The Most Beautiful Theory Part III

Disappear like smoke

For the first time, general relativity was explaining new phenomena in the world. Bright young minds rushed into the field; wild ideas that had been speculated on in the fallow decades were buffed up and taken further. There was talk of “wormholes” in space-time that could connect seemingly distant parts of the universe. There were “closed time-like curves” that seemed as though they might make possible travel into the past. Less speculatively, but with more profound impact, Stephen Hawking, a physicist (pictured, with a quasar), and Roger Penrose, a mathematician, showed that relativistic descriptions of the singularities in black holes could be used to describe the Big Bang in which the expansion of the universe began—that they were, in fact, the only way to make sense of it. General relativity gave humans their first physical account of the creation.

Dr Hawking went on to bring elements of quantum theory into science’s understanding of the black hole. Quantum mechanics says that if you look at space on the tiniest of scales you will see a constant ferment in which pairs of particles pop into existence and then recombine into nothingness. Dr Hawking argued that when this happens at the event horizon of a black hole, some of the particles will be swallowed up, while some will escape. These escaping particles mean, in Dr Hawking’s words, that “black holes ain’t so black”—they give off what is now called “Hawking radiation”. The energy lost this way comes ultimately from the black hole itself, which gives up mass in the process. Thus, it seems, a black hole must eventually evaporate away to nothingness.

Adding quantum mechanics to the description of black holes was a step towards what has become perhaps the greatest challenge in theoretical physics: reconciling the theory used to describe all the fields and particles within the universe with the one that explains its overall shape. The two theories view reality in very different ways. In quantum theory everything is, at some scale, bitty. The equations of relativity are fundamentally smooth. Quantum mechanics deals exclusively in probabilities—not because of a lack of information, but because that is the way the world actually is. In relativity all is certain. And quantum mechanics is “non-local”; an object’s behaviour in one place can be “entangled” with that of an object kilometres or light-years away. Relativity is proudly local; Einstein was sure that the “spooky action at a distance” implied by quantum mechanics would disappear when a better understanding was reached.

It hasn’t. Experiment after experiment confirms the non-local nature of the physical world. Quantum theory has been stunningly successful in other ways, too. Quantum theories give richly interlinked accounts of electromagnetism and of the strong and weak nuclear forces—the processes that hold most atoms together and split some apart. This unified “standard model” now covers all observable forms of matter and all their interactions—except those due to gravity.

Some people might be satisfied just to let each theory be used for what it is good for and to worry no further. But people like that do not become theoretical physicists. Nor will they ever explain the intricacies of the Big Bang—a crucible to which grandiose theory-unifiers are ceaselessly drawn. In the very early universe space-time itself seems to have been subject to the sort of fluctuations fundamental to the quantum world (like those responsible for Hawking radiation). Getting to the heart of such shenanigans requires a theory that combines the two approaches. There have been many rich and subtle attempts at this. Dr Penrose has spent decades elaborating an elegant way of looking at all fields and particles as new mathematical entities called “twistors”. Others have pursued a way of adding quantum bittiness to the fabric of space-time under the rubric of “loop quantum gravity”. Then there is the “Exceptionally Simple Theory of Everything”—which isn’t. As Steven Weinberg, one of the unifiers whose work built the standard model, puts it, “There are so many theories and so few observations that we’re not getting very far.” Dr Weinberg, like many of his colleagues, fancies an approach called superstring theory. It is an outgrowth of an outgrowth of the standard model with various added features that seem as though they would help in the understanding of space-time and which its proponents find mathematically beguiling. Ed Witten of the Institute for Advanced Study (IAS) in Princeton, Einstein’s institutional home for the last 22 years of his life, is one of those who has raised it to its current favoured status. But he warns that much of the theory remains to be discovered, and that no-one knows how much. “We only understand bits and pieces—but the bits and pieces are staggeringly beautiful.”

This piecemeal progress, as Dr Witten tells it, offers a nice counterpoint to the process which led up to November 1915. “Einstein had the conception behind general relativity before he had the theory. That’s in part why it has stood: it was complete when it was formulated,” he says. “String theory is the opposite, with many manifestations discovered by happy accident decades ago.”

Entangled up in blue

And the happy accidents continue. In 1997 Juan Maldacena, an Argentine theoretician who now also works at the IAS, showed that there is a deep connection between formulations of quantum mechanics known as conformal field theories and solutions to the Einstein equations called anti-de Sitter spaces (similar to the expanding-universe solution derived by Willem de Sitter, but static and much favoured by string theorists). Neither provides an account of the real world, but the connection between them lets physicists recast intractable problems in quantum mechanics into the sort of equations found in general relativity, making them easier to crack.

This approach is being gainfully employed solving problems in materials science, superconductivity and quantum computing. It is also “influencing the field in a totally unexpected way,” says Leonard Susskind, of Stanford University. “It’s a shift in our tools and our methodology and our way of thinking about how phenomena are connected.” One possibility Dr Maldacena and Dr Susskind have developed by looking at things this way is that the “wormholes” relativity allows (which can be found in the anti-de Sitter space) may be the same thing as the entanglement between distant particles in quantum mechanics (which is part of the conformal field theory). The irony of Einstein’s spooky quantum bête noire playing such a crucial role has not gone unremarked.

There is more to the future of relativity, though, than its eventual subsumption into some still unforeseeable follow-up theory. As well as offering new ways of understanding the universe, it is also providing new ways of observing it.

End of the Most Beautiful Theory

Another form of relativity-assisted astronomy uses gravitation directly. Einstein’s equations predict that when masses accelerate around each other they will create ripples in space-time: gravitational waves. As with black holes and the expanding universe, Einstein was not keen on this idea. Again, later work has shown it to be true. A pair of neutron stars discovered spinning round each other in the 1970s are exactly the sort of system that should produce such waves. Because producing gravitational waves requires energy, it was realised that these neutron stars should be losing some. And so they proved to be—at exactly the rate that relativity predicts. This indirect but convincing discovery garnered a Nobel prize in 1993.

As yet, though, no one has seen a wave in action by catching the expansion and contraction of space that should be seen as one goes by, because the effects involved are ludicrously small. But researchers at America’s recently upgraded Laser Interferometer Gravitational-wave Observatory (LIGO) now think they can do it. At LIGO’s two facilities, one in Louisiana and one in Washington state, laser beams bounce up and down 4km-long tubes dozens of times before being combined in a detector to make a pattern. A passing gravitational wave that squashes space-time by a tiny fraction of the radius of an atomic nucleus in one arm but not the other will make a discernible change to that pattern. Comparing measurements at the two sites could give a sense of the wave’s direction.

Step into the light

The aim is not just to detect gravitational waves—though that would be a spectacular achievement—but to learn about the processes that produce them, such as mergers of neutron stars and black holes. The strengths of the warping effects in such cataclysms are unlike anything seen to date; their observation would provide a whole new type of test for the theory.

And history suggests there should be completely unanticipated discoveries, too. Kip Thorne, a specialist in relativity at the California Institute of Technology and co-founder of LIGO, says that “every time we’ve opened a new window on the cosmos with new radiation, there have been unexpected surprises”. For example, the pioneers of radio astronomy had no inkling that they would discover a universe full of quasars—and thus black holes. A future global array of gravitational-wave observatories could open a whole new branch of observational astronomy.

A century ago general relativity answered no-one’s questions except its creator’s. Many theories are hit upon by two or more people at almost the same time; but if Einstein had not devoted years to it, the curvature of space-time which is the essence of gravity might not have been discovered for decades. Now it has changed the way astronomers think about the universe, has challenged them to try and build theories to explain its origin, and even offered them new ways to inspect its contents. And still it retains what most commended it to Einstein: its singular beauty, revealed first to his eyes alone but appreciated today by all who have followed. “The Einstein equations of general relativity are his best epitaph and memorial,” Stephen Hawking has written. “They should last as long as the universe.”