More than a billion years ago, in a galaxy that sits more than a billion light-years away, two black holes spiraled together and collided. We can’t see this collision, but we know it happened because, as Albert Einstein predicted a century ago, gravitational waves rippled out from it and traveled across the universe to an ultra-sensitive detector here on Earth.
This discovery, announced today by researchers with the Laser Interferometer Gravitational-wave Observatory (LIGO), marks another triumph for Einstein’s general theory of relativity. And more importantly, it marks the beginning of a new era in the study of the universe: the advent of gravitational-wave astronomy. The universe has just become a much more interesting place.
The discovery was a bit of a surprise: The collision of two black holes is a rare event, although it’s a powerful one. As the two black holes spiraled in toward each other, they churned up spacetime, generating an ever-louder and faster cycle of waves, strong enough for LIGO to spot it from 1.3 billion light-years away, far beyond the expectations of the researchers.
LIGO sensed this collision way back on September 14, 2015—four days before official operations were set to begin. LIGO is a big, expensive experiment, and its scientists are under tremendous pressure to get the science right. Between September and now, the researchers worked hard to make sure the signal was legitimate and to determine its source.
Few physicists doubt gravitational waves exist: Their effects have been visible for decades in astronomical observations. But some doubted we’d ever detect them directly, because although they ripple spacetime, like rings moving outward in a pond, they are extremely faint. It’s especially difficult to detect them on Earth where there is a great deal of seismic activity. This discovery could only be made with an exquisite instrument like LIGO, which required astronomical amounts of money, decades of careful planning, and a group of researchers numbering in the thousands. And it was all worth it, for LIGO’s detection is much more significant than a single observation. The entire cosmos is awash in gravitational waves, and astronomers finally have a way to see them.
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Gravitational-wave astronomy is fundamentally different than ordinary astronomy. Most astronomy is based on light, which is an electromagnetic wave: a vibration in electric and magnetic fields. Gravitational waves are vibrations in the structure of space-time, which travel at the speed of light. Like sound, the frequency—the “tone”—of a gravitational wave often depends on the size of the system producing it. LIGO is particularly attuned to “high-pitched” waves made by pairs of black holes or pulsars right before they collide.
It’s easy to make gravitational waves. A planet orbiting the Sun will generate them, and so can you by spinning in your desk chair. But you won’t be able to produce powerful waves because your idle spinning doesn’t involve much energy. The farther from the source, the weaker those waves become (just like a star appears fainter when seen from a great distance). To produce gravitational waves strong enough and powerful enough to be seen across the vastness of space, you need cosmic catastrophes like merging black holes or supernova explosions.
Even then, detecting gravitational waves is difficult because the force of gravity is extremely weak. (If you doubt that, remember that humans regularly climb stairs, fly in airplanes, and lift weights heavier than themselves. Gravity is weak sauce.) In practice, that means a catastrophic event produces gravitational waves that can carry a lot of energy, yet still barely nudge a detector.
LIGO uses a pair of detectors, which are separated by nearly 2,000 miles. One is located close to Livingston, Louisiana and the other, Hanford, Washington. The distance between the two detectors helps reduce the possibility of false detections and eliminate some sources of “noise”: random vibrations that could hide signals.
The detectors are L-shaped. Each has two concrete tubes 2.5 miles (4 kilometers) long. The researchers shine powerful lasers down these tubes. When a gravitational wave passes through, mirrors at the far end of each arm move, changing the length of the arm. The electronics running LIGO are sensitive enough to detect a nudge of that mirror by one-ten-thousandth of the diameter of a proton, an almost unimaginably tiny length.
LIGO’s current version is known as “advanced LIGO” or aLIGO, which began operations in September 2015. Upgrades to nearly every aspect of the experiment will increase its sensitivity tenfold over the next few years—which means the observatory can “see” a volume a thousand times greater than previously, encompassing a much bigger chunk of the universe. At present, aLIGO can spot colliding pulsars at a distance of 260 million light-years away; when the upgrades are done, that distance will be more like 650 million light-years. (For comparison, the nearest large galaxy to us, Andromeda, is 2.5 million light-years away.) Black holes can be more massive and therefore much “louder” than pulsars, so they can be detected at much greater distances—as the new LIGO announcement attests.
The “loudness” of a gravitational wave depends on what’s producing it, and how close the source is to Earth. Strong gravity makes for loud waves, so objects like binary pulsars are the good sources, and black holes are best because they are even more extreme. The collision LIGO spotted consisted of two black holes respectively 36 and 29 times the mass of the Sun, but much denser. Despite its sun-dwarfing mass, the larger of the two black holes is less than 300 miles across.
When those two black holes crashed together 1.3 billion years ago, they sent out an amazingly powerful burst of gravitational waves, loud enough for LIGO to detect from that astounding distance. If you could hear the waves, they would start on a low note and rapidly sweep up the scale to higher and higher pitches (technically known as a “chirp”, since it resembles the sound many birds make) as the black holes spiral inward, sweeping toward each other inexorably, all the while increasing in volume until the actual collision—and a phenomenally intense burst of waves.
Since the signal itself doesn’t say “I was caused by black holes!,” LIGO scientists had to compare it to various “templates.” A gravitational-wave template is calculated theoretically, based on a few simple assumptions: what sort of objects (black holes, pulsars, etc.) are making the waves, how massive they are, and so forth. Once the best match between the signal and the template is made, researchers can identify the source and even how far away it is. What they can’t do is tell exactly where it is in the sky: LIGO is not very accurate as a telescope.
Since calculations show that collisions of black holes like this are rare, many thought they weren’t great sources for LIGO. That this was the very first thing LIGO ever saw means either we were lucky, or black-hole collisions are more common than we thought. Either is a fascinating possibility, which LIGO researchers will sort out in the coming years.
The road to this discovery has been a long one. Einstein first proposed gravitational waves in a presentation he gave to his fellow scientists in 1913, two years before he finished work on the general theory of relativity. Once he had completed the theory, he wrote a full article on gravitational waves published in 1916, which means the LIGO announcement comes a full century after Einstein first published on them.
The first indirect observation of gravitational waves came in 1974, when a student named Russell Hulse and his professor Joseph Taylor discovered changes in a pair of pulsars orbiting each other. Pulsars are the rapidly-rotating remnants of massive stars, which send powerful beams of light as they spin. When those beams sweep across Earth, our telescopes pick them up as pulses.
Hulse and Taylor saw changes in the flashes that indicated the pulsars were getting closer together, meaning they were losing energy, and the energy loss perfectly matched the predictions of general relativity if they were emitting gravitational waves. (The waves carry energy out of the binary pulsar system.) For this discovery, Hulse and Taylor won the Nobel Prize in physics in 1993.
Detecting gravitational waves directly is much harder than studying the Hulse-Taylor binary. The basic concept for LIGO dates back to the early 1960s, and researchers built the first early prototype experiments at the end of that decade. However, small detectors must be exceedingly lucky to catch any gravitational waves at all, as they only can pick up powerful bursts originating fairly close by. Since only a few major gravitational wave events probably happen in our galaxy in a century, the solution was to build big.
Starting in the 1980s, the National Science Foundation (NSF) decided to invest in the LIGO project. It was a risky venture, but as NSF director France Córdova says, part of the agency’s mission is to fund “fundamental science at a point in the road to discovery where that path is anything but clear.”
The initial phase of LIGO began operations in 2002. Like the prototype experiments, the first iteration would have needed a bit of luck to detect anything, and unfortunately it didn’t strike. Starting in 2010 and continuing over the next five years, researchers replaced and upgraded nearly all the equipment to make a far more sensitive observatory, capable of listening for gravitational-wave sources like black-hole collisions much farther out in space.
Gravitational-wave astronomy is a powerful way to determine how rare these collisions are, since light-based astronomy can’t see pulsar or black hole binaries as readily at large distances. Currently, astronomers rely on computer models to predict how often collisions occur, but LIGO will be able to tell us whether those models are right or not.
But this is just the beginning. The similarly-sized Virgo observatory in Europe—undergoing a similar upgrade as aLIGO—and the KAGRA experiment (still in the planning stages) in Japan will provide two more observatories to form a global gravitational-wave network. And plans are in the works for a space-based detector that could “hear” frequencies that LIGO and other Earth-based observatories can’t—including those that might be emanating from the very beginning of the universe. But those will come in the future. Today, we can savor the very first detection. A little more than a century on, Einstein has been vindicated—and humans now have a new way to see the vast, strange cosmos that surrounds us.