LIGO signal reveals first observation of two massive black holes colliding, proves Einstein right
Jennifer Chu • MIT News Office • February 11, 2016
Almost 100 years ago today, Albert Einstein predicted the existence of gravitational waves — ripples in the fabric of space-time that are set off by extremely violent, cosmic cataclysms in the early universe. With his knowledge of the universe and the technology available in 1916, Einstein assumed that such ripples would be “vanishingly small” and nearly impossible to detect. The astronomical discoveries and technological advances over the past century have changed those prospects.
Now for the first time, scientists in the LIGO Scientific Collaboration — with a prominent role played by researchers at MIT and Caltech — have directly observed the ripples of gravitational waves in an instrument on Earth. In so doing, they have again dramatically confirmed Einstein’s theory of general relativity and opened up a new way in which to view the universe.
But there’s more: The scientists have also decoded the gravitational wave signal and determined its source. According to their calculations, the gravitational wave is the product of a collision between two massive black holes, 1.3 billion light years away — a remarkably extreme event that has not been observed until now.
The researchers detected the signal with the Laser Interferometer Gravitational-wave Observatory (LIGO) — twin detectors carefully constructed to detect incredibly tiny vibrations from passing gravitational waves. Once the researchers obtained a gravitational signal, they converted it into audio waves and listened to the sound of two black holes spiraling together, then merging into a larger single black hole.
“We’re actually hearing them go thump in the night,” says Matthew Evans, an assistant professor of physics at MIT. “We’re getting a signal which arrives at Earth, and we can put it on a speaker, and we can hear these black holes go, ‘Whoop.’ There’s a very visceral connection to this observation. You’re really listening to these things which before were somehow fantastic.”
By further analyzing the gravitational signal, the team was able to trace the final milliseconds before the black holes collided. They determined that the black holes, 30 times as massive as our sun, circled each other at close to the speed of light before fusing in a collision and giving off an enormous amount of energy equivalent to about three solar masses — according to Einstein’s equation E=mc2 — in the form of gravitational waves.
“Most of that energy is released in just a few tenths of a second,” says Peter Fritschel, LIGO’s chief detector scientist and a senior research scientist at MIT’s Kavli Institute for Astrophysics and Space Research. “For a very short amount of time, the actual power in gravitational waves was higher than all the light in the visible universe.”
These waves then rippled through the universe, effectively warping the fabric of space-time, before passing through Earth more than a billion years later as faint traces of their former, violent origins.
“It’s a spectacular signal,” says Rainer Weiss, a professor emeritus of physics at MIT. “It’s a signal many of us have wanted to observe since the time LIGO was proposed. It shows the dynamics of objects in the strongest gravitational fields imaginable, a domain where Newton’s gravity doesn’t work at all, and one needs the fully non-linear Einstein field equations to explain the phenomena. The triumph is that the waveform we measure is very well-represented by solutions of these equations. Einstein is right in a regime where his theory has never been tested before.”
The new results are published today in the journal Physical Review Letters.
“Magnificently in alignment”
The first evidence for gravitational waves came in 1974, when physicists Russell Hulse and Joseph Taylor discovered a pair of neutron stars, 21,000 light years from Earth, that seemed to behave in a curious pattern. They deduced that the stars were orbiting each other in such a way that they must be losing energy in the form of gravitational waves — a detection that earned the researchers the Nobel Prize in physics in 1993.
Now LIGO has made the first direct observation of gravitational waves with an instrument on Earth. The researchers detected the gravitational waves on September 14, 2015, at 5:51 a.m. EDT, using the twin LIGO interferometers, located in Livingston, Louisiana and Hanford, Washington.
Each L-shaped interferometer spans 4 kilometers in length and uses laser light split into two beams that travel back and forth through each arm, bouncing between precisely configured mirrors. Each beam monitors the distance between these mirrors, which, according to Einstein’s theory, will change infinitesimally when a gravitational wave passes by the instrument.
“You can almost visualize it as if you dropped a rock on the surface of a pond, and the ripple goes out,” says Nergis Malvalvala, the Curtis and Kathleen Marble Professor of Astrophysics at MIT. “[It’s] something that distorts the space time around it, and that distortion propagates outward and reaches us on Earth, hundreds of millions of light years later.”
Last March, researchers completed major upgrades to the interferometers, known as Advanced LIGO, increasing the instruments’ sensitivity and enabling them to detect a change in the length of each arm, smaller than one-ten-thousandth the diameter of a proton. By September, they were ready to start observing with them.
“The effect we’re measuring on Earth is equivalent to measuring the distance to the closest star, Alpha Centauri, to within a few microns,” Evans says. “It’s a very tough measurement to make. Einstein expected this to never have been pulled off.”
Nevertheless, a signal came through. Using Einstein’s equations, the team analyzed the signal and determined that it originated from a collision between two massive black holes.
“We thought it was going to be a huge challenge to prove to ourselves and others that the first few signals that we saw were not just flukes and random noise,” says David Shoemaker, director of the MIT LIGO Laboratory. “But nature was just unbelievably kind in delivering to us a signal that’s very large, extremely easy to understand, and absolutely, magnificently in alignment with Einstein’s theory.”
For LIGO’s hundreds of scientists, this new detection of gravitational waves marks not only a culmination of a decades-long search, but also the beginning of a new way to look at the universe.
“This really opens up a whole new area for astrophysics,” Evans says. “We always look to the sky with telescopes and look for electromagnetic radiation like light, radio waves, or X-rays. Now gravitational waves are a completely new way in which we can get to know the universe around us.”
LIGO Detects Gravitational Waves (MIT Video)
Tiny detection, massive payoff
LIGO research is carried out by the LIGO Scientific Collaboration (LSC), a group of some 950 scientists at universities around the United States, including MIT, and in 15 other countries. The LIGO Observatories are operated by MIT and Caltech. The instruments were first explored as a means to detect gravitational waves in the 1970s by Weiss, who along with Kip Thorne and Ronald Drever from Caltech proposed LIGO in the 1980s.
“This has been 20 years of work, and for some of us, even more,” Evans says. “It’s been a long time working on these detectors, without seeing anything. So it’s a real sea change and an interesting psychological change for the whole collaboration.”
“The project represents a triumph for federally funded research,” says Maria Zuber, vice president for research and E. A. Griswold Professor of Geophysics at MIT. “LIGO is an example of a high-risk, high-return investment in discovery-driven science. In this case the investment was major and sustained over many years, with a successful outcome far from assured. But the scientific payoff is shaping up to be extraordinary. While the discoveries reported here are already magnificent, they represent the tip of the iceberg of what will be learned about fundamental physics and the nature of the universe.”
The LIGO Observatories are due for more upgrades in the near future. Currently, the instruments are performing at one-third of their projected sensitivity. Once they are fully optimized, Shoemaker predicts that scientists will be able to detect gravitational waves emanating “from the edge of the universe.”
“In a few years, when this is fully commissioned, we should be seeing events from a whole variety of objects: black holes, neutron stars, supernova, as well as things we haven’t imagined yet, on the frequency of once a day or once a week, depending on how many surprises are out there.” Shoemaker says. “That’s our dream, and so far we don’t have any reason to know that that’s not true.”
As for this new gravitational signal, Weiss, who first came up with the rudimentary design for LIGO in the 1970s as part of an experimental exercise for one of his MIT courses, sees the tiny detection as a massive payoff.
“This is the first real evidence that we’ve seen now of high-gravitational field strengths: monstrous things like stars, moving at the velocity of light, smashing into each other and making the geometry of space-time turn into some sort of washing machine,” Weiss says. “And this horrendously strong thing made a very tiny effect in our apparatus, a relative motion of 10 to the minus 18 meters between the mirrors in the interferometer arms. It’s sort of unbelievable to think about.”
This research was funded by the National Science Foundation.
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Letter regarding the first direct detection of gravitational waves
MIT News Office
February 11, 2016
The following email was sent today to the MIT community by President L. Rafael Reif.
To the members of the MIT community,
At about 10:30 this morning in Washington, D.C., MIT, Caltech and the National Science Foundation (NSF) will make a historic announcement in physics: the first direct detection of gravitational waves, a disturbance of space-time that Albert Einstein predicted a century ago.
You may want to watch the announcement live now. Following the NSF event, you can watch our on-campus announcement event.
You can read an overview of the discovery here as well as an interview with MIT Professor Emeritus Rainer Weiss PhD ’62, instigator and a leader of the Laser Interferometer Gravitational-Wave Observatory (LIGO) effort.
The beauty and power of basic science
I do not typically write to you to celebrate individual research achievements, no matter how impressive; our community produces important work all the time. But I urge you to reflect on today’s announcement because it demonstrates, on a grand scale, why and how human beings pursue deep scientific questions – and why it matters.
Today’s news encompasses at least two compelling stories.
First is the one the science tells: that with his theory of general relativity, Einstein correctly predicted the behavior of gravitational waves, space-time ripples that travel to us from places in the universe where gravity is immensely strong. Those rippling messages are imperceptibly faint; until now, they had defied direct observation. Because LIGO succeeded in detecting these faint messages – from two black holes that crashed together to form a still larger one – we have remarkable evidence that the system behaves exactly as Einstein foretold.
With even the most advanced telescopes that rely on light, we could not have seen this spectacular collision, because we expect black holes to emit no light at all. With LIGO’s instrumentation, however, we now have the «ears» to hear it. Equipped with this new sense, the LIGO team encountered and recorded a fundamental truth about nature that no one ever has before. And their explorations with this new tool have only just begun. This is why human beings do science!
The second story is of human achievement. It begins with Einstein: an expansive human consciousness that could form a concept so far beyond the experimental capabilities of his day that inventing the tools to prove its validity took a hundred years.
That story extends to the scientific creativity and perseverance of Rai Weiss and his collaborators. Working for decades at the edge of what was technologically possible, against the odds Rai led a global collaboration to turn a brilliant thought experiment into a triumph of scientific discovery.
Important characters in that narrative include the dozens of outside scientists and NSF administrators who, also over decades, systematically assessed the merits of this ambitious project and determined the grand investment was worth it. The most recent chapter recounts the scrupulous care the LIGO team took in presenting these findings to the physics community. Through the sacred step-by-step process of careful analysis and peer-reviewed publication, they brought us the confidence to share this news – and they opened a frontier of exploration.
At a place like MIT, where so many are engaged in solving real-world problems, we sometimes justify our nation’s investment in basic science by its practical byproducts. In this case, that appears nearly irrelevant. Yet immediately useful «results» are here, too: LIGO has been a strenuous training ground for thousands of undergraduates and hundreds of PhDs – two of them now members of our faculty.
What’s more, the LIGO team’s technological inventiveness and creative appropriation of tools from other fields produced instrumentation of unprecedented precision. As we know so well at MIT, human beings cannot resist the lure of a new tool. LIGO technology will surely be adapted and developed, «paying off» in ways no one can yet predict. It will be fun to see where this goes.
* * *
The discovery we celebrate today embodies the paradox of fundamental science: that it is painstaking, rigorous and slow – and electrifying, revolutionary and catalytic. Without basic science, our best guess never gets any better, and «innovation» is tinkering around the edges. With the advance of basic science, society advances, too.
I am proud and grateful to belong to a community so well equipped to appreciate the beauty and meaning of this achievement – and primed to unlock its opportunities.
In wonder and admiration,
L. Rafael Reif
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