In 1916, as he worked on his theory of relativity, Albert Einstein predicted that, as a mass moved, its gravity would cause ripples in the fabric of spacetime. Every mass would do this, although larger masses would make bigger waves, in the same way that a pebble dropped in a pond makes an ever-increasing circle of ripples, while a meteor impacting the ocean creates tsunami-sized waves.
In 2016, 100 years after Einstein’s predictions, a collaboration between scientists working under the name LIGO announced that they had discovered these ripples, or gravitational waves. Their decades-long search had revealed the gravitational equivalent of tsunamis created by two black holes spiraling around each other and then colliding.
It is hoped that the discovery of gravitational waves will provide a new way of observing the universe. Instead of using light or other electromagnetic radiation, astronomers are hoping to map the universe by the gravitational effects of its contents. While radiation is obscured in many ways, including by the opaque plasma of the early universe up to 380,000 years after the Big Bang, gravitational waves pass through everything. This means that gravitational astronomy could see back to the very beginning of time, a trillionth of a second after the Big Bang.
LIGO stands for Laser Interferometer Gravitational-Wave Observatory. It is a remarkable set of instruments for measuring expansions and contractions in space itself. This is no easy task. A ruler cannot do it because, as space changes in size, so does the ruler, so the observer measures no change at all. LIGO succeeded using the benchmark that remains constant whatever space is doing: the speed of light. Light behaves like a wave, but it does not require a medium through which to travel. Instead, light (and any kind of electromagnetic radiation) is an oscillation of an electromagnetic field: in other words, light is a disturbance in a field permeating all of space.
Gravitational waves can be understood as disturbances in the gravitational field that permeates the universe. Einstein described how these disturbances are caused by the mass of objects curving the space around them. What is understood as the “pull of gravity” is a small mass appearing to alter its motion and “fall” toward a larger mass as it encounters a region of warped space.
All masses are in motion—planets, stars, even galaxies—and as they move, they leave a trail of gravitational disturbances in their wake. Gravity waves propagate in a comparable way to sound waves, by distorting the medium through which they travel. In the case of sound waves, that medium is made of molecules, which are made to oscillate. In the case of gravity, the medium is spacetime, the very fabric of the universe. Einstein predicted that the speed of gravity would be the same as the speed of light, and that the ripples in spacetime would move outward in all directions. The intensity of these ripples diminishes rapidly with distance (by a square of the distance), so detecting a distinct gravity wave from a known object far out in space would require a very powerful source of waves and a very sensitive instrument.
As its name suggests, LIGO employs a technique called laser interferometry. This makes use of a property of waves called interference. When two waves meet, they interfere with one another to create a single wave. How they do this depends on their phase—the relative timing of their oscillations. If the waves are exactly in phase—rising and falling perfectly in sync—they will interfere constructively, merging to create a wave with double the intensity. By contrast, if the waves are exactly out of phase—one rising as the other falls—the interference will be destructive. The two waves will merge and cancel one another out, disappearing completely.
LIGO’s source of waves is a laser, which is a light beam that contains a single color, or wavelength, of light. In addition, the light in a laser beam is coherent, which means that its oscillations are all perfectly in time. Such beams can be made to interfere with one another in very precise ways.
The laser beam is split in two and the resulting beams are sent off perpendicular to one another. They both hit a mirror and bounce straight back to the starting point. The distance traveled by each beam is very precisely controlled so that one has to travel exactly half a wavelength farther than the other (a difference of a few hundred billionths of a meter). When the beams meet each other again, they are exactly out of phase as they interfere, and promptly disappear—unless a gravitational wave has passed through space while the beams were traveling. If present, a gravitational wave would stretch one of the laser tracks and compress the other, so the beams would end up traveling slightly altered distances.
The laser beams are split and sent on a 695-mile (1,120-km) journey up and down LIGO’s 2.5-mile (4-km) long arms before being recombined. This gives LIGO the sensitivity to detect minute perturbations in space that add up to a few thousandths of the width of a proton. With the distances put very slightly out of sync, the interfering beams would no longer cancel each other out. Instead, they would create a flickering pattern of light, perhaps indicating a gravity wave passing through LIGO’s corner of space.
The difficulty was that such a sensitive detector was prone to distortions from the frequent seismic waves that run through Earth’s surface. To be sure that a laser flicker was not an earth tremor, two identical detectors were built at opposite ends of the United States: one in Louisiana, the other in Washington state. Only signals registering on both detectors were gravitational waves (the signals are in fact 10 milliseconds apart—the time it takes for light, and gravitational waves, to travel from Louisiana to Washington). Ligo operated from 2002–2010 with no success, then started up again in 2015 with enhanced sensitivity.
Colliding black holes
On September 14, 2015, at 9:50:45 GMT, two black holes a billion lightyears away collided and unleashed huge warps in the fabric of space. In fact, this event occurred a billion years ago but it had taken that long for the ripples they had released to reach Earth—where they were detected by both LIGO detectors. The researchers took another few months to check their result and went public in February 2016.
The search is now on for more gravitational waves, and the best place to do it is from space. In December 2015, the spacecraft LISA Pathfinder was launched. It is headed to an orbit at L1, which is a gravitationally stable position between the sun and Earth. There, the spacecraft will test laser interferometry instruments in space, in the hope that they can be used in an ambitious experiment called eLISA (evolved Laser Interferometer Space Antenna). Provisionally scheduled for 2034, eLISA will use three spacecraft triangulated around the sun. Lasers will be fired between the spacecraft,
making a laser track 2 million miles (3 million km) long that is many times more sensitive to gravitational waves than LIGO.
The discovery of gravitational waves has the potential to transform astronomers’ view of the universe. The patterns in the fluctuations in the light signals from LIGO and future projects will produce new information, providing a detailed map of mass across the universe.
“Gravitational waves will bring us exquisitely accurate maps of black holes—maps of their spacetime.” Kip Thorne
RAI WEISS AND KIP THORN
LIGO is a collaboration between Caltech and MIT, and also shares its data with a similar experiment called Virgo, which is running in France and Poland. Hundreds of researchers have contributed to the discovery of gravitational waves. However, there are two people, both Americans, who stand out among them all: Rainer “Rai” Weiss (1932–) and Kip Thorne (1940–). In 1967, while at MIT, Weiss developed the laser interferometry technique used by LIGO, working from the initial ideas of Joseph Weber, one of the inventors of the laser. In 1984, Weiss cofounded LIGO with Thorne, a counterpart at Caltech, who is a leading expert on the theory of relativity. LIGO is the most expensive science project ever funded by the US government, with a current cost of $1.1 billion. After 32 years of trying, in 2016 Weiss and Thorne announced their discovery of gravitational waves at a news conference in Washington, D.C.