5 Things We Know About Gravitational Waves—And 2 That Are a Mystery
By Dan Falk
Gravitational waves, first detected in fall 2015 and then again a few months later, are making headlines this week following the detection of a third pair of colliding black holes. This particular duo is located a whopping 3 billion light years from Earth, making it the most distant source of gravitational waves discovered so far.
The signal from this latest black hole merger tripped the detectors at the twin LIGO facilities on January 4 of this year (the acronym stands for Laser Interferometer Gravitational-wave Observatory). The newly created black hole—the result of this latest cosmic collision—weighs in at about 49 times the mass of the Sun, putting it in-between the two earlier black hole collisions that LIGO recorded, in terms of size. There’s now ample evidence that black holes can weigh more than 20 solar masses—a finding that challenges the traditional understanding of black hole formation. “These are objects we didn’t know existed before LIGO detected them,” David Shoemaker, an MIT physicist and spokesperson for the LIGO collaboration, said in a statement.
Gravitational waves are shaping up to be the hot new astronomical tool of the 21st century, offering glimpses into the universe’s darkest corners and providing insights into the workings of the cosmos that we can’t get by any other means. Here, then, are five things we know about these cosmic ripples, and a couple more things that we haven’t quite figured out yet:
1. THEY'D HAVE MADE EINSTEIN SMILE.
We knew, or at least strongly suspected, that gravitational waves existed long before their discovery in 2015. They were predicted by Einstein’s theory of gravity, known as general relativity, published just over 100 years ago. The first black hole mergers observed by LIGO produced tell-tale cosmic signatures that meshed perfectly with what Einstein’s theory predicted. But the black hole collision announced this week may yield yet another feather for Einstein’s cap. It involves something called “dispersion.” When waves of different wavelengths pass through a physical medium—like light passing through glass, for example—the rays of light diverge (this is the how a prism creates a rainbow). But Einstein’s theory says gravitational waves ought to be immune to this sort of dispersion—and this is exactly what the observations suggest, with this latest black hole merger providing the strongest confirmation so far. (This Einstein fellow was pretty bright!)
2. THEY'RE RIPPLES IN THE FABRIC OF SPACE-TIME.
According to Einstein’s theory, whenever a massive object is accelerated, it creates ripples in space-time. Typically, these cosmic disturbances are too small to notice; but when the objects are massive enough—a pair of colliding black holes, for example—then the signal may be large enough to trigger a “blip” at the LIGO detectors, the pair of gravitational wave laboratories located in Louisiana and in Washington state. Even with colliding black holes, however, the ripples are mind-bogglingly small: When a gravitational wave passes by, each 2.5-mile-long arm of the L-shaped LIGO detectors gets stretched and squeezed by a distance equivalent to just 1/1000th of the width of a proton.
3. THEY LET US "LISTEN" TO THE UNIVERSE.
At least in a figurative sense, gravitational waves let us “listen in” on some of the universe’s most violent happenings. In fact, the way that gravitational waves work is closely analogous to sound waves or water waves. In each case, you have a disturbance in a particular medium that causes waves to spread outward, in ever-increasing circles. (Sound waves are a disturbance in the air; water waves are a disturbance in water—and in the case of gravitational waves, it’s a disturbance in the fabric of space itself.) To “hear” gravitational waves, you just have to convert the signals received by LIGO into sound waves. So what do we actually hear? In the case of colliding black holes, it’s something like a cosmic “chirp”—a kind of whooping sound that progresses quickly from low pitch to high.
4. THEY'VE SHOWN US THAT YOU REALLY DON'T WANT TO GET TOO CLOSE TO A PAIR OF COLLIDING BLACK HOLES.
Thanks to gravitational waves, we’re learning a lot about that most mysterious of objects, the black hole. When two black holes collide, they form an even bigger black hole—but not quite as large as you’d expect from simply adding up the masses of the two original black holes. That’s because some of the mass gets converted into energy, via Einstein’s famous equation, E=mc2. The magnitude of the explosion is truly staggering.
As astronomer Duncan Brown told Mental Floss last June: “When a nuclear bomb explodes, you’re converting about a gram of matter—about the weight of a thumb-tack—into energy. Here, you’re converting the equivalent of the mass of the Sun into energy, in a tiny fraction of a second.” The blast could produce more energy than all the stars in the universe—for a split-second.
5. THEY MIGHT BE POWERFUL ENOUGH TO KICK A BLACK HOLE OUT OF A GALAXY.
This spring, astronomers discovered a “rogue” black hole moving speedily away from a distant galaxy known as 3C186, located some 8 billion light years from Earth. The black hole is believed to weigh as much as 1 billion Suns—which means it must have received quite a kick, to set it in motion (its speed was determined to be around 5 million miles per hour, or a bit less than 1 percent of the speed of light). Astronomers have suggested that the necessary energy may have come from gravitational waves produced by a pair of very heavy black holes that collided near the galaxy’s center.
But there’s still plenty we’d like to know about gravitational waves—and about the objects they let us probe. For example …
6. WE DON'T KNOW IF GRAVITATIONAL WAVES CONTRIBUTE TO "DARK MATTER."
Most of the mass of the universe—about 85 percent—is stuff we can’t see; astronomers call this unseen material “dark matter.” Exactly what this dark stuff is has been the subject of intense debate for decades. The leading theory is that dark matter is made up of exotic particles created soon after the big bang. But some physicists have speculated that so-called “primordial black holes”—black holes created in the first second of the universe’s existence—might make up a significant fraction of the mysterious dark matter. The theorists who back this idea say that it could help to explain the unusually high masses of the black hole binary systems that LIGO has detected so far.
7. WE DON'T KNOW IF THEY ARE EVIDENCE OF DIMENSIONS BEYOND THE ONES WE PERCEIVE.
Particle physicists and cosmologists have long speculated about the existence of “extra dimensions” beyond the four we experience (three for space and one for time). It was hoped that experiments at the Large Hadron Collider would offer hints of these dimensions, but no such evidence has turned up so far. Some physicists, however, suggest that gravitational waves might provide a clue. They speculate that gravity could freely spread out over all of the dimensions, perhaps explaining why gravity is such a weak force (it’s by far the weakest of the four known forces in nature). Further, they say that the existence of extra dimensions would leave their mark on the gravitational waves that we measure here on Earth. So, stay tuned: It’s only been a bit more than a year since we first detected gravitational waves; no doubt they have much more to tell us about our universe.