Nobel Prize in Physics 2017: How gravitational waves termed 'undetectable' by Einstein were discovered at LIGO

"Once upon a time, in a galaxy far, far away, two massive black holes engaged in a deadly dance. They revolved around each other, spiraling faster and faster until they were whirling at half the velocity of light when they collided and merged, forming an even massive black hole," began Olga Botner on Tuesday.

Botner was part of the panel announcing the Nobel Prize in Physics for 2017, which was awarded to Rainer Weiss, Barry Barish and Kip Thorne "for decisive contributions to the LIGO detector and the observation of gravitational waves".

The discovery that 'shook the world'

Predicted by Albert Einstein a century ago as part of his theory of general relativity, gravitational waves are "ripples" in space-time — the theoretical fabric of the cosmos. They are the aftermath of violent galactic events such as colliding black holes or imploding massive stars, and can reveal events that took place billions of years ago.

The first detection of gravitational waves happened in September 2015 at the US-based Laser Interferometer Gravitational-wave Observatory (LIGO), where Weiss, Barish and Thorne worked. Announced in February 2016 to great excitement in the scientific community, the discovery was hailed as the historic culmination of decades of research.

The gravitational waves that were detected in 2015 were none other than those that came out of the momentous colliding of the two black holes which Botner was mentioning. These waves travelled through space and time, carrying information about what had just happened.

"This event took place about 1.3 billion years ago, at a time when the first multi-cellular life emerged on Earth. Ever since then, these gravitational waves have sped through the universe, reaching our cosmic neighbourhood some 2,00,000 years ago, when early humans walked in Africa. They finally swept through the Earth on 14 September, 2015, when the waves were detected by perhaps the most sensitive instrument ever devised by man," said Botman, a physicist in the Nobel Committee for this year.

Artist's conception shows two merging black holes similar to those detected by LIGO. Image courtesy: LIGO

Artist's conception shows two merging black holes similar to those detected by LIGO. Image courtesy: LIGO

"Their discovery shook the world," said Goran Hansson, the head of the Swedish Royal Academy of Sciences, which selects the Nobel laureates.

'Universe full of music'

Black holes emit no light, and can be observed only through gravitational waves that occur when they collide and violently merge — offering scientists a means of studying them. Such violent events, LIGO's official website says, create ripples which travel outward as gravitational waves, carrying information about their cataclysmic origins as well as invaluable clues to the nature of gravity itself.

Artist's impression of gravitational waves generated by binary neutron stars. Image courtesy: NASA

Artist's impression of gravitational waves generated by binary neutron stars. Image courtesy: NASA

The strongest gravitational waves are produced during catastrophic events such as colliding black holes, the collapse of star cores, coalescing neutron stars, the slightly wobbly rotation of neutron stars, and the remnants of gravitational radiation created by the birth of the universe itself.

"If we could hear all the waves and not only the strongest ones, the entire universe would be full of music, like birds chirping in a forest, with a louder tone here and a quieter one there," the Nobel academy said on Tuesday.

But Gravitational waves are minuscule and near-undetectable because they interact very weakly with matter and travel through the universe at the speed of light unimpeded.

Although Einstein's mathematics allowed us to understand how gravitational waves operate and travel, proof of their existence wouldn't arrive for another 20 years until two astronomers discovered a binary pulsar — two extremely dense and heavy stars in orbit around each other.

Since then, astronomers around the world and through the decades studied the timing of pulsar radio emissions and found similar effects, further confirming the existence of gravitational waves.

Physical contact

These confirmations had always come indirectly or mathematically, and not through actual 'physical' contact with the waves.

This changed on 14 September, 2015. LIGO, for the first time in history, physically sensed distortions in spacetime itself caused by passing gravitational waves generated by two colliding black holes over a billion light years away.

Aerial view of the LIGO Virgo detector. Image courtesy: LIGO

Aerial view of the LIGO Virgo detector. Image courtesy: LIGO

For this, LIGO used instruments called interferometers, whose design was based on work Weiss conducted in the 1960s.

During regular laboratory experiments, a laser beam is split equally between the two extremely long arms of an interferometer. At the end of each arm is a mirror, which reflects the laser back to the starting point.

LIGO realised it made the discovery when when the gravitational waves distorted spacetime just enough so that one of the arms becomes temporarily and very slightly longer than the other. The difference in the lengths of the two arms was smaller than the width of a proton.

It is this sensitivity achieved by LIGO that won Weiss, Barish and Thorne the Nobel prize this year.

Einstein was convinced such detection was impossible

In 1984, Thorne, now 77, and Weiss, 85, co-created LIGO at the prestigious California Institute of Technology, which has taken home 18 Nobels since the prizes were first awarded in 1901. Barish, 81, joined the project in 1994 and helped bring it to completion. LIGO is now a collaboration between more than 1,000 researchers from 20 countries.

Gravitational waves detected by LIGO were extremely hse

Gravitational waves detected by LIGO were extremely short and weak, which required the LIGO detectors to operate at very high sensitivity. Image courtesy: LIGO

"Although the signal was extremely weak when it reached Earth, it is already promising a revolution in astrophysics," the Nobel academy said. "Gravitational waves are an entirely new way of following the most violent events in space and testing the limits of our knowledge."

In an interview on the Nobel prize website, Thorne said the discovery will enable scientists to see an "enormous number of things" in coming decades. "We will see neutron stars collide, tear each other apart, we will see black holes tearing neutron stars apart, we will see spinning neutron stars, pulsars ... We'll be exploring basically the birth of the universe."

Since 2015, the enigmatic ripples of gravitational waves have been detected three more times: twice by LIGO and once by the Virgo detector at the European Gravitational Observatory (EGO) in Cascina, Italy.

"Einstein was convinced it would never be possible to measure them," the jury said.

"The LIGO project's achievement was using a pair of gigantic laser interferometers to measure a change thousands of times smaller than an atomic nucleus, as the gravitational wave passed the Earth" it said.

'Thrilled and humbled'

Weiss was awarded half the prize of nine million Swedish kronor (about $1.1 million), while Barish and Thorne shared the rest. "It's really wonderful. I view this more as a thing that recognises the work of about a thousand people," Weiss said shortly after the announcement.

"It took us a long time... two months... to convince ourselves that we had seen (something) that came from the outside and was truly a gravitational wave."

Barish, who went to his laptop to see who won the Nobel when he didn't get a call, suddenly heard his cellphone ringing — wondering how Stockholm got that number. "My feelings at the time were... a complicated mixture between being thrilled and being humbled," he said.

"I had hoped it would go to the team, it didn't, it went to us," Thorne said after learning of the prize. "We had been expecting it, so I thought I would be blase but in fact I was overwhelmed."

Caroline Crawford, an astronomer at Cambridge University, said the discovery "holds the potential for a completely new way of observing parts of the cosmos, the parts... completely obscured from our view."

With inputs from AFP

Published Date: Oct 04, 2017 12:55 pm | Updated Date: Oct 08, 2017 10:07 am

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