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Time, Information and Perspective in the Quantum World

Sydney :: 14-15 December 2005

Abstracts and slides from the talks

Wednesday 14 December

Thursday 15 December

9:30 - 11:00

Huw Price
To Be Announced

9:30 - 11:00

Amit Hagar
Quantum Computing: Lessons from two Halting Problems

Morning Tea

11:30 - 1:00

Rod Sutherland
A Time-symmetric Bohm model

11:30 - 1:00

Guido Bacciagaluppi
Time-symmetry argument for v being a gradient in Nelson's theory

Lunch

2:30 - 3:30

Veiko Palge
Open future and quantum delayed choice experiments in McCall's Universe

2:30 - 4:00

Alexei Grinbaum
Information-theoretic derivation of quantum theory via quantum logic

Afternoon Tea

4:00 - 5:30

Guido Bacciagaluppi
Probability, arrow of time and decoherence

4:30 - 6:00

Michael Dickson
The Role of the Observer in Quantum Computation

7:00 - bedtime

Conference dinner





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Abstracts

Huw Price
To Be Announced: the Ontological Significance of Ignorance About the Future
slides

According to an appealing conception, physics aims for a complete, perspective-free description of physical reality; or at least for theories couched in the right conceptual vocabulary for such a description (leaving the details to be filled in by God). However, we human practitioners of physics are chronically ignorant, in two senses. Firstly, obviously, we'll never know more than a tiny fraction of the available facts (hence the task for God). Secondly, and more interestingly, our epistemic viewpoint is chronically or temporally constrained, in striking ways: in particular, we're especially ignorant about the future.

In this talk I argue that the second kind of ignorance is still deeply embedded in the conceptual foundations of physics (of contemporary theoretical physics, including quantum mechanics, as well as of folk physics). In particular, I argue that dispositionsal and counterfactual facts are fundamentally epistemic. From the point of view of God, for whom nothing is ever TBA, there are no facts of this kind.


Rod Sutherland
A Time-symmetric Bohm Model

A time-symmetric version of David Bohmís well-known model will be summarized as an explicit example of a theory involving backwards-in-time effects. The rationale is that such effects can be examined more easily in the context of a specific model. Furthermore, the fairly classical ontology employed in Bohm-type models helps to highlight more clearly the consequences of imposing time-symmetry. In particular, it is seen that we are able to (i) avoid a preferred reference frame, (ii) explain EPR nonlocality and (iii) work with separate 3D wavefunctions in the correlated multiparticle case. Of course, there are trade-offs and the pros and cons of the time-symmetric versus the usual Bohm model will be compared.


Veiko Palge
Open future and quantum delayed choice experiments in McCall's Universe

In the debate between the A- and B-theory of time, the A-theory is associated with ontological becoming and open future. McCall proposes a tree-like model of the Universe which seeks to do justice to both these features. The goal is to give a representation of a world in which objective chance arises out of branching spacetime structure. In such a world, an indeterministic process like quantum measurement results in the world's choosing a particular path through the tree-like structure. In this paper, we discuss compatibility of McCall's model with quantum delayed choice experiments.


Michael Dickson
The Role of the Observer in Quantum Computation
slides

The main point of this talk is to say something preliminary, from a philosopher's point of view, about the role of the observer in quantum computation. I will begin with some more abstract considerations about the nature and role of observers in quantum theory generally, reviewing a group-theoretic account of the notion of an observer based in part on Mackey's imprimitivity theorem. I will then say something about the application of this general account to the case of quantum computation. This talk is thus intended to be a philosophical commentary on the question that is often put in terms of 'the role of measurement in quantum computation'. Given time, I will comment on recent movements in quantum computation that speak to this issue: 'measurement-based quantum computation', and specifically 'one-way quantum computation'.


Guido Bacciagaluppi
Probability, arrow of time and decoherence
slides

This paper relates both to the metaphysics of probability and to the physics of time asymmetry. It investigates whether and how intuitions about 'open future' often associated with probability might be justified in the context of the decoherent histories formalism of quantum mechanics. At the same time it investigates whether and if so under which conditions time asymmetry at the level of histories can be said to emerge from time symmetry at the level of the universal Schroedinger equation.


Amit Hagar
Quantum Computing: Lessons from two Halting Problems
slides

Starting as a visionary idea, quantum computing has become a small industry; by far one of the most fascinating domains in quantum mechanics to this date. The common view is that by harnessing some essentially quantum features, quantum computers, if built, may solve computational problems that are believed to be intractable for classical computers. Concentrating on theoretical aspects rather than on practical ones, I shall discuss two halting problems that constrain any attempt to rewrite the abstract notion of computability using quantum mechanics. I will then use these problems to raise some good natured philosophical doubts about the putative power of quantum computers.


Guido Bacciagaluppi
Time-symmetry argument for v being a gradient in Nelson's theory

Nelson aims to derive the wave function and the Schroedinger equation from conditions on a time-symmetric diffusion process in configuration space. One of the assumptions that are needed is that the current velocity v of the process be the gradient of some function S. We give a new interpretation of this assumption as a further time-symmetry condition. This talk is work in progress.


Alexei Grinbaum
Information-theoretic derivation of quantum theory via quantum logic
slides - papers

In this paper we look at quantum mechanics as a theory about information. We propose a system of information-theoretic axioms and we use quantum logical techniques for deriving the Hilbert space structure. Lattice orthomodularity follows from the postulate of finite amount of relevant information, while quantumness is shown to be due to the postulated possibility to obtain new information. Reconstruction of the formalism is completed by a derivation of the state space and the time evolution from information-theoretically intepreted principles.