at the speed of light, just as electromagnetic waves generated by accelerating electric charges spread.

Except not quite: rather than propagating through space-time as electromagnetic waves do, gravitational waves are contractions and expansions of space-time itself. Because gravity is much weaker than electromagnetism, they are also minuscule by comparison.

All of that makes calculations with gravitational waves rather hairy. Einstein realised straight away that the equations of general relativity had wave-like solutions, and in 1918 he derived a formula that allowed him to estimate how much energy these waves should carry. But he regarded the waves as unphysical, and general relativity’s core equations are so intractable that controversy persisted for decades over whether the formula was even theoretically sound.

“With gravitational waves we could peer back, maybe even to the big bang”

Even if sound, the formula suggested that only the most massive objects in the universe could produce a detectable signal: two black holes or neutron stars in a tango, for example. A typical gravitational wave would distort surrounding objects by less than one part in a billion trillion as it passed through Earth. Detecting such small displacements is akin to measuring the distance between Earth and the sun to the accuracy of an atomic radius.

Wave hello

It was only after Einstein’s death that gravitational waves became widely accepted. Experimentalists duly built detectors, initially large suspended cylinders that might be nudged by a passing wave. In the late 1960s, US physicist Joseph Weber was the first to claim a sighting. More than a dozen such claims followed, but none stood up to scrutiny.

To a theorist such as Buonanno, our failure to detect gravitational waves directly is academic: we already have overwhelming indirect proof that they exist. In 1974, astronomers Russell Hulse and Joseph Taylor discovered a binary pulsar – an orbiting pair of neutron stars beaming out radio waves at precise intervals – and started tracking its rotation rate. By the early 1990s they had shown that the stars were losing energy at precisely the rate Einstein predicted they would if they were emitting gravitational waves. A handful of similar binary systems studied since then have confirmed this view.

The observations could just be spurious: perhaps some strange astrophysical process is tricking us into thinking the binaries are slowing down, says theorist of Cardiff University in the UK. Or perhaps gravitational waves are being emitted as predicted, but aren’t reaching us. But in this case, “it is not easy to come up with a theory that does one thing and not the other”, says Sathyaprakash.

The search for alternative proof continues. Pulsar timing arrays are one relatively new method. They chart the precise arrival times of radio waves from a series of fast-rotating binaries. If space-time momentarily wobbles, we ought to see a distinctive radio pattern – one now being hunted by the International Pulsar Timing Array, a global network of radio telescopes.

The Advanced Laser Interferometer Gravitational-Wave Observatory (Advanced LIGO), which turned on last month after a five-year upgrade of a previous detector, uses a more direct method. It bounces laser beams up and down detector arms that are kilometres long to spy distortions caused by passing gravitational waves. Its detectors in Louisiana and Washington state will work in sync with instruments in Germany (GEO600), Italy (VIRGO) and Japan (KAGRA).

Advanced LIGO is 10 times more sensitive than the detector it replaces, and can scan a volume of space more than a thousand times bigger. That means we’re now almost certain to strike success, believes of the University of Glasgow, UK: “I personally believe the advanced detectors will make a discovery.”

But that depends on there being enough sources of detectable gravitational waves out there that can be picked up on the ground. Astrophysical models put the expected annual “event rate” anywhere between less than one and more than 200. To guarantee a detection, says Hough, we need to go into space. The European Space Agency’s Evolved Laser Interferometer Space Antenna (eLISA) should do just that. Planned to fly in the mid-2030s, its three detectors will form a triangle with sides a million kilometres long. LISA Pathfinder, a probe to test the technology, is due to launch next month.

eLISA should be swamped with signals from even the weakest sources, and will be the make-or-break test. “We would need to wait for eLISA not seeing gravitational waves from well-defined binary systems before we could be definitive about general relativity being wrong,” says Hough.

The prize from a sighting would not just be further confirmation of Einstein’s theory, but a new type of astronomy that uses gravitational waves to peer much further back into the universe’s history than is possible with light – right to the big bang, perhaps. That would also allow us to witness the birth of black holes and other processes where a much-sought quantum theory of gravity would otherwise be needed to make sense of things. In anticipation of the space-time fog lifting, Hough and his collaborators are already planning upgrades to existing interferometers and new generations of instruments.

If we don’t see anything, the consequences are huge, and not just for general relativity. Gravitational waves are actually a subtle consequence of the special theory of relativity derived by Einstein in 1905, where they serve to prevent gravitational influences propagating across space-time instantaneously. Special relativity’s precepts have also been incorporated into other queries, such as the quantum field theories that describe the other forces of nature. “If we fail to discover gravitational waves from a source which we know for sure is within our horizon, then that would be a massive blow to not just general relativity, but many of its alternatives,” says Sathyaprakash.

Read more: ” General relativity at 100: Einstein’s unfinished masterpiece”

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