by Thea Singer
On Thursday, scientists with the Laser Interferometer Gravitational-Wave Observatory, or LIGO, announced that they had detected gravitational waves, fierce ripples of energy across space-time sparked by the merging of two black holes 1.3 billion years ago that had finally, with a “chirp,” reached Earth. The servers of the journal publishing their report crashed. David Reitze, executive director of LIGO, the system of twin antennas that “heard” the crash, rejoiced: “This was a scientific moonshot, and we did it! We landed on the moon.” A century ago, Einstein predicted the existence of gravitational waves in his general theory of relativity. We asked Northeastern’s Pran Nath, Matthews Distinguished University Professor in the Department of Physics, how LIGO works, why the discovery matters, and what we can learn from “listening” to the universe.
How did the antennas of LIGO enable the scientists to detect the gravitational waves? What indicated to them that the signals the antennas picked up were, in fact, gravitational waves?
LIGO has two detectors, one in Livingston, Louisiana and the other in Hanford, Washington and they are about 3000 km apart. The gravitational waves are created by colliding blackholes and their merger releases a huge amount of energy in the form of gravitational waves which then spread outwards in the universe reaching us years later. However, since the two detectors are located a distance apart the waves do not arrive at the detectors exactly at the same instant. Rather one of the detectors picks up the signal about a few milliseconds later. This is what you expect since the gravitational waves travel at the velocity of light. In fact using the time lapse in the observation by the two detectors you can calculate approximate distance between Livingston and Hanover. This is what gives us confidence that the signal one sees is from gravitational waves and not from a random background noise.
In The New York Times Kip Thorne, one of the physicists responsible for the discovery, compared the effect of gravitational waves on space-time to an ocean’s surface “roiled in a storm, with crashing waves.” Before, he said, we had only seen “warped space-time when it’s calm.” The detection is important as it confirms Einstein’s prediction that such waves exist. Why else is the discovery important? What does it mean for science and for the public in general?
The description of gravitational waves by Kip Thorne is excellent. Einstein’s theory of gravitation has many predictions aside from reproducing Newton’s universal law of gravity. For example, it predicted the bending of light in a gravitational field—a phenomena that has led to the confirmation of dark matter in the universe.
However, another prediction of the Einstein theory is that there must be gravitational waves. Such waves are produced by accelerating matter. The discovery is important since it opens a new window to the study of the universe that is not accessible to optical telescopes such as Hubble, X-ray telescopes such as Chandra, and radio telescopes. So LIGO will allow us to probe into far reaches of the universe that have remained unknown to us. It will deepen our knowledge of the universe we live in.
At the moment, this is an intellectual journey to learn what the constituents of the universe are. But in the future there may be indirect benefits. For example, the devices being developed for space exploration could find their way into industry in different guises. It is difficult to predict the implications of advances in fundamental
knowledge on everyday life, but from history we know that they can be enormous.
LIGO detected a definite “chirp,” signaling the merging of the two black holes 1.3 billion years ago and the release of energy equal to three times the mass of the sun. What does it mean to “hear” the result of that collision? What does it represent, and what can scientists learn now and in the future from “listening” to it?
Gravitational waves originating from a source far from Earth reach the two detectors, which are situated some distance apart, at slightly different times. Each of the detectors has to observe the signal. This is done using a laser interferometer, which splits a light beam into two light beams, which travel perpendicular to each other for a given distance, and are then reflected back and combined to interfere with each other, creating a light pattern—hence the name interferometer. A gravitational wave passing through the interferometer will affect the path of each beam a bit differently. This change, or “phase difference,” shows up as a shift in the interference pattern. The LIGO detectors can also measure the frequency of the gravitational waves, which contain clues about the masses of the colliding black holes. The frequencies observed can be described as listening to the universe chirping.
For more than 40 years, Nath was a leader in developing theories regarding supergravity, which combines supersymmetry with general relativity. He shared a personal anecdote related to the recent burst in discoveries in fundamental physics.
At a conference in 1998 at Texas A&M University, I was party to a bet regarding what would be discovered first: the Higgs boson, gravitational waves, or dark matter. Two physicists—myself and Mary K. Gaillard, from the University of California, Berkeley, bet that the Higgs boson would be first, while Charles Misner, from the University of Maryland, bet on gravitational waves and David Cline, from the University of California, Los Angeles, on dark matter. Of course, Gaillard and I won the bet as the Higgs boson was discovered in 2012. But it is interesting that within the past four years two of the three major predictions of fundamental physics mentioned above, that is, the Higgs boson and gravitational waves, have been confirmed. Experiments like LIGO and high-energy experiments such as those leading to the discovery of the Higgs bison are like building cathedrals—they take decades. LIGO was conceived four decades ago and the Large Haldron Collider, which led to the Higgs discovery, was more than 25 years in development. The fact that these discoveries came so close is a welcome surprise. It gives us hope that in the near future we may be lucky to have laboratory detection of dark matter, too.