Merger and acquisition: Gravitational waves have been detected for the first time
【《经济学人》原文】TWO black holes circle one another. Both are about 100km across. One contains 36 times as much mass as the sun; the other, 29. They are locked in an orbital dance, a kilometre or so apart, that is accelerating rapidly to within a whisker of the speed of light. Their event horizons—the spheres defining their points-of-no-return—touch. There is a violent wobble as, for an instant, quintillions upon quintillions of kilograms redistribute themselves. Then there is calm. In under a second, a larger black hole has been born.
It is, however, a hole that is less than the sum of its parts. Three suns’ worth of mass has been turned into energy, in the form of gravitational waves: travelling ripples that stretch and compress space, and thereby all in their path. During the merger’s final fifth of a second, envisaged in an artist’s impression above, the coalescing holes pumped 50 times more energy into space this way than the whole of the rest of the universe emitted in light, radio waves, X-rays and gamma rays combined.
And then, 1.3 billion years later, in September 2015, on a small planet orbiting an unregarded yellow sun, at facilities known to the planet’s inhabitants as the Advanced Laser Interferometer Gravitational-wave Observatory (LIGO), the faintest slice of those waves was caught. That slice, called GW150914 by LIGO’s masters and announced to the world on February 11th, is the first gravitational wave to be detected directly by human scientists. It is a triumph that has been a century in the making, opening a new window onto the universe and giving researchers a means to peer at hitherto inaccessible happenings, perhaps as far back in time as the Big Bang.
Finger on the pulsar
The idea of gravitational waves emerged from the general theory of relativity, Albert Einstein’s fundamental exposition of gravity, unveiled almost exactly 100 years before GW150914’s discovery. Mass, Einstein realised, deforms the space and time around itself. Gravity is the effect of this, the behaviour of objects dutifully moving along the curves of mass-warped spacetime. It is a simple idea, but the equations that give it mathematical heft are damnably hard to solve. Only by making certain approximations can solutions be found. And one such approximation led Einstein to an odd prediction: any accelerating mass should make ripples in spacetime.
Einstein was not happy with this idea. He would, himself, oscillate like a wave on the topic—rescinding and remaking his case, arguing for such waves and then, after redoing the sums, against them. But, while he and others stretched and squeezed the maths, experimentalists set about trying to catch the putative waves in the act of stretching and squeezing matter.
Their problem was that the expected effect was a transient change in dimensions equivalent to perhaps a thousandth of the width of a proton in an apparatus several kilometres across. Indirect proof of gravitational waves’ existence has been found over the years, most notably by measuring radio emissions from pairs of dead stars called pulsars that are orbiting one another, and deducing from this how the distance between them is shrinking as they broadcast gravitational waves into the cosmos. But the waves themselves proved elusive until the construction of LIGO.
As its name states, LIGO is an interferometer. It works by splitting a laser beam in two, sending the halves to and fro along paths identical in length but set at right angles to one another, and then looking for interference patterns when the halves are recombined (see diagram). If the half-beams’ paths are undisturbed, the waves will arrive at the detector in lock-step. But a passing gravitational wave will alternately stretch and compress the half-beams’ paths. Those half-beams, now out of step, will then interfere with each other at the detector in a way that tells of their experience. The shape of the resulting interference pattern contains all manner of information about the wave’s source, including what masses were involved and how far away it was.
To make absolutely certain that what is seen really is a gravitational wave requires taking great care. First, LIGO is actually two facilities, one in Louisiana and the other in Washington state. Only something which is observed almost, but not quite, simultaneously by both could possibly be a gravitational wave. Secondly, nearly everything in the interferometers’ arms is delicately suspended to isolate it as far as possible from distant seismic rumblings and the vibrations of passing traffic.
Moreover, in order to achieve the required sensitivity, each arm of each interferometer is 4km long and the half-beam in it is bounced 100 times between the mirrors at either end of the arm, to amplify any discrepancy when the half-beams are recombined. Even so, between 2002 when LIGO opened and 2010, when it was closed for upgrades, nary a wave was seen.
Those improvements, including doubling the bulk of the devices’ mirrors, suspending them yet more delicately, and increasing the laser power by a factor of 75, have made Advanced LIGO, as the revamped apparatus is known, four times as sensitive as the previous incarnation. That extra sensitivity paid off almost immediately. Indeed, the system’s operators were still kicking its metaphorical tyres and had yet to begin its official first run when GW150914 turned up, first at the Louisiana site, and about a hundredth of a second later in Washington—a difference which places the outburst somewhere in the sky’s southern hemisphere. Since then, the team have been checking their sums and counting their lucky stars. As they outline in Physical Review Letters, the likelihood that the signal was a fluke is infinitesimal.
When one result comes so quickly, others seem sure to follow—particularly as the four months of data the experiment went on to gather as part of the first official run have yet to be analysed fully. A rough estimate suggests one or two other signals as striking as GW150914 may lie within them.
For gravitational astronomy, this is just the beginning. Soon, LIGO will not be alone. By the end of the year VIRGO, a gravitational-wave observatory in Italy, should join it in its search. Another is under construction in Japan and talks are under way to create a fourth, in India. Most ambitiously, a fifth, orbiting, observatory, the Evolved Laser Interferometer Space Antenna, or e-LISA, is on the cards. The first pieces of apparatus designed to test the idea of e-LISA are already in space.
Together, by jointly forming a telescope that will permit astronomers to pinpoint whence the waves come, these devices will open a new vista on the universe. As technology improves, waves of lower frequency—corresponding to events involving larger masses—will become detectable. Eventually, astronomers should be able to peer at the first 380,000 years after the Big Bang, an epoch of history that remains inaccessible to every other kind of telescope yet designed.
The real prize, though, lies in proving Einstein wrong. For all its prescience, the theory of relativity is known to be incomplete because it is inconsistent with the other great 20th-century theory of physics, quantum mechanics. Many physicists suspect that it is in places where conditions are most extreme—the very places which launch gravitational waves—that the first chinks in relativity’s armour will be found, and with them a glimpse of a more all-embracing theory.
Gravitational waves, of which Einstein remained so uncertain, have provided direct evidence for black holes, about which he was long uncomfortable, and may yet yield a peek at the Big Bang, an event he knew his theory was inadequate to describe. They may now lead to his theory’s unseating. If so, its epitaph will be that in predicting gravitational waves, it predicted the means of its own demise.