Ripples in spacetime: Science's 2016 Breakthrough of the Year

The discovery of ripples in spacetime—gravitational waves—shook the scientific world this year. It fulfilled a prediction made 100 years ago by Albert Einstein and capped a 40-year quest to spot the infinitesimal ripples. But instead of the end of the story, scientists see the discovery as the birth of a new field: gravitational wave astronomy.

In 1915, Einstein explained that gravity arises because massive bodies warp space and time, or spacetime, causing free- falling objects to follow curved paths such as the arc of a thrown ball or the elliptical orbit of a planet around its sun. Einstein then calculated that a barbell-shaped distribution of mass whirling end-to-end like a baton should radiate ripples in spacetime that zip along at light speed—gravitational waves. On 11 February, physicists working with the Laser Interferometer Gravitational-Wave Observatory (LIGO)—twin instruments in Hanford, Washington, and Livingston, Louisiana—announced that they had seen just what Einstein predicted: a burst of waves created as two black holes spiraled into each other 1.3 billion light- years away. The triumph was hard earned. Einstein himself vacillated for decades over whether gravitational waves should exist. Even if they did, the only source Einstein could imagine, two orbiting stars, would produce waves too feeble to detect. By the late 1960s, however, astrophysicists knew of much denser concentrations of mass. They had spotted neutron stars and dreamed up black holes, the ultraintense gravitational fields left behind when massive stars collapse to nothing. Spiraling together, such things could, in theory, produce observable waves. In 1972, Rainer Weiss, a physicist at the Massachusetts Institute of Technology in Cambridge, set out a scheme to detect them with L-shaped optical instruments called interferometers, sowing the seed for LIGO.

People’s choice Visitors to Science’s website were offered the chance to vote on a list of candidates for Breakthrough of the Year while Science editors and writers were finalizing their choices. A first round of voting narrowed the top candidates to five, and a second round, in which some 225,000 votes were cast, determined the final People’s Choice. In the end, a breakthrough in culture techniques that enabled researchers to keep human embryos developing in the lab for almost 2 weeks edged out Science’s top choice, the detection of gravitational waves. The full results: Human embryo culture 43% Gravitational waves 32% Portable DNA sequencers 13% AI beats Go champ 7% Worn-out cells and aging 5%

The discovery of ripples in spacetime—gravitational waves—shook the scientific world this year. It fulfilled a prediction made 100 years ago by Albert Einstein and capped a 40-year quest to spot the infinitesimal ripples. But instead of the end of the story, scientists see the discovery as the birth of a new field: gravitational wave astronomy.

In 1915, Einstein explained that gravity arises because massive bodies warp space and time, or spacetime, causing free- falling objects to follow curved paths such as the arc of a thrown ball or the elliptical orbit of a planet around its sun. Einstein then calculated that a barbell-shaped distribution of mass whirling end-to-end like a baton should radiate ripples in spacetime that zip along at light speed—gravitational waves. On 11 February, physicists working with the Laser Interferometer Gravitational-Wave Observatory (LIGO)—twin instruments in Hanford, Washington, and Livingston, Louisiana—announced that they had seen just what Einstein predicted: a burst of waves created as two black holes spiraled into each other 1.3 billion light- years away. The triumph was hard earned. Einstein himself vacillated for decades over whether gravitational waves should exist. Even if they did, the only source Einstein could imagine, two orbiting stars, would produce waves too feeble to detect. By the late 1960s, however, astrophysicists knew of much denser concentrations of mass. They had spotted neutron stars and dreamed up black holes, the ultraintense gravitational fields left behind when massive stars collapse to nothing. Spiraling together, such things could, in theory, produce observable waves. In 1972, Rainer Weiss, a physicist at the Massachusetts Institute of Technology in Cambridge, set out a scheme to detect them with L-shaped optical instruments called interferometers, sowing the seed for LIGO.

Each LIGO interferometer has two 4- kilometer-long arms with mirrors at either end, housed in a giant vacuum chamber. By bouncing laser light between the mirrors, physicists can compare the arms’ lengths to within 1/10,000 the width of a proton. A passing gravitational wave would generally stretch the arms by different amounts, and that’s what the LIGO team spotted. The tight fit between that first signal and computer modeling validated Einstein’s theory of gravity, known as general relativity, as never before.

Now, physicists are eagerly anticipating what may come next, because gravitational waves promise an entirely new way to peer into the cosmos. First, physicists hope to spot many more events. LIGO already has detected a second black hole merger and a third, weaker signal. The interferometers resumed taking data last month, and if they can reach their design sensitivity, they may eventually see a black hole merger on average once a day.

Other instruments will soon join the hunt. The upgraded VIRGO detector in Italy should turn on early next year. Physicists in Japan are building a detector called the Kamioka Gravitational Wave Detector, and LIGO physicists plan to add a detector in India in the early 2020s. Three or more detectors together should be able to pinpoint a source in the sky by triangulation. That could help telescopes home in on the same event, and perhaps detect other signals from it. For example, if gravitational wave detectors sense the merger of two neutron stars and telescopes pick up light or x-rays from it, the signals together might offer clues about the exotic matter in neutron stars.

The detectors might even test wilder ideas about black holes. Quantum theory suggests a black hole might contain a hidden “firewall” that would obliterate anything that falls in. If so, merging black holes should produce gravitational wave echoes, some theorists predict. Others speculate that a spinning black hole could generate a cloud of hypothetical particles called axions, which could generate gravitational waves by annihilating one another en masse.

Meanwhile, some astronomers are trying to detect gravitational waves in a different way. Within the hearts of large galaxies lurk supermassive black holes weighing hundreds of millions or billions of solar masses. When two such monsters merge, they radiate hugely powerful waves with wavelengths light-years long—thousands of times longer than instruments like LIGO can detect. To spot those waves, astronomers are turning to stellar timepieces called millisecond pulsars.

Pulsars—spinning neutron stars—emit incredibly regular pulses of radio waves. As long-wavelength gravitational waves buffet Earth, they should push the planet toward some pulsars and away from others. That motion would, in turn, shorten or stretch the time between pulses from pulsars in different directions, in an effect akin to the Doppler shift. The resulting variations and correlations among pulsars’ timing should reveal the cacophony of long-wavelength gravitational waves, and the spectrum of longer and shorter waves would help physicists trace the rate at which galaxies formed and merged throughout cosmic history. Teams in the United States, Europe, and Australia hope to see a signal within 2 or 3 years— although the U.S. effort is threatened by plans at the National Science Foundation to defund the two radio telescopes it uses.