I’ll admit it. I am partial to neutrinos. And I always have been.
Neutrinos alone, among all the known particles, have ethereal properties that are striking and romantic enough both to have inspired a poem by John Updike and to have sent teams of scientists deep underground for 50 years to build huge science-fictionlike contraptions to unravel their mysteries.
It never ceases to amaze me that every second of every day, more than 6,000 billion neutrinos coming from nuclear reactions inside the sun whiz through my body, almost all of which will travel right through the earth without interruption. But I am even more amazed that in spite of their ghostliness, we can detect them, probe them and unravel their mysteries.
That is why, during the 30-odd years in which I have been a practicing physicist, my research has continually returned to these astonishing particles. And over the past months neutrinos have again reminded me in a very personal way of how daring science allows us to be in our imagination.
Emboldened by the remarkable experimental detection of solar neutrinos by the late Raymond Davis, Jr., 26 years ago, several colleagues and I started to think about other natural sources of neutrinos. One was right below our feet, literally. Radioactive elements sometimes produce antineutrinos (the antiparticles of neutrinos), and when we calculated how many such antineutrinos might be produced by all the radioactive materials thought to be in the earth, the number was almost as large as the solar neutrino flux across a small energy range. But as we tried to think of ways to detect these particles—which, as I began to learn a little geophysics, I recognized might reveal a lot about the makeup of the earth—we also realized that it would be much harder than it had been for Davis to detect solar neutrinos (which was plenty hard). So we wrote up the paper, figuring such a study would never be done.
But we also proposed an even more esoteric source. We knew that when stars explode as supernovae, 10,000 times more energy in the explosion should go into a stunning burst of neutrinos than into the emitted light. We also knew that astrophysical arguments suggested about one star would explode per galaxy every century. Although no one had ever measured a neutrino from such an explosion, and there was no way at the time to get a direct observational handle on the very small presumed supernova rate in galaxies, we nonetheless decided to estimate what the flux of neutrinos on the earth should be from all stars that have exploded over cosmic history. I remember thinking at the time that it was surely the most impractical estimate I might ever make.
Flash forward to the present. This year the Borexino detector in Gran Sasso, Italy, a gargantuan liquid scintillation counter designed to catch solar neutrinos, reported an observation of “geo-neutrinos” from the earth (with a less than one-in-10,000 chance of coming from other backgrounds), confirming an earlier, somewhat more tentative result from Japan. The observed rate is remarkably compatible with estimates from those indirect and theoretical geophysical arguments about the interior of the earth.
Meanwhile the Japanese instrument—the mammoth, 50,000-ton Super-Kamiokande (Super-K) water neutrino detector—has increased its sensitivity to be able to detect even a single supernova antineutrino-induced event per year, which is within striking distance of the rate from the cosmic background we had so casually estimated a generation ago. Remarkably, in the intervening time, however, neutrinos from a supernova explosion at the edge of our galaxy in 1987 were observed, and sophisticated imaging and data-analysis techniques now allow us to detect supernovae in distant galaxies, thereby refining our knowledge of their frequency. It turns out that the predicted rate at which Super-K should detect antineutrino events is strikingly consistent with our original guesstimate.