Cosmology Group Podcasts Unknown

David summarizes his distinction between inference by prediction/retrodiction and inference by measurement, and reiterates that this can make plausible the claim that the special status of the Past-Hypothesis can ground the asymmetry in our epistemic relations to the past and future. He goes through some objections to this view that came up in the class before, which he and Tim then discuss.
David relates this epistemic asymmetry to the time-asymmetries in causation, counterfactual conditionals, and our apparent ability to elicit change in the world. He argues that the very facts that make the past epistemically more accessible than the future also makes our causal handle on the future far far weaker than our handle on the past. There is a discussion of how this is supposed to fit with our pre-theoretic intuition that we have NO causal handle on the past.
Tim weighs in on some of these points, criticizing David's desire for a "mechanical" explanation for temporal asymmetries. He argues that our notion of a mechanical explanation already presupposes a substantive temporal asymmetry. David discusses the relation between the Mentaculus (which he takes to be the source of mechanical explanations) and the notion that time passes/has a direction. Tim responds and puts some pressure on David's claim that these two are largely independent. There is a lengthy discussion of this point.

Tim opens with a brief point about the postulation of a very low entropy past state. He contrasts the claim that, if this state is not the initial state of the world, that the entropy profile of the universe will be symmetric (intuitively, with a global minimum at the moment of the past hypothesis) and the claim that the macroscopic profile of the universe will be symmetric in the same way. David takes some time to respond to this point.
David then turns the discussion to the inferential role of the past hypothesis. He starts by identifying two kinds of inferences that can be used to determine the state of the world at times other than the present: Inference by Prediction/Retrodiction (in which one takes facts about the present and applies the laws to determine facts about the past or future) and Inference by Measurement (in which one takes facts about the present plus facts about certain past events and determines facts about the past or future). Inference by Measurement is, David points out, much stronger, but its appeal to certain past events is in need of justification. David argues that the event postulated by the past hypothesis is precisely the privileged past state that can do this inferential work. Tim asks whether these inference types really exhaust the relevantly available inference procedures. He also wonders whether the sorts of inferences David wants to call inferences by measurement really always have to appeal to facts about states other than the present, specifically the past hypothesis. There is a long discussion of the predictive centrality/fundamentality of the past hypothesis and of the nature of records.

Barry Loewer is the guest lecturer for this class. Tim starts by recapitulating the notion of typicality introduced in the previous lecture. Tim, David and Barry then engage in a debate about Tim's view of the nature of statistical explanation. Barry then gives a summary of the difference between his and David's view of statistical mechanics and Tim's view. He starts by arguing that one main point of contention is the "imperialistic" nature of his and David's project. A second main point of contention is the nature of laws and time, which is the topic for the remainder of the lecture. Barry contrasts two conceptions of laws. The first one is the Humean view of laws on which laws of nature are reducible to facts about the Humean mosaic, which goes hand-in-hand (Barry claims) with a view of time on which time doesn't have an intrinsic direction. The second is Tim's view, on which laws of nature are a primitive category in our ontology, and which (Barry argues) goes naturally with a view of time on which it has an intrinsic direction.

Tim and David begin by continuing their discussion of the 'Stosszahlansatz', the assumption that the number of particle collisions in a given subregion is just proportional to the volume of that region. They summarize the debate up to this point and attempt to clarify the nature of their disagreement about its explanatory value. Tim then takes over and gives a short primer on the mathematics of measure theory, connecting it with the purely mathematical notion of a "probability measure" and contrasting that with the physical notion of something being probabilistic in any real sense.
Tim relates this discussion by example to the notion of Typicality. A behavior of a system is typical of that system iff the set of initial conditions that yield that behavior has measure 1 (on any normalized continuous measure). When a certain limit frequency is typical, then we have something very close in form and content to an objective, deterministic probability. Tim goes on to discuss how this model can be
extended to adequately include finite systems or finite, and then he and David debate at length whether this extension preserves the virtues of the view as first presented.