Abstract
Recently I published an article in this journal entitled “Less interpretation and more decoherence in quantum gravity and inflationary cosmology” (Crull in Found Phys 45(9):1019–1045, 2015). This article generated responses from three pairs of authors: Vassallo and Esfeld (Found Phys 45(12):1533–1536, 2015), Okon and Sudarsky (Found Phys 46(7):852–879, 2016) and Fortin and Lombardi (Found Phys, 2017). In what follows, I reply to the criticisms raised by these authors.
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Notes
This latter point explains in part why I did not include equations in the original paper—a fact that Okon and Sudarsky mention more than once as a defect of my account. I had assumed (wrongly, it seems) that the basic aspects of decoherence were well enough known and appreciated from the prodigious and respected literature—much of which I cited in my paper—that instead of presenting it all yet again, I might more profitably focus on certain philosophical aspects. I will try to remedy this lack of formalism to my critics’ satisfaction in what follows, but again I direct the reader to references cited in this and the original paper.
Those who, prior to decoherence considerations, are already standard-bearers for a particular interpretation ought to acknowledge that decoherence importantly alters both the motivations for and implementations of the various candidate interpretations. However, cataloguing those differences is not my aim here, nor was it in the original paper. Instead, my aim was to emphasize that one can invoke decoherence processes from a standpoint that is neutral with respect to the interpretation debate, and still gain significant explanatory benefits.
Qualifications: (i) mixed states needn’t be represented this way; e.g., one might equally well employ the reduced Feynman path integral (RPI) approach described in Mensky [55]. Also, (ii) while both proper and improper mixtures may have identical density matrices, they nevertheless represent vastly different physical situations: the former represents a classical statistical distribution of possible states (where the system definitely occupies one state but it is unknown which), while the latter represents an entangled case (where the system cannot be said to occupy a single state from the ensemble). Because entanglement is a necessary condition for decoherence, whenever one uses the density matrix formalism to study decoherence processes, one is dealing with improper mixtures. One must not lose sight of this failure of uniqueness in the formalism.
Qualifications: again, this proposition isn’t strictly necessary within the standard formalism. Observables—or operators more generally—are derivative entities used to link the formalism to empirical data. This point was made in the original paper in discussion of Shimony’s epistemic circle and by referring to research conducted using non-Hermitian operators.
More precisely: I used decoherence to explain the cat’s apparently well-defined state within the alive-dead basis, but was careful to say decoherence does not explain why a given observation yields the specific state that it does.
Instead of defending in detail what I name with confidence “the received view”, I direct skeptics to the wealth of literature—spanning nearly half a century—supporting this claim. For starters: Zeh [79], Kübler and Zeh [49], Zurek [82], Zurek [83], Giulini et al. [32], Giulini et al. [31], Diósi and Kiefer [24], Mensky [55], Zeh [80], Zurek [84], Duplantier et al. [26], Joos [45], Zurek [85], Stamp [73], Castagnino et al. [17], Janssen [43], Lombardi et al. [52], Gell-Mann and Hartle [30].
In addition to research from Haroche’s Paris team cited above in Sect. 2.2, the Institute of Quantum Optics and Quantum Information in Austria performed a suite of experiments measuring interference patterns with fullerenes: Arndt et al. [3], Arndt et al. [4], Brezger et al. [16], Hackermüller et al. [35], Hackermüller et al. [34], Hornberger et al. [41].
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Acknowledgements
I’d like to thank Max Schlosshauer, Don Howard, Claus Kiefer, Carlo Rovelli, Guido Bacciagaluppi and an anonymous referee for their support and helpful discussions.
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Crull, E.M. Yes, More Decoherence: A Reply to Critics. Found Phys 47, 1428–1463 (2017). https://doi.org/10.1007/s10701-017-0116-1
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DOI: https://doi.org/10.1007/s10701-017-0116-1
Keywords
- Decoherence
- Quantum mechanics
- Interpretation
- Entanglement