
4 2 Physics World September 1993
Physics began with the study of simple models that became more
complicated as they became more realistic. Biology has followed the
opposite path but the two disciplines are now converging in the
study of complex systems
Statistical physics
and biology
The relationship between
biology and physics has
often been close and, at
times, uneasy. During this
century many physicists
have moved to work in
biology. Amongst the ^^^^^^^^^^^^^^^^^
most famous are Francis ^^^^^^^^^^^^^^^^B
Crick (the joint discoverer
of the DNA double helix with Jim Watson), and Max
Delbriick and Salvatore Luria (Nobel prize-winners for
their work on mutations). However, after these scientists
changed their research field, they worked in the same way
as other biologists and used their physics training to a
reduced extent.
An intermediate discipline is biophysics, but here physics
is often used as a tool to serve biology (a similar situation
exists in biochemistry). In both cases, chemistry and
physics provide explanations of what is happening at the
lowest level, that of molecules and forces, but these are
used in a biological framework.
The situation is now changing and scientists are using
physics methods and developments in theoretical physics,
such as statistical mechanics, to study certain fundamental
problems in biology. This phenomenon arises from reasons
common to both disciplines.
Current physics
One cycle in the history of physics has finished and a new
one with different problems is emerging. One of the key
problems in physics has been to discover the fundamental
laws of nature, that is the elementary constituents of matter
and the forces between them. Twenty years ago the
structure of the components of the nucleus (protons and
neutrons) and the origin of nuclear forces were unknown.
Intense debates took place about whether quarks were the
constituents of the proton and there was no clear idea of
the nature of the forces between these hypothetical quarks.
Now almost everything is known about quarks and their
interactions. The laws of
physics,
from the atomic nucleus
to the galaxy, appear to be firmly worked out and most
scientists do not expect the future to hold many surprises.
However, at scales much smaller than the atomic nucleus
and as large as the entire Universe, many things are still not
understood. In some cases we are still in almost total
ignorance. In physics possibly the greatest mystery
still
to be
unravelled is the origin of gravitational forces and their
behaviour over very short distances. This is a difficult
problem, since the crucial experiments may involve
particles with energies many billions of times greater than
currently produced in the laboratory. But, within the range
that affects normal human activities, from the physics of
elementary particles to the study of stellar evolution, we
GIORGIO PARISI
have a satisfactory formula-
tion of the laws.
However, a knowledge of
the laws that govern the
behaviour of the constitu-
ent elements of the system
^^^^^^^^^^^^^^^^ does not necessarily imply
^^^^^^^^^^^^^^^â„¢ an understanding of the
overall behaviour. For ex-
ample, it is not easy to deduce from the forces that act
between molecules of water why ice is lighter than water.
The answer to such questions can be obtained from
statistical mechanics. This discipline, which arose in the
late 19th century from the work of Boltzmann and Gibbs,
studies systems of many particles using probability
methods rather than by determining the trajectories of
the individual particles.
Statistical mechanics has provided the fullest possible
understanding of the emergence of collective behaviour.
We cannot say whether a few atoms of water will form a
solid or a liquid and what the transition temperature is.
Such statements only become precise when many atoms are
being considered (or, more accurately, when the number of
atoms tends to infinity). Phase transitions therefore emerge
as the effect of the collective behaviour of many
components - for example, the transition in which a
normal metal suddenly becomes a superconductor - as the
temperature is lowered. Statistical mechanics has been used
to study
a
variety of phase transitions from which a range of
collective behaviours emerge.
The predictive capabilities of statistical mechanics has
increased greatly over the last 20 years, both through
refinements of the theoretical analyses and through the use
of computers. In particular, interesting results have been
obtained from the study of systems in which the laws have
been selected by chance - so-called disordered systems.
Effects of computers
The computer has produced great changes in theoretical
physics. Current computers can perform about a billion
operations a second on seven figure numbers, and tasks
that were once considered impossible have now become
routine. If one wanted to calculate theoretically the
liquefaction temperature of a gas (argon, for example)
and one already knew the form of the forces between the
atoms, then one had to make very rough approximations
and only under the above conditions could a prediction of
the liquefaction temperature be produced by simple
calculations. However, the prediction would not agree
with the experimental data (typically the error was about
10%).
The only way to improve the agreement between the
predictions and experiment was to remove the approxima-
tions gradually. This theoretical approach, known as