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Zenith Grant Awardee

Dr. Julian Barbour

Oxford University


Joseph Silk, <i>University of Oxford</i><br>Hans Westman, <i>Perimeter Institute, Waterloo, Canada</i><br>Edward Anderson, <i>Pembroke, University of Cambridge, UK</i><br>Sean Gryb, <i>Perimeter Institute, Waterloo, Canada</i>

Project Title

Machian Quantum Gravity

Project Summary

Einstein's general relativity and quantum theory describe different things, gravity and atoms, and have remarkably different structures. To overcome this disharmony, theoreticians must unify the two theories in quantum gravity. This is the aspiration of string theory and loop quantum gravity, but I believe that both these leading projects fail to take proper account of an essential issue. I have spent many years studying the foundations of general relativity, in which Einstein sought to find an alternative to the absolute space introduced by Newton to define the motion of bodies. Being invisible, this problematic concept was criticized by Mach (1883), who argued that the positions of bodies are determined relative to each other. Einstein attempted to implement this idea, now known as Mach's Principle, but did so indirectly and thus created confusion despite the great success of his theory. My collaborators and I have clarified the precise manner in which motion is relative in Einstein's theory and thereby identified its irreducible essential principle. The aim of the Machian Quantum Gravity Project is to use this insight to unify the principles of quantum theory and general relativity. It will be a third route to quantum gravity.

Technical Abstract

Mach's Principle addresses the most basic issue in dynamics: the definition of motion. In 1982 Bertotti and I developed the universal method of best matching for constructing theories that are relational in their treatment of motion and thereby implement Mach's Principle. My recent work with O Murchadha and others has shown that general relativity just fails to be fully Machian because it is not quite scale invariant. The two main approaches to the creation of a quantum theory of gravity – string theory and loop quantum gravity – take little or no account of the specific way in which general relativity is Machian or of its curious failure to be fully relational. They largely ignore its essential dynamical structure. The Machian Quantum Gravity Project aims to rectify this shortcoming by taking the relational structure of Einstein's theory as its guiding principle. It is proposed as a more fundamental third route to quantum gravity.

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