Originally published 16 March 1987
As Lewis Mumford put it, “if man had not encountered dragons and hippogriffs in dream, he might never have conceived of the atom.”
According to Mumford, a historian and critic of Western culture, it was from the experience of dreams that humans came to believe that there is more to reality than meets the eye. Dreams gave sleepers access to an unseen world, veiled from our senses and daily experience, but as apparently real as the food we eat.
We no longer believe in the literal reality of dream images. But belief in an unseen world veiled from our senses is an important part of modern science. We believe, for instance, in atoms. Atoms are as real to us as were dragons and hippogriffs to our ancestors.
Sometimes the unseen realities of science seem no less bizarre than the creatures of dream. Read the following paragraph, and then ask yourself if any dragon or hippogriff is stranger than the solar neutrino.
As you sit at the breakfast table reading this newspaper, a flood of neutrinos from the sun is pouring through the roof of your house. Every second, hundreds of billions of these unseen subatomic particles pass through every square inch of your body! At night, equal numbers of solar neutrinos enter the Earth on the opposite side, zip through the body of the planet (and your mattress) and pierce you in your sleep, from underneath the bed, in one side and out the other at the speed of light. To these elusive, fast-traveling particles, your body (like the Earth itself) is as transparent as is glass to light.
An indispensable particle
What is this unseen, unfelt, unceasing wind of particles blowing from the sun? It is easier, I suppose, to believe in dragons. Neutrinos have no charge, and perhaps no mass. They interact with ordinary matter very rarely (which is why they pass harmlessly through your body). They are as close to being nothing as something can be and still be something. But neutrinos have been an indispensable part of physical theory since Wolfgang Pauli proposed their existence in 1931, to balance the books on energy in certain nuclear reactions.
Neutrinos have been produced and detected experimentally since 1956. These will‑o’-the-wisp neutrinos are securely established as a very real part of the unseen world of physics.
According to present theories, neutrinos are produced in copious quantities at the core of the sun, as a by-product of the nuclear reactions that make the sun burn. They zip up and out of the sun at the speed of light, their numbers only slightly diminished by interaction with the sun’s mass. Eight minutes later, the neutrinos coming in our direction encounter the Earth — and pass through it. The number of neutrinos that reaches the Earth per square inch per second can be predicted. If we could catch these solar neutrinos and compare their number with prediction, we could test our theories for what transpires at the core of the sun.
But because of the neutrino’s elusiveness, catching and counting them is not easy (no less demanding than the task, in dreamtime, of capturing a unicorn). Since 1967 a solar neutrino detector has been operating in a mine deep under South Dakota. The detector is a tank containing 100,000 gallons of perchloroethylene, a common cleaning fluid containing chlorine as a principle ingredient. When — on rare occasion! — a solar neutrino hits the nucleus of a chlorine atom head-on, it converts the chlorine atom into a radioactive atom of argon. The argon atoms accumulate with time, and can be detected and counted by ordinary methods of nuclear chemistry. The detector is placed deep in a mine to shield it from everything but neutrinos from the sun.
Our theories for what goes on at the core of the sun predict that the South Dakota neutrino trap will snare about one of the elusive particles a day. But to everyone’s chagrin, in the twenty years since the detector has been in operation, the actual capture rate has been only one-third of what is predicted.
Setting the traps
The trap has been scrutinized and tested over the years, and it is thought to be reliable. The deficiency of captured neutrinos means one of two things: Either we do not understand the sun as well as we thought we did, or we do not understand neutrinos as well as we thought we did. Either conclusion has wide-ranging repercussions for physics.
So now labs all over the globe are anxious to get into the business of snaring solar neutrinos. The Europeans, with Israeli and U.S. cooperation, are installing a neutrino trap under a mountain in Italy, with gallium rather than chlorine as the stopping element. The Soviets are building a similar trap, using sixty metric tons of metallic gallium. Several groups have proposed exploiting naturally-occurring ore bodies to search for evidence of neutrinos that were stopped in the past. And the Japanese are converting a huge detector that was designed to look for disintegrating protons into an instrument to count solar neutrinos.
(Neutrino detectors worldwide were given an unexpected test several weeks ago when a star exploded in the Large Magellanic Cloud, a satellite galaxy of our own Milky Way Galaxy. For a few seconds, that stellar detonation, or supernova, bathed the Earth with a flood of neutrinos two hundred thousand times more intense than what comes from the sun. This astonishing event was apparently recorded by neutrino detectors in this country and in a Japan.)
During the next few years we will be hearing the results of the new experiments to detect neutrinos from the sun. As strange as these experiments seem, they are designed to help us answer one of the most profound questions humans can ask about the world: What makes the sun burn? And since the sun is a typical star, this is equivalent to asking: Why is the universe luminous with light?
To think about such questions, and to answer them, requires the ability to think deeply and seriously about unseen realities. It cannot hurt to have practiced on dreams of hippogriffs and dragons.
The Homestake experiment, which collected neutrinos emitted from nuclear fusion within the Sun, earned physicist Raymond Davis Jr. a share of the 2002 Nobel Prize in Physics. ‑Ed.
