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Home / Opinion / Online-views /  The neutrino hunter

Amol Dighe picks up a stapler from his wooden table, which—except for a computer monitor, speakers and a file—is bare. “How do you know a stapler is here?" asks Dighe, 43, a slim, thoughtful man with a soft voice. “Because it reflects light."

Reflections are at the heart of a great scientific search for Dighe’s object of interest, an invisible, elemental speck of matter that travels from deep within the heart of exploding stars, across unfathomably countless billions of kilometres to Earth, to reveal itself as a ghostly blue trail in water, or a blip on a computer monitor.

These trails and pings reflect the arrival of a neutrino, the tiniest, lightest—almost weightless—particle ever discovered. No one really knows a neutrino’s mass. Only in 1998 did science discover it had mass. Before that, Dighe confesses, even he did not believe a neutrino, which is invisible, weighed anything. What we do now know is that it is millions of times lighter than an electron, itself a tiny subatomic particle that is, however, large enough to be give off light.

The nuclear fusion reactions that power stars release neutrinos, as do the cosmic rays that heat the earth’s atmosphere, and when stars explode, a process called going supernova, great barrages of neutrinos stream into the cosmos, at the rate of 1058 (that’s 10 followed by 58 zeros) neutrinos in 10 seconds.

“These 10 seconds are the only time in the universe that gold is created," says Dighe, with a smile. “The supernova is a gold mine." What Dighe means is that under the extreme pressures and temperatures in a supernova explosion, every element in the universe—including gold—is created. The earth and the sun are the leftovers of a supernova explosion in some incredibly distant past. “Neutrinos make matter, they made the earth and they make the sun shine," says Dighe, an engineering physics graduate of IIT Bombay, holder of a Phd and three post-doctoral stints.

When a supernova explodes, the only thing that escapes its immense violence is the neutrino, which also behaves very differently inside the explosion as it does outside. Dighe—who knows seven languages, including Sanskrit, and can deliver scientific lectures in Marathi, English or Italian—studies what happens when a supernova explodes, how neutrinos are created and, specifically, how they change at such densities. For this work, he was awarded the 2013 Bhatnagar prize, India’s national award for scientists under 45.

Dighe works on a special kind of supernovae, where the core collapses. Stars are usually stable because nuclear fusion reactions in their core push their material outwards, even as gravity pulls it inwards. When a star’s nuclear fuel is exhausted, a big ball of iron—as large, or larger, than the sun—forms at its core. At this point, fusion stops. Gravity has no opposition and so, says Dighe, the iron core collapses, something that the Nobel-prize-winning Indian-American scientists Subrahmanyan Chandrasekhar explained in the 1930s.

The collapsing core becomes immensely dense; even light cannot squeeze through. But, like a rubber ball bouncing back after being squeezed, the still-simmering nuclear furnace pushes it back, creating a shock wave that blows up the star. “Neutrinos are trapped behind the shock wave and they push the wave out," explains Dighe.

The neutrinos that stream out of a dying star are so small that once they reach earth, they pass through things, you and me, iron and steel, water and ice. As one astrophysicist recently put it, they are like bullets in a fog—meaning, you see the trail, not the bullet.

Neutrinos are simple objects, and they require only a medium to be discerned. Their trails are picked up by a variety of detectors, which use a variety of media, including water, kerosene, iron and argon. Great masses of neutrinos pass through the earth every day, about 1033, but scientists detect only a handful. A water detector, the size of an Olympic swimming pool, will sometimes reveal a handful of eerie, blue trails—the arrival of a neutrino. What you see is not the neutrino itself but its reflection, much like the light on a stapler, the neutrino pushing an electron when it hits the water.

Neutrino detectors help scientists understand how stars are born and how they die, the violent creation of the universe and its very nature, including vast missing pieces called dark matter. In a recent book, Neutrino Hunters: The thrilling chase for a ghostly particle to unlock the secrets of the Universe, US astrophysicist Ray Jayawardhana says neutrino detectors could also help understand the furnace at the centre of the earth, perhaps to find oil and mineral deposits, even to detect illegal nuclear plants, which send forth pulses of neutrinos.

Dighe’s special interest is detectors made of iron because, under the forests of the Bodi West Hills in southern Tamil Nadu, India is preparing to build the world’s largest electromagnet, 50,000 tonnes of magnetized iron, larger than a Boeing 747 and 10 times larger than the next biggest such detector. There are two reasons for the size, explains Dighe, who is closely involved in the design of the detector: You need a magnetic field if you want to see the neutrino’s mirror image, the anti-neutrino; and bigger the detector the more neutrinos you will see. “If we want to distinguish neutrinos from anti-neutrinos, we have to find if the neutrino makes a negative particle or a positive particle (for example, electron or positron) in a detector," says Dighe. “Positive and negative particles bend differently in a magnetic field, so a magnetised detector can do this job."

Dighe sees the India-based Neutrino Observatory (INO)—digging will begin this year—as an incubator of future high-energy research. His own desk at the Tata Institute of Fundamental Research in Mumbai is uncluttered by any signs of things scientific, but in a campus laboratory, he shows me a prototype of the detector, which will eventually be a 150-layered club sandwich of iron and glass plates, with gas and electricity pumped through. When neutrinos pass through the iron, they slow, almost imperceptibly, enough to leave soft electronic pings. A joint effort between many scientists and scientific institutions, the detector will be built by Indian companies and buried below hard rock, which stops many unwanted particles but cannot stop the neutrino. The instrument—actually a collection of 29,000 detectors—will be placed at the base of the rock mountain in such a way that, on all sides, there will be a km of rock

“And why should I care about this?" Again, Dighe asks his own question. Although neutrino-research is “esoteric", as he puts it, the great, underground magnet might do for India what the giant collider of atomic particles at CERN in Geneva did—spark a series of advances building detectors for high-energy physics. “That excites me," says Dighe, “You need a high level of technology to build this…to create a place where people can test out new ideas." Tracking the speck from the nuclear fires of the known universe requires at least this much.

Samar Halarnkar is a Bangalore-based journalist. This is a fortnightly column that explores the cutting edge of science and technology. Comments are welcome at frontiermail@livemint.com.

To read Samar Halarnkar’s previous columns, go to www.livemint.com/frontiermail-

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