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About 1.4 billion light-years from Earth, two black holes were on a dance of death. One was about 14 times the mass of the Sun; the other eight. For a long time their orbits had been decaying, approaching each other ever more rapidly. And then, finally, so close they were whipping each other around at very nearly the speed of light, they merged. The event was catastrophic, sending out a blast of energy that literally shook the very fabric of the Universe itself.
Eons later, that death cry was seen by astronomers here on Earth. By the time it got here it was vanishingly feeble, but strong enough to shake the sensors in LIGO, the Laser Interferometer Gravitational-Wave Observatory. Two facilities comprise the observatory, one in Livingston, Louisiana, and the other in Hanford, Washington. Each one uses a system of lasers to measure the distance between a set of mirrors, and when the ripples in space-time emanating from the black hole merger passed through the Earth, they changed the distance between mirrors ever so slightly.
Perhaps I’d better explain. In fact, I already have, when the first event was announced in February 2016:
One of the outcomes of Einstein’s General Relativity theory is that space and time are two facets of the same thing, which we call space-time. There are lots of analogies for it, but you can think of it as the fabric of space, a four-dimensional tapestry (three of space and one of time) in which we are all embedded. Remember, it’s not literally like this; we’re using an analogy. But it’ll help you picture it …
… if a massive object is accelerated, it will cause ripples, waves, to move away from itself as it moves. These are actually ripples in the fabric of space-time itself! Space-time expands and contracts in complicated ways as a wave passes, a bit like how ripples will move out from a rock dropped into a pond, distorting the surface of the water.
In other words, when a massive object accelerates, it emits what’s called a gravitational wave that quite literally stretches and shrinks space. The more massive the object and the higher the acceleration, the more powerful the gravitational wave is, and the more space gets distorted. Most objects in the Universe are way too placid to do this, but when two black holes merge, the masses are high and the acceleration fierce.
Even then, by the time the waves get here (moving at the speed of light across the Universe), the ripples are incredibly tiny. The ripples from this new event, called GW 151226 (for the gravitational wave source detected on Dec. 26, 2015), stretched space by only a factor of about 10-22 by the time they reached Earth. That’s so small it’s hard to imagine, so let me put it this way: If you had a ruler a kilometer long, as a ripple passed through, it would change its length by less than the width of a proton!
Still, that’s measurable! Barely. As I described in my earlier article, LIGO is designed to see incredibly small strains in the fabric of space-time. The biggest problem is noise; in this case the detectors are so sensitive that they can detect molecules of air hitting the mirrors!
It’s taken many years, but last year LIGO was finally made sensitive enough to detect the more powerful gravitational waves passing through it. The first detection, made on Sep. 14, 2015, was from two pretty beefy black holes, roughly 36 and 29 times the mass of the Sun. The event lasted two-tenths of a second.
In this second case, the entire detected event lasted about a full second. As the black holes fell in those last few kilometers, their fierce gravity swung them around faster and faster, causing the gravitational waves to increase in frequency and strength. When sound waves do this, you get a sharp, short “chirp,” and that’s what astronomers call this event, too.
The detection itself is pretty amazing. Automated software checks the signal from the LIGO setup and was the first to notice something was up. It alerted astronomers, who checked to make sure the signal was real. Part of that was looking at the signal from both facilities in Washington and Louisiana, and they both saw it (it was first seen in Louisiana, then in Washington 1.1 milliseconds later; that has to do with the speed of light and the angle to the merging black holes).
Note that in the plot above, a pair of up and down cycles is one orbit of the black holes around each other. These objects combined are 20 times the mass of our entire star, but they were whipping around each other hundreds of times per second before the end.
The exact shape of that signal is predicted by Einstein’s Theory of General Relativity. Using computers, the astronomers then generated literally millions of theoretical signals, comparing them to the observed one. They change the masses of the black holes, as well as many other parameters, giving them a range of values for their masses and distances. In the end, the masses found were 14.2 ± 8.3/3.7 (so as much as 8.3 more and 3.7 less) and 7.5 ± 2.3/2.3 times the mass of the Sun, and the distance roughly 1.4 billion light-years.
This means the black holes were probably created in the usual way. A long time ago, two very massive, hot stars were in orbit around each other. One blew up, expelling its outer layers, and its core collapsed to form a black hole. Sometime after that the second one blew, creating the other black hole. They would have orbited each other stably forever, but Einstein has something to say about that: As they moved, they emitted those gravitational waves. The emission was very weak at first, but it removed energy from the system, and the black holes spiraled every so slightly closer together. As they got closer they moved faster, were accelerated more, and emitted more waves. This was a positive feedback loop, and when they got close enough together, BLOOP! They merged.
The 14 and eight solar mass black holes combined to form a single black hole with 21 times the mass of the Sun.
Of course, 14 + 8 = 22. What happened to the missing mass? That mass was converted into the energy of gravitational waves.
I’m not going to lie to you: Just writing that gave me a chill down the back of neck.
That amount of energy is beyond staggering: It’s equivalent of all the energy the Sun emits over a period of about 15 trillion years. That’s a thousand times the lifetime of the Sun! Or, if you prefer, it’s about the same amount of energy emitted by a billion galaxies like ours over the same time interval as the merger.
And now you can see why astronomers are so excited by this. These are among the most energetic events in the Universe, and until last year we were completely blind to them.
The first event showed we could do it. This second event shows that the age of gravitational wave astronomy is truly here. Mind you, both events were detected just a few months after LIGO became sensitive enough to detect them; many more will be seen, and soon. As data are gathered, we’ll learn more about this entirely new field of astronomy, one completely divorced from the usual detection of electromagnetic waves—light. Certainly, conventional telescopes will help; it’s suspected there may be a brief flash of light accompanying the release of gravitational waves, but it’s not certain. And just on their own, gravitational waves yield a treasure of information.
This is an amazing event. Predicted by esoteric physics a century ago, detected by physics even older than that, we finally have the technology that allows us to hear the faint whispers the results from these deafening roars. And it will allow us understand the universe in a whole new way.