Three Roads To Quantum Gravity

AREA AND INFORMATION
97
tion that matter and radiation were continuous. The experiments
that detected atoms came later because their very
conceptualization required ideas that were invented as part
of the same process. Had the experiments been done twenty
years earlier, the results may not even have been interpreted
as evidence for the existence of atoms.
The crucial arguments that convinced people of the existence
of atoms had to do with understanding the laws
governing heat, temperature and entropy ± the part of physics
called thermodynamics. Among the laws of thermodynamics
are the second law, which we have already discussed, which
states that entropy never decreases, and the so-called zeroth
law, which states that when the entropy of a system is as high
as possible, it has a single uniform temperature. Between
them comes the ®rst law, which asserts that energy is never
created or destroyed.
During most of the nineteenth century most physicists did
not believe in atoms. It is true that the chemists had found
that different substances combine in ®xed ratios, which was
suggestive of the existence of atoms. But the physicists were
not very impressed. Until 1905 most of them thought either
that matter was continuous, or that the question of whether
there were atoms or not lay outside science, because even if
they existed atoms would be forever unobservable. These
scientists developed the laws of thermodynamics in a form
that made no reference to atoms or their motions. They did
not believe the basic de®nitions of temperature and entropy
that I introduced in earlier chapters: that temperature is a
measure of the energy of random motion, and that entropy is a
measure of information. Instead, they understood temperature
and entropy as essential properties of matter: matter was
just a continuous ¯uid or substance, and temperature and
entropy were among its basic properties.
Not only did the laws of thermodynamics make no
reference to atoms, but the nineteenth-century founders of
the theory even believed there was a reason why there could
be no relation between atoms and thermodynamics. This is
because the second law, by saying that entropy increases
towards the future, introduces an asymmetry in time. Accord-