Nearly a century after their existence was predicted by Albert Einstein’s General Theory Of Relativity, a global scientific collaboration, including scientists from across Australia, has proven the existence of gravitational waves for the first time.
With the help of lasers and mirrors, scientists have directly observed wrinkles in the fabric of spacetime itself.
Two colliding black holes, one with 36 times the mass of the sun, and the other with 29, emitted those gravitational waves as they spiralled into one another and eventually collided.
From roughly 1.3 billion light-years away, these waves spread like ripples in the cosmic pond and washed over Earth on September 14, causing a minuscule but measurable change in the distance between four sets of mirrors – two in Louisiana, and two in Washington State.
In the last second before the black holes merged, they released 50 times more energy than all the stars in all the galaxies in the universe were releasing, combined.
More than 1,000 scientists around the world have been involved in the research, including a collaboration of Australian universities through the Australian Consortium for Interferometric Gravitational Astronomy (ACIGA), led by Professor David McClelland from The Australian National University (ANU).
“For the first time, we’ve been able to observe a gravitational wave, created 1.3 billion years ago by the collision of two massive black holes,” said Professor David McClelland from The Australian National University
“This observation confirms that gravitational waves do exist. It is a moment that will be remembered for a thousand years.”
To scientists monitoring that mirror-based experiment at the Laser Interferometer Gravitational-wave Observatory (LIGO), the signal received on Earth carried the characteristic “chirp” predicted to accompany the death and unification of two black holes.
“We can hear gravitational waves, we can hear the universe,” said Gabriela Gonzalez of Louisiana State University. “We are not only going to be seeing the universe, we are going to be listening to it.”
It’s a discovery that many say is likely to earn a Nobel Prize, and an announcement that has been hinted at for weeks, if not months, as tantalizing rumours of the LIGO team’s find circulated on social media.
Feel the Vibrations
First predicted by Einstein in 1916, gravitational waves are among the most paradoxical parts of his theory of general relativity. They’re produced by extreme events—such as colliding black holes, merging neutron stars, or exploding stars—that are energetic and violent enough to warp the tough, stiff fabric of spacetime, causing it to expand and contract.
But as you might imagine, those shifts aren’t normally perceptible. If they were, we’d see clocks running inconsistently and landscapes stretching and compressing all the time.
Yet “gravitational waves are going through us right now,” says Alan Weinstein, who leads the LIGO team at Caltech. “I’d bet my left arm that’s true. And I’m a lefty.”
That means that as these exceedingly powerful waves sweep through Earth, their effect is, appropriately, exceedingly difficult to measure. “The stretching and squeezing of space is insanely small,” Weinstein says, noting that a passing gravitational wave might change the distance between two people sitting a meter apart by just 10-21 meters. That’s on the order of a millionth of the diameter of a proton, one of the particles that make up an atom’s nucleus.
But put two mirrors four kilometres apart, as LIGO has done, and the effect of that gravitational wave is on the order of a ten-thousandth of the diameter of a proton. “That, we can do,” Weinstein says.
Scientists detected gravitational waves produced by the merger of two black holes (simulated here), an event so intense that in the moment before the colliding black holes swallowed each other, they emitted more energy than the rest of the universe combined [Image: SXS Collaboration]
LIGO uses two identical L-shaped detectors set a continent apart, in Livingston, Louisiana, and Hanford, Washington. For a gravitational wave signal to be counted as real, it must show up in both detectors, which are made of two sets of mirrors set perpendicularly to one another. A passing gravitational wave will stretch spacetime in one direction and compress it in another, causing an inconceivably small change in the length of the detectors’ arms, which is measured by a laser.
The apparatus is the most sensitive measuring device on the planet, and in addition to gravitational waves, can detect vibrations from passing trucks, earthquakes, lightning strikes six states away, signals from global positioning satellites, and electromagnetic pulses in Earth’s upper atmosphere. All that noise has to be filtered out to pick up the minuscule signal from gravitational waves.
After decades of planning and political drama, the LIGO detectors first tried to hear gravitational waves in 2002; after eight years of quiet, the detectors were shut down in 2010 and further insulated against interfering noise.
So, when Advanced LIGO observations began again on September 18, scientists were optimistic that they’d come up with something.
In a strange twist of fate, they already had a detection in hand. The detectors had been up and running before the official observing run began, and had already bagged an extremely tantalizing signal. It arrived first at the detector in Louisiana, and seven milliseconds later, showed up in Washington.
“We were quite confident when this event came in that it was a good event. Were we surprised that it was too good to be true? Absolutely. My reaction was, wow. I couldn’t believe it,” Reitze says.
When Black Holes Collide
Using a handful of Einstein’s equations, scientists backtracked from the observable waves to determine what kind of astrophysical event was to blame. In this case, those equations suggested that two colliding black holes were the culprit—and that when they coalesced, they formed a new black hole, one with a little more than 60 solar masses.
Gravitational Waves Explained
Formed by the death and collapse of massive stars, black holes are among the strangest objects in the known universe—if you can call them “objects.” It’s easy to think of a black hole as being a clump of matter so dense that its gravity traps everything that gets too close, even light. But black holes are less “things” or “objects” than regions of intensely curved, bottomless spacetime. Thus, when two black holes merge, the event is anything but ordinary.
“It’s kind of a roiling mess of curved space, rapidly changing,” Weinstein describes.
In the collision LIGO detected, the two black holes had slowly spiralled around each other for millions or billions of years. But as the two bodies inched closer and closer, their orbits sped up until eventually, they were whirling around one another at roughly half the speed of light and emitting gargantuan amounts of energy in the form of space-warping gravitational waves.
Then the black holes merged. In the last second before that event, the whirling black holes emitted more energy than the entire universe emits in all forms of radiation. Once they’d merged, the resulting amalgamated black hole wobbled around for a bit before settling down, emitting what’s known as ringdown, or a kind of last gasp before going quiet.
It’s an impressive story, told by the infinitesimally small shifts in distance between mirrors on Earth.
Scientists on the LIGO team are confident the signal is real; in fact, they calculated that such a convincing false alarm wouldn’t arrive more than once every 200,000 years. That’s not true for all the potential gravitational wave detections the team has collected so far. LIGO found at least one more candidate signal—on October 12—produced by merging black holes, but scientists can’t say for sure that it isn’t a false alarm.