One hundred years ago, Albert Einstein presented a paper to the Prussian Academy of Sciences that explained gravity. It is one of the four funda- mental forces in the universe, although in 1915 only one of the others—the electromagnetic force—was known. (The other two act inside the atomic nucleus.) But Einstein’s paper offered a radically different way of thinking about gravity. Rather than being an invisible force between two massive objects, he described it as a distortion induced by the masses in time and space themselves—which is to say, in the smooth fabric called spacetime.
This warping dictates the paths that objects take under gravity’s influence: Isaac Newton’s apple fell to Earth because it was, in effect, slipping down the slope of the dent that the planet’s mass induces in the fabric of spacetime. In the space curved by the sun’s gravity, the planets execute orbits rather like marbles running around the rim of a bowl.
This geometric interpretation of gravity is the central idea of Einstein’s theory of general relativity. It is widely considered to be not only his greatest intellectual achievement but also the epitome of a beautiful theory. The physicist Ernest Rutherford said that the theory “cannot but be regarded as a magnificent work of art.” Einstein was not shy of advertising its virtues himself: “Scarcely anyone who fully understands this theory can escape from its magic,” he wrote. But the centenary celebrations for general relativity will not simply be looking back, for 2015 will be a banner year for some big, ambitious experiments that aim to probe the theory. They are looking for one of the most spectacular of its predictions: ripples in spacetime called gravitational waves.
General relativity (GR) was the fruit of Einstein’s efforts to incorporate gravity into his theory of special relativity, which he had unveiled in 1905. Special relativity introduced the idea of spacetime as a unified fabric, and it explained the consequences of Einstein’s insight that the speed of light in a vacuum remains the same for all observers, irrespective of their own speed. It led to some extraordinary conclusions: that mass and energy are interconvertible, and that at very high speeds objects gain mass and contract in shape while experiencing time as passing more slowly. But all this was true only for steady speeds, whereas gravity induces changes of speed: acceleration. So GR was an extension—a generalisation—of special relativity. Einstein concluded that gravity too dilates time and space: close to a very massive body, clocks run slower. The theory was “verified” very quickly, for in 1919 the British astronomer Arthur Eddington showed during a total eclipse of the sun that starlight is bent by the sun’s gravitational field just as Einstein predicted. The finding sealed Einstein’s international celebrity, but he himself was cockily phlegmatic about Eddington’s result, saying that if the observations hadn’t proved him right then he should “feel sorry for the dear Lord, for the theory is correct.”
"Finding a theory of quantum gravity—is one of the greatest challenges facing modern science"Yet it wasn’t until the 1960s that scientists—particularly astronomers and astrophysicists—started to get to grips with its implications. That, perhaps, explains why Einstein never won a second Nobel Prize for work that, in physicist Paul Dirac’s assessment, was “probably the greatest scientific discovery that was ever made.”
Yet even a century after the theory’s inception, scientists are looking for new ways to test it. That’s not because anyone doubts its validity. But for one thing, general relativity remains stubbornly incompatible with quantum theory, the other conceptual framework underpinning modern fundamental physics. So by pushing GR to its limits, it should be possible to see cracks that might offer clues about where quantum theory comes in. Solving that question—finding a theory of quantum gravity—is one of the greatest challenges facing modern science. String theory, which attempts to describe fundamental particles as one-dimensional vibrating objects called strings, provides one avenue, but it’s not alone and not without problems.
The other reason general relativity is very much an active field of research is that it makes predictions that are still to be verified. One of them is black holes. When some stars run out of fuel, they are predicted to collapse in a runaway process that ends up concentrating the mass in an infinitesimally small volume of space, creating a so-called singularity in spacetime. The gravitational field—the curvature of spacetime—around such an object is so intense that not even light can escape. Black holes are almost universally accepted among astronomers and there is plenty of indirect evidence for them. But there are still many mysteries about how they behave, and scientists of the calibre of Stephen Hawking and Roger Penrose made their reputations investigating them.
The other key prediction of general relativity is gravitational waves (GWs). If extremely massive bodies undergo violent events—if two black holes or neutron stars (super-dense, partly collapsed stars) were to collide, say—GR predicts that the resulting distortions in spacetime should ripple outwards as waves.
There is already indirect evidence that GWs exist. In particular, when in 1974 astronomers Russell Hulse and Joseph Taylor discovered binary pulsars—paired neutron stars orbiting each other while one of them sends out beams of radio waves that look pulsed because the star is spinning like a lighthouse—they realised that the pulsar’s orbit should be steadily shrinking, thanks to the energy radiated away as GWs. Taylor and his co-workers measured this orbital decay (which can be deduced by precise measurement of the pulsar’s pulsing rate) and reported in 1978 that it fits exactly with what is expected of GW emission as predicted by general relativity. It was this, as much as the discovery itself, that won Hulse and Taylor the 1993 physics Nobel Prize.
There was great excitement last March when observations made using a telescope at the South Pole called BICEP2 were said to show evidence of GWs produced in the Big Bang itself. The idea is that a period of unspeakably rapid expansion of the universe in its first instants, known as inflation, left spacetime ringing with such wild GWs that their signature is even now imprinted in the microwave radiation that permeates all of space as a kind of afterglow of this cosmic creation. The BICEP2 team thought that they had detected this imprint of primordial GWs, but it now seems likely that they were misled by the obscuring effects of dust scattered through our own galaxy. Doubts about the BICEP2 claims stemmed largely from measurements made by a satellite instrument called Planck, operated by the European Space Agency, which is mapping out the cosmic microwave background in great detail and can distinguish the effects of galactic dust. The BICEP2 and Planck teams are now collaborating to see if any GW signature can be detected after all.
Even if the BICEP2 results had stacked up, the evidence for GWs would be indirect. But several experiments are hoping to identify them directly as they ripple past our own neighbourhood. As a gravitational wave passes by, space itself changes shape, getting compressed or stretched. These changes of shape would be incredibly tiny for GWs reaching us from distant cosmic cataclysms: they would typically alter lengths by about a thousandth of the diameter of a proton, one of the particles in an atom’s minuscule nucleus. But this might be detectable by comparing changes in the distance a light beam travels perpendicular to the wave and parallel to it. Tiny changes in this distance can be detected by looking for interference effects between light waves in a laser beam as they travel for perhaps several kilometres through straight channels and bounce back from a mirror at the far end.
That’s the principle behind GW detectors such as the Laser Interferometer Gravitational-Wave Observatory (Ligo), operated by the US National Science Foundation. The project has two observatories: one in Livingston, Louisiana, the other at Hanford in Washington state. The arms of the L-shaped instruments are 4km long, and with these two detectors 3,000km apart it should be possible to locate the source of a GW in the sky by triangulation.
Although Ligo has been operating for over 10 years, the fact that so far it has failed to see GWs surprises no one: the instruments needed better sensitivity. To this end, both the Livingston and Hanford sites have just finished installing new, improved detectors housed in the original arms, collectively known as Advanced Ligo. The refitting, involving more powerful lasers, better isolation from seismic disturbances and other improvements, began in 2010 and is now complete. The new instruments are being gradually brought up to their design specification—a process that will take two or three years, according to David Reitze, Executive Director of the Ligo Laboratory at the California Institute of Technology in Pasadena. Nonetheless, the Livingston instrument, at least, will be ready to start doing real science by the summer or autumn of 2015.
Reitze is confident that Advanced Ligo should see GWs once it reaches full sensitivity. He says that they expect to see around 10 to 50 sources consisting of twinned (binary) neutron stars, as well as an uncertain number of black-hole pairs merging together.
Ligo, and other interferometric detectors such as the European Virgo facility in Italy, which is also restarting this year after an upgrade, aren’t our only hope of seeing GWs. We can look for them from space, too. The Laser Interferometer Space Antenna (Lisa) was a satellite designed to search for GWs using the same principles as Ligo: measuring tiny distance changes via interference effects in laser light bouncing along two arms. But in space, Lisa’s arms could be a few million kilometres long, improving the sensitivity. The beams would pass through empty space between three spacecraft arranged at the corners of an immense equilateral triangle while circling the sun in an Earth-like orbit.
But Lisa ran into problems in 2011 when the US space agency Nasa, which had been collaborating with the European Space Agency (ESA) on the mission, withdrew because of funding problems. The ESA has elected to go it alone with a related mission called the Gravitational Universe, tentatively scheduled for 2034 and with precise goals yet to be decided.
We don’t have to wait quite that long for things to kick off—this summer the European Space Agency plans to launch a pilot mission called Lisa Pathfinder. As curtain-raisers go, it could hardly be more modest: the detector has only one arm, and that’s just 38cm long. The point, however, is to test the basic instrumentation: that the spacecraft can be controlled in the proposed way, that the detection equipment will work, and that it will be robust enough.
With Ligo and the other GW detectors already up and running, one might wonder about the point of pursuing a big GW space mission two decades away. But it’s not just about proving that the waves exist: they could provide a new tool for astronomy. Just as radio, ultraviolet, X-ray and gamma-ray astronomy have all opened up new windows on the cosmos by searching the skies at new wavelengths, so too should GW observatories enable new modes of astronomy, telling us about the most extreme events in the universe: immense black holes at the centres of galaxies, and possibly even weird “cosmic strings” that are predicted by some theories to thread through the universe like gigantic kinks in spacetime. “The science return from a space-borne gravitational wave observatory would be outstanding”, says Paul McNamara, project scientist for Lisa Pathfinder. “Not only do we get to do astrophysics in a completely new way, but we also get to test general relativity in the strong gravitational fields around black holes.” Reitze adds that space-based GW detectors would be complementary to those on Earth, because they will look for waves of different frequency, produced by different types of source: binary star systems composed of white dwarfs rather than neutron stars, say.
It would not at all surprise me if general relativity turned out to be perfectly valid at all scales, from the cosmological to the astrophysical to the microscopic, failing only at the Planck scale.Gravitational waves aren’t the only means of testing general relativity. For example, four years ago a space-based experiment called Gravity Probe B finally delivered a verdict that had been four decades in the making: the experiment was first proposed in the 1960s, but not launched by Nasa until 2004. The spacecraft contained four spinning gyroscopes, to all intents and purposes just ordinary familiar gyroscopes of the kind children used to play with (albeit obviously a bit more stable and dependable). These were predicted by GR to develop two wobbles: one caused by the warping of spacetime in the Earth’s gravitational field, the other by the “dragging” of spacetime caused by the Earth’s rotation. Changes in the orientation of Gravity Probe B’s gyroscopes were measured by making the gyros magnetic and detecting tiny shifts in their poles. While the warping was confirmed quite quickly, verifying the dragging effect took some time because it was so small compared to the random “noise” in the instruments. Not until 2011 did the analysis confirm that the results matched the predictions of GR to within a few per cent.
The status of GR offers one of the best rejoinders to the misconceived accusation that scientific theories are in any sense “only a theory”—that is, just an idea. It has been tested far beyond any reasonable doubt. And yet we know it can’t be the final word, for it will surely have to be made compatible with quantum theory. It is generally assumed that this must make gravity a force mediated by quantum particles, just as electromagnetism is conveyed by particles of light radiation (photons). Those hypothetical particles are dubbed gravitons, and are expected, like photons, to have no mass. String theory offers a possible description of them, but there’s no immediate prospect of testing whether it is right. Either way, quantum gravity would be to GR what GR was to Newtonian gravity: it wouldn’t replace GR but would reveal it as an approximation that works well enough in most circumstances but breaks down at the smallest scales (the so-called Planck scale, much less than the size of atoms) where quantum effects must dominate. As physicist Clifford Will, one of the foremost experts on the modern theory of gravitation, says, “it would not at all surprise me if general relativity turned out to be perfectly valid at all scales, from the cosmological to the astrophysical to the microscopic, failing only at the Planck scale.” Einstein didn’t have the final theory of gravity, but he won’t be proved wrong.
