As you look up at the Sun for a second, a billion neutrinos pass through your eyeball, as nuclear fusion is eating the stars from inside and they irradiate flux of light and neutrinos.
Neutrinos, like photons, weigh nothing and travel at the speed of light.
The conversion of hydrogen into helium in the center of the stars accounts for their brightness in visible light. But this conversion also produces neutrinos, a radiance of mysterious and ghostly kind which make any star glow faintly…
Neutrinos are not a kind of light, thus they are not photons. Matter is transparent to neutrinos, which pass effortlessly through the Earth and through the stars. These elements, like protons, electrons and neutrons, carry an intrinsic angular momentum, or spin, while protons have no spin at all. Only a tiny fraction of neutrinos is stopped by the intervening matter.
Of course, when neutrino reaches you eyeball, it is not stopped at the retina as ordinary photons but it continues unmolested through the back of your head.
The curious part is that if at night you look down at the ground, toward the place where the Sun would be seen if the Earth were not in the way, the same number of solar neutrinos pass through your eyeball, pouring through an interposed Earth which is as transparent to neutrinos as a pane of clear glass is to visible light.
We understand the nuclear physics enough to calculate with fair accuracy the number of solar neutrinos which pass through a given area in a given unit of time. Experimental confirmation of the calculation is however much more difficult. Indeed, since neutrinos pass through the Earth without noticing it, we cannot catch a neutrino.
For a very vast number of neutrinos, however, a very small fraction of them will interact with matter. In the appropriate circumstances they might be detected. They can also on rare occasion convert protons and neutrons.
A few experiments using chlorine, a cleaning fluid, have been performed to neutrinos, as to detect the predicted solar neutrino flux, you need chlorine (American physicists have poured a huge quantity of chlorine into the Homestake Mine in Lead, South Dakota for their experiments). The chlorine is micro-chemically swept for the newly produced argon. The more argon found, the more neutrinos inferred.
These experiments have proved that the Sun is dimmer in neutrinos than the calculations predict. If this is true (and it seems it is true), there is a real and unsolved mystery here.
This low solar neutrino flux does not put our view of stellar nucleo-synthesis in jeopardy. Proposed explanations range from the hypothesis that the nuclear fires in the solar interior are temporarily banked, sunlight being generated partly by slow gravitational contraction, to the idea that neutrinos fall to pieces during their passage between the Sun and the Earth.
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 for the Humans to prove nuclear fusion in the deep interiors of the nearby stars.
The Earth is as transparent to neutrinos as a pane of clear glass is to visible light. Image: © Megan Jorgensen