Talks

Our universe has a split personality: quantum and relativity. Understanding how the two can coexist, i.e. how our universe can exist, is one of the greatest challenges facing theoretical physicists in the 21st century. The presentation focuses on a simple but mind-bending thought experiment that hints at some fascinating new ways of thinking that may be required to unravel this mystery. Could the world be like a hologram? The half hour multimedia presentation provides refreshing insight into how science works – how theoretical physicists search for the ultimate nature of reality, and is followed by a half hour question and answer session with PI researcher Sean Gryb.

The quantum equations for bosonic fields may be derived using an 'exact uncertainty' approach [1]. This method of quantization can be applied to fields with Hamiltonian functionals that are quadratic in the momentum density, such as the electromagnetic and gravitational fields. The approach, when applied to gravity [2], may be described as a Hamilton-Jacobi quantization of the gravitational field. It differs from previous approaches that take the classical Hamilton-Jacobi equation as their starting point in that it incorporates some new elements, in particular the use of a formalism of ensembles on configuration space and the postulate of an exact uncertainty relation. These provide the fundamental elements needed for the transition to the quantum theory. The formalism of ensembles on configuration space is general enough to describe classical, quantum, and interacting classical-quantum systems in a consistent way. This is of some relevance to gravity: although there are many physical arguments in favour of a quantum theory of gravity, it appears that the justification for such a theory does not follow from logical arguments alone [3]. It is therefore of interest to consider the coupling of quantum fields to a classical gravitational field. This leads to a theory that is fundamentally different from standard semiclassical gravity. 1. Michael J W Hall, Kailash Kumar and Marcel Reginatto, Bosonic field equations from an exact uncertainty principle, J. Phys. A 36 (2003) 9779-9794 (http://arxiv.org/abs/hep-th/0307259). 2. M. Reginatto, Exact Uncertainty Principle and Quantization: Implications for the Gravitational Field, Proceedings of DICE2004 in: Braz. J. Phys. 35 (2005) 476-480 (http://arxiv.org/abs/gr-qc/0501030). 3. Mark Albers, Claus Kiefer and Marcel Reginatto, Measurement analysis and quantum gravity, Phys. Rev. D 78 (2008) 064051 (http://arxiv.org/abs/0802.1978)

An introduction to the mathematics necessary to fully appreciate the ISSYP relativity and quantum lectures. Binomial theorem, series expansions of common functions, complex numbers, and real and complex waves.

The fact that quantum mechanics admits exact uncertainty relations is used to motivate an ‘exact uncertainty’ approach to obtaining the Schrödinger equation. In this approach it is assumed that an ensemble of classical particles is subject to momentum fluctuations, with the strength of the fluctuations determined by the classical probability density [1]. The approach may be applied to any classical system for which the Hamiltonian is quadratic with respect to the momentum, including all physical particles and fields [2]. The approach is based on a general formalism that describes physical ensembles via a probability density P on configuration space, together with a canonically conjugate quantity S [3]. Quantum and classical ensembles are particular cases of interest, but one can also ask more general questions within this formalism, such as (i) Can one consistently describe interactions between quantum and classical systems? and (ii) Can one obtain local nonlinear modifications of quantum mechanics? These questions will be briefly discussed, with respect to measurement interactions and spin-1/2 systems respectively. 1. M.J.W. Hall and M. Reginatto, “Schroedinger equation from an exact uncertainty principle”, J. Phys. A 35 (2002) 3289 (http://lanl.arxiv.org/abs/quant-ph/0102069). 2. M.J.W. Hall, “Exact uncertainty approach in quantum mechanics and quantum gravity”, Gen. Relativ. Gravit. 37 (2005) 1505 (http://lanl.arxiv.org/abs/gr-qc/0408098). 3. M.J.W. Hall and M. Reginatto, “Interacting classical and quantum systems”, Phys. Rev. A 72 (2005) 062109 (http://lanl.arxiv.org/abs/quant-ph/0509134).

Non-relativistic quantum theory is derived from information codified into an appropriate statistical model. The basic assumption is that there is an irreducible uncertainty in the location of particles so that the configuration space is a statistical manifold with a natural information metric. The dynamics then follows from a principle of inference, the method of Maximum Entropy: entropic dynamics is an instance of law without law. The concept of time is introduced as a convenient device to keep track of the accumulation of changes. The resulting formalism is close to Nelson's stochastic mechanics. The statistical manifold is a dynamical entity: its (information) geometry determines the evolution of the probability distribution which, in its turn, reacts back and determines the evolution of the geometry. As in General Relativity there is a kind of equivalence principle in that “fictitious” forces – in this case diffusive “osmotic” forces – turn out to be “real”. This equivalence of quantum and statistical fluctuations – or of quantum and classical probabilities – leads to a natural explanation of the equality of inertial and “osmotic” masses and allows explaining quantum theory as a sophisticated example of entropic inference. Mass and the phase of the wave function are explained as features of purely statistical origin. Recommended Reading: arXiv:0907.4335 "From Entropic Dynamics to Quantum Theory" (2009)

What belongs to quantum theory is no more than what is needed for its derivation. Keeping to this maxim, we record a paradigmatic shift in the foundations of quantum mechanics, where the focus has recently moved from interpreting to reconstructing quantum theory. We present a quantum logical derivation based on Rovelli's information-theoretic axioms. Its strengths and weaknesses will be studied in the light of recent developments, focusing on the subsystems rule, continuity assumptions, and the definition of observer. Publications: * "Reconstruction of quantum theory," British Journal for the Philosophy of Science, 58, 2007, pp. 387-408. * "Information-theoretic principle entails orthomodularity of a lattice," Foundations of Physics Letters 18 (6), 2005, pp. 563-572. * "Elements of information-theoretic derivation of the formalism of quantum theory", International Journal of Quantum Information 1(3), 2003, pp. 289-300.

I will consider physical theories which describe systems with limited information content. This limit is not due observer's ignorance about some “hidden” properties of the system - the view that would have to be confronted with Bell's theorem - but is of fundamental nature. I will show how the mathematical structure of these theories can be reconstructed from a set of reasonable axioms about probabilities for measurement outcomes. Among others these include the “locality” assumption according to which the global state of a composite system is completely determined by correlations between local measurements. I will demonstrate that quantum mechanics is the only theory from the set in which composite systems can be in entangled (non-separable) states. Within Hardy's approach this feature allows to single out quantum theory from other probabilistic theories without a need to assume the “simplicity” axiom. 1. Borivoje Dakic, Caslav Brukner (in preparation) 2. Caslav Brukner, Anton Zeilinger, Information Invariance and Quantum Probabilities, arXiv:0905.0653 3. Tomasz Paterek, Borivoje Dakic, Caslav Brukner, Theories of systems with limited information content, arXiv:0804.1423

Quantum theory is the most accurate scientific theory humanity has ever devised. But it is also the most mysterious. No one knows what the underlying picture of reality at quantum level is. This presentation will introduce you to some of the many interpretations of quantum theory that scientists have devised and discuss the infamous 'measurement problem'.