On May 6, 1981, the physicists Kip Thorne and Jeremiah Ostriker made a bet*. If extraterrestrial gravitational waves were detected before January 1, 2000, by at least two experimental groups, Thorne would win the bet and a case of red wine. If not, then Ostriker would have it. As it happens, Thorne would’ve smelled victory if only they’d decided to wait 16 years more.
At a press conference in Washington, DC on February 11, a global collaboration of scientists and engineers officially declared that they’d detected gravitational waves, ripples of energy flying through the fabric of space-time.
“The first direct detection of gravitational waves by the LIGO science collaboration is a breathtaking discovery because it opens a brand new window on the universe,” said Abhay Ashtekar, the director of the Institute for Gravitational Physics and Geometry at Pennsylvania State University. “It will reveal secrets from the farthest regions of the universe that we cannot access by conventional astronomy.”
Apart from better understanding how neutron stars and black-holes evolve and merge, the finding will allow astronomers to compare them against predictions made by Albert Einstein’s theory of general relativity. In short, such studies will help determine if the theory presents a perfect picture of gravity. It was first published 100 years ago.
In the month leading up to the declaration, rumours of the detection’s details had been making the rounds and were frequently hailed as “huge”; Martin Rees, the United Kingdom’s Astronomer Royal, wrote it would be the “scientific highlight of the decade”. If they are verified by other experiments in the future – even though the collaboration claims a very reliable result – February 11, 2016, could be the first day of the era of gravitational-wave astronomy.
And the time at which the observation, designated GW150914, was made was September 14, 2015, 3.21 pm IST, by the twin Advanced Laser Interferometer Gravitational-wave Observatories (aLIGO) in Hanford, Washington, and Livingston, Louisiana. The observatories are funded by the National Science Foundation of the US. Their working principles are based on ideas formulated by Thorne, Rainer Weiss and Ronald Drever in the 1980s.
The gravity of massive objects in the universe deforms the space-time around them. The motion of other objects in the vicinity is influenced by this deformation and they feel it as the force of gravity. The work of Einstein as well as a group of other mathematicians and physicists in the early 20th century helped elucidate this picture as the way gravity works.
However, it was Einstein’s theory of general relativity that predicted that when massive objects accelerate, they set off disturbances in space-time that propagate outward, and throughout the universe. These disturbances are the manifestations of the objects losing gravitational energy, and the energy being carried off in the form of gravitational waves. As the waves move through the continuum, they temporarily distort distances in the regions they pass through.
The LIGO project was set up in 1992 for detecting these passing distortions, and upgraded to a more sensitive ‘Advanced’ avatar, aLIGO, by 2014. Each of its observatories has a common design: two long tunnels connected at a vertex, shaped like an ‘L’. A source at the vertex fires a laser pulse down each tunnel and waits for them to be reflected back by a mirror at the end. When the pulses reconvene, they form an interference pattern registered by a detector. In the absence of a gravitational wave, the interference is fully destructive and the detector draws a blank.
When a gravitational wave passes through aLIGO, it temporarily (and alternately) contracts and expands the length of the arms by a tiny amount. As a result, one of the laser pulses ends up travelling a longer distance than the other. When they reconvene, one pulse is slightly out of step relative to the other and their interference isn’t destructive. The detector lodges an interference pattern. According to the February 11 announcement, that’s what happened on September 14, 2015.
A simulation of GW150914 by the Numerical Relativity group at the Georgia Institute of Technology.
According to the data released, the waves likely originated from a pair of black-holes 1.2-1.3 billion lightyears away. They were orbiting each other, reaching speeds of about 180 million metres per second, and eventually merged to form a larger black-hole. While they initially weighed 29 and 36 solar masses, the resulting monster weighed 62 solar masses. The remaining 3 solar masses (equivalent to 178.7 billion trillion trillion trillion joules of energy) were released as gravitational waves during the merger and subsequent ringdown, when the resultant settles down to form a stable shape. The entire event spanned a few seconds, which means – as Thorne figured during the press conference – the power output was 50-times as much as the output of all the stars in the universe put together.
“The coolest thing for me is that the signal was emitted some 1.3 billion years ago. Back then, there was no major life-form on Earth. The signal travelled for 1.3 billion years and passed through Earth in less than half a second,” said Karan Jani, a PhD candidate at the Georgia Institute of Technology and an analyst with the LIGO collaboration.
Clifford Burgess, a theoretical physicist at the McMaster University in Hamilton, Canada, had leaked in an email to his students – eventually circulated on the Internet – ahead of the announcement that the signals registered at aLIGO were made at a statistical significance of more than 5 sigma. This means that the odds that the detection was a false signal were at most 1 in 3.5 million, sufficient among physicists to claim a discovery.
As a summary of results accompanying the announcement noted, “We expect an event as strong as GW150914 to appear by chance only once in about 200,000 years of such data.”
The detection took as long as it did to be made because, of the four fundamental forces in nature, gravity is the weakest by far. As a result, the effect of a gravitational wave is also extremely small and requires super-sensitive instruments to pick up on it. At the same time, any gravitational wave detector needs to be at least as large as the source of the wave it’s detecting.
Because two black-holes orbiting each other can be separated by only a few kilometres before smashing, the aLIGO’s arms are 4 km long, and are maintained with a perfect vacuum. The lasers and the detectors are configured to pick up on changes in the length of space of the order of 10-20 metres – that’s about 10,000 times smaller than the nucleus of a hydrogen atom. Such sensitivity means the detectors pick up on a lot of noise as well – from vehicles moving on the surface in the vicinity, minor seismic disturbances underground, disturbances left behind by ancient cosmic events, and other activity that for pretty much any other purposes wouldn’t be bothersome.
So even when a bona fide detection is made, scientists will have to apply advanced data-filtering techniques to spot it in the sea of noise logged by the detectors. Satya Mohapatra, a staff technician at the LIGO Lab at the Massachusetts Institute of Technology, Boston, explained that different groups within the collaboration studied “thousands of channels in the LIGO instruments to characterise different noise sources that could affect a potential gravitational wave signal”. Additional groups also studied how gravitational waves originating from sources other than black-hole-mergers would look like so they could be filtered out better.
Mohapatra continued, “The exact shape of the gravitational wave that comes from the collision of two black-holes remained elusive until 2005 as general relativity is a very non-linear theory.” In that year, “the first complete simulation of the merger of two black-holes was demonstrated by Frans Pretorius.” Pretorius is now a professor of physics at Princeton University, New Jersey.
But that wasn’t the end of that road. “Black-holes and neutron stars also have spins. So the shapes of waveforms for different combinations of masses and spins have not all been simulated,” Mohapatra said. So the February 11 announcement was effectively the result of great advancements in numerical astrophysics.
The existence of gravitational waves was assured since the 1970s, when two astronomers from the University of Massachusetts-Amherst discovered a pair of neutron stars orbiting each other whose orbits were shrinking at a rate predicted by Einstein’s equations for general relativity. The astronomers would go on to win the 1993 Nobel Prize for physics for making the connection that the neutron stars were losing gravitational energy – probably by emitting gravitational waves.
So a great part of the excitement now isn’t because the waves have finally been directly detected but because we now have an instrument that can probe deeper into the mysterious sources of the waves themselves.
For example, though Einstein was satisfied by how his theory of general relativity seemed able to explain the behaviour of gravity in the universe, he wasn’t comfortable with one of its direct consequences: black-holes. The ability of these freaks of nature to distort space-time to the point of bending electromagnetic radiation into themselves has made it very difficult to study them using telescopes that ‘see’ using electromagnetic radiation.
Gravitational-wave observatories, on the other hand, ‘hear’ using the nature of gravity, which “couples to everything and cannot be masked”, according to Ghanashyam Date, a professor at the Institute for Mathematical Sciences, Chennai. And configuring detectors like aLIGO to better detect and investigate the waves opens up a new way to investigate the cosmos. As David Reitze, the executive director of the LIGO Laboratory, California Institute of Technology, said at the press conference, “This is the first time the universe has spoken to us – through gravitational waves.”
For one, general relativity predicts that gravitational waves should set off at the speed of light, which means the hypothesised particles carrying gravitational energy – gravitons – should have no mass. The waves in GW150914 arrived at the Louisiana and Washington detectors some seven-thousandths of a second apart, consistent with the time light would take to travel the same distance.
However, if the waves are detected to be passing through slower in the future, then theoretical physicists will have to return to the proverbial drawing board for new ideas of particulate gravity.
In another case, given how sensitive to gravitational waves the current generation of LIGO is, astronomers can also measure how many black-holes there are of different masses and how often they’re involved in intense events like mergers. “Black-holes in astrophysics were thought to belong in two extreme classes – stellar black-holes weighing less than 20 solar masses and those at the centres of galaxies weighing millions to billions of solar masses,” said Jani. There’s circumstantial evidence for these black-holes as well from conventional telescopes – which is what made the current detection more unlikely.
“We just didn’t have strong astrophysical bounds on whether black-holes of such masses can exist in the universe,” explained Jani. They weigh an intermediary 50-10,000 solar masses and haven’t been studied much with telescopes. But at LIGO, they generate the ‘loudest’ signals. “With [this finding] of black-holes just lighter than the intermediate mass, we now have a smooth range of possible masses for black-holes in the universe,” said Jani.
Currently, there are five gravitational-wave observatories: two in the US and one each in Italy (Virgo), Germany (GEO600) and Japan (KAGRA). The Japanese observatory has a different detection technique. Meanwhile, the American and German observatories form a network of observatories that’s blind to about a few hundred degrees of the sky. That is, the network won’t be able to pinpoint the source of gravitational waves from this patch of the sky.
As Jani explained in the context of GW150914, which was logged by the two American observatories: the gravitational waves “that we observed came from 1.2 billion lightyears away. Based on the mass of the two black holes, each almost 30-times the mass of the Sun, they must’ve been formed through the evolution of very-heavy stars. This implies the black-holes must be residing in some host galaxy, but it’s difficult to locate it.”
Many upgrades have been proposed for the aLIGO network to become better in this sense. One is the Evolved Laser Interferometer Space Antenna (ELISA), comprising three spacecraft orbiting the Sun in an equilateral triangle. Because of the distances between them, ELISA will be able to look for gravitational waves from very large sources. Ahead of its 2034 launch, a test mission called LISA Pathfinder was launched on December 3, 2015.
The other is an aLIGO in India. According to Tarun Souradeep, of the Inter-University Centre for Astronomy and Astrophysics, Pune, its location would reduce the aLIGO network’s blindspot by an order of magnitude. The project, worth about Rs.1,500 crore, is being funded by the Department of Atomic Energy and received clearance from the erstwhile Planning Commission in its 12th Five Year Plan. At the moment, it’s waiting for clearance from the Union Cabinet.
In the meantime, pending future tests confirming the detection, the aLIGO announcement is unquestionably a Nobel moment. What’s questionable is whom the eventual Nobel Prize will end up overlooking. The LIGO Scientific Collaboration involves over 1,000 scientists from 19 countries, with over 250 research institutes involved in developing technology and analysing results. The observatories are operated by the Massachusetts Institute of Technology, Boston, and the California Institute of Technology.
With inputs from Nithyanand Rao and Raghu Karnad.
*Set out in the book 300 Years of Gravitation (1989).