It’s Nobels week so it would be remiss of me not to mention this year’s poster boys and girls of science. Yesterday I talked about the prize for medicine and physiology, today it’s physics. The 2015 prize has been jointly awarded to Takaaki Kajita and Arthur B. McDonald for their work on discovering that neutrinos have mass and come in three different varieties.
Through the 1960s the Standard Model of physics had been steadily fleshed out and by the 80s it had withstood all experimental attempts to break it. But there was a problem. We knew that the sun produced huge quantities of neutrinos during its nuclear fusion reactions but we were unable to detect nearly as many as we expected here on earth, we were only seeing about one third the predicted value. Kajita and McDonald were the leaders of large teams that had built enormous detectors at the bottoms of mines hoping to catch glimpses of these elusive elementary particles. Part of the problem is that neutrinos are just so damned hard to spot. They have no electric charge, move at nearly the speed of light, interact very rarely with ordinary matter and, at the time, were thought to be without mass. First postulated by the legendary Wolfgang Pauli (of exclusion principle fame) even he himself didn’t find them an especially satisfactory solution to the mathematics that had led him to conclude their existence. He once wrote to colleagues: ‘I have done a terrible thing. I have postulated a particle that cannot be detected.’ So it was that these huge experiments came online at the turn of the millennium.
Take a quick look at your little finger nail. Every single second of the day 60 billion neutrinos are passing through it at nearly the speed of light. Despite their overwhelming abundance the teams were initially only detecting a few interactions per day. They noticed, though, that there was a difference in the readings depending on whether or not the particles were coming straight down through the atmosphere or straight up having passed through the earth first. This wasn’t because the neutrinos were interacting with the earth on the way through, they realised that the explanation was a little more obscure than that and needed the help of quantum physics, specifically the concept of wave-particle duality.
In quantum theory particles and waves are two halves of the same coin. A particle with a certain energy can be described by a corresponding wave with a certain frequency. The genius of McDonald and Kajita was to realise that neutrinos come in three distinct types; this is because the neutrino wave function is actually a composite of three waves that are superposed but not in phase with each other. As the waves travel through space they can get out of sync with each other and depending on what stage in the cycle the waves are in when they hit the detector will determine whether or not they are able to be detected at all (the detectors were only able to detect one kind of neutrino, hence two thirds of them appearing to be ‘missing’).
They had solved the decades old Solar Neutrino Problem, something that had long been a thorn in the side of particle physicists. Today we no longer have just neutrinos, we know we have electron, tau and muon neutrinos. The Standard Model had been shown to be wrong. Today we also know that there are actually multiple problems with the Standard Model and there is literally an army of brainy folk out there striving to find the new physics that will let us fill in the gaps. There is still a lot to learn about these elusive elements of the universe, but thanks to Kajita and McDonald and their teams we at least have the fundamentals right.