At 5:51 a.m. EDT (9:51 a.m. GMT) on September 14, 2015, history was made. At that exact moment, the twin Laser Interferometer Gravitational-Wave Observatory (LIGO) detectors in Louisiana and Washington state independently saw evidence for gravitational waves, originating from two merging black holes 1.3 billion light-years away. It was pretty big news. You might have heard about it.
The discovery is undoubtedly one of the biggest moments in astronomy, not only confirming Einstein’s theory of general relativity from 100 years ago, but also providing us with a new way to observe the universe. But you’ve no doubt still got many questions about gravitational waves. What are they, exactly? Why did the detection have to be made at that moment? How do we know it was two black holes merging?
Well, wonder no more. Below, we run through some of the outstanding questions you might have about the biggest discovery of the century.
What is a gravitational wave?
A gravitational wave is essentially a ripple in the fabric of space-time caused by a massive object, or objects in this case, moving or merging. For this pair of black holes, their moment of becoming one caused them to lose mass equal to three times our Sun. This release of energy caused a ripple in space-time, a gravitational wave. Then 1.3 billion years later, this ripple arrived at Earth – and we detected it.
“The space-time continuum is often explained with the analogy of steel balls on a rubber sheet,” Justin Greenhalgh from Harwell Campus’ Rutherford Appleton Laboratory, a contributor to LIGO, told IFLScience. “To extend this, gravitational waves can be viewed as ripples in this rubber sheet.”
How did we get so much information from a single “blip”?
As the gravitational wave passed over Earth, it produced a noticeable signal at both LIGO detectors, which had only recently been upgraded as part of the Advanced LIGO project. It is the nature of the blip here that is key.
Essentially, scientists have an idea of what sort of events will produce different kinds of blips. Once the signal had been received, they simply needed to plug the various values – frequency and time, for instance – into equations, which told them that it must have been two merging black holes, one 36 times the mass of our Sun and one 29, about 1.3 billion years ago.
“The analysis of this signal is consistent with two black holes merging – different sources have different characteristics,” CERN physicist Jon Butterworth told The Guardian. “Apparently they have a 'library' of expected spectral characteristics from different sources.”
Above, the "blip" produced at both LIGO detectors, from which the size, mass, and age of the black holes could be determined. LIGO Laboratory
How big was the wave?
Prepare to have your mind mildly blown. When the black holes merged, the release of energy sent out a wave in all directions as a sphere expanding at the speed of light. This means that, by the time the wave reached us, it was the edge of a sphere that spanned 1.3 billion light-years in radius, extending from Earth to the same distance on the other side of the black holes. Yeah.
Do gravitational waves travel forever?
Like light, gravitational waves are thought to propagate seemingly forever, but they do also get weaker over time, albeit at a slower rate than light.
How fast do they move?
At the speed of light, based on the difference in time of blips between the two detectors – 7 milliseconds apart. This has the further consequence of telling us that the “graviton,” the hypothetical particle of gravity, must be massless, since particles with mass cannot reach the speed of light. However, this could be verified further with more measurements.
“The theory suggests that these waves travel at the speed of light, so if we were to identify an event that produces light and gravitational waves simultaneously, we could confirm this,” said Greenhalgh. “It is possible that a supernova would provide the right amount of light and gravitational waves to do so, but we are not yet able to confirm this.”
Each LIGO detector uses tunnels 4 kilometers (2.5 miles) long. LIGO Laboratory
Why was it important the detector was turned on at a specific time to find this signal?
Everything in the universe is producing gravitational waves, but only the most massive events warp space-time to any noticeable degree. Thus, we must rely on massive events producing powerful waves for us to measure, and that’s what happened here.
The moment the two black holes merged produced a sudden, measurable burst of gravitational waves, with 50 times the power of all the stars in the universe combined. This wave traveled over 1.3 billion light-years to Earth and, thanks to LIGO being switched on, it was able to catch the incredibly brief moment this wave passed over our planet.
“Like a camera, or telescope, you have to have LIGO on at the right time to witness the wave,” Greenhalgh said. “While waves are always being produced, most are not of a scale that would allow them to be measured.”
Now that Advanced LIGO is up and running, there could be many more of these events for us to discover.
Is it possible an alien race in another galaxy could have used this same event to discover gravitational waves?
There’s no reason why not. “They do propagate evenly in all directions, so it is entirely possible that an alien race could discover gravitational waves from the same event,” said Greenhalgh. “Indeed, it is possible that one already has, or will in a hundred years time.”
“We have detected gravitational waves. We did it!” David Reitze, executive director of LIGO, said at a press conference (pictured). SAUL LOEB/AFP/Getty Images
What could we use gravitational waves for?
Like radio waves, gravitational waves are a form of information, and by detecting them we could get information from previously unobservable parts of the universe.
Take these two black holes, for instance. Both are less than 150 kilometers (93 miles) across, but located 1.3 billion light-years away. We have no other instruments in existence that could detect information from objects so small and far away. In visible light, for instance, we could barely see an entire galaxy at this distance.
Perhaps most interestingly, finding more of these merging black holes could allow us to see into the history of the universe – possibly closer to the Big Bang than ever before. “A cosmic distance ladder using these black holes would be extremely accurate and compliment existing distance ladders based, for example, on supernovas,” Stephen Hawking told the BBC. “We may even see relics of the very early universe during the Big Bang, at the most extreme energies possible.”
How will future missions improve on LIGO?
The key thing we’re missing at the moment is location. As we only had two detectors, we were only able to tell the direction of the signal – somewhere in the southern sky. A third detector will help us to triangulate where future signals are coming from – something that could be achieved with Italy’s upcoming VIRGO detector.