Peering inside the sun: the elusive neutrino

There's not much in nature that's more familiar than the sun. With its warmth and light, it seems strange to think that the big glowing ball above us is still quite a mystery. So much so, that this year's Nobel Prize was given to a group of people who finally proved, about 30 years ago, why the sun shines.

In the late 19th century, the most widely accepted theory was that the sun generates its energy by gravitational contraction. The sun's immense gravitational field, so the theory went, kept the interior of the sun under constant pressure. And, as one of the most basic laws of chemistry states, when you compress a gas, its temperature rises. Eventually, this gravitational pressure-cooker got the temperature hot enough (about 6,000 degrees) to produce visible light, and the sun turned on.

This seemed a simple, elegant way to produce energy, and this energy should have lasted the sun a long time - at least ten million years. Eventually, the gases would get as compressed as they were going to get, and the whole thing would begin to cool off.

The sun probably did heat up, originally, from gravity bringing the gases of an interstellar cloud together and pressing them into a large, glowing ball. But eventually, the very center of the sun became hot enough (many millions of degrees) to ignite nuclear fusion. This process fuses single protons into helium nuclei (which have two protons and two neutrons), producing a generous amount of heat and light in the process.

Fusion is a much more efficient way to produce energy than gravitational compression, and thus the new estimate of the sun's lifetime has risen to about 10 billion years. But how do we know we're right this time? Any nuclear fusion going on inside the sun happens about 400,000 miles beneath the solar surface. We've never seen the nuclear fusion happen.

Well, doesn't the nuclear fusion produce the light? Isn't that proof enough? In reality, the light we see coming from the sun doesn't tell us much about how it was produced. Nuclear fusion reactions in the center of the sun do produce light, but the light has a terrible time getting out of the core of the sun. When the photons are produced, they have almost half a million miles of hot gas to move through before they reach the surface and can sail away into space.

In the hot, super-dense gases inside the sun, the light is continuously absorbed by atoms and then re-emitted in a random direction. This "scattering" effect of the sun's interior gases causes the light to bounce around inside the sun almost indefinitely. On average, it takes about a million years for a photon to reach the surface of the sun after it's been created in the core. That's right. The sunlight coming down on us now was actually created in fusion reactions millions of years ago. And after all that time, the light has lost most of its initial energy and pretty much all the information about how, where, and when it was created.

But this year's Nobel Prize was given to people who didn't despair of ever seeing the inside of the sun. They just got a lot more clever about it.

As it turns out, photons are not the only things that are produced in fusion reactions. Remember that during fusion, protons are rammed together to make helium? Helium nuclei are made of two protons and two neutrons, so where do the neutrons come from? Amazingly, two protons are converted into neutrons during this process. Now, the main difference between protons and neutrons is charge; protons have positive charge and neutrons are neutral. In order to turn into a neutron, a proton has to get rid of its charge. It does this by emitting a positron, which is an anti-matter electron with a positive charge. Once released, the positron sails off and annihilates with an electron, and yes, some of the sun's energy is really generated by matter-antimatter annihilation.

Now here's the clever part.

Scientists knew that the equations of nuclear reactions have to be balanced. If you emit an anti-particle, the universe demands that a regular-matter particle also be created. So what sort of particle was balancing the positron? The particle had to have no charge (as the charge problem has already been handled by the positron), and it would have to have a very tiny mass, if any mass at all. The only particle that fit all those conditions was a neutrino; and the nature of neutrinos is what finally allowed scientists to peer into the very core of the sun.

Neutrinos are just about as close to nothing as matter gets. As the word "neutrino" suggests, these particles have no charge (neutral) and are very, very tiny (the "ino" ending). Neutrinos have almost no mass, travel through space very near the speed of light, and really don't pay much attention to other matter in the universe at all.

Almost nothing in the universe interacts with neutrinos. A neutrino could sail right through a million miles of solid lead without a single particle-sized hiccup. It stands to reason that if neutrinos can sail through most normal matter, then unlike the photons produced by fusion reactions deep inside the solar core, the neutrinos can speed directly out of the sun. Instead of reaching us after millions of years of random interactions, neutrinos from the sun's fusion reaction would reach the Earth about eight minutes after they were created (eight minutes being the light-speed travel time from the Earth to the sun). Solar neutrinos, therefore, are a simple, direct probe of fusions reactions hidden from us in the superdense core of the sun.

Of course, while it's convenient that neutrinos are almost unstoppable, allowing us to peer into the sun, it's a real challenge to actually catch them. Neutrinos can't be stopped easily; after all, they can fly freely through the sun's core, which is denser than any possible material on Earth.

What they will react with, I kid you not, is dry-cleaning fluid. Perchloroethylene is a common, chlorine-rich fluid used in dry cleaning. When neutrinos pass through this fluid, there is a very small chance that a neutrino will bump up against the nucleus of a chlorine atom and change it into argon. It might only happen one time out of trillion trillion chances, but that problem gets easier if you can get a LOT of dry-cleaning fluid together in one place.

Dr. Ray Davis (one of the recipients of this year's Nobel Prize in physics) did just that. His Nobel Prize-winning idea was to get about 100,000 gallons of perchloroethylene and bury the whole lot deep in an old South Dakota gold mine. The idea behind putting the tank in the mine was to clean up any contamination from other high-energy particles from space, namely cosmic rays. Cosmic rays, which are produced by all kinds of processes in space, might produce false-positive results in the neutrino detector, so the couple-thousand feet of rock above the mine was used to screen them out.

Davis and his team did detect a few chlorine atoms turning into argon (on average, one event was observed every two days or so), and scientists had their first readings directly from the solar core.

Interestingly, the number of events, as statistically small as they seem, was far below what particle physicists had predicted. Over many years, this result became impossible to ignore; for some reason, the detectors were only finding about one third of the number of neutrinos that should be there. This was an important cause for concern, as the neutrino rate should be linked to the energy production inside the sun. Could it be possible that the sun's nuclear furnace was slowing down, maybe even stopping?

In the end, it was discovered that neutrinos come in three "species," of which the dry-cleaning fluid could only detect one. If the neutrinos had mass, no matter how tiny, it would be possible for the neutrinos created in the sun to change on their way to Earth. That would mean we'd only detect about one third of the total neutrinos out there, which matched the results exactly!

Now the search is on for how much mass neutrinos have. There are so many neutrinos, careening freely through space, that even if they have a mass less than a billionth the mass of an electron, neutrinos may make up the majority of the mass of the universe. And that may allow the humble neutrino to go from a convenient tool to understand our sun, to a factor that may change the fate or our universe.

Michelle Thaller is an astronomer at the California Institute of Technology.