Talks

The presumed irreversibility of quantum measurements (whatever they are) leads, in conventional approaches to quantum theory, to an asymmetry between state preparation and post-selection. Is it possible that a trajectory can be predicted from the former, yet not inferred from the latter? Especially in light of the exciting applications of non-unitary operations (i.e., postselection) in quantum information, it becomes timely to reconsider how much one can say about a post-selected subensemble. I will review the weak-measurement formalism of Aharonov, Vaidman et al., and discuss some applications and extensions. These will include a proposed experiment to study the duration of the tunneling process (a question controversial since the 1930s) and a recently completed experiment aiming to \'resolve\' Hardy\'s retrodiction paradox.

My favorite version of quantum mechanics is Bohmian mechanics, a theory about particle trajectories. What is so great about it is that it removes all the mystery from quantum mechanics. I will provide a Bohmian perspective on some issues about time, including time measurements (Why is there no time operator?), tunnelling times (How long did the particle stay inside the barrier?), and the problem of time in quantum gravity (How can it be that the wave function of the Wheeler-de Witt equation is time-independent?). I will particularly address the arrow of time, including the question whether quantum measurements are examples of fundamental time asymmetry (like PCT violations, as Roger Penrose has suggested) or merely of irreversibility (like thermodynamics), and the problem how to transfer Boltzmann\'s explanation of the arrow of time from classical to quantum mechanics.

A brief review of the Two State Vector Formalism (TSVF) will be presented. It will be argued that we need to consider also backwards evolving quantum state because information given by forwards evolving quantum states is not complete. Both past and future measurements are required for providing complete information about quantum systems. Peculiar properties of pre- and post-selected quantum systems which can be efficiently analyzed in the framework of the TSVF and which can be observed using weak measurements will be described. An example is a particle reaching a certain location without being on the path that leads to and from this location. An extension of the TSVF to multiple space-time points will be discussed.

Are time and space independently existing entities? Or is their existence secondary in that they are merely properties of other, more fundamental physical systems? The parameters of this enduring debate have shifted according to the physical theory in which it are set. In the 17th century, Newton\'s notions of Absolute Time and Space strongly favored the idea of independently existing times and spaces. Yet Leibniz famously plagued Newton by pointing to changes that Newton must suppose real even though they issued in no observable differences. Most recently, with the advent of general relativity and quantum theories of gravity that seek to incorporate it, the balance has shifted once again. Is the independence of space and time now finally revealed by the metric field of space and time absorbing the matter of the gravitational fields? Or has time and space has lost its independence from matter in so far as space and time have been absorbed into the matter of the gravitational field? The focus of my talk will be an intermediate episode of this debate that plays out in the context of special relativity. Lorentz noted that moving electrodynamical systems slow in time and shrink in space. The realist tradition explains this slowing and shrinking through the adaptation of matter fields to a real, independently existing Minkowski spacetime. A dissident constructive tradition has long felt that the reverse is the case. These spatio-temporal effects are best explained by the properties of matter theories, most notably, their Lorentz covariance. Harvey Brown has advocated a form of this latter constructivism in his Physical Relativity: Space- time Structure from a Dynamical Perspective. This debate between these two views has proven hard to resolve. That is largely because the notion of explanation is not sufficiently understood for us to adjudicate cleanly between competing claims of what explains what better. In my talk, I will review a new approach to the debate. Constructivists have tacitly assumed a technical result, that it is indeed possible to construct a Minkowski spacetime from Lorentz covariant matter theories. I will show that this is incorrect. This construction project can succeed only in so far as constructivists presume antecedently the basic tenets of the realist view of spacetime. Hence constructivism fails as an alternative to realism about spacetime.

I will examine a number of time-related issues arising in quantum theory, and in particular attempt to address the following basic questions from a quantum perspective: 1. What is a clock? 2. Why do uniformly moving clocks dilate? 3. What is the behaviour of accelerating clocks?

In 1898, Poincaré identified two fundamental issues in the theory of time: 1)What is the basis for saying that a second today is the same as a second tomorrow? 2) How can one define simultaneity at spatially separated points? Poincaré outlined the solution to the first problem { which amounts to a theory of duration { in his 1898 paper, and in 1905 he and Einstein simultaneously solved the second problem. Einstein\'s daring and elegant approach so gripped the imagination of theoreticians, especially after Minkowski\'s introduction of spacetime,that the definition of duration, and with it the theory of clocks, has received virtually no attention for over a century. This is a remarkable state of affairs and is a major cause of the conceptual confusion surrounding the problem of time in the canonical approach to the creation of a quantum theory of gravity. In my talk I shall develop Poincaré\'s outline into a potentially definitive theory of duration and clocks.

This course is aimed at advanced undergraduate and beginning graduate students, and is inspired by a book by the same title, written by Padmanabhan. Each session consists of solving one or two pre-determined problems, which is done by a randomly picked student. While the problems introduce various subjects in Astrophysics and Cosmology, they do not serve as replacement for standard courses in these subjects, and are rather aimed at educating students with hands-on analytic/numerical skills to attack new problems.

This course is aimed at advanced undergraduate and beginning graduate students, and is inspired by a book by the same title, written by Padmanabhan. Each session consists of solving one or two pre-determined problems, which is done by a randomly picked student. While the problems introduce various subjects in Astrophysics and Cosmology, they do not serve as replacement for standard courses in these subjects, and are rather aimed at educating students with hands-on analytic/numerical skills to attack new problems.

A quantum channel models a physical process in which noise is added to a quantum system via interaction with its environment. Protecting quantum systems from such noise can be viewed as an extension of the classical communication problem introduced by Shannon sixty years ago. A fundamental quantity of interest is the quantum capacity of a given channel, which measures the amount of quantum information which can be protected, in the limit of many transmissions over the channel. In this talk, I will show that certain pairs of channels, each with a capacity of zero, can have a strictly positive capacity when used together, implying that the quantum capacity does not completely characterize a channel\'s ability to transmit quantum information. As a corollary, I will show that a commonly used lower bound on the quantum capacity - the coherent information, or hashing bound - is an overly pessimistic benchmark against which to measure the performance of quantum error correction because the gap between this bound and the capacity can be arbitrarily large.