“As I look back… I am impressed even more by the progress that experimenters have made in the quest to invent, design, and build detectors of ever greater sensitivity. That the quest ultimately will succeed seems almost assured. The question is when, and with how much further effort.”
- K.S. Thorne, in 300 Years of Gravitation, 1987
“I was blind and behold, now I see.”
- John 9:25
After decades of intense research, physicists announce the discovery of ripples in spacetime for the first time, opening a new window upon the cosmos.
Kip Thorne is famous for making bets. In 1981, Kip made a bet with his colleague Jerry Ostriker, an astrophysicist at Princeton University. The subject of the bet was an exotic prediction of Einstein’s Theory of General Relativity — namely, that some of the most compact stars and black holes in the universe should send out minute ripples in space and time throughout the cosmos. If the gravitational waves predicted by Albert were seen by the start of the new millennium, January 1, 2000, then Kip would win a good case of French red wine. If the waves were not seen, then Kip had to pony up a case of California red.
Needless to say, in the absence of any discoveries, Kip found himself out of a good case of wine sixteen years ago. The story of why his timing was off by over a decade, but yet ultimately vindicated, is a fascinating tale in which fundamental physics is deeply entangled with large-scale science.
Black holes have captivated both the public and scientists alike for decades. Nonetheless, the evidence in support of their existence has all been gathered far away from the boundary of the black holes, called their event horizons, beyond which nothing — not even light — may return. For instance, astronomers have measured the tremendous speeds of stars whizzing by our galactic center at speeds up to three million miles per hour. While the observations strongly suggest that the galactic center must harbor a supermassive black hole nearly five million times greater in mass than our own sun, they have been made far away from the event horizon. By directly revealing the final moments in the mergers of black holes, the detection of gravitational waves provides “smoking gun” evidence that black holes do indeed exist.
The types of black hole mergers being discussed by the LIGO team are hundreds of times more energetic than even the most energetic stellar explosions which astronomers have seen in their conventional telescopes. The merger of black holes creates intense distortions in the structure of spacetime surrounding the black holes. The amount of power released during the final death throes of such black hole mergers is absolutely astonishing — for a brief instant, exceeding the energy output by all visible stars in the universe by a large factor. However, this enormous amount of energy is radiated away in gravitational waves which are pure distortions of spacetime, and can only be detected by a gravitational wave detector like LIGO.
One century ago, in a paper published in 1916, Einstein predicted that gravity sends out ripples in spacetime. His paper followed right upon the heels of his historic breakthroughs of 1915 on the nature of gravity as curvature in space and time, unified in a single mathematical four-dimensional spacetime manifold. Like visible light, these ripples of spacetime, known as gravitational waves, propagate at the speed of light. However, gravity is fundamentally an extremely weak force. Hence, unlike visible light, gravitational waves couple only extremely weakly to matter, and have eluded detection until now.
The experimental detection of gravitational waves was first pioneered by the physicist Joseph Weber at the University of Maryland. Weber sought to detect the tell-tale minute distortions in spacetime brought about by the passage of a gravitational wave in a large bar of solid aluminum, two meters in length and one in diameter, cooled to low temperatures to reduce thermal vibrations. Gravitational waves at a particular frequency, around 1660 cycles per second, would set the bar in resonant vibration. Minute resonant vibrations could then be picked up by sensitive piezoelectric detectors. Weber claimed detections of gravitational waves in 1969 and 1970 using his apparatus. These claims were disputed, but led to the realization that gravitational waves could be experimentally detected, sparking substantial interest in the field in the decades which followed.
Early on, LIGO scientists realized that Weber bars could not achieve the sensitivity they sought out over a wide range of frequencies. Instead, LIGO works by utilizing an amazing piece of technology, the optical interferometer, invented by Robert Michelson in the late 19th century. Within an optical interferometer, a single beam of light is split into two beams, which are then recombined to form an interference pattern. This technology enabled incredibly precise measurements of the speed of light, and ruled out the existence of the so-called “luminiferous ether,” leading to Michelson’s Nobel Prize in 1907 — the first awarded to an American physicist. Two experimental physicists, Ron Drever and Rainer Weiss, are credited with pioneering the technically-challenging application of interferometers to gravitational wave detection.
Over its twenty-five year history as one of the largest ongoing physics projects in the world, the LIGO project has had its ups and downs — most notably the ejection of Ron Drever from the project in 1993 (http://lat.ms/20MWfzS). Drever sought out technical perfection, and advocated for a slower development of the technically-demanding technology which underpinned the project. Drever was vocal in his criticism of the project leadership, who sought to keep their project — the largest ever funded by the National Science Foundation — on time and within budget, even if it meant deploying a somewhat suboptimal design. Over two decades later, the LIGO project has remained on time and under budget over its long development history.
On February 11, 2016, David Reitze, Gabriela Gonzalez, Rainer Weiss, and Kip Thorne held a press conference announcing the discovery of gravitational waves for the first time. “We did it!” Reitze proclaimed, “We have detected gravitational waves!” A beautifully clean, almost textbook signal was detected nearly simultaneously at both the Hanford and Livingston LIGO sites, consistent with the merger of a massive binary black hole system, with 36 and 29 solar masses, respectively. During the final merger, nearly three solar masses was directly converted into the pure energy of gravitational waves.
What we will be able to learn from the detection of gravitational waves? The initial discovery already tells us quite a lot.
It is the first discovery of gravitational waves, confirming the final pièce de résistance of Einstein’s Theory of General Relativity.
It is the first discovery of a black hole merger.
It is the discovery of the most massive stellar-mass black hole yet seen.
It is a direct measurement of the spin of a black hole.
It sheds light on how black holes form in the collapse of massive stars.
It places the tightest bounds on the mass of the graviton.
The region outside of some gravitational wave events may not be entirely black. If you’ve seen the film Interstellar, you will recall the beautifully-portrayed visualizations of the gaseous disk surrounding the fictional supermassive black hole Gargantua. Merging black holes detected by LIGO may also be surrounded by similar gaseous disks, powering energetic processes which are observable to astronomers working with conventional telescopes on the Earth. More probably, electromagnetic counterparts will be observed in neutron stars merging with black holes and other neutron stars, as well as in core-collapse supernovae.
Indeed, the LIGO scientists are working closely with a worldwide network of astronomers, sending out rapid alerts whenever a possible gravitational wave signal is detected. These astronomers are poised at a moment’s notice to deploy their expansive array of instruments spanning the globe and Earth orbit to search for counterparts to gravitational wave signals. From these combined observations, scientists may soon be able to crack a number of outstanding mysteries surrounding black holes, merging neutron stars, exploding stars, and much more.
At the same time, new generations of gravitational wave detectors coming online, including Laser Interferometer Space Antenna (LISA), and the International Pulsar Timing Array (IPTA) will open up new windows of gravitational waves at much lower frequencies. These frequencies are drowned out by ground noise on the Earth, but will be detectable through detectors mounted on space satellites (as with LISA), or by using compact spinning pulsars in space as detectors (as with IPTA).
Just as new technologies enabled new forms of astronomy in bands apart from the optical — in the radio, millimeter, infrared, ultraviolet, X-ray, and gamma ray — so too will these new gravitational wave bands open up new windows of discovery onto the universe. As Kip Thorne said during the NSF press conference, “We are going to have a huge richness of gravitational wave signals.”
The future, as seen with gravitational wave eyes, is bright indeed.
