CHAPTER 8 | THE SUN 153
It can take a million years for the energy from a single
gamma ray to work its way outward fi rst as radiation and then as
convection. When the energy fi nally reaches the photosphere, it
is radiated into space as about 1800 photons of visible light.
It is time to ask the critical question that lies at the heart of
science. What is the evidence to support this theoretical explana-
tion of how the sun makes its energy?
Counting Solar Neutrinos
Nuclear reactions in the sun’s core produce fl oods of neutrinos that
rush out of the sun and off into space. Over 10^12 solar neutrinos
fl ow through your body every second, but you never feel them
because you are almost perfectly transparent to neutrinos. If you
could detect these neutrinos, you could probe the sun’s interior.
You can’t focus neutrinos with a lens or mirror, and they zip right
through detectors used to count other atomic particles, but neutri-
nos of certain energies can trigger the radioactive decay of certain
atoms. Th at gives astronomers a way to count solar neutrinos.
In the 1960s, chemist Raymond Davis Jr. devised a way to
count neutrinos produced by hydrogen fusion in the sun. He
buried a 100,000-gallon tank of cleaning fl uid (perchloroethyl-
ene, C 2 Cl 4 ) in the bottom of a South Dakota gold mine where
cosmic rays could not reach it (■ Figure 8-11a) and counted the
number of times a neutrino triggered a chlorine atom into
becoming an argon atom. He expected to detect one neutrino a
day, but he actually counted one-third as many as expected, only
one every three days.
Th e Davis neutrino experiment created a huge controversy.
Were scientists wrong about nuclear fusion in the sun? Did they
misunderstand how neutrinos behave? Was the detector not work-
ing properly? Because astronomers had great confi dence in their
understanding of the solar interior, they didn’t abandon their
theories immediately (How Do We Know? 8-1). It took
over 30 years, but eventually physicists were able to build better
detectors, and they discovered that neutrinos oscillate among three
diff erent types, which physicists call fl avors. Nuclear reactions inScientifi c Confi dence
How can scientists be certain of something?
Sometimes scientists stick so fi rmly to their ideas
in the face of contradictory claims that it sounds
as if they are stubbornly refusing to consider
alternatives. To see why they do this, consider
the perpetual motion machine, a device that
supposedly runs continuously with no source of
energy. If you could invest in a real perpetual
motion machine, you could sell cars that would
run without any fuel. That’s good mileage.
For centuries people have claimed to have
invented perpetual motion machines, and for
just as long scientists have been dismissing
these claims as impossible. The problem with
a perpetual motion machine is that it violates
the law of conservation of energy, and scientists
are not willing to accept that the law could be
wrong. In fact, the Royal Academy of Sciences
in Paris was so sure that a perpetual motion
machine was impossible, and so tired of debunk-
ing hoaxes, that in 1775 they issued a formal
statement refusing to deal with them. The U.S.
Patent Offi ce is so skeptical that they won’t even
consider granting a patent for one without see-
ing a working model fi rst. Why do scientists seem
so stubborn and close-minded on this issue?
Why isn’t one person’s belief in perpetual
motion just as valid as another person’s belief
in the law of conservation of energy? In fact,
the two positions are not equally valid. The
confi dence physicists have in their law is not
a belief or even an opinion; it is an under-
standing founded on the fact that the law has
been tested uncountable times and has never
failed. The law is a fundamental truth about
nature and can be used to understand what is
possible and what is impossible. In contrast,
no one has ever successfully demonstrated a
perpetual motion machine.
When the fi rst observations of solar neu-
trinos detected fewer than were predicted,
some scientists speculated that astronomers
misunderstood how the sun makes its energy
or that they misunderstood the internal
structure of the sun. But many astronomers
stubbornly refused to reject their model
because the nuclear physics of the proton–
proton chain is well understood, and models
of the sun’s structure have been tested
successfully many times. The confi dence
astronomers felt in their understanding of the
sun was an example of scientifi c certainty,
and that confi dence in basic natural laws
prevented them from abandoning decades
of work in the face of a single contradictory
observation.What seems to be stubbornness among
scientists is really their confi dence in basic
principles that have been tested over and
over. Those principles are the keel that keeps
the ship of science from rocking before every
little breeze. Without even looking at that per-
petual motion machine, your physicist friends
can warn you not to invest.8-1
For centuries, people have tried to design a
perpetual motion machine, but not a single one
has ever worked. Scientists understand why.