The stars orbited each other like a pair of dancers, their sequined costumes glowing against a dark stage. Round and round they went, until the distance between them began to shrink. The closer they got, the faster they spun. And then, smack! The stars collided.
About 500 million years later, Mansi Kasliwal’s phone rang in the middle of the night in April. “Dear human,” a robotic voice said when she picked up. “You have received a new gravitational-wave alert.”
The signal from the cosmic dance had reached her at last.
Kasliwal, an astronomy professor at Caltech, jumped out of bed. Gravitational waves are ripples in the very fabric of the universe. It sounds bizarre, but space is elastic, and can be bent, warped, and squished. These gymnastics require some extremely powerful motions, such as the furious spinning of massive astrophysical objects. Their rotation is so intense that it sends waves coursing through the universe at the speed of light. The ripples move through everything they pass—galaxies, stars, even planets. And when they reach us, ultrasensitive instruments are now waiting to detect them.
Research stations broadcast the detections to astronomers such as Kasliwal and her team at Caltech, a rapid-response group trained to respond to sudden and intriguing observations in the night sky. When she gets the call, Kasliwal commandeers telescopes around the world to drop what they’re doing and search for the origin of the gravitational waves. The ripples are invisible, but sometimes cosmic collisions give off light. Astronomers can learn a lot more if they see that.
A day later, Kasliwal got another call. The universe, the same robotic voice said, was on the line again. This time, the evidence of cosmic dance moves had taken even longer to reach Earth, about 1.2 billion years.
“It’s so remarkable for two of these things to go off in the space of 30 hours,” Kasliwal says.
It’s remarkable how many of these events astronomers have recorded at all. Nearly 100 years passed between Albert Einstein’s prediction of gravitational waves and the first direct proof of their existence, in 2015. The discovery proved the wild-haired scientist right and opened the door to a brand-new field in astronomy. Astronomers detected gravitational waves again, and then again, and again. As of this week, gravitational waves have been detected more than a dozen times.
The discoveries don’t rely on brawny telescopes with broad antennas, but on delicate laser beams inside steel tubes in the ground, located at two sites in the United States and one in Italy. The beams travel up and down the tubes, bouncing off mirrors at each end. When no gravitational waves are present, the distance the light travels remains the same. But when the ripples wash over Earth, they stretch and shrink the steel of the tubes. The distortions are unfathomably imperceptible—the distance shifts by one-thousandth of the width of a proton—but these experiments are sensitive enough to detect them.
From these tiny signals, astronomers can discern some properties of their sources. Einstein’s equations predicted that massive, fast-moving objects could be capable of distorting space, and he was right. Astronomers have traced gravitational waves to black holes, the mysterious, invisible objects that vacuum up anything that comes near, and neutron stars, the bright, leftover cores of big stars that ran out of fuel and imploded, hundreds of millions of light-years away.
The organizations that operate the laser experiments, LIGO and Virgo, now share detections of gravitational waves in real time so that astronomers such as Kasliwal can chase the signals. The first few discoveries came from collisions between two black holes, which, as their name suggests, produce no light. Scientists were obviously thrilled about them, but they couldn’t follow up and pin down the source.
That changed about two years ago. LIGO picked up a wave of cosmic ripples. Two seconds later, a pair of space telescopes observed a sudden burst of gamma rays, the most energetic wave in the electromagnetic spectrum. The timing didn’t seem coincidental. Astronomers scrambled to pivot dozens of telescopes, in space and on the ground, toward the direction from which the gamma rays had come.
Their effort was worth it: They ended up observing a dazzling light show, the product of a collision between two neutron stars, in nearly every wavelength.
“It was absolutely majestic,” Kasliwal says. “It lit up the electromagnetic spectrum.” For the first time, astronomers saw the very source of gravitational waves.
After Kasliwal receives alerts for black-hole mergers at night, she goes back to sleep. She leaps into action for the other collisions, as she did for the two calls she received last month. The first was about a neutron-star merger. The second, if confirmed, is something scientists have never detected before: a mashup of a black hole and a neutron star. Kasliwal summoned an army of telescopes to search for both, but no corresponding light was found.
These days, the merger of a black hole and a neutron star is high on astronomers’ wish list.
They can only guess at the outcome in theoretical models. “Some say that they could be very, very luminous, but maybe in the infrared bands—not so much in the optical,” Kasliwal says. “Some models say maybe the black hole will just swallow the neutron star and you’re left with nothing, so there’s no electromagnetic emission at all.”
And what’s left over? Likely the same object that emerges when the dust settles after the other collisions: a new black hole. In cosmic standoffs, even between two neutron stars, black holes always win.
Aside from the inherent drama of space collisions, astronomers are looking to gravitational waves and the tremendous astrophysical objects that radiate them for answers to some big questions about our universe. They have long suspected that neutron-star collisions produce a stream of heavy elements, such as gold, silver, and platinum, and send them hurtling through the universe. The much-studied merger of 2017 showed some evidence of this.
“Most of the universe is pretty boring; it’s just hydrogen and helium,” Kasliwal says. “It’s [from] these spectacular, rare events that the rest of the periodic table is actually synthesized.” They might be responsible for infusing our world with “the elements that we have taken for granted and put on our wedding rings,” she says.
Scientists also want evidence for more phenomena that exist only in theory, and some of them sound pretty trippy. “There is something called the gravitational-wave memory effect, which is basically a kind of permanent displacement in space-time, which is still there after the gravitational waves have gone by,” says Salvatore Vitale, a physics professor at MIT and a LIGO scientist. “That comes out of Einstein’s theory of relativity, but it would be nice to see that directly.”
The key, as with most scientific work, is more. More cosmic mashups, more ruffles in the fabric of space, more beacons of radiation that broadcast the message Something incredible is happening. Kasliwal is ready.
“My phone’s always on and charged,” Kasliwal says. “Unfortunately, my husband may also wake up, but he enjoys the astrophysics enough that he’s putting up with it.”
After all, when it’s the universe calling, you kind of have to pick up.