arXiv subjects

Huw Price

Publications and source records attributed to Huw Price.

28 records · Page 2Linked to original sources

arXiv2011

Action Duality: A Constructive Principle for Quantum Foundations

An analysis of the path-integral approach to quantum theory motivates the hypothesis that two experiments with the same classical action should have dual ontological descriptions. If correct, this hypothesis would not only constrain realistic interpretations of quantum theory, but would also act as a constructive principle, allowing any realistic model of one experiment to generate a corresponding model for its action-dual. Two pairs of action-dual experiments are presented, including one experiment that violates the Bell inequality and yet is action-dual to a single particle. The implications generally support retrodictive and retrocausal interpretations.

quant-ph↗

arXiv2010

New Slant on the EPR-Bell Experiment

The best case for thinking that quantum mechanics is nonlocal rests on Bell's Theorem, and later results of the same kind. However, the correlations characteristic of EPR-Bell (EPRB) experiments also arise in familiar cases elsewhere in QM, where the two measurements involved are timelike rather than spacelike separated; and in which the correlations are usually assumed to have a local causal explanation, requiring no action-at-a-distance. It is interesting to ask how this is possible, in the light of Bell's Theorem. We investigate this question, and present two options. Either (i) the new cases are nonlocal, too, in which case action-at-a-distance is more widespread in QM than has previously been appreciated (and does not depend on entanglement, as usually construed); or (ii) the means of avoiding action-at-a-distance in the new cases extends in a natural way to EPRB, removing action-at-a-distance in these cases, too. There is a third option, viz., that the new cases are strongly disanalogous to EPRB. But this option requires an argument, so far missing, that the physical world breaks the symmetries which otherwise support the analogy. In the absence of such an argument, the orthodox combination of views -- action-at-a-distance in EPRB, but local causality in its timelike analogue -- is less well established than it is usually assumed to be.

quant-ph↗

arXiv2008

Toy Models for Retrocausality

A number of writers have been attracted to the idea that some of the peculiarities of quantum theory might be manifestations of 'backward' or 'retro' causality, underlying the quantum description. This idea has been explored in the literature in two main ways: firstly in a variety of explicit models of quantum systems, and secondly at a conceptual level. This note introduces a third approach, intended to complement the other two. It describes a simple toy model, which, under a natural interpretation, shows how retrocausality can emerge from simple global constraints. The model is also useful in permitting a clear distinction between the kind of retrocausality likely to be of interest in QM, and a different kind of reverse causality, with which it is liable to be confused. The model is proposed in the hope that future elaborations might throw light on the potential of retrocausality to account for quantum phenomena.

quant-ph↗

arXiv2008

Decisions, Decisions, Decisions: Can Savage Salvage Everettian Probability?

Critics object that the Everett view cannot make sense of quantum probabilities, in one or both of two ways: either it cannot make sense of probability at all, or it cannot explain why probability should be governed by the Born rule. David Deutsch has attempted to meet these objections by appealing to an Everettian version of Savage's rational decision theory. Deutsch argues not only that an analogue of classical decision under uncertainty makes sense in an Everett world; but also that under reasonable assumptions, the betting odds of a rational Everettian agent should be constrained by the Born rule. Deutsch's proposal has been defended and developed by David Wallace, and in a different form by Hilary Greaves. In this paper I offer some objections to the Deutsch-Wallace-Greaves argument, focussing in particular on the supposed analogy with classical decision under uncertainty.

quant-ph↗

arXiv2006

Probability in the Everett World: Comments on Wallace and Greaves

It is often objected that the Everett interpretation of QM cannot make sense of quantum probabilities, in one or both of two ways: either it can't make sense of probability at all, or it can't explain why probability should be governed by the Born rule. David Deutsch has attempted to meet these objections. He argues not only that rational decision under uncertainty makes sense in the Everett interpretation, but also that under reasonable assumptions, the credences of a rational agent in an Everett world should be constrained by the Born rule. David Wallace has developed and defended Deutsch's proposal, and greatly clarified its conceptual basis. In particular, he has stressed its reliance on the distinguishing symmetry of the Everett view, viz., that all possible outcomes of a quantum measurement are treated as equally real. The argument thus tries to make a virtue of what has usually been seen as the main obstacle to making sense of probability in the Everett world. In this note I outline some objections to the Deutsch-Wallace argument, and to related proposals by Hilary Greaves about the epistemology of Everettian QM. (In the latter case, my arguments include an appeal to an Everettian analogue of the Sleeping Beauty problem.) The common thread to these objections is that the symmetry in question remains a very significant obstacle to making sense of probability in the Everett interpretation.

quant-ph↗

arXiv2004

The Thermodynamic Arrow: Puzzles and Pseudo-puzzles

For more than a century, physics has known of a puzzling conflict between the T-asymmetry of thermodynamic phenomena and the T-symmetry of the underlying microphysics on which these phenomena depend. This paper provides a guide to the current status of this puzzle, distinguishing the central issue from various issues with which it may be confused. It is shown that there are two competing conceptions of what is needed to resolve the puzzle of the thermodynamic asymmetry, which differ with respect to the number of distinct T-asymmetries they take to be manifest in the physical world. On the preferable one-asymmetry conception, the remaining puzzle concerns the ordered distribution of matter in the early universe. The puzzle of the thermodynamic arrow thus becomes a puzzle for cosmology.

physics.class-ph↗

arXiv1996

Time Symmetry in Microphysics

Physics takes for granted that interacting physical systems with no common history are independent, before their interaction. This principle is time-asymmetric, for no such restriction applies to systems with no common future, after an interaction. The time-asymmetry is normally attributed to boundary conditions. I argue that there are two distinct independence principles of this kind at work in contemporary physics, one of which cannot be attributed to boundary conditions, and therefore conflicts with the assumed T (or CPT) symmetry of microphysics. I note that this may have interesting ramifications in quantum mechanics.

quant-ph↗

arXiv1996

Locality, Independence and the Pro-Liberty Bell

Construed as an argument against hidden variable theories, Bell's Theorem assumes that hidden variables would be independent of future measurement settings. This Independence Assumption (IA) is rarely questioned. Bell considered relaxing it to avoid non-locality, but thought that the resulting view left no room for free will. However, Bell seems to have failed to distinguish two different strategies for giving up IA. One strategy takes for granted the Principle of the Common Cause, which requires that a correlation between hidden variables and measurement settings be explained by a joint correlation with some unknown factor in their common past. The other strategy rejects the Principle of the Common Cause, and argues that the required correlation might be due to the known interaction between the object system and the measuring device in their common future. Bell and most others who have discussed these issues have focussed on the former strategy, but because the two approaches have not been properly distinguished, it has not been well appreciated that there is a quite different way to relax IA. This paper distinguishes the two strategies, and argues that the latter is considerably more appealing than the former.

quant-ph↗

arXiv1994

A Neglected Route to Realism About Quantum Mechanics

Bell's Theorem assumes that hidden variables are not influenced by future measurement settings. The assumption has sometimes been questioned, but the suggestion has been thought outlandish, even by the taxed standards of the discipline. (Bell thought that it led to fatalism.) The case for this reaction turns out to be surprisingly weak, however. We show that QM easily evades the standard objections to advanced action. And the approach has striking advantages, especially in avoiding the apparent conflict between Bell's Theorem and special relativity. The second part of the paper considers the broader question as to why advanced action seems so counterintuitive. We investigate the origins of our ordinary intuitions about causal asymmetry. It is argued that the view that the past does not depend on the future is largely anthropocentric, a kind of projection of our own temporal asymmetry. Many physicists have also reached this conclusion, but have thought that if causation has no objective direction, there is no objective content to an advanced action interpretation of QM. This turns out to be a mistake. From the ordinary subjective perspective, we can distinguish two sorts of objective world: one "looks as if" it contains only forward causation, the other ``looks as if'' it involves a mix of backward and forward causation. This clarifies the objective core of an advanced action interpretation of QM, and shows that there is an independent symmetry argument in favour of the approach.

gr-qc↗

arXiv1993

Cosmology, Time's Arrow, and That Old Double Standard

It is widely accepted that temporal asymmetry is largely a cosmological problem; the task of explaining temporal asymmetry reduces in the main to that of explaining an aspect of the condition of the early universe. However, cosmologists who discuss these issues often make mistakes similar to those that plagued nineteenth century discussions of the statistical foundations of thermodynamics. In particular, they are often guilty of applying temporal "double standards" of various kinds---e.g., in failing to recognise that certain statistical arguments apply with equal force in either temporal direction. This paper aims to clarify the issue as to what would count as adequate explanation of cosmological time asymmetry. A particular concern is the question whether it is possible to explain why entropy is low near the Big Bang without showing that it must also be low near a Big Crunch, in the event that the universe recollapses. I criticise some of the objections raised to this possibility, showing that these too often depend on a temporal double standard. I also discuss briefly some issues that arise if we take the view seriously. (Could we observe a time- reversing future, for example?)

gr-qc↗