**Decoherence in the Philosophy of Quantum Physics**

University of Leeds

Friday 2^{nd} September 2016

**Schedule:**

9.30 Arrival

10.00-10.40 Session 1a Nahuel Sznajderhaus (University of Leeds), “Decoherence in the Received Realist View of Quantum Mechanics”

10.40-11.20 Session 1b Simon Newey (University of Leeds), “A decoherent multiverse; consistent or incoherent?”

11.20-11.40 Coffee

11.45-12.55 Session 2 Guido Bacciagaluppi (University of Utrecht), “Classicality, decoherence and time-symmetry”

12.55-13.45 Lunch

13.50-15.00 Session 3 Chris Timpson (Univeristy of Oxford), “Decoherence and… the beginning of time!”

15.00-15.10 Coffee

15.15-16.25 Session 4 Elise Crull (The City College of New York), “Quantum Rods and Clocks from Decoherence?”

16.25-16.45 Coffee

16.50-18.00 Session 5 Claus Kiefer (University of Cologne), “Decoherence in situations involving the gravitational field”

18.10 Pub and dinner

**Organising Committee: **Simon Newey and Nahuel Sznajderhaus

**Venue:**

Room G36, Baines Wing, School of PRHS, Woodhouse Lane, Leeds LS2 9JT

For advice about getting to the university please see: https://www.leeds.ac.uk/info/5000/about/131/find_us

There is no registration fee, but please contact Simon Newey at prsmtn@leeds.ac.uk and Nahuel Sznajderhaus at phns@leeds.ac.uk by 25th August if you plan to attend or have any questions about the event so that we can arrange catering.

The workshop is part of the ARHC Scientific Realism and the Quantum Project

**Abstracts:**

**Classicality, decoherence and time-symmetry
**(Guido Bacciagaluppi, University of Ultrecht)

Decoherence is the crucial ingredient for deriving classical behaviour from quantum mechanics, but there is a mismatch between thinking of decoherence as time-asymmetric and thinking of classical mechanics as time-symmetric. In this talk I explore a toy model of time-symmetric decoherence.

**Quantum Rods and Clocks from Decoherence?
**(Elise Crull, The City College of New York)

It has long been argued (perhaps most forcibly by Harvey Brown in his 2005 book, *Physical Relativity*) that if one wishes to consider quantum theory as fundamental, then one must provide a quantum-dynamical description of rods and clocks — entities treated as ontological primitives in relativistic measurements. In keeping with recent theoretical work by Rodolfo Gambini and collaborators, this talk explores whether the microdynamics of decoherence might explain the emergence of suitably robust rods and clocks in gravitational fields.

** **

**Decoherence in situations involving the gravitational field
**(Claus Kiefer, University of Cologne)

The quantum-to-classical transition through decoherence arises from the entanglement between the system under consideration and irrelevant degrees of freedom (“environment”) to which it is coupled. Because of the ubiquitous nature of gravity, two questions arise. First, what is the effect of decoherence on a quantized gravitational field? And second, what is the role of gravity in the decoherence of non-gravitational degrees of freedom? In my talk, I shall address both questions. I shall discuss decoherence in quantum cosmology, the decoherence of primordial cosmological fluctuations, and the possible decohering influence of gravity in laboratory situations.

**A decoherent multiverse: consistent or incoherent?
**(Simon Newey, University of Leeds)

Modern Everettians rely on decoherence to provide a non-arbitrary basis of branching quasi-classical worlds. Zurek (2005) showed deriving such a basis of worlds in this way to be circular. This has been thought to represent a significant problem for this approach. Wallace (2012) avoids this problem by presenting the branching structure of worlds as emergent from, rather than derivative of, linear quantum mechanics. Longstanding circular derivations of the branching structure therefore represent a demonstration of consistency, rather than a putative proof.

Dawid and Thebault (2015) argue that the decision-theoretic Born rule, which Wallace derives for an agent in an Everettian branching structure, is insufficient to ground decoherence. Rather, they believe, decoherence rests on a stronger objective formulation of the Born rule, which is incompatible with (fundamentally deterministic) Everettian QM. If true, this would seem to show Wallace’s multiverse to be incoherent.

I will argue that Dawid and Thebault are mistaken in suggesting that the decision-theoretic Born rule is inadequate for obtaining decoherence. I will also argue, however, that, although they are wrong to suggest that Wallace’s position is incoherent, they do identify significant concerns relating to its evidential basis.

References:

Dawid, R. & Thebault, K. [2015]. “Many worlds: Decoherent or incoherent?” Synthese 192: 1559-1580.

Wallace, D. [2012] The Emergent Multiverse: Quantum Theory According to the Everett Inter-pretation. Oxford: University Press.

** **

**Decoherence and…the beginning of time!
**(Chris Timpson, University of Oxford)

As discussed in Wallace’s canonical 2012 *The Emergent Multiverse*, there are clear links between the appeal to decoherence to define branching structure in Everettian quantum mechanics and the traditional problematic in thermal physics regarding the emergence of time-asymmetric behaviour from underlying time-symmetric dynamics. I will review these thoughts, focussing my remarks around Kastner’s 2014 contention (doi:10.1016/j.shpsb.2014.06.004) that there is real problem here for the Everettian, indeed a problem worse than in the traditional thermal physics case. It seems to me that this isn’t so.

**Decoherence in the Received Realist View of Quantum Mechanics
**(Nahuel Sznajderhaus, University of Leeds)

I present a realist framework built upon very basic realist intuitions in order to question the role decoherence plays in what I call the Received View of quantum mechanics. As a response to worries about decoherence, such as those raised by d’Espagnat (1995, p. 257), Bacciagaluppi (2014), Kastner (2014), and Landsman (2007), it has been advocated that decoherence is valid only “for all practical purposes” (FAPP). Now, the realist is aware of the character of this kind of arguments: they resemble an instrumentalism which is content with empirical predictive power. This tension is dissolved within my realism, whereby the real question is not to explain the appearance of classicalities but to realistically describe quantum systems (even before the coherence is delocalised).

References:

G. Bacciagaluppi. Measurement and classical regime in quantum mechanics. In R. Batterman, editor, The Oxford Handbook of Philosophy of Physics, pages 416–459. OUP, 2014.

B. d’Espagnat. Veiled Reality. Reading-MA: Addison-Wesley Publishers, 1995.

R. E. Kastner. Einselection of pointer observables: The new H-theorem? Studies in History and Philosophy of Science Part B: Studies in History and Philosophy of Modern Physics, 48: 56–58, 2014.

N. P. Landsman. Between classical and quantum. In B. Jeremy and E. John, editors, Handbook of the Philosophy of Science: Philosophy of Physics, pages 417–554. Elsevier, 2007.