PERSPECTIVE: ORIGINS 183How do astronomers know the sun isn’t made
of burning coal? People say dismissively of a
theory they dislike, “That’s only a theory,” as if
a theory were just a random guess. A hypothesis
is something like a guess, but of course not a
random one. What scientists mean by the word
theory is a hypothesis that has “graduated”
to being confi dently considered a well-tested
truth. You can think of a hypothesis as equiva-
lent to having a suspect in a criminal case and
a theory as equivalent to having fi nished the
trial and convicted someone of the crime.
Of course, no matter how many tests and
experiments you conduct, you can never prove
that any scientifi c theory is absolutely true. It
is always possible that the next observation
you make will disprove the theory. And it is
unfortunately sometimes true that innocent
people go to jail and guilty people are free,
but, occasionally, with further evidence, those
legal mistakes can be fi xed.
There have always been hypotheses about
why the sun is hot. Some scientists once
thought the sun was a ball of burning coal.
Only a century ago, most astronomers accepted
the hypothesis that the sun was hot because
gravity was making it contract. In the late
19th century, geologists showed that Earth
was much older than the sun could be if it was
powered by gravity, so the gravity hypothesis
had to be wrong. It wasn’t until 1920 that a
new hypothesis was proposed by Sir Arthur
Eddington, who suggested that the sun is pow-
ered somehow by the energy in atomic nuclei.
In 1938 the German-American astrophysicist
Hans Bethe showed how nuclear fusion could
power the sun. He won the Nobel Prize in 1967.
The fusion hypothesis is now so completely
confi rmed that it is fair to call it a theory.
No one will ever go to the center of the sun,
so you can’t prove the fusion theory is right.
Many observations and model calculations
support this theory, and in Chapter 8 you saw
further evidence in the neutrinos that have
been detected coming from the sun’s core.
Nevertheless there remains some tiny possibility
that all the observations and models are misun-
derstood and that the theory will be overturned
by some future discovery. Astronomers have tre-
mendous confi dence that the sun is powered by
fusion and not gravity or coal, but a scientifi c
theory can never be proven conclusively correct.
There is a great difference between a
theory in the colloquial sense of a far-fetched
guess and a scientifi c theory that has under-
gone decades of testing and confi rmation withIt is only a theory, but astronomers have
tremendous confi dence that the sun generates its
energy by hydrogen fusion and not by burning
coal. (SOHO/MDI)P-3 Theories and Proof
observations, experiments, and models. But no
theory can ever be proven absolutely true. It
is up to you as a consumer of knowledge and
a responsible citizen to distinguish between
a fl imsy guess and a well-tested theory that
deserves to be treated like truth—at least
pending further information.■ Figure P-11
Data from the WMAP spacecraft were used to make this far-infrared map of the sky. This infrared radiation was emitted when the universe was very young
and fi lled with the hot gas of the big bang. The statistical distribution of the tiny irregularities in brightness allows astronomers to determine the age of the
universe plus its rate of expansion. (NASA/WMAP Science Team)