Since 2002 Perimeter Institute has been recording seminars, conference talks, and public outreach events using video cameras installed in our lecture theatres. Perimeter now has 7 formal presentation spaces for its many scientific conferences, seminars, workshops and educational outreach activities, all with advanced audio-visual technical capabilities. Recordings of events in these areas are all available On-Demand from this Video Library and on Perimeter Institute Recorded Seminar Archive (PIRSA). PIRSA is a permanent, free, searchable, and citable archive of recorded seminars from relevant bodies in physics. This resource has been partially modelled after Cornell University's arXiv.org.
The most remarkable recent discovery in fundamental physics is that the Universe is undergoing accelerated expansion. A proper understanding of its physical origin forces us to make a hard choice between dynamical and environmental scenarios. The former approach predicts the existence of a new long distance physics in the gravitational sector, while the second relies on the vast landscape of vacua with different values of the cosmological constant. I will discuss achievements and shortcomings of both approaches, and illustrate them in the concrete examples.
Decoherence attempts to explain the emergent classical behaviour of a
quantum system interacting with its quantum environment. In order to
formalize this mechanism we introduce the idea that the information
preserved in an open quantum evolution (or channel) can be
characterized in terms of observables of the initial system. We use
this approach to show that information which is broadcast into many
parts of the environment can be encoded in a single observable. This
supports a model of decoherence where the pointer observable can be an
It is common to assert that the discovery of quantum theory overthrew our classical conception of nature. But what, precisely, was overthrown? Providing a rigorous answer to this question is of practical concern, as it helps to identify quantum technologies that outperform their classical counterparts, and of significance for modern physics, where progress may be slowed by poor physical intuitions and where the ability to apply quantum theory in a new realm or to move beyond quantum theory necessitates a deep understanding of the principles upon which it is based.
We prove that all non-conspiratorial/retro-causal hidden variable theories has to be measurement ordering contextual, i.e. there exists
*commuting* operator pair (A,B) and a hidden state \\\\lambda such that the outcome of A depends on whether we measure B before or after.
Interestingly this rules out a recent proposal for a psi-epistemic due to Barrett, Hardy, and Spekkens. We also show that the model was in fact partly discovered already by vanFraassen 1973; the only thing missing was giving a probability distribution on the space of ontic states (the hidden variables).