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.
See what we're tweeting about
7 Comments
Add CommentMy dad, a geologist, cosmologist, and philosopher, would keep us around the dinner table for hours every night, talking about wondrous things in the universe. He, too, especially loved neutrinos. That was over 40 years ago. I am pleased as punch to know that work has progressed to such a degree!
Reply | Report Abuse | Link to thisWhy I Love Scientific American: Articles like these.
Reply | Report Abuse | Link to thisI was recently thinking about how the neutrino inspires awe and amazement in me.
I had been particularly entranced by photons since I first learned of particle-wave duality and the concepts of interpretation and entanglement. But now, I've become entirely mystified by the Neutrino. The shear magnitude of weakly-interacting solar neutrinos that are stated to pass through us constantly amazed me. I have since developed this deep ethereal attachment to Neutrinos - as if they personify characteristic ideals in particle physics.
If era of the photon is expected to follow the current era of the electron, this is my official vote for the Neutrino (in all its flavors) as the next super-star particle! And articles like these, are a part of why I love Scientific American!
Fifty years of research should have resulted in a more efficient detector than Super-K. Has anyone attempted using laser interferometry (assuming the loss or phase change of individual photons occurs due to collision with neutrinos)?
Reply | Report Abuse | Link to this@DaveInAustinTx: neutrinos are electrically neutral, and therefore don't interact with photons (except through a couple of extremely unlikely channels-- and by unlikely I mean far less likely than neutrinos interacting w/nuclei). So, laser interferometry isn't a meaningful way to detect neutrinos.
Reply | Report Abuse | Link to thisIf you could make a Z-particle "laser" (which, given their lifetime, seems unlikely, never mind the part where you'd have to create A and B states which transition by emitting Z's preferentially), you could do interferometry with that.
As a young fellow at the ETH (Switzerland) I was present at Wolfgang Pauli's lecture to the first detection of a neutrino at CERN (1956). I did'nt understand much of this particle, named by Fermi (an Italian) the "little neutron". This year I was present in a lecture about the today know properties of the Neutrinos (&) 60 years after it's postulation by Pauli. An other most amazing particle that's forms the universe.
Reply | Report Abuse | Link to thisNeutrinos are the younger cousins of photons..They share an imaginary life and are killed by the same poison :mathematics
Reply | Report Abuse | Link to thisNeutrinos move at light speed. Photons move slower. What we call light speed is the ultimate physical speed 1/c = Sqrt[eps_0 mu_0]. Einstein's Special Relativistic formulae remain valid. Photons are bosons consisting of an electron-positron assembly. Physical mass is zero, but the mass of the composing particles not. Therefore a photon is subject to decay, which manifests itself as a slight dispersion of the photon's wave, causing a very tiny reduction of its speed. Light, manifest as photons, is measured by light in the format of photons. Actually light should be measured by neutrinos, which are fermions, not subject to dispersion in its wave front. So, the result of the CERN-scientists is correct: a neutrino is faster than a photon.
Reply | Report Abuse | Link to this