Great excitement rippled through the physics world on Thursday at the announcement that gravitational waves have been detected after a 100-year search. The first-ever detection of gravitational waves will open a new window on the universe and its most violent phenomena.
Scientists have for the first time detected gravitational waves, ripples in space and time hypothesized by Albert Einstein a century ago, in a landmark discovery announced on Thursday which opens a new window for studying the cosmos.
The researchers said they detected gravitational waves coming from two distant black holes - extraordinarily dense objects whose existence also was foreseen by Einstein - that orbited one another, spiraled inward and smashed together. They said the waves were the product of a collision between two black holes roughly 30 times the mass of the Sun, located 1.3 billion light years from Earth.
"Ladies and gentlemen, we have detected gravitational waves. We did it," said California Institute of Technology physicist David Reitze, triggering applause at a packed news conference in Washington.
The announcement of a press conference on Thursday revived rumors that have been circulating in the scientific community for months that the LIGO team may have indeed directly detected gravitational waves for the first time.
The scientific milestone was achieved using a pair of giant laser detectors in the United States, located in Louisiana and Washington state, capping a decades-long quest to find these waves.
"The colliding black holes that produced these gravitational waves created a violent storm in the fabric of space and time, a storm in which time speeded up, and slowed down, and speeded up again, a storm in which the shape of space was bent in this way and that way," Caltech physicist Kip Thorne said.
The two laser instruments, which work in unison, are known as the Laser Interferometer Gravitational-Wave Observatory (LIGO). They were able to detect remarkably small vibrations from passing gravitational waves. After detecting the gravitational wave signal, the scientists said they converted it into audio waves and were able to listen to the sounds of the two black holes merging.
"We're actually hearing them go thump in the night," Massachusetts Institute of Technology physicist Matthew Evans said. "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."
The scientists said they first detected the gravitational waves last 14 September.
But what are gravitational waves and why should you care?
Einstein predicted gravitational waves in his general theory of relativity a century ago. They are ripples in space-time, the very fabric of the Universe.
The game-changing theory states that mass warps space and time, much like placing a bowling ball on a trampoline. Other objects on the surface will "fall" towards the centre — a metaphor for gravity in which the trampoline is space-time.
When objects accelerate, they send ripples along the curved space-time fabric at the speed of light — the more massive the object, the larger the wave and the easier it would be for scientists to detect.
Gravitational waves do not interact with matter and travel through the Universe completely unimpeded.
The strongest waves are caused by the most cataclysmic processes in the Universe — two black holes colliding, massive stars exploding, or the very birth of the Universe some 13.8 billion years ago.
Why would detection of gravitational waves be important?
Finding proof of gravitational waves will end the search for a key prediction in Einstein's theory, which changed the way that humanity perceived key concepts like space and time.
If gravitational waves become detectable, this would open up exciting new avenues in astronomy — allowing measurements of faraway stars, galaxies and black holes based on the waves they make.
So-called primordial gravitational waves, the hardest kind to detect, would boost another leading theory of cosmology, that of "inflation" or exponential expansion of the infant Universe.
Primordial waves are theorised to still be resonating throughout the Universe today, though feebly.
If they are found, they would tell us about the energy scale at which inflation ocurred, shedding light on the Big Bang itself.
Why are they so hard to find?
Einstein himself doubted gravitational waves would ever be detected given how tiny they are.
Ripples emitted by a pair of orbiting black holes, for example, would stretch a one-million-kilometre (621,000-mile) ruler on Earth by less than the size of an atom.
Waves coming from tens of millions of lightyears away would stretch and squeeze a four-kilometre light beam such as the ones used at the Advanced Laser Interferometer Gravitational Wave Observatory (LIGO) at the centre of Thursday's announcement, by about the width of a proton.
How have we looked for them?
So far, gravitational waves have only been detected indirectly.
In 1974, scientists found that the orbits of a pair of neutron stars in our galaxy, circling a common centre of mass, were getting smaller at a rate consistent with a loss of energy through gravitational waves.
That discovery earned the Nobel Physics Prize in 1993. Experts say the first direct detection of gravitational waves is likely to be bestowed the same honour.
After American physicist Joseph Weber built the first aluminium cylinder-based detectors in the 1960s, decades of effort followed using telescopes, satellites and laser beams.
Earth and space based telescopes have been trained on cosmic microwave background, a faint glow of light left over from the Big Bang, for evidence of it being curved and stretched by gravitational waves.
Two years ago, American astrophysicists announced they had finally identified gravitational waves using a telescope called BICEP2, stationed at the South Pole.
But they later had to admit they made an error.
Another method involves detecting small changes in distances between objects.
Gravitational waves passing through an object distort its shape, stretching and squeezing it in the direction the wave is travelling, leaving a telltale, though miniscule, effect.
Detectors such as LIGO and its sister detector Virgo in Italy, are designed to pick up such distortions.
At LIGO, scientists split laser light into two perpendicular beams that travel over several kilometres to be reflected by mirrors back to the point where they started.
Any difference in their length would point to the influence of gravitational waves.
Last December, the European Space Agency launched a lab called Lisa Pathfinder to seek out gravitational waves in space.
New look at the Universe
The ability to observe these gravitational waves would offer astronomers and physicists a new look at the most mysterious workings of the universe, including the fusion of neutron stars and the behaviour of black holes, which are often found in the centres of galaxies.
"The driving force of the universe is gravity," said Tuck Stebbins, Gravitational Astrophysics Lab Chief at NASA's Goddard Space Flight Center.
"These waves are streaming to you all the time and if you could see them, you could see back to the first one trillionth of a second of the Big Bang," he told AFP.
"There is no other way for humanity to see the origin of the universe."
Stebbins said he believes "we stand at a threshold of a revolutionary period in our understanding, our view of the universe."
The LIGO detectors — one in Washington and one in Louisiana — can "measure changes of spacetime at the level of 1/1000 diameter of a proton," he added.
Catherine Man, an astronomer at the Cote d'Azur Observatory in France, said the detection of these waves — if confirmed — would allow astronomers to probe the interior of stars and perhaps resolve the mystery of gamma rays, which are among the most powerful explosions in the universe and whose cause remains poorly understood.
"Now we are no longer observing the universe with telescopes using ultraviolet light or visible light but we are listening to the noises produced by the effects of the gravitation of celestial bodies on the fabric of space-time, which could come from stars or black holes," she told AFP.
"And since the star or black hole does not stop these waves, which move at the speed of light, they come right to us and we can therefore make models... to distinguish and detect their signatures."
Previously, two Princeton scientists won the Nobel Prize for Physics in 1993 for discovering a new type of pulsar that offered indirect proof of the existence of gravitational waves.
The LIGO team is collaborating with a French-Italian team on another detector, called VIRGO, that should become operational soon.
What is LIGO?
LIGO was originally proposed as a means of detecting gravitational waves in the 1980s by physicists from MIT and Caltech. LIGO research is carried out by the LIGO Scientific Collaboration (LSC), a group of more than 1000 scientists from universities around the United States and in 14 other countries including India. More than 90 universities and research institutes in the LSC developed detector technology and analyzed data. About 250 students are strong contributing members of the collaboration.
What was Indian participation in LIGO?
The Indian participation in the LSC, under the umbrella of the Indian Initiative in Gravitational-Wave Observations (IndIGO), involves 61 scientists from nine institutions - Chennai Mathematical Institute, International Centre for Theoretical Sciences, Bangalore, IISER-Kolkata, IISER-Trivandrum, IIT Gandhinagar, Institute for Plasma Research Gandhinagar, Inter-University Centre for Astronomy and Astrophysics (IUCAA) Pune, Raja Ramanna Centre for Advanced Technology Indore and TIFR Mumbai. The discovery paper has 35 authors from these institutions.
How did Indian scientists contribute to the discovery?
Indian groups contributed significantly in understanding the response of the detector to signals and terrestrial influences, in developing the method used for detecting the signal, bounding the orbital eccentricity, estimating the mass and spin of the final black hole and the energy and power radiated during merger confirming that observed signal agrees with Einstein's General Theory of Relativity, and to the search for a possible electromagnetic counterpart using optical telescopes. Some of these works were carried out on high performance computing facilities at IUCAA, Pune, and ICTS, Bengaluru.
With inputs from agencies