Fun with Neutrinos (Part 1)
I have been thinking about developing a public outreach talk on science for a couple of years. I have not made any progress, so I thought I would start putting down some ideas in the blog. If I am lucky, I might get some useful suggestions. If not I will still have something down in writing.
My goal is less to inform people about some topic in physics, than to illustrate how scientists work. The topic that I
want to discuss is the solar neutrino problem and its solution of matter-enhanced neutrino oscilltions. I find the amount of time and effort that it took for physicists to resolve the solar neutrino problem says a lot about what makes science successful. Even the fact that they discovered the problem to begin with is
Neutrinos are fascinating particles in their own right, since they are so different from other particles. They have very small masses and interact very weakly. In the talk I would explain these facts, but the real topic is what experiments physicists chose to do and why they felt they were necessary and how these choices led to completely unexpected discoveries.
Of the several aspects of the solar neutrino problem that make it such an interesting example of doing science, the first is the fact that physicists felt a need to measure the flux of neutrinos of neutrinos coming from the sun. Using some relatively simple order of magitude arguments, you can show that there is only one known process that can power the sun. It is nuclear fusion, and theoretical physicists were able to construct a model of the sun that did a very good job of accounting for the sun’s properties, such as its temperature and the amount of energy that radiates. Nothing observable contradicted this model of the sun, and many people would be satisfied with that. That level of evidence is quite satisfactory for everyday activities. I put gas in my car when the gas gauge is near empty, and it keeps running for me. I see a parking lot full of cars, and I decide to not go to the store later since it is crowded. I do not takevapart my car to see how the gasoline flows, not do I go into the store to check if they were really crowded, since those cars could have been moved there from somewhere else. Doing science is different than everyday life, you have check out other possible explanations. Physicists could not be happy that the solar model worked as well as it did. They wanted to check all of its predictions.
A very simple predcition of the solar model is that neutrinos are produced, and with some detailed calculations you get a prediction of very definite number of neutrinos reaching the earth. Physicists wanted to measure these neutrinos so they could be sure that fusion was powering the sun and not some new unknown mechanism. Now
neutrinos are very hard to detect. If you had a stack of steel plates that went from the earth to the moon only about half of the neutrinos sent through them would be stopped.
Ray Davis led a group of scientists that built a detector, shown at the right, to detect and count the number of neutrinos that come from the sun. It was a 100,000 gallon tank of cleaning fluid located deep in a gold mine. That may not seem like the obvious thing to do, but particle physicists are an interesting example of scientists. Some scientist work in labs, while others work in the field. Most of particle physicists work at particle accelerator labs like Fermilab or CERN, but some go an find other places to do their work. Mines are common for measurements that are sensitive to cosmic rays. There are some who have built a neutrino detector in the ice at the South Pole. They actually use the ice as part of the dectector. Another group has built a giant cosmic ray detector on the high desert of Western Argentina. The need a large fairly flat area without many people since they need a very dark sky. Unlike a biologist or geologist who goes out into the field because what they want to study is there, these physicists use parts of the earth to build customized detectors to study something else.
To be continued in my next entry