Ten summers ago,
Sanjit Mitra and his wife travelled from Nice in southern France to Pisa in
Italy, taking in the view of the Mediterranean coast from their train window. A
doctoral student at the Inter-University Centre for Astronomy and Astrophysics
(IUCAA) in Pune then, Mitra was working on a project at the Observatoire de la
Côte d’Azur, which stands on the summit of Mont Gros in Nice. The observatory
commanded the best view of the deep blue waters of the Mediterranean and from
Mont Gros he could see planes land on the beaches along the French Riviera.
Mitra, who was researching the gravitational waves predicted by Einstein’s Theory of Relativity, was part of the VIRGO collaboration, a European project to detect gravitational waves using a detector in Italy. Gravitational waves are ripples produced in the fabric of space-time, usually by cataclysmic events in the universe, like the collision of massive black holes.
Mitra did the usual tourist scene and visited the Leaning Tower of Pisa and attended a conference of gravitational wave physicists from VIRGO and LIGO, another international collaboration searching for gravitational waves in the USA. In September 2015, LIGO detectors confirmed the existence of gravitational waves, further proof of Einstein’s theory.
The story of modern gravity begins in Pisa. Legend has it that Galileo Galilei, the 17th century Italian astronomer dropped two metal balls of differing weight from the top of the leaning tower. He overturned Aristotle’s ideas of gravity that said heavier bodies fall to the earth faster. Both balls reached the ground at the same time. Galileo’s discovery eventually led to Newton’s theory of gravity in the 18th century.
After the meeting, Mitra saw something that left a deeper impression than the Mediterranean. A senior scientist took him to see the Virgo detector, a 15-minute drive from Pisa. It consisted of a three-kilometre long tunnel through which physicists shot laser beams at the speed of light to strike a set of state-of-the-art mirrors, whose vibrations could be measured to sub-atomic distances. “It was stunning. A kilometre-class detector is something to see. We cannot even see till the end of it. To most people it would just look like a big gas pipeline. But it is different for someone like me who knows how everything in there works. When I saw it, my first thought was, I want something like this in India.”
Mitra, along with other colleagues at IUCAA, contributed to LIGO’s eventual discovery of gravitational waves using twin detectors in the US. And he got his wish when India signed a Memorandum of Understanding (MoU) with LIGO on March 31, 2016, to build a detector in India.
He will be one of the scientists crucially involved in building and operating the India detector. LIGO USA will ship most of the components to India free of cost, while Indian scientists will supervise its assembly. The Union government allotted Rs 1,250 crore for the project.
But in 2006, even Mitra, a self-confessed dreamer would not have been optimistic about the chances of India building a gravitational wave detector. Next year, he went to the California Institute of Technology (Caltech), for a post-doctorate. There he found reasons to be optimistic after all.
A young German migrant
to the US named Albert Michelson, a non-believer of Jewish parentage and a
future physics Nobel laureate, became deeply interested in measuring the speed
of light. Physicists had discovered that light acted in the same way as sound
waves. Just as the sounds we hear are actually waves in the air, it was thought
that light waves must move in an invisible medium called ether. In 1877, along
with a chemist-turned physicist named Edward Morley, he set up an
interferometer in Cleveland to accurately measure a peculiar property of light
waves. What the young Albert was looking for was the drag that the ether caused
on light when it moved through it. The experiment failed and became one of the
most famous failed experiments in the history of science. If the apparatus had
registered the drag of ether on light waves it was looking for, no one would be
searching for gravitational waves today. Physicists abandoned the ether theory
and concluded that for some reason the speed of light in vacuum was a constant,
and did not change even when the person measuring it sped up or slowed down.
The results would inspire the work of another German Jewish physicist, who shared Michelson’s first name, and eventually migrated to America. Albert Einstein in 1905 discovered the theory of relativity, which showed that the flow of time depended on how close to the speed of light an observer was travelling. The theory of relativity replaced the absolute space and time of Newton with a unified space-time. Gravity itself would come to be understood not as two bodies pulling at each other, but as the result of bodies warping the space between them. The greater the mass, the more it dents space.
IUCAA is a small
independent research institute exclusively dedicated to astronomy and
astrophysics, sitting in the middle of the vast Pune University campus. The
main part of the campus consists of a few living quarters and office blocks
around a central lawn. At various corners, white-coated statues of scientific
luminaries stare at their successors. A foppish looking Einstein with a
prominent waistline and missing the familiar shock of wild hair, Newton
half-sitting, a stern looking Galileo who reminds one of the ancient mariner,
and Aryabhata with a sacred thread around his torso.
The first step is to acquire land to house the detector complex. Given the sensitivity of the instruments and the precision required it has to be in an area that has a minimum of seismic activity and terrestrial noise. The search has now narrowed to two sites in Maharashtra and Rajasthan.
Returning to India after his stint at Caltech, Mitra joined IUCAA as faculty. LIGO India has scientists from several institutes, working on data provided by the detectors in Washington and Louisiana. Now that a third detector will come up in India, many of these scientists will oversee the construction and eventual operation and data gathering of the India detector. IUCAA will oversee the scientific part, while the construction of the four kilometre-long laser tunnels and infrastructure will be the job of scientists and engineers from the Institute of Plasma Research, Gandhinagar. LIGO members from the Raja Ramanna Centre for Advanced Technology (RRCAT), Indore, will assemble the interferometer and the optics once they are shipped from the US. When the detector is operational by the expected date of 2022, IUCAA will be in charge of computing the data.
IUCAA has already set up a new data centre, which has started working on real-time data from the twin detectors in the USA. With 2,500 cores and around 100 teraflops processing speed, the centre will give Indian scientists hands-on experience. LIGO India has also started sending more scientists, doctorate and post-doctorate students to European and US research institutes to gain greater technical expertise in the area.
“The first step is to acquire land to house the detector complex. Given the sensitivity of the instruments and the precision required it has to be in an area that has a minimum of seismic activity and terrestrial noise. The search has now narrowed to two sites in Maharashtra and Rajasthan,” says Tarun Souradeep, who is acting as spokesperson for LIGO India.
“Building the vacuum tunnels will be a great technological challenge. This has to be done from the Indian side. Two high-power lasers will be directed through tubes in the tunnel, at mirrors held in a vacuum chamber in the detector site. The level of vacuum to be maintained in the tubes is of a very high order. While it is not difficult to produce these conditions in smaller areas, the two LIGO detectors are probably the largest such vacuums in the world. And the vacuum has to be maintained at that level perfectly throughout the life of the detector. That is a big challenge but I am confident we have the expertise,” he says.
Talking to me in his
office, Mitra recalls that the whole thing started in IUCAA. And he was bang in
the middle of it. In December 2007, he was sitting in the rose garden with
Caltech professor Rana Adhikari. They had both come down from California to
attend a conference at IUCAA. Mitra remembers asking Adhikari as they were
chatting over beer, “Why can’t we have a third detector in India?” It had been
in Mitra’s mind for a long time.
At Caltech, Mitra had been interacting with a number of leading gravitational wave physicists, including Kip Thorne, one of the architects of the LIGO programme. Every evening when returning from the lab Mitra would look in at the 40 metre prototype detector that Caltech had built and were operating. Many of the experiments and technologies eventually implemented in LIGO were first developed at the Caltech prototype. Studying the work there helped Mitra to transition from the theory of detecting gravitational waves to its experimental and technological dimensions.
Around the same time, several Indian gravitational wave physicists were at Caltech as doctoral and post-doctoral researchers. All were second generation scientists, having been brought into the field by a small number of pioneering gravitational wave scientists working in India from the Seventies through the Nineties. Like Mitra, many of them chose to return to India. Mitra was beginning to be convinced that India had the manpower and the know-how to run a gravitational wave detector on its soil.
Rana Adhikari, an American physicist of Indian origin was also an enthusiast for LIGO’s third detector being located in India. He recalls the conversations in IUCAA on the sidelines of the conference with Sanjit and Anand Sengupta, another Indian post-doc at Caltech. Soon, more senior gravitational wave scientists from India were pulled into the discussion.
“After giving a talk suggesting the idea, we had the first Indigo meeting. Bala Iyer was there and I remember that his emotional and emphatic endorsement of the idea is what gave the rest of us the motivation to be serious about it. In the following days, Anand and I worked on some of the calculations for how this detector would complement the network, etc.,” Adhikari says.
Bala Iyer and Sanjeev Dhurandar are credited with being the moving forces behind gravitational wave research in India. Once the proposals got serious, networking started between senior Indian scientists and their counterparts in LIGO USA, lubricated by Indian students in US research institutions.
predicted the existence of ripples in spacetime, he wrote that they were too
tiny to be ever experimentally detected. He was proved wrong almost exactly 100
years later, but if more pragmatic voices had prevailed, he might not have
been. When the National Science Foundation (NSF), the US government body which
funds mega science projects, decided to fund LIGO in the Nineties, it knew it
was taking a big risk. It was the most expensive project ever undertaken by the
NSF and the frequency of gravitational wave events were then unknown. Worse,
much of the technology that was needed to build the LIGO detectors was not
available. It was hoped that it would be developed alongside the process of
building the detector.
But NSF took a bold decision that the science was worth it. The lack of technology was also a double-edged sword. NSF gambled on the fact that if LIGO was built successfully, the technological spin-offs for industry would be tremendous. This proved to be the case. In 2010, NSF approved an investment of $620 million to upgrade the sensitivity of the two detectors. Advanced LIGO came online after five years, operating at three times more sensitivity. It detected gravitational waves from a pair of colliding black holes actually before its science run began. The signal strength was slightly weaker than required for the old LIGO to pick it up. When LIGO announced the discovery, it prompted France Cordova, the NSF director to joke: “NSF was very pleased and relieved. On behalf of all our tax payers.”
In 2009, Adhikari and Mitra were back in IUCAA to attend the first meeting of Indigo, a consortium for Indian gravitational wave physicists. Indigo is a separate entity from LIGO India, being only a group of scientists interested in gravitational wave research. Bala Iyer held the council chair, while Dhuarandar acted as scientific adviser. The informal setup allowed Indigo to drive the Indian LIGO collaboration, giving it the flexibility to work outside institutional structures.
“Indian universities do not have systems to facilitate international collaborations. Often, they have things that obstruct them. That is why we decided to work through Indigo,” says Tarun Souradeep, the Indigo spokesperson. The year before Indigo’s formation had been one of intense activity for Mitra. “The busiest period of my life,” he says. And he was not the only one.
Adhikari, Mitra, Sengupta and Ajith Parameswaran, another Indian doctorate student at Caltech, were all hard at work drawing up detailed presentations and technical proposals for the India detector. “Professors have little time, so most of the work had to be carried out by students. I had to juggle my own thesis work, publication and lab work and the work on the Indian detector. We got a lot of help from doctoral students who were part of LIGO. There was a student from Iran, one from Bangladesh and another from Nepal. They all contributed towards a detector in India. That is the nature of international scientific collaborations.”
Indigo also started to build the research manpower that India would eventually need if the plan for an Indian detector succeeded. There was a migration from other fields of physics. Many people working in areas like cosmic microwave background, photonics, cold atoms and precision measurement, moved to research in gravitational waves.
The advanced LIGO
detectors that caught the signal from the black hole were three times more
sensitive after the upgrade. They could make measurements as small as the
diameter of a proton. In a few years, when they reach full design sensitivity,
they will be 10 times as sensitive as the original detectors. But the two
detectors can locate the direction of the black hole only very roughly. The
patch of sky in which LIGO detectors know the gravitational waves originated is
as big as the Saptarishi (Big Dipper) constellation. With a third detector, the
section of the sky where the scientists search for the source becomes much
smaller. Given the location of the current detectors in US, the ideal position
for the third detector was near the Indian Ocean.
The LIGO collaboration’s first choice to set up the new detector was Australia. “Australian scientists also lobbied hard. But the government had another mega science astronomy project before it, which it decided to fund. It could not afford money for both. In fact the reason we were in a position to move swiftly on our own proposal was our experience of collaborating with the Australians. They had asked for our help in preparing feasibility reports for the Australian detector. This gave us knowledge about the experimental side of gravitational wave interferometers,” says Souradeep.
But it was barely fast enough. LIGO’s proposal to Indigo was conditional upon the Indian government approving the project within a stipulated time. Concerns about India’s readiness as well as bureaucratic hurdles were an initial source of anxiety to both LIGO and the NSF.
Stan Whitcomb, former director of LIGO flew down to India to interact with scientists in IUCAA, RRCAT and other institutes. “I was impressed and encouraged by what I saw. We were reassured that the Indian gravitational wave scientists had the knowledge to operate a detector for LIGO,” he says. The advantage of a third detector for the science was the deciding argument in convincing the NSF to approve the LIGO India project, he says.
In 2011, Indigo submitted the proposal to the planning commission. All mega science projects are jointly funded by the Ministry of Atomic Energy and the Ministry of Science and Technology. The proposal was put up for Cabinet approval during the last year of the second UPA government. With the global enthusiasm generated by the announcement of LIGO’s discovery of gravitational waves in February, the present government gave in-principle approval. The final step, the MoU with the NSF, was signed when Prime Minister Narendra Modi visited the US in March.
Albert Michelson’s and Edward Morley’s interferometer experiment, led to the first postulates of Einstein’s theory of relativity. But it had a further role to play, in confirming one of relativity theory’s last predictions. The laser interferometer that LIGO used to detect gravitational waves is basically the same kind of interferometer that Michelson created for his famous experiment. The biggest difference is that the two arms of the LIGO interferometer are massive, four kilometres each, making them the biggest such devices ever built. The detector coming up in India will be identical to the ones in Washington and Louisiana.
Einstein’s theory of
gravitation explains space as curved. Interestingly enough, most of the space
in the universe is flat. We can think of space-time as a calm pond, the surface
of whose water is perfectly even. If a number of beach balls were to float on
the surface, the water around the balls would be warped. Now, if the balls were
to move quickly along the surface of the water, ripples would radiate outwards
from the ball. When massive bodies accelerate, they send out ripples along the
space-time curvature as gravitational waves.
These ripples or perturbations are not waves that travel from one point in space to another point in space. They are changes that propagate in the fabric of space-time itself. Suppose a gravity wave were to pass through you as you are reading this article, then space and time will shift around you. How will you measure such a thing? No object around you would have moved relative to each other. But the distance between the objects would have increased or decreased, instantaneously, as if by magic. Luckily, the effects of gravitational waves are local rather than global. Outside the area through which the wave passed spacetime would retain its normal character, which allows an observer there to see what happened.
Now suppose that when you are reading this article you are sipping tea from a teacup on the floor. And using a ruler, you measured the distance between your arm and the teacup. If a gravitational wave invades your home, passing through the teacup but leaving everything else alone, you will find that the cup has moved a bit away, or nearer to you. If you use your ruler again, you can calculate how much the distance has changed, and find the strength of the gravitational wave. But in real life, gravitational waves are much smaller. Any change they cause are at sub-atomic levels, millions of times smaller than an atom. It is technologically impossible to monitor such infinitesimally small changes.
The detector that will be built in India, like the USA ones, is designed to solve this problem in a very clever way. It uses light itself as a ruler. The chambers housing the equipment will be vacuumised to an air pressure that is one trillionth of the atmosphere. It takes 40 days of continual pumping down to evacuate air to the necessary vacuum. Two ultra high-precision mirrors weighing 40 kg are placed in two 4 km-long tunnels (the arms) at right angles to each other. In the middle, at the right angle, sits a beam splitter. A laser is fired at the beam splitter, which splits it into two. Each beam travels towards the two mirrors and is reflected back towards the beam splitter. They will both hit the beam splitter at the same time and recombine to form a single laser beam. The new beam will then go on to hit a screen.
A laser is a highly focused beam of light. And we know light travels in waves. Just like waves in high tide swell and fall, light waves rise and fall in a very regular motion. Imagine two waves from opposite directions approach a cliff, the sea rising and falling along their path. When the waves run into each other, if the crests of the two waves collide, they combine and the single wave that crashes into the cliff would be doubly as powerful. But if the crests of one wave and the hollows of the other collide, only a weak wave will wash up at the foot of the cliff.
The laser beam in the detector is split into identical beams, which are then reflected back by the mirrors. The crests and troughs of the two new beams are in perfect sync with each other. Both the mirrors are placed precisely at the same distance from the beam splitter. Since light travels in vacuum at a constant speed, both the beams will return to the beam splitter at precisely the same instant. The crests and troughs of the two lasers will still be in perfect sync. When the combined laser hits the screen, it will create a bright spot that is brighter than both the constituent beams.
When a gravitational wave passes through the mirrors, it squeezes space in one direction and stretches it in the other. The atoms on the surface of the mirror vibrate, by orders of distance that is trillions of times less than the width of a hair. The distance both the light beams have to travel are altered ever so slightly, and when the two beams recombine at the beam splitter, they are now slightly out of phase. Some of the crests and troughs cancel each other out, so that spot on the screen is less bright. By measuring the variation in brightness over time, LIGO scientists can calculate the extent of displacement of the two mirrors. From that the strength and form of the gravitational wave is derived.
But for this to work, the mirrors and the lasers have to be isolated from every conceivable disturbance possible. This is the enormous technological challenge that LIGO India scientists will face. They have to isolate the detector from all seismic, electromagnetic and terrestrial disturbances. In addition to isolating the various kinds of potential disturbances through vacuum and shielding, a bevy of instruments in the detector complex will continually measure external disturbances (noise) that contaminate the gravitational wave signal.
Seismographs measure any seismic disturbances, as well as disturbances due to things like passing trucks. Thermal noise is also continuously measured, since heat can increase the vibration of the mirrors. And since gravity varies slightly across the earth, the gravity gradient has to be estimated too. One of the upgrades that advanced LIGO made is to continuously monitor each and every component of the detector complex, so that any disturbances produced in the detector apparatus can be cancelled out. While LIGO detected gravitational waves in September 2015, they worked on the data till February before announcing the discovery to the world. Much of the work that was done during that period went into checking the data from thousands of monitors to ensure that the signal was not the result of an instrumental artifact.
wave physicists like Sanjit Mitra, Tarun Souradeep and Bala Iyer are excited at
the prospect of discovering more gravitational waves from various sources, the
people who are going to benefit the most are the astronomers and
astrophysicists. This is particularly true once the Indian detector comes
online. Varun Bhalerao, an astronomer doing post-doctoral research in IUCAA
says “Galileo invented the telescope in the 17th century. It took almost 200
years before something completely unknown was discovered with it—the planet
Uranus. All these years, we have had only one way of studying astrophysical objects.
Through telescopes, radio telescopes and X-ray telescopes. All of them detect
electromagnetic radiation from sources in the cosmos. But with the ability to
detect gravitational waves, an entirely new window has opened up into the
universe. And just like with any other new way of looking, we are bound to find
completely unknown things.”
But there are more exciting things around the corner, in the shorter term. With a third detector both confidence in the signal as well as noise to signal ratio becomes better. Bhalerao explains the difference. “Suppose you have a blurred picture. Being sure that the face is that of a man is to be sure of your signal. (You know that it is a gravitational wave). When the noise to signal ratio improves, you are able to identify the face (source of the wave),” he says.
But the most important addition, the Indian detector will bring to the LIGO collaboration, is the ability to localise the source much better. LIGO collaborates with a number of telescopes around the world. Once a gravitational wave event is located, LIGO alerts these observatories. They have the job of doing an optical follow up of the gravitational event. But as things stand now, it is a daunting chance. The localisation attainable by the two detectors of LIGO plus Virgo, is still an enormous sky patch. “When the India detector becomes operational, with the right kind of source, the sky patch will become smaller by a factor of twenty to one hundred,” he says.
Now, if the event has been caused by a black hole, as happened with the event in September 2015, astronomers do not expect to find any electromagnetic signature. The gravitational field of black holes is so strong that not even light can escape it. But many other sources of gravitational waves, like the collision of neutron stars, emit light. Studying them is a rich source of information about these objects. “The gravitational wave signal carries information about some properties of the source. Electromagnetic follow-up—in optical, radio and X-ray bands—can provide complementary information about the nature of the source, its environment, and details about the explosion creating the gravitational waves. It is this confluence of two messengers of the universe: gravitational waves and light—that will give us significantly more information than either messenger individually can,” Bhalerao says.
The field of gravitational wave astronomy is just taking off. Virgo is scheduled to upgrade its sensitivity to Advanced Virgo. A Japanese detector called Karga is also being built. There are plans for a gravitational wave observatory based in space. As the field comes of age, gravitational wave astronomy is expected to provide data valuable to astrophysicists and cosmologists. It will eventually help them get more accurate values of parameters like the Hubble Constant, understand how the universe looked like in its early stages and solve some of the most enduring problems of cosmology.
(Corrections, June 30, 2016: The name of Tarun Souradeep, the Indigo spokesperson, was misspelt. The MoU between in India and LIGO was signed on March 31, 2016. The 40 metre prototype detector was built by Caltech, and not Thorne and his team as stated in an earlier version of this story.)
(Published in the May 2016 edition of Fountain Ink.)