Gravitational Waves: What You Should Know
On October 3, the Royal Swedish Academy of Sciences awarded physicists Rainer Weiss, Kip Thorne, and Barry Barish the Nobel Prize in physics for directly detecting gravitational waves—wrinkles in space-time predicted more than a century ago by Einstein’s theory of general relativity, but which had been stubbornly elusive until 2015.
Judging from the fanfare that surrounded the first detection's 2016 announcement, this is perhaps the least surprising physics Nobel since 2013, when physicists François Englert and Peter Higgs won for theorizing the Higgs boson.
“For as long as 40 years, people have been thinking about this, trying to make a detection, sometimes failing in the early days, and then slowly but surely getting the technology together to be able to do it,” Weiss said. “It’s very, very exciting that it worked out in the end that we are actually detecting things, and actually adding to the knowledge, through gravitational waves, of what goes on in the universe.”
Weiss, of MIT, and Caltech’s Thorne and Barish played an instrumental role in bringing to fruition one of the most ambitious (and expensive) experiments of the last couple of decades: The Laser Interferometer Gravitational-Wave Observatory. In September 2015, LIGO’s two sprawling detectors heard the gentle chirp caused by two black holes that collided more than a billion years ago.
The power of that collision kinked the fabric of space-time, producing ripples in it that, traveling at the speed of light, took more than a billion years to nearly imperceptibly change the distances between two sets of mirrors in each of LIGO’s detectors.
“This year’s prize is about a discovery that shook the world,” says Göran Hanssen, the Swedish Academy’s secretary general. The Nobel Foundation awarded half of the million-dollar prize to Weiss, and the other half to Barish and Thorne, “for decisive contributions to the LIGO detector and the observation of gravitational waves.”
Here’s a short primer on these cosmic ripples.
What are gravitational waves?
Put simply, gravitational waves are ripples in the fabric of space-time produced by the most violent phenomena the cosmos can offer – things like exploding stars, collisions between ultra-dense neutron stars or merging black holes.
These cosmic cataclysms are so energetic they radiate gravitational waves, which we can directly observe as distortions in the otherwise tough, stiff fabric of space-time. Gravitational waves are washing over Earth all the time, but experiments have not been sensitive enough to detect them until very recently.
Why are they so hard to detect?
By the time gravitational waves reach us from the distant events that spawn them, they distort space-time by an utterly minuscule amount. A gravitational wave passing through Earth will alternately stretch and squeeze space along two axes, but this distortion is many times smaller than the width of a proton, one of the particles in an atom’s nucleus. Measuring such minute changes in length is pretty much impossible for most instruments.
So how do scientists detect gravitational waves?
Scientists first directly observed gravitational waves with LIGO, the Laser Interferometer Gravitational-Wave Observatory, which is funded by the National Science Foundation.
This U.S. facility consists of two identical L-shaped detectors in Washington state and Louisiana, each of which employs lasers and mirrors to measure tiny changes in space-time made by passing gravitational radiation. It’s the most sensitive measuring device on the planet, with each arm of the L measuring roughly 2.5 miles end-to-end. The LIGO Science Collaboration is similarly huge, comprising more than 1,000 scientists.
For detecting gravitational waves, the name of the game is the change in distance between mirrors parked at each end of those perpendicular, 2.5-mile-long arms that matters.
One mirror is set at the tip of each L-arm, and there’s another at the arms' intersection. As gravitational waves wash over Earth, they’ll first distort the distance between one pair of mirrors, and then distort the distance between the perpendicular pair. A laser bouncing back and forth between the mirrors keeps track of how far apart they are to an almost impossibly precise degree (the detectors are sensitive to such things as passing trucks, lightning strikes, ocean waves, and earthquakes). For a signal to be real, it should show up in both detectors.
So far, at least four such signals have been picked up by LIGO, all of which are the work of colliding black holes. A fifth, rumored to be produced by merging neutron stars, is reportedly awaiting an imminent announcement. (Read more about the tantalizing rumor and why it could revolutionize astronomy.)
“We have now witnessed the dawn of a new field, gravitational wave astronomy,” said the Nobel Committee’’s Nils Mårtensson. “This will teach us about the most violent processes in the universe, and it will lead to new insights about the nature of extreme gravity.”
Now, the European Gravitational Observatory’s Virgo detector, which is similar to LIGO in design, is live: In fact, it also detected the fourth black hole-black hole collision spotted by LIGO’s twin detectors. With three such working observatories on the ground, scientists can now more precisely identify the region on the sky where a gravitational wave source is located. Soon, similar experiments are anticipated to come online in Japan and India.
Are there other ways to detect them?
Other teams, notably the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) and two similar collaborations in Europe and Australia, are using spinning stellar corpses called pulsars to record a passing gravitational wave. Pulsars are among the most precise clocks in the cosmos: These pirouetting objects emit powerful beams of electromagnetic radiation that shine on Earth with a regular cadence, as if the pulsars were lighthouses.
Astronomers can use changes in pulsars’ time-keeping to point to gravitational radiation, which sweeps through an array of these dead stars in a telltale way. Unlike LIGO, pulsar timing arrays can detect the gravitational waves released by colliding supermassive black holes, or the bruisers churning away at the centers of galaxies.
In addition, NASA and the European Space Agency are developing a mission called LISA—the Laser Interferometer Space Antenna—that would use three detectors in space, set millions of miles apart, to pick up these minute vibrations in space-time.
Who first came up with the idea of gravitational waves?
In 1916, Albert Einstein suggested that gravitational waves could be a natural outcome of his theory of general relativity. Though many other scientists accepted his prediction, Einstein wasn’t totally convinced that he was right; over the next several decades, he continually waffled over the question of gravitational waves and occasionally published papers refuting his original idea.
In the 1970s, scientists observing a pair of pulsars orbiting one another indirectly detected gravitational waves for the first time. Using the giant radio telescope in Arecibo, Puerto Rico, the team had measured the orbits of the two pulsars and determined that the pulsars were moving closer together. For that to happen, the system must have been radiating energy in the form of gravitational waves—an insight that earned Joe Taylor and Russell Hulse the 1993 Nobel Prize in physics.
And then, of course, the LIGO team directly detected gravitational waves in September 2015—ending a century of speculation and confirming Einstein’s original prediction.
“This event caused a sensation worldwide,” says the Nobel Committee’s Olga Botner. “We knew that gravitational waves existed indirectly, but this was the first time ever they had been directly observed.”
Aside from the fact that they prove (once again) that Einstein was right, why do we care about these things?
Since LIGO first announced the detection of gravitational waves, we’ve gained unexpected insight into the cosmos—namely, oddly huge black holes seem to be colliding more often than we first thought.
But even more importantly, gravitational waves are a new way of seeing the cosmos: We can now detect events that would otherwise leave little to no observable light, like black hole collisions. It’s a bit like seeing the sky in radio waves, infrared, and optical wavelengths; we learn something new from looking at it through all those filters. Gravitational waves are adding yet another pair of glasses to look through.
“Most of us fully expect that we’re going to learn things we didn’t know about,” Weiss says. “We knew about black holes in other ways, and we knew about neutron stars—well those are the two things that ultimately got seen.
"But we hope there are all sorts of other phenomena that you can see mostly because of the gravitational waves they emit. That will open a new science.”