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Less Interpretation and More Decoherence in Quantum Gravity and Inflationary Cosmology

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Abstract

I argue that quantum decoherence—understood as a dynamical process entailed by the standard formalism alone—carries us beyond conceptual aspects of non-relativistic quantum mechanics deemed insurmountable by many contributors to the recent quantum gravity and cosmology literature. These aspects include various incarnations of the measurement problem and of the quantum-to-classical puzzle. Not only can such problems be largely bypassed or dissolved without default to a particular interpretation, but theoretical work in relativistic arenas stands to gain substantial physical and philosophical insight by incorporating decoherence phenomena.

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Notes

  1. A representative sample of this sort of claim can be found in [3, 4], in which Anderson argues that decoherence eliminates the need to invoke a collapse of the wave function. See also the response to Anderson by Adler [1]; more glimpses of the question of decoherence solving the measurement problem can be found in [2, 7, 40, 42].

  2. Though I follow Schlosshauer’s analysis of the measurement problem, my arguments for what decoherence has to say about each one are not necessarily his.

  3. Granted, it would be hard to say just what a superposition like that would look like: would the molecule be smeared out between the two positions? or flash back and forth between them? or phase in and out of existence? The question might then arise: how do we know these superposed states are still part of the system’s description if we cannot really measure them, and do not even know what it would look like if we could? In response, recall that interference terms are the physical manifestation of interference among different states of a superposition, i.e., phase relations among component states. There have been a number of significant experiments done in recent years in which physicists were able to observe interference phenomena in systems even larger than sugar and ammonia molecules. That is to say, experimenters have been able to stave off decoherence long enough in highly-engineered situations that they could observe interference—which can only be explained as arising from superposed states—in systems that are enormous compared to the sorts of things typically described using quantum mechanics. In fact, the 2012 Nobel Prize in Physics was jointly awarded to Haroche and Wineland, whose respective labs have made extensive progress in measuring the quantum behavior of systems at the microscopic and mesoscopic scale (cf. [8, 31, 33, 35] for details). Physicists have now observed superpositions in systems as large as Rydberg atoms and ‘Bucky balls’—\(C_{60}\) and \(C_{70}\) fullerenes. One wonders whether our engineering might ever become so clever that we could send an elephant into a double-slit apparatus and get a pachydermatous interference pattern out. It is, in principle, possible.

  4. For more experiments in the mesoscopic regime, see [5, 6] or the numerous experiments discussing the production of stable mesoscopic superpositions, or so-called “Schrödinger kittens.”

  5. Although for certain interactions there is a time of recurrence, or a time after which nonlocal aspects of the system will re-achieve their initial values, this length of time is so great in most situations involving uncontrolled decoherence as to be ignored.

  6. By “empirically indistinguishable” I mean there exists no executable test or event or measurement, etc., that would likely ever—even if repeated over the span of several life-times of the universe—yield results differing from what we usually expect in this regard, e.g., a macroscopic system occupying a superposition of position eigenstates.

  7. Independent of decoherence considerations, one might point out that 1C* not only assumes that quantum systems (“whatever they are”) are nevertheless distinct entities from the macroscopic systems they comprise—which is false due to widespread entanglement—but this assumption subjects the whole proposition to the fallacy of composition.

  8. For more details cf. [37].

  9. The authors even quote another cosmology textbook [32] as specifically denying decoherence’s ability to satisfactorily explain translational invariance. However, the quote used by OS is from the manuscript version of Mukhanov’s book, and has been omitted from the published version. One naturally speculates.

  10. For example, [21–24, 26].

  11. This observable, \(\alpha _{lm}\), is a complicated entity described briefly in [34, pp. 122–123].

  12. And you can take your pick - the conceptual point remains. Though the particular values one might calculate for parameters like the decoherence timescale will depend, of course, on the nature of the environment one chooses and on the interaction Hamiltonian for that specific coupling with the system.

  13. Kiefer and Polarski do not in this paper justify their assumption that field amplitude will be the dynamically-emerging pointer basis, but instead refer the reader to such proofs in [26] and [24].

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    Elise M. Crull

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Crull, E.M. Less Interpretation and More Decoherence in Quantum Gravity and Inflationary Cosmology. Found Phys 45, 1019–1045 (2015). https://doi.org/10.1007/s10701-014-9847-4

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