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Time and Background Independence

Alexis de Saint-Ours


Background independence is one of the leading principles in quantum gravity, particularly in canonical quantum gravity. It originates from Leibniz relational ideas on space and time in his critique of Newton’s metaphysical conceptions. Those ideas have come to Einstein through Mach and influenced him in his search for general relativity. Indeed, Einstein’s hole argument -­‐ that leads to the idea that only spacetime points coincidences have reality -­ but not the points themselves -­ can be seen as a modern analogue of Leibniz principle of the identity of indiscernibles.

As a guiding principle, background independence has had much influence on the work done in physics. But it also had a non-­‐trivial impact on physicist’s representation of the world. In particular concerning the issue of time. We will draw attention on how those relational ideas on time can be differently understood. Carlo Rovelli and Julian Barbour -­‐ both claiming for relationalism, do not agree on how to implement background independent time. If both claim that quantum gravity leads to a machian shift in which “It is utterly beyond our power to measure the changes of things by time. Quite the contrary, time is an abstraction at which we arrive through the changes of things”, they don’t agree on which kind of change leads to an operational definition of time (cf. Edward Anderson 2012). At the Planck scale, Rovelli conjectures that there is no good clocks whereas Barbour argues that there is no change nor time. At the classical level, Barbour claims that that some kind of generalized ephemeris time can emerge from timeless laws that governs change where Rovelli proposes a thermal time hypotheses that tries to identify a time in a background independent context.

Finally, we will examine Lee Smolin's recent idea on time. Relying on Bergson’s philosophy, Smolin has argued that “time is real.” Not so paradoxically, we will try to show that Rovelli’s physics without time is closer to Bergson’s intuitions.