The existence of gravitational waves was first predicted by Albert Einstein in 1916. According to Einstein’s theory of general relativity, gravity results from how mass warps the fabric of space and time. When any object with mass moves, it should generate gravitational waves that travel at the speed of light, stretching and squeezing space-time along the way.
Gravitational waves are extraordinarily weak, making them extremely difficult to detect, and even Einstein was uncertain whether they really existed. A century later, in 2016, researchers successfully detected the first direct evidence of gravitational waves, using the Laser Interferometer Gravitational-Wave Observatory (LIGO). This work earned three scientists the 2017 Nobel Prize in physics in October 2017.
Scientists announced Monday they have observed gravitational waves for the fifth time—and they’ve seen the light from the cosmic crash that produced them. The waves came from the collision of two neutron stars in a galaxy called NGC 4993, located about 130 million light-years from Earth.
Some 130 million years ago, in a galaxy far away, the smoldering cores of two collapsed stars smashed into each other. The resulting explosion sent a burst of gamma rays streaming through space and rippled the very fabric of the universe. On Aug. 17, those signals reached Earth — and sparked an astronomy revolution.
The distant collision created a “kilonova,” an astronomical gape that scientists have never seen before. It was the first cosmic event in history to be witnessed via both traditional telescopes, which can observe electromagnetic radiation like gamma rays, and gravitational wave detectors, which sense the wrinkles in space-time produced by distant cataclysms. The detection, which involved thousands of researchers working at more than 70 laboratories and telescopes on every continent, heralds a new era in space research known as “multimessenger astrophysics.”
Laser Interferometer Gravitational-Wave Observatory (LIGO) uses a pair of detectors in the United States — one in Livingston, Louisiana, and the other in Hanford, Washington — to sense the warping that gravitational waves cause as they move through matter. Each detector is shaped like a gigantic L, with legs about 2.5 miles (4 kilometers) long. The legs of each detector are normally the same length, so laser beams take the same time to travel down each. However, if gravitational waves pass through Earth — and they make the detector legs expand and contract by as much as one-ten-thousandth the diameter of a proton — these space-time distortions allow each detector’s instruments to detect the split-second differences in time it would take for the laser beams to zip down one leg of the detector versus the other.
Because LIGO’s detectors are separated by about 1,865 miles (3,000 km), it can take up to 10 milliseconds for a gravitational wave to cross from one detector to the other. Scientists can use this difference in arrival times to gauge where the gravitational waves come from. As more gravitational-wave detectors come online — such as the Virgo facility near Pisa, Italy — researchers can do a better job of pinpointing the sources of gravitational waves.
Here’s what happened. On Thursday, August 17th, at 12:41:04 UT, LIGO bagged its fifth confirmed gravitational-wave signal, now designated GW170817. But this signal lasted much longer than the first four: instead of a fraction of a second, like the earlier detections, the spacetime ripples lasted for a whopping ninety seconds, increasing in frequency from a few tens of hertz to about one kilohertz — the maximum frequency that LIGO can observe.
This is the gravitational-wave signal expected from closely orbiting neutron stars, both less than two times the mass of the Sun. Eventually they whirled around each other hundreds of times per second (faster than your kitchen blender), at a fair fraction of the speed of light. The waves emitted by the accelerating masses kept draining the system of orbital energy, and before long, the two neutron stars collided. The collision took place at a distance of roughly 150 million light-years from Earth.
Just 1.7 seconds after the initial gravitational wave detection, NASA’s Fermi Space Telescope registered a brief flash of gamma radiation coming from the constellation Hydra. Half an hour later, McEnery, the telescope’s project scientist, got an email from a colleague with the subject line, “WAKE UP.”
All told, about 70 observatories captured the event, named GW170817 for the day it made itself known to Earth. The collision’s aftermath was recorded at nearly every wavelength. O’Shaughnessy described the discovery as a Rosetta stone for astronomy; the observation produced reams of data with richness seemingly unprecedented for a single astronomical event. The findings, which are spread across many papers in several journals, provide evidence for several theories in astronomy.
The discovery supports the theory that neutron-star collisions produce short gamma-ray bursts, brief streams of light that shine brighter than a million trillion times the sun. Gamma-ray bursts have been detected and imaged before, but without gravitational-wave detectors like LIGO and Virgo, astronomers couldn’t know whether they came from cosmic collisions.
The presence of the short gamma ray-burst suggests the merger led to a kilonova, a powerful explosion 1,000 times brighter than a supernova. Astronomers have long suspected kilonovae follow neutron-star collisions, spewing material out into space. In the case of GW170817, scientists estimate the kilonova ejected material at one-fifth the speed of light, faster than a typical supernova.
Scientists don’t yet know what happened in the wake of the kilonova. Neutron stars are too faint to be seen from so far away, so researchers can’t tell if the merger produced one large neutron star, or if the bodies collapsed to form a black hole, which emits no light at all.
But after two months of analysis, the collaborators were ready to inform the world about what they have so far. Their results were announced Monday in more than a dozen papers in the journals Nature, Science and the Astrophysical Journal Letters.
The collaboration’s capstone paper in Astrophysical Journal Letters lists roughly 3,500 authors, approaching the record set in 2015 by 5,154 Large Hadron Collider physicists who estimated the mass of the Higgs boson. If gravitational wave research had already weakened the stereotype of a lone astronomer genius, the dawn of multi-messenger astrophysics dealt it a fatal blow.
Adding observations physicists have combined the gravitational wave signal with observations from some 70 different observatories, covering the electromagnetic spectrum from bottom (radio waves) to top (gamma rays) to piece together an unprecedented picture of the neutron star collision. “Studying black holes tells us how space-time behaves in its most extreme limits, and studying neutron tells us how matter behaves in its most extreme limits,” Owen says. Physicists have used data from the event to verify their working models of neutron stars, ruling out some wilder variants, and to confirm Einstein’s prediction that gravitational waves should travel at the speed of light.
They have also parsed the gravitational wave readings to reveal a remarkably detailed narrative of the neutron stars’ final moments, says Wynn Ho, an associate professor of mathematics and physics and astronomy at the University of Southampton, where he studies gravitational wave sources and is part of the LIGO Scientific Collaboration. At the moment LIGO first felt the gravitational waves, the neutron stars were circling each other about 15 times per second. For the next minute or so, they danced closer and closer, faster and faster. With less than a second left to go, they were whipping around each other 100 times each second.
Meanwhile, researchers have also confirmed the signatures of heavy elements like lead and gold near the kilonova, providing the best evidence yet that Earth’s store of these metals really does come from neutron star mergers.
The new discovery establishes gravitational-wave science as a new emerging field. And it’s emerging fast, too. But within 20 years or so, gravitational-wave measurements may be just as routine as X-ray observations have become over the past 40 years. It’s really beyond my wildest dreams.