Catching cosmic chirps

This year’s Nobel Prize in Physics has been awarded to three U.S. scientists, Rainer Weiss, Kip S. Thorne and Barry C. Barish, for their contribution to the discovery of gravitational waves.

Kip S. Thorne, born in 1940 in Logan, Utah, U.S. PhD in 1965 from Princeton University, New Jersey. Feynman Professor of Theoretical Physics, California Institute of Technology, Pasadena, California.

THE faint “chirp” lasted just one-fifth of a second. But the feeble signal, picked up by sophisticated detectors situated in two different geographical locations in the United States about two years ago, has given scientists enough evidence of the existence of the elusive gravitational waves, predicted by Albert Einstein about a century ago.

In 1915, Einstein theorised about the possible existence of gravitational waves when he formulated his monumental General Theory of Relativity. But two decades later, he himself expressed doubts about science ever recording these waves as they would be weak by the time they reached the earth.

Now, science has shown that his doubts were “misplaced”. On September 14, 2015, two detectors, known as the Laser Interferometer Gravitational-wave Observatory (LIGO) and located some 3,000 kilometres apart in the U.S., captured gravitational waves that had become extremely feeble by the time they passed the earth (see “Ripples from the past”, Frontline , March 18, 2016). The signals had emanated from a violent collision and merger of two black holes 1.3 billion years ago in cosmic hinterland.

Window to the universe

This opening of the gravitational wave window to the universe has won three U.S. scientists, Rainer Weiss, Kip S. Thorne and Barry C. Barish, the Nobel Prize in Physics. They were instrumental in conceptualising and building LIGO—which has now grown into a mammoth collaborative project involving over a thousand scientists from 20 countries. Weiss and Thorne are pioneers who firmly believed that the detection of gravitational waves was possible and have made serious efforts to do so since the mid 1970s, and Barish successfully set up the LIGO detectors outside Hanford, Washington, in the north-west of the U.S., and in Livingston, in the southern State of Louisiana.

Weiss, who is with the Massachusetts Institute of Technology, will get one half of the prize. Thorne and Barish—both with the California Institute of Technology—will share the other half. Incidentally, Thorne was one of the executive producers of the popular 2014 sci-fi movie Interstellar , directed by Christopher Nolan.

The September 2015 discovery by the LIGO researchers set several records. Apart from being the first ever observation of gravitational waves, it indicated for the first time the existence of medium-sized black holes, which have 30 to 60 times the mass of the sun, that they could merge by colliding with each other, and that the gravitational radiation released at the time of the collision could be so powerful that it could surpass the light of all the stars in the visible universe.

Thorne, in an interview to the Nobel Prize Foundation immediately after the announcement, said he wished the prize had been awarded to the entire team rather than just the three scientists. “I was hoping that the prize would go to the LIGO-Virgo collaboration [Virgo is a similar facility set up by European scientists near Pisa in Italy] which made the discovery, or to the LIGO laboratory, the scientists of the LIGO laboratory, who designed and built and perfected the gravitational wave detectors and not to Barish, Weiss and me,” he said.

“We live in an era where some huge discoveries are really the result of giant collaborations, with major contributions coming from very large numbers of people. I hope that in the future the Nobel Prize Committee finds a way to award the prize to the large collaborations... and not just to the people who may have been seminal to the beginning of the project, as we were,” he observed.

Over 40 scientists from 13 Indian institutes were actively involved in the LIGO experiments and they contributed to the 2015 discovery too (see “The Indian role”, Frontline , March 18, 2016). Each of the LIGO detectors is four kilometres long and has an L-shaped structure. They are designed to measure only gravitational waves, not light or other electromagnetic radiation. Even though gravitational waves are not sound waves, their frequency is equivalent to those we can hear with our ears. The detectors are designed to measure a change that is thousands of times smaller than an atomic nucleus as the gravitational wave passes the earth.

Ripples in space-time

Unlike light and other electromagnetic radiation, gravitational waves do not travel through space. But, like ripples created in water by a moving object, say, a boat, they transfer distortions created by massive astrophysical phenomena to neighbouring regions, and so on.

For decades, physicists have tried to detect these gravitational waves that shake the universe. According to Einstein, space and time are malleable, and the combined four-dimensional space-time (the three dimensions of space and the one dimension of time) vibrates with gravitational waves that are created when a mass accelerates—like when an ice skater pirouettes, a star explodes in a distant galaxy, or two black holes rotate around each other. The U.S. physicist Joseph Weber commenced the first set of experiments to detect gravitational waves in the 1960s.

One piece of indirect evidence came in the 1970s, when the U.S. astronomers Joseph Taylor and Russell Hulse, recipients of the Nobel Prize in Physics in 1993, used a large radio telescope to observe a pair of extremely dense stars, a double pulsar. They were able to show that the stars rotated around each other at increasing speeds while losing energy and moving closer together. The amount of lost energy corresponded to the theoretical calculations for gravitational waves.

Weiss entered the scene in the mid 1970s. He designed a laser-based interferometer that drowned all the background noise that disturbed measurements. Weiss and Thorne were firm believers in the existence of gravitational waves and convinced that they could be detected, and that the detection would bring about a revolution in the understanding of the universe.

So far, all sorts of electromagnetic radiation and particles, such as cosmic rays and neutrinos, have been used to explore the universe. However, gravitational waves are a direct testimony to disruptions in space-time itself. This is something completely new and different and opens up unseen worlds. A wealth of discoveries awaits those who succeed in capturing the waves and interpreting their message, said the Nobel Prize Committee.

Yet another discovery

The scientists at the LIGO-Virgo collaboration celebrated the laurels their pioneering leaders won by making yet another discovery. On October 16, the scientists announced that they had detected gravitational waves once again on August 17 this year. The source of the gravitational waves was different this year. They emanated from the merger of two neutron stars.