Presentation Speech by Professor S. Claesson of the Royal Swedish Academy of Sciences

Your Majesty, Your Royal Highnesses, Ladies
and Gentlemen.

Professor Lars Onsager has been awarded this year's Nobel Prize
for Chemistry for the discovery of the reciprocal relations,
named after him, and basic to irreversible thermodynamics. On
hearing this motivation for the award one immediately gets a
strong impression that Onsager's contribution concerns a
difficult theoretical field. A closer study shows this indeed to
be the case. Onsager's reciprocal relations can be described as a
universal natural law, the scope and importance of which becomes
clear only after being put in proper relation to complicated
questions in border areas between physics and chemistry. A short
historical review emphasizes this.

Onsager presented his fundamental discovery at a Scandinavian
scientific meeting in Copenhagen in 1929. It was published in its
final form in 1931 in the wellknown journal *Physical
Review* in two parts with the title "Reciprocal relations in
irreversible processes". The elegant presentation meant that the
size of the two papers was no more than 22 and 15 pages
respectively. Judged from the number of pages this work is thus
one of the smallest ever to be rewarded with a Nobel Prize.

One could have expected that the importance of this work would
have been immediately obvious to the scientific community.
Instead it turned out that Onsager was far ahead of his
time.

The reciprocal relations, which were thus published more than a
third of a century ago, attracted for a long time almost no
attention whatsoever. It was first after the second world war
that they became more widely known. During the last decade they
have played a dominant role in the rapid development of
irreversible thermodynamics with numerous applications not only
in physics and chemistry but also in biology and technology. Here
we thus have a case to which a special Rule of the Nobel
Foundation is of more than usual applicability. It reads: "Work
done in the past may be selected for the award only on the
supposition that its significance has until recently not been
fully appreciated."

The great importance of irreversible thermodynamics becomes
apparent if we realize that almost all common processes are
irreversible and cannot by themselves go backwards. As examples
can be mentioned conduction of heat from a hot to a cold body and
mixing or diffusion. When we dissolve a cold lump of sugar in a
cup of hot tea these processes take place simultaneously.

Earlier attempts to treat such processes by means of classical
thermodynamics gave little success. Despite its name it was not
suited to the treatment of dynamic processes. It is instead a
perfect tool for the study of static states and chemical
equilibria. This science was developed during the nineteenth and
the beginning of this century. In this work many of the most
renowned scientists of that time took part. The Three Laws of
Thermodynamics gradually emerged and formed the basis of this
science. These are among our most generally known laws of nature.
The First Law is the Law of Conservation of Energy. The Second
and the Third Laws define the important quantity entropy which
among other things provides a connection between thermodynamics
and statistics. The study of the random motion of molecules by
means of statistical methods has been decisive for the
development of thermodynamics. The American scientist J. Willard
Gibbs (1839-1903) who made so many important contributions to
statistical thermodynamics, has his name attached to the special
professorship which Onsager now holds.

It can be said that Onsager's reciprocal relations represent a
further law making possible a thermodynamic study of irreversible
processes.

In the previously mentioned case with sugar and tea it is the
transport of sugar and heat during the dissolution process which
is of interest in this connection. When such processes occur
simultaneously they influence each other: a temperature
difference will not only cause a flow of heat but also a flow of
molecules and so on.

Onsager's great contribution was that he could prove that if the
equations governing the flows are written in an appropriate form,
then there exist certain simple connections between the
coefficients in these equations. These connections - the
reciprocal relations - make possible a complete theoretical
description of irreversible processes.

The proof of the reciprocal relations was brilliant. Onsager
started from a statistical mechanical calculation of the
fluctuations in a system, which could be directly based on the
simple laws of motion which are symmetrical with regard to time.
Furthermore he made the independent assumption that the return of
a fluctuation to equilibrium in the mean occurs according to the
transport equations mentioned earlier. By means of this
combination of macroscopic and microscopic concepts in
conjunction with an extremely skilful mathematical analysis he
obtained those relationships which are now called Onsager's
Reciprocal Relations.

Professor Lars Onsager. You have made a
number of contributions to physics and chemistry which can be
regarded as milestones in the development of science. For
example, your equation for the conductivity of solutions of
strong electrolytes, your famous solution of the Ising problem,
making possible a theoretical treatment of phase changes, or your
quantisation of vortices in liquid helium. However, your
discovery of the reciprocal relations takes a special place. It
represents one of the great advances in science during this
century.

I have the honour to convey to you the congratulations of the
Royal Academy of Sciences and to ask you to receive the Nobel
Prize for Chemistry for 1968 from the hands of His Majesty the
King.

From *Les Prix Nobel en 1968*, Editor Wilhelm Odelberg, [Nobel Foundation], Stockholm, 1969

Copyright © The Nobel Foundation 1968

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