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Yes, More Decoherence: a Reply to Critics

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Yes, More Decoherence: a Reply to Critics Yes, More Decoherence: A Reply to Critics Elise M. Crull∗ Submitted 11 July 2017; revised 24 August 2017 1 Introduction A few years ago I published an article in this journal titled \Less interpretation and more decoherence in quantum gravity and inflationary cosmology" (Crull, 2015) that generated replies from three pairs of authors: Vassallo and Esfeld (2015), Okon and Sudarsky (2016) and Fortin and Lombardi (2017). As a philosopher of physics it is my chief aim to engage physicists and philosophers alike in deeper conversation regarding scientific theories and their implications. In as much as my earlier paper provoked a suite of responses and thereby brought into sharper relief numerous misconceptions regarding decoherence, I welcome the occasion provided by the editors of this journal to continue the discussion. In what follows, I respond to my critics in some detail (wherein the devil is often found). I must be clear at the outset, however, that due to the nature of these criticisms, much of what I say below can be categorized as one or both of the following: (a) a repetition of points made in the original paper, and (b) a reiteration of formal and dynamical aspects of quantum decoherence considered uncontroversial by experts working on theoretical and experimental applications of this process.1 I begin with a few paragraphs describing what my 2015 paper both was, and was not, about. I then briefly address Vassallo's and Esfeld's (hereafter VE) relatively short response to me, dedicating the bulk of my reply to the lengthy critique of Okon and Sudarsky (hereafter OS). The contribution to this debate by Fortin and Lombardi (hereafter FL) largely substantiates my points; I will call upon their paper at various times in my defense. ∗The City College of New York, CUNY. [email protected] 1This 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. 1 1.1 What my paper was about The aim of my original paper was to urge researchers investigating applications of quan- tum theory (especially, but not exclusively, relativistically) to consider the fruitfulness of decoherence studies and incorporate these results in their own pursuits. Doing so, I ar- gued, would have several benefits. It would significantly delay the moment when various interpretations of quantum mechanics, on which there is no consensus, were called upon to explain certain dynamical facts. This thereby lessens the significance of betting on the wrong horse (or indeed on any one horse) in a race of indeterminate length.2 My motivation for presenting a breakdown of the measurement problem similar to that of Max Schlosshauer in his textbook on decoherence (Schlosshauer, 2007) was to make clear that while decoherence cannot provide an answer to the question of why one specific, apparently determinate outcome is measured rather than another, it can provide satisfactory explanations for kindred questions like the selection of preferred bases and the lack of observable superpositions. With these explanations forming my premises, I argued that troubles encountered within relativistic applications of quantum theory frequently attributed to the measurement problem stem from those aspects of the problem decoherence in fact explains. Hence, progress can be made despite the intractability of the problem of specific measurement outcomes. 1.2 What my paper was not about Nowhere in the original paper was it said that interpretations of quantum mechanics could be eliminated altogether (pace OS and VE), nor that decoherence solves all problems to completeness (pace FL); my claims were more tempered than that. Regarding the latter, FL understood me to be offering a too-rosy view of decoherence as a concept free from controversy and perfectly well understood. Certainly not. But it should be involved in far less controversy than it is, and what is more, what the consensus view consists in is sufficient physics to run my arguments. Perhaps these misunderstandings arose because the claim has been made elsewhere (though never by me) that decoherence solves the measurement problem tout court. How- ever, since in the original paper I explicitly denied this incorrect view of decoherence, I am unsure what to say to my critics. Likewise, perhaps some of the misunderstanding of my points was due to the widespread and persistent conflation of \decoherence the- ory" or \environment-induced decoherence", or simply \decoherence processes" { none of which exceed the implications of the bare quantum formalism { with explicitly interpre- 2Those who, prior to decoherence considerations, are already standard-bearers for a particular interpre- tation ought to acknowledge that decoherence importantly alters both the motivations for and implemen- tations 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 pro- cesses from a standpoint that is neutral with respect to the interpretation debate, and still gain significant explanatory benefits. 2 tive projects that take decoherence (qua physical process) as their starting point, like the decoherent histories interpretation of Gell-Mann and Hartle (1996) and Halliwell (1995). But once again, in the original paper I was careful to place myself firmly in the former camp regarding my employment of the term \decoherence"; such misunderstandings are unnecessary. 2 Reply to Vassallo and Esfeld VE raise two primary objections that overlap somewhat with comments made by OS; I will respond to them together here, as this will prove a useful starting point. The first objection concerns what precisely I meant by \the standard quantum formalism", and whether or not this requires assuming wave function collapse or the Born rule. The second objection concerns Schr¨odinger's cat. 2.1 The Bare Formalism VE write: \While we agree with the author that decoherence is a powerful tool that effectively helps us dealing with the conceptual asperities of quantum physics { the mea- surement problem above all {, we do not see how this fact might dispense us with the urge to interpret the bare quantum formalism" (Vassallo and Esfeld, 2015, p. 1533).3 Here VE misunderstand my general point, which (as reiterated above) is not to say one can do away with interpretation entirely, but rather that one might profitably post- pone the invocation of particular interpretations of quantum mechanics. Admittedly I ought to have been explicit about what I took the standard formalism to be. OS crit- icize both Max Schlosshauer (who, recall, literally wrote the book on decoherence) and myself on this point: \...by not stating precisely what is the concrete quantum formalism he [Schlosshauer] endorses, he allows for some `wiggle room' at the moment of physically interpreting the formal results obtained by decoherence" (OS p. 858). Schlosshauer does in fact state in later chapters which aspects of the formalism he requires. I, on the other hand, need only the propositions OS themselves allow { but with important qualifications: the complete statistics of mixed states can be represented by density operators in a Hilbert 3Hereafter, unless otherwise noted, all references to VE are to this article; likewise, all references to OS are to Okon and Sudarsky (2016) and all references to FL are to Fortin and Lombardi (2017). 3 space,4 observables (\properties") are typically self-adjoint operators,5 and the Schr¨odinger equation describes the unitary evolution of closed systems. Add to these three propositions certain widely accepted facts about the superposition rule, entanglement, and the ubiquity of quantum interactions (conversely, the nonexistence of a truly closed system aside from the universe as a whole), and one has enough to get decoherence dynamics. I disagree with OS when they claim that I cannot arrive at decoherence without also as- suming wave function collapse and the Born rule (ergo the eigenstate-eigenvalue link). This is not the case, though why I do not require these assumptions understandably becomes murky owing to OS's conflation of \standard formalism" with \standard interpretation" at various times in their exposition (where the latter is meant to refer to von Neumann's view or the mythical Copenhagen interpretation, perhaps? Neither of which, please note, I endorse). Regarding the question of collapse, VE and OS are certainly not the first to elide com- ponents of physical collapse theories like GRW with the effects of decoherence. In fact, this mistake is so frequent that the Stanford Encyclopedia of Philosophy entry on decoherence contains a caveat listing two physical assumptions that collapse theories require but that decoherence does not. It will be instructive to repeat them here. First, collapse theories require true, physical collapse of the wave function; decoherence only entails effective col- lapse, in that this process renders practically impossible measurement results that are not eigenstates of an unperturbed basis (Bacciagaluppi, 2012). The experimental fact that cer- tain decohered systems exhibit coherence revivals demonstrates that no physical collapse occurred in these cases.6 The second assumption collapse theories require but decoherence does not is the exis- tence of an as-yet undiscovered collapse mechanism that introduces nonlinearity apart from the system-environment Hamiltonian. Decoherence introduces no additional nonlinearity but \simply takes real, inevitable, interaction with external degrees of freedom to bear in the Hamiltonian" (Bacciagaluppi, 2012).
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