Gravitational waves have struck again. Scientists who in February announced their landmark discovery of these ripples in spacetime revealed on Wednesday that they had detected more—again caused by a pair of crashing black holes. The gargantuan gravitational forces involved when two such incredibly dense objects ram into each other are so catastrophic that they wrench spacetime out of shape, curving it in powerful waves that travel clear across the cosmos.
This second find shows that the initial discovery was not a rare windfall, but rather a preview of many more to come, ushering in an era where astronomers can use gravitational waves, rather than light, to “see” black holes and other invisible components of the hidden universe.
These waves were predicted by Albert Einstein’s general theory of relativity but were never directly detected until the Laser Interferometer Gravitational-Wave Observatory (LIGO) team observed them last September. Their sequel discovery stokes physicists’ hopes that they will soon collect enough findings to study the frequency of black hole fender benders and how they originate. More discoveries will also help researchers use gravitational waves to test relativity in extreme environments—possibly confirming the theory or even pointing to a deeper law of nature.
“Our intent was not to just detect the first gravitational wave or prove Einstein right or wrong—it’s to create an observatory,” says LIGO spokesperson Gabriela González of Louisiana State University. “Now we can really say the goal of LIGO has been justified.”
The discovery, accepted for publication in Physical Review Letters, is the second firm detection from LIGO; the team also encountered one “candidate” event that was too weak to confirm, and had reported it in February along with the first conclusive finding. LIGO’s success so far is a good indication that it will be able to find gravitational waves at a steady clip.
“It allows us to explore literally the dark side of the universe,” says Arizona State University theoretical physicist Lawrence Krauss, who is not involved in LIGO. “Gravitational-wave astronomy will become the astronomy of the 21st century.”
Smaller black holes
The newfound gravitational waves began about 1.4 billion years ago in the merger of two black holes—one about 14 times and the other about eight times the mass of the sun—that had gradually circled closer and closer to each other and eventually smashed together, according to scientists’ calculations. The crash produced a new black hole containing 21 times the mass of the sun—the missing mass from the parent black holes was converted to energy in the form of gravitational waves.
Compared with LIGO’s first detection, which came from two larger colliding black holes (each roughly 30 solar masses), this merger created gravitational waves of a higher frequency that were “visible” longer than those involved in the initial discovery. In that case, scientists witnessed just one or two orbits of the black holes around each other but here they were able to track the objects’ final 27 orbits before they crashed.
“That allows for better tests of general relativity and better characterization of the black holes’ parameters,” González says.
This time the researchers were also able to measure the black holes’ spin rates and found that at least the larger one was definitely twirling, likely at some 20 percent of the maximum theoretical spin for a black hole.
“With the first detection it looked like the two black holes could be non-spinning,” says LIGO team member Vicky Kalogera of Northwestern University, “so this is a new finding.”
The gravitational waves announced Wednesday reached LIGO on December 26, 2015—just under three months after the observatory saw its first signal on September 14. LIGO uses two detectors—one in Louisiana and the other in Washington State—to capture the squeezing and expanding of spacetime that occurs when a gravitational wave passes through Earth.
Both detectors are giant L shapes with four-kilometer-long legs. Scientists use mirrors to bounce laser beams back and forth through the legs and measure how long it takes to make the trip. Under normal circumstances the two legs are the same length and the travel times of the two light beams are exactly the same duration. But if a gravitational wave passes, the space between the mirrors will expand and contract minutely in one direction and the two perpendicular legs will briefly have unequal lengths, causing one of the laser beams to arrive a fraction of a second later than the other.
The change is infinitesimal—LIGO must be able to measure a difference in length smaller than one ten-thousandth the diameter of a proton in order to detect the waves. The $1-billion experiment, now officially called Advanced LIGO, is an upgraded version of a project that has been in the making since the 1960s and was first turned on in 2002. Its initial discovery earlier this year electrified the science community as well as the public and won the experiment’s founders the 2016 Kavli Prize in Astrophysics and a Breakthrough Prize as well as many other accolades. It has inspired dozens of follow-up theoretical papers analyzing all aspects of the discovery, from explorations of a possible connection between the black holes and dark matter to a discussion of whether they were not black holes at all, but wormholes.
“The most interesting work was done outside of LIGO,” says LIGO team member Szabolcs Márka of Columbia University. “That’s how science should work.”
After the dawn of gravitational astronomy
Advanced LIGO has now completed its initial run of observations, which lasted from September through January. Its detectors are currently offline for upgrades, and scientists plan a test run in July. If that goes well, a second live run lasting about six months could start in the late summer.
Meanwhile researchers continue to analyze the data from the first run. In addition to black hole collisions, physicists hope to find gravitational waves produced by neutron stars—the extremely small and dense hulks of former stars in which all the protons and electrons have been crammed so tightly that they essentially merge to form neutrons. If two neutron stars crashed together, they would theoretically trigger gravitational waves, which could also result from one spinning neutron star that is a bit lopsided, possibly with a protrusion on one side.
“That’s not an explosive event like the collision of black holes—it would produce gravitational waves that are much fainter,” says Georgia Institute of Technology physicist Laura Cadonati, who chairs the LIGO Data Analysis Council. “That’s a long-term search—it takes time—that’s still being run now.”
As the team gathers more data, the researchers hope to be able to learn more about how black hole binary pairs originate. Perhaps most come from stars that were originally in pairs and then died, becoming black holes that stayed in orbit around each other. Another scenario suggests that binaries are born in tight stellar clusters, when black holes that might have started off as single stars before they died get caught up in each other’s gravity.
“This is my primary interest—can we tell how these binary black holes actually form in reality?” Kalogera asks. “Is there one of these mechanisms dominating or is it more of a mixture?”
And the more gravitational waves LIGO finds, the better it can test whether they seem to fit predictions from general relativity. Although most scientists expect they likely will—after all, the theory has passed every test thrown at it so far—physicists would love to see some kind of deviation from relativity that points to a subtler truth about the universe. Such a discrepancy might provide a clue that helps devise a theory of gravity compatible with quantum mechanics, the current reigning rules of the microscopic realm.
“So far we have found no inconsistencies with general relativity,” Cadonati says, “but if we start seeing anomalies—which can only come with higher statistics—we may start exploring beyond general relativity.”
In any case, scientists hope LIGO’s first two findings are just the beginning of a long and productive future for the experiment.
“Three generations have already worked on this,” Márka says, “and there will be three generations more at least. We are only in the middle. Isn’t that gorgeous?”