By Prasad Ganti

Astronomy started out as optical, viewing the light emitted by astronomical bodies in space. Our ancestors looked at the night sky and imagined patterns of stars and distant bodies. Several advances occurred in the last four hundred years in identifying stars, galaxies, clusters of galaxies, and finally, our universe. Along the way, the optical limitation was overcome and astronomy branched out to other areas.

Firstly, into radiation-related multi-messenger astronomy: beyond the visible light, radio astronomy, infrared astronomy, X-ray astronomy, and gamma-ray astronomy ensued. Each of them provided different perspectives and images of the Universe compared to the limited visible spectrum. As a whole, a composite mosaic of the Universe started emerging.

Recently, two more candidates were added to the multi-messenger astronomy club: gravitational waves and neutrinos. Neutrinos are elementary particles with no charge that are nearly massless. They travel without interacting with any matter or any radiation, as if the matter did not exist. It is an extremely rare event that a neutrino produces a trace of its interaction with the material universe.

There are copious numbers of neutrinos produced by violent nuclear reactions in the stars, supernovae, neutron star mergers, active galactic nuclei, and even the nuclear reactors on our own planet Earth and our Sun. They were also produced in the Big Bang which created our universe. Trillions of neutrinos pass through our bodies every second and yet we have no clue about them. Many come from cosmic rays which are very energetic and travel great distances to convey a picture of our Universe to us. Neutrinos offer a unique opportunity to observe processes that are inaccessible to optical telescopes.

With a great number of them and their ability to travel astronomical distances without any impediment, they are the perfect messengers from distant astronomical events. From the cores of the stars and passing through intervening objects, they are constantly on the move. But perfection comes at a cost. What makes them ideal to travel long distances unimpeded, also makes them very difficult to detect. Neutrinos were a theoretical prediction by Wolfgang Pauli in the 1930s to explain the case of missing mass-energy in a nuclear reaction called beta decay. Beta decay is the emission of an electron from the nucleus of some radioactive elements. Their existence was confirmed in 1956 by Clyde Cowan and Frederick Reines.

The largest neutrino detector is called IceCube. Located at the South Pole, it consists of about 5,000 electronic sensors buried at depths between roughly 1,450 meters (4,760 feet) and 2,450 meters (8,040 feet) below the ice surface. This shields the detectors from other radiation. The sensors detect light emitted by charged particles that are produced when a single neutrino collides with a proton or neutron inside an atom. Although a rare event, a quadrillion such neutrinos produce a collision every few days. The resulting nuclear reaction produces secondary particles traveling at high speeds that give off a blue light called Cherenkov radiation. Such a reaction is called “inverse beta decay.” Such tiny flashes of light are multiplied by thousands of photomultiplier tubes to produce a detectable signal.

There are other neutrino detectors which consist of thousands of gallons of a liquid like chlorine or argon or heavy water housed deep underground, like the Super-Kamiokande in Japan or the Sudbury Neutrino Observatory in Canada. Kamiokande-II, the predecessor of Super-Kamiokande, detected the first extrasolar neutrinos coming from Supernova 1987A, an explosion in the Large Magellanic Cloud, a satellite galaxy of our own Milky Way.

The lexicon of astronomy is getting richer while we get one step closer to understanding our Universe.