Doing astronomy with invisible particles
In the end, nature provided, and experiments performing scientists discovered, supported by calculations from theorists. First came decades of searching by many experiments, with important hints to encourage the chase.Then, in 1998, the Super-Kamiokande experiment in Japan announced strong evidence that muon neutrinos produced in Earth’s atmosphere change to another type (now thought to be tauon neutrinos). The proof was seeing this happen for neutrinos that came from “below,” having traveled a long distance through Earth, but not for those from “above,” having traveled just the short distance through the atmosphere. Because the neutrino flux is (nearly) the same at different places on Earth, this allowed a “before” and “after” measurement.
View from the bottom of the Sudbury Neutrino Observatory acrylic vessel and PMT array. image credit: Ernest Orlando Lawrence Berkeley National Laboratory
In 2001 and 2002, the Sudbury Neutrino Observatory in Canada announced strong evidence that electron neutrinos produced in the core of the sun also change flavors. This time the proof was seeing that electron flavor neutrinos that disappeared then reappeared as other types (now thought to be a mix of muon and tauon neutrinos).
Each of those experiments saw about half as many neutrinos as expected from theoretical predictions. And, perhaps fittingly, Takaaki Kajita and Arthur McDonald each got half a Nobel Prize.
In both cases, quantum-mechanical effects, which normally operate only at microscopic distances, were observed on terrestrial and astronomical distance scales.
As the front page of The New York Times said in 1998, “Mass Found in Elusive Particle; Universe May Never Be the Same.” These clear indications of neutrino flavor change, since confirmed and measured in detail in laboratory experiments, show that neutrinos have mass and that these masses are different for different types of neutrino. Interestingly, we don’t yet know what the values of the masses are, though other experiments show that they must be about a million times smaller than the mass of an electron, and perhaps smaller.
That’s the headline. The rest of the story is that the mixing between different neutrino flavors is in fact quite large. You might think it’s bad news when predictions fail – for example, that we would never be able to observe neutrino flavor change – but this kind of failure is good, because we learn something new.
International society of neutrino hunters
Arthur B. McDonald, professor Emeritus at Queen’s University in Canada, speaks to reporters at Queen’s University in Kingston, Ontario, October 6, 2015. McDonald and Japan’s Takaaki Kajita were co-winners of the 2015 Nobel Prize for Physics for their discovery that neutrinos, labelled nature’s most elusive particles, have mass, the award-giving body said on Tuesday. REUTERS/Lars Hagberg – RTS3AOV
Takaaki Kajita at a news conference after the announcement he’s won the Nobel Prize for Physics. Photo credit: Kato/Reuters
I’m delighted to see this recognition for my friends Taka and Art. I wish that several key people, both experiments performing scientists and theorists, who contributed in essential ways had been similarly recognized. It took many years to construct and operate those experiments, which themselves built on slow, difficult and largely unrewarding work going back decades, requiring the effort of hundreds of people. That includes major US participation in both Super-Kamiokande and the Sudbury Neutrino Observatory. So, congratulations to neutrinos, to Taka and Art, and to the many others who made this possible!
When I first started working on neutrinos, over 20 years ago, many people, including prominent scientists, told me I was wasting my time. Later, others urged me to work on something else, because “people who worked on neutrinos don’t get jobs.” And, even now, plenty of physicists and astronomers think we’re chasing something almost imaginary.
But we’re not. Neutrinos are real. They’re an essential part of physics, shedding light on the origin of mass, the particle-antiparticle asymmetry of the universe, and perhaps the existence of new forces that are too feeble to test with other particles. And they are an essential part of astronomy, revealing the highest-energy accelerators in the Universe, what’s inside the densest stars, and perhaps new and otherwise unseen astrophysical objects.