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.
In frustrated systems, competing interactions lead to complex phase diagrams and sometimes entirely new states of matter. Frustration often arises from the lattice geometry, leaving the system delicately balanced between a variety of possible orders. A number of normally weak effects can lead to a lifting of this degeneracy. For example, I will discuss how quantum fluctuations can stabilize a supersolid phase, where the system is at once both a crystal and a superfluid.
In this talk I will discuss a feature of quantum state evolution in a relativistic spacetime, the feature that David Albert has recently dubbed \'non-narratability.\' This is: a complete state history given along one foliation does not always, by itself (that is, without specification of the dynamics of the system), determine the history along another foliation. The question arises: is this a deep distinction between quantum and classical state evolution, that deserves our fuller attention? I will discuss some results relevant to this question.
It has been a common viewpoint that the process of quantization ought to replace the singularities of classical general relativity by some chaotic-looking structure at the scale of the Planck length. In this talk I shall argue that whereas this is to be expected at black-hole singularities, Nature\'s true picture of what goes on at the Big Bang is very different, where clocks cannot exist and the conformal geometry is completely smooth.
The underlying motivation for rejecting Everett\'s many-worlds interpretation of quantum mechanics and instead exploring single-world interpretations is to make physical theory concordant with human experience. From this perspective, the wave function collapse and Bohm-de Broglie interpretations are anthropocentric in origin. But this does not lessen their importance. Indeed accounting for our human experience of the physical world is a key element of any physical theory.
The unparalleled empirical success of quantum theory strongly suggests that it accurately captures fundamental aspects of the workings of the physical world. The clear articulation of these aspects is of inestimable value --- not only for the deeper understanding of quantum theory in itself, but for its further development, particularly for the development of a theory of quantum gravity.
A reference frame can be treated as a physical quantum object internal to the theory. Quantum reference frames whose size, and therefore accuracy, are bounded in some way necessarily limit one\'s ability to prepare states and to perform quantum operations and measurements on a system. The nature of these limitations is similar in many ways to that of decoherence. We investigate how a quantum reference frame of bounded size can be \'dequantized\', i.e., treated as external to the quantum formalism, in such a way as to induce an effective decoherence on any system described relative to it.
It is widely believed that the dynamical mechanism of decoherence plays a key role in understanding the emergence of classicality from quantum systems, via the environment-induced superselection of a preferred set of subsystem states, the density matrices for which are approximately diagonal in the pointer basis. In this talk, I prove that the vast majority of subsystems do *not* exhibit this behavior, regardless of the Hamiltonian. This shows that the emergence of classicality is highly state-dependent (as suggested by related work of Hartle and others).
In deBroglie-Bohm theory the quantum state plays the role of a guiding agent. In this seminar we will explore whether this is a universal feature shared by all hidden variable theories, or merely a peculiarity of the deBroglie-Bohm theory. We present the bare bones of a theory in which the quantum state represents a probability distribution and does not act as a guiding agent. The theory is also psi-epistemic according to Spekken\'s and Harrigan\'s definition. For simplicity we develop the model for a 1D discrete lattice but the generalization to higher dimensions is straightforward.
One approach to the problem of time in canonical quantum gravity is to use correlations between a carefully chosen physical system and all other physical systems to provide a simulacrum of time. Time emerges as an ordering of correlated measurement results. In many ways this is an echo of an idea introduced by Poincare to give a geometric description of dynamical systems. Pullin and Gambini have addressed some objections to this approach using a consistent discretization, but in so doing introduce an intrinsic decoherence mechanism into physical theories.
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.