— Amol Dighe
Needless to say, the sun started shining somewhen around 4.5 billion years before I was born (keep aside my past births for the moment). So, of course, it had a headstart in our relationship. Indeed, even before I had a choice in the matter, it had already collected a lot of baggage: what with nine (or maybe eight, now that one has been deemed unworthy) kids hovering around it all the time and those numerous fleeting affairs with comets.
Anyway, my initial dealings with the sun were cordial (its rising version was a different story, but we managed to stay out of each other’s way), but not too close. During the school days in Mumbai, it was a constant presence in the sky during the day, often to be avoided by walking in shadows, but rather welcome during the evenings on the cricket ground. The textbooks described the sun as a great ball of fire that gave us heat and light. The words in the textbooks are sacrosanct, and the balls of fire are better left alone (except during the eclipses, when they form beautiful diamond rings). Overall, the sun was a rather mundane occurrence.
Unbeknownst to me, another drama was taking place in parallel, this time casting the sun in the role of a nuclear fusion reactor. And as each such reactor should, it was sending out neutrinos at an astronomically large rate: a few million million per second through every human being, in spite of being so far away. Most of them went unnoticed, just passing through us like ghosts in movies. It seems there were people crazy/ambitious (strike off whichever is not applicable) enough to build huge detectors that would see a handful of neutrinos every month. And moreover, they missed about half of them – or so it seemed. This parallel chapter started in the sixties, well before I was born, and continued well into my professional life. Neutrinos may travel almost at the speed of light, but neutrino physicists pursuing them don’t seem to be in a hurry.
The graduate school days brought about a change in the equation between me and the sun. Here, the sun would not appear in the sky for days and weeks together – in fact the sun-tracking solar panel on the top of the Chicago physics department terrace was a legend, it would keep on pointing West from November to March – and sunshine was now something to look forward to. Moreover, this was the first time I studied some particle physics, and learnt that even when the sun was absent from the sky – be it cloudy days or night – it was still sending neutrinos. Half of them still seemed to be missing, but the blame was put squarely on the solar physicists who calculated the number of neutrinos coming from the sun. After all, we don’t understand the hydrodynamics and temperatures inside the sun so accurately, it was said.
Over the postdoc years, the sky over il Adriatico became clearer. The sun started shining more regularly. It was realized that the neutrinos were fine, some of them were just getting converted from electron flavour to muon or tau flavour. The solar physicists were correct after all, and particle physicists had seen a signal from beyond the Standard Model. This is when yours truly got converted to the religion, which believed that neutrinos had mass.
The particle physicists had tasted blood and were not satisfied merely with explaining the missing neutrinos. Now one had to find out if there was any role played by the magnetic field of the sun, by the movement of matter inside the sun, by possible sterile neutrinos, by impossible mass-varying neutrinos, and so on. The basic nuclear fusion reaction inside the sun is the merging of two protons. The neutrinos seen till now were not the ones produced directly from two protons merging, but were later products of chain reactions. Better experiments needed to be designed to observe the neutrinos from the primary reaction of proton-proton fusion, which are lower in energy individually, but collectively carry almost all the neutrino energy from the sun. Only when we match it with the light output from the sun, would one be able to say that we really understand how the sun shines.
Walking along the TIFR seashore around the sunset, one often remembers that the light we see is actually coming from nuclear reactions a million years ago, but the neutrinos passing through us had started from the sun just eight minutes ago. Neutrinos thus show us a much more recent sun. The light produced by them will arrive a million years later, when maybe humans will have ceased to exist.
The sun has appeared in different guises in various myths. It was Ra to the Egyptians, Tonatiuh to the Aztecs, Lugh to the Celts, Helios to the Greeks, Sol to the Romans, Amatarasu to the Japanese. To Hanuman, Surya was a red apple. To Kunti, he was a celestial lover. To Yajnavalkya, he was the teacher. To a particle physicist with open eyes (and working detectors), he is a source of wonder, and of immense information about the world around us.