What do neutrinos tell us about the sun?
The sun taught us about neutrinos, now neutrinos teach us about the Sun.
Neutrinos oscillate, they turn into each other. This oscillation between the three species of neutrinos explains the low number of observed neutrinos coming from the sun.
The Gamma rays produced by fusion takes thousands of years to reach the surface of Sun. But neutrinos travel at the speed of light and reach us. So the light is telling us about the fusion which took place millions of years back. But the neutrinos tells us the current status.
It all started with Beta decay. An electron is emitted in the decay of a nucleus. There are no electrons in the nucleus, lepton number is not conserved unless another lepton is formed. Since electric charge is conserved, the particle needed to be neutral, it was called a neutrino. Associated with each lepton, we have one neutrino. One for the electron, one for the muon, and one for the Tau.
Neutrinos interact via gravity, weak interaction, but not electromagnetic interaction. Because they are neutral, their mean free path is larger than that of charged particles. Neutrinos are produced during Proton-Proton reactions. These reaction are fusion reactions, by which nuclei of increasingly large atomic number are produced. Starting with Hydrogen and moving up the Mendeleev ladder.
Fusion is the process that heats up the sun. It also prevents it from collapsing under its own weight. In the sun, conversion Hydrogen into Deuterium takes place through the chain PP I. P P III generates the most energetic neutrinos. It contains the reaction
These reactions were the ones for which John Bacall designed his experiment. When he observed about 1/3 of the expected number of neutrinos, the standard model had to be revised and a tiny mass was shown to give rise to neutrino oscillations. Electron neutrinos can turn into muon neutrinos. In general, all species of neutrinos can turn into each other. With a detector targeting only one species, the neutrino count was off by 2/3.
With this problem now solved, we can and we need to use neutrinos to probe the sun's interior.
Why we need neutrinos to know what is going on inside the sun is because the sun's core has 150 times the density of water. Assuming the path of a photon to be a random walk, the average size of each step was estimated to be 9/100 centimeters. Considering that the photon has almost 700000 kms to travel to reach the surface and that it does so through a diffusion process, one finds that it takes 1.7 10^5 years for photons to emerge out of the sun,
Photons are a poor way of probing the sun's interior. Neutrinos travel close to the speed of light and reach detectors much faster.