Saturday, June 23, 2012

Nothingness and decoherence

More than twenty years ago, philosopher of science David Albert suggested that the global state of the universe might be akin to a vacuum state in relativistic quantum field theory. Albert argued that if one endorsed the many-worlds interpretation of quantum theory, then whilst the global state might correspond to the vacuum, the different local branches of the universe would possess physical substance:

"Observers such as ourselves cannot establish, by any practical means, that our experience is not merely a constituent, merely a branch, of that vacuum...What if the Creator, the Selector of Initial Conditions, had decided not to create; to create nothing, to create the vacuum? That vacuum would already have contained us and what we see around us. The option not to create some world like ours, given the physics [of] relativistic quantum field theory,...is not a logical possibility." (Philosophy of Science Association Symposium 1988, Volume 2, p129).

Albert has since abandoned the many-worlds interpretation, and indeed, now throws scorn on those, such as Lawrence Krauss, who suggest that quantum theory can explain why there is something rather than nothing. However, I'd like to suggest that Albert's proposal can now be resurrected.

The many-worlds interpretation has become increasingly popular in recent years, not only amongst philosophers of physics, but also amongst quantum cosmologists, who are driven to this approach by the fact that the universe has no external observers or measurement devices to collapse the quantum state.

Indeed, in the last month the many-worlds interpretation has received perhaps its most sophisticated defence with the publication of The Emergent Multiverse, the culmination of research pursued by philosopher of physics David Wallace over the past decade.

Central to the modern understanding of the many-worlds interpretation is the notion of decoherence, a process which effectively eliminates interference between macroscopically distinct branches of the quantum state. As Wallace writes, "there is, in fact, every reason to think that the microscopic degrees of freedom of even an isolated system suffice to destroy coherence between macroscopic superpositions of that system's macroscopic degrees of freedom," (p81).

Wallace argues that the quasi-classical, macroscopic branches of the world are an emergent, approximate structure, instantiated in the quantum state of the universe.

In addition to this modern understanding of the many-worlds interpretation, the notion of decoherence can explain how a stationary (i.e., unchanging) global state is consistent with the passage of time in the different macroscopic branches:

The candidate fundamental equation [in canonical quantum cosmology]—the Wheeler-DeWitt equation—is an analogue of a time-independent Schr√∂dinger equation, and does not contain time at all. The problem is thus in a sense simply: where does time come from? In the context of decoherence theory, one can construct toy models in which the analogue of the Wheeler-DeWitt wave function decomposes into non-interfering components (for a suitable sub-system) each satisfying a time-dependent Schr√∂dinger equation, so that decoherence appears in fact as the source of time.

(n29). An analogy from standard quantum mechanics may be helpful here. Take a harmonic oscillator in equilibrium with its environment. An equilibrium state is by definition a stationary state under the dynamics, i.e. it is itself time-independent. However, one can decompose the equilibrium state of the oscillator as a mixture of localised components each carrying out one of the oscillator's possible classical motions (time-dependent!). (Guido Bacciagaluppi, The Role of Decoherence in Quantum Mechanics, Stanford Encyclopedia of Philosophy).

Thus, whilst Krauss attempts to explain the existence of the universe by supposing that the global vacuum state must have been unstable, decoherence explains how the existence of evolving, quasi-classical branches of the universe is consistent with the global state of the universe being the stationary, vacuum state.

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