Neutrinos are produced in the cores of stars by processes such as the PP chain.
Since neutrinos interact only weakly with matter, they speed out of the core of the
star essentially unimpeded. Thus, if neutrinos can be detected from a star, they
provide a glimpse directly into the processes going on now in the core of the star,
while the visible light emitted at the surface may correspond to energies produced
hundreds of thousands of years ago.
Neutrino experiments are difficult because the neutrinos
interact so weakly with matter, but now several independent experiments have
confirmed what was already indicated by the earliest such experiments: the Sun
is producing approximately a factor of 2 fewer neutrinos than we expect that it
should be. This is called the solar neutrino problem. (The standard unit
of neutrino flux used in this discussion is called a "solar neutrino unit" or
snu. Thus, one says that the Sun is not producing enough "snus".)
Possible Explanations of the Solar Neutrino Anomaly
Once experimental difficulties have been ruled out, the
scarcity of snus could be explained in two general ways:
At present it is difficult to accept (1) because the standard solar model is very
successful at describing many other aspects of the Sun (for example, the results
from helioseismology mentioned in an
earlier section). Thus much recent attention has
been focussed on the possibility that neutrinos do something unusual. Most
speculation centers on some varation of a theory that there are three kinds
(called "flavors") of neutrinos, and that their passage through matter can cause
one neutrino flavor to "oscillate" into another.
- Perhaps we don't understand the Sun well enough. Maybe a better theory of the
internal structure of the Sun would predict fewer neutrinos, in agreement with the
Perhaps we don't understand neutrinos well enough; maybe they have some features
beyond the standard theory of neutrinos that account for the anomaly.
It can be shown that if this is arranged in
just the right way it could account for the observed deficit of solar neutrinos.
There is now convincing evidence that neutrinos oscillate in this manner:
The Solar Neutrino Problem