Three Roads To Quantum Gravity

120 THREE ROADS TO QUANTUM GRAVITY
applied this wisdom to modern physics. He was I believe the
®rst person to gain a deep understanding of the role this idea
plays in the mathematical structure of Einstein's theory of
relativity. In a series of papers, ®rst alone and then with an
Italian friend, Bruno Bertotti, he showed how to formulate
mathematically a theory in which space and time were
nothing but aspects of relationships. Had Leibniz or anyone
else done this before the twentieth century, it would have
changed the course of science.
As it happened, general relativity already existed, but ± and
this is a strange thing to say ± it was widely misunderstood,
even by many of the physicists who specialized in its study.
Unfortunately, general relativity was commonly regarded as a
machine that produces spacetime geometries, which are then
to be treated as Newton treated his absolute space and time: as
®xed and absolute entities within which things move. The
question then to be answered was which of these absolute
spacetimes describes the universe. The only difference between
this and Newton's absolute space and time is that there
is no choice in Newton's theory, while general relativity offers
a selection of possible spacetimes. This is how the theory is
presented in some textbooks, and there are even some
philosophers, who should know better, who seem to interpret
it this way. Julian Barbour's important contribution was to
show convincingly that this was not at all the right way to
understand the theory. Instead, the theory has to be understood
as describing a dynamically evolving network of relationships.
Julian was of course not the only person to learn to see
general relativity in this way. John Stachel also came to this
understanding, at least partly through his work as the ®rst
head of the project to prepare Einstein's collected papers for
publication. But Julian came to the study of general relativity
equipped with a tool that no one else had ± the general
mathematical formulation of a theory in which space and
time are nothing but dynamically evolving relationships.
Julian was then able to show how Einstein's theory of general
relativity could be understood as an example of just such a
theory. This demonstration laid bare the relational nature of
the description of space and time in general relativity.