Neutrinos

Neutrinos

The conversion of hydrogen into helium in the center of the Sun not only accounts for the Sun’s brightness in photos of visible light; it also produces a radiance or a more mysterious and ghostly kind: the Sun glows faintly in neutrinos, which, like photons, weigh nothing and travel at the speed of light. But neutrinos are not photons. They are not a kind of light. Neutrinos, like protons, electrons and neutrons, carry an intrinsic angular momentum, or spin, while protons have no spin at all. Matter is transparent to neutrinos, which pass almost effortlessly through the Earth and through the Sun. Only a tiny fraction of them is stopped by the intervening matter. As I look up at the Sun for a second, a billion neutrinos pass through my eyeball. Of course, they are not stopped at the retina as ordinary photons but they continue unmolested through the back of my head. The curious part is that if at night I look down at the ground, toward the place where the Sun would be (if the Earth were not in the way), almost exactly the same number of solar neutrinos pass through my eyeball, pouring through an interposed Earth which is as transparent to neutrinos as a pane of clear glass is to visible light.

If our knowledge of the solar interior is as complete as we think, and if we also understand the nuclear physics that makes neutrinos, then we should be able to calculate with fair accuracy how many solar neutrinos we should receive in a given area – such as my eyeball – in a given unit of time, such as a second. Experimental confirmation of the calculation is much more difficult.

Nuclear fusion is eating the stars from inside. Image: © Elena

Since neutrinos pass directly through the Earth, we cannot catch a given one. But for a vast number of neutrinos, a small fraction will interact with matter and in the appropriate circumstances might be detected. Neutrinos can on rare occasion convert protons and neutrons. To detect the predicted solar neutrino flux, you need an immense amount of chlorine, so American physicists have poured a huge quantity of cleaning fluid into the Homestake Mine in Lead, South Dakota. The chlorine is microchemically swept for the newly produced argon. The more argon found, the more neutrinos inferred. These experiments imply that the Sun is dimmer in neutrinos than the calculations predict.

There is a real and unsolved mystery here. The low solar neutrino flux probably does not put our view of stellar nucleosynthesis in jeopardy. But it surely means something important. Proposed explanations range from the hypothesis that neutrinos fall to pieces during their passage between the Sun and the Earth to the idea that the nuclear fires in the solar interior are temporarily banked, sunlight being generated in our time partly by slow gravitational contraction. But neutrino astronomy is very new. For the moment we stand amazed at having created a tool that can peer directly into the blazing heart of the Sun. As the sensitivity of the neutrino telescope improves, it may become possible to prove nuclear fusion in the deep interiors of the nearby stars.