Huw Price

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.

The so-called Canberra Plan is a grandchild of the Ramsey-Carnap treatment of theoretical terms. In its original form, the Ramsey-Carnap approach provided a method for analysing the meaning of scientific terms, such as “electron”, “gene” and “quark”—terms whose meanings could plausibly be delineated by their roles within scientific theories. But in the hands of David Lewis (1970, 1972), the original approach begat a more ambitious descendant, generalised and extended in two distinct ways: first, Lewis applied the technique to analyse the meaning of terms introduced not just by explicit scientific theories, but also by implicit folk theories such as folk psychology; second, he supplemented the theory to provide an account of the way in which the referents of the analysed terms might be identified on the basis of empirical investigation.

The status and respectability of alethic modality was always a point of contention and divergence between naturalism and empiricism. It poses no problems in principle for naturalism, since modal vocabulary is an integral part of all the candidate naturalistic base vocabularies. Fundamental physics is above all a language of laws; the special sciences distinguish between true and false counterfactual claims; and ordinary empirical talk is richly dispositional. By contrast, modality has been a stumbling-block for the empiricist tradition ever since Hume forcefully formulated his epistemological and ultimately semantic objections to the concepts of law and necessary connection. (, Lecture , §).

Proponents of causal decision theories argue that classical Bayesian decision theory (BDT) gives the wrong advice in certain types of cases, of which the clearest and commonest are the medical Newcomb problems. I defend BDT, invoking a familiar principle of statistical inference to show that in such cases a free agent cannot take the contemplated action to be probabilistically relevant to its causes (so that BDT gives the right answer). I argue that my defence does better than those of Ellery Eells and Richard Jeffrey; and that it applies, where necessary, to other types of Newcomb problem.

In a recent paper, Richard Rorty begins by telling us why pragmatists such as himself are inclined to identify truth with justification: ‘Pragmatists think that if something makes no difference to practice, it should make no difference to philosophy. This conviction makes them suspicious of the distinction between justification and truth, for that distinction makes no difference to my decisions about what to do.’ (1995, p. 19) Rorty goes on to discuss the claim, defended by Crispin Wright, that truth is a normative constraint on assertion. He argues that this claim runs foul of this principle of no difference without a practical difference: ‘The need to justify our beliefs to ourselves and our fellow agents subjects us to norms, and obedience to these norms produces a behavioural pattern that we must detect in others before confidently attributing beliefs to them. But there seems to be no occasion to look for obedience to an additional norm – the commandment to seek the truth. For – to return to the pragmatist doubt with which I began – obedience to that commandment will produce no behaviour not produced by the need to offer justification.’ (1995, p. 26) Again, then, Rorty appeals to the claim that a commitment to a norm of truth rather than a norm of justification makes no behavioural difference. This is an empirical claim, testable in principle by comparing the behaviour of a community of realists (in Rorty’s sense) to that of a community of pragmatists. In my view, the experiment would show that the claim is unjustified, indeed false. I think that there is an important and widespread behavioural pattern that depends on the fact that speakers do take themselves to be subject to such an additional norm. Moreover, it is a..

It has often been suggested that retrocausality offers a solution to some of the puzzles of quantum mechanics: e.g., that it allows a Lorentz-invariant explanation of Bell correlations, and other manifestations of quantum nonlocality, without action-at-a-distance. Some writers have argued that time-symmetry counts in favour of such a view, in the sense that retrocausality would be a natural consequence of a truly time-symmetric theory of the quantum world. Critics object that there is complete time-symmetry in classical physics, and yet no apparent retrocausality. Why should the quantum world be any different? This note throws some new light on these matters. I call attention to a respect in which quantum mechanics is different, under some assumptions about quantum ontology. Under these assumptions, the combination of time-symmetry without retrocausality is unavailable in quantum mechanics, for reasons intimately connected with the differences between classical and quantum physics (especially the role of discreteness in the latter). Not all interpretations of quantum mechanics share these assumptions, however, and in those that do not, time-symmetry does not entail retrocausality.

In many physical systems, coupling forces provide a way of carrying the energy stored in adjacent harmonic oscillators from place to place, in the form of waves. The wave equations governing such phenomena are time-symmetric: they permit the opposite processes, in which energy arrives at a point in the form of incoming concentric waves, to be lost to some external system. But these processes seem rare in nature. What explains this temporal asymmetry, and how is it related to the thermodynamic asymmetry? This paper attempts to clarify these old issues, in the light of recent contributions. After brief introductory remarks (§1), the paper is in three main parts. §2 examines the so-called ‘Sommerfeld Radiation Condition’, arguing that its link to the observed asymmetry is much less direct than commonly supposed. §3 begins with Zeh's proposal to make the Sommerfeld condition an ingredient in an explanation of the observed asymmetry, and makes explicit a useful distinction between two ways in which the thermodynamic asymmetry might connect to the radiation asymmetry. §4 reviews a proposal I have defended in earlier work about the relation of the radiative asymmetry to that of thermodynamics, and defends it against recent objections by Zeh and Frisch. I also distinguish it from a recent proposal due to North. I agree with North that the observed asymmetry of radiation stems from the low entropy history, but argue that she mis-characterises the asymmetry, and hence misses a crucial element in a proper account of the role of the low entropy past.