Conference

One of the most remarkable features of our quantum universe is the wide range of time, place, scale, and epoch on which the deterministic laws of classical physics apply to an excellent approximation. This talk reviews the origin of such a quasiclassical realm in a universe governed fundamentally by quantum mechanical laws characterized by indeterminacy and distributed probabilities. We stress the important roles in this origin played by classical spacetime, coarse-graining in terms of approximately conserved quantities, local equilibrium, and the initial quantum state of the universe. The discussion is carried out first in the decoherent (or consistent) histories formulation of the quantum mechanics of closed systems (most generally the universe) assuming spacetime geometry is fixed. This is an Everettian generalization of the usual Copenhagen text book quantum mechanics of measurement situations that assumes the quasiclassical realm. Conversely we isolate the assumptions and approximations necessary to derive Copenhagen quantum mechanics from the more general quantum mechanics of closed systems. We describe a further generalization of usual quantum theory that is necessary to deal with quantum spacetime and describe under what conditions it predicts our observed coarse-grained classical spacetime that is a prerequisite for a quasiclassical realm.

I analyze a series of common objections to Everett\'s Many Worlds Interpretation. I discuss which ones are unique to quantum mechanics, and which have nothing to do with quantum mechanics per se as they can also be debated in the context of other areas of physics

Probability is often regarded as a problem for the many-worlds interpretation: if all branches of the splitting wavefunction are equally real, what sense does it make to say that the branches have different probabilities? In the decision-theoretic approach due to Deutsch and Wallace, probabilities acquire a meaning through the preferences of a rational agent. This talk reviews the decision-theoretic approach to probability in classical physics and quantum mechanics and shows that its application to the many-world interpretation creates a new difficulty for the latter.

I will rehearse and try to sharpen some of the perennial worries about making sense of probabilities in Everettian interpretations of quantum mechanics, with particular attention to the recent Decision-Theoretic proposals of Deutsch and others

I will review the current state of the probability problem. My main focus will be on the attempts by David Deutsch and myself to provide a proof of the Born Rule starting from Everettian assumptions, but I will also attempt to locate these attempts within the more general framework of the probability problem.

Everett explained “collapse of the wavepacket” by noting that observer will perceive the state of the measured quantum system relative to the state of his own records. Two elements (missing in this simple and compelling explanation of effective collapse) are required to complete relative state interpretation: (i) A preferred basis for states of at least some systems in the wholly quantum Universe must be identified, so that apparatus pointers and other recording devices can persist over time. This implies breaking of the unitary symmetry in the original (more egalitarian) relative state interpretation, so that it can successfully account for classicality of macroscopic objects in accord with Bohr’s view of the role of measuring apparatus, and with our everyday experience. It is now widely accepted that decoherence (caused by the monitoring of systems by their environments) leads to einselection of pointer states, accounting for the emergence of preferred states. However, tools used by decoherence rely on the second missing link between quantum substrate and reality; (ii) A prescription that connects probabilities of outcomes with amplitudes of quantum states – such as Born’s rule is still needed. Born’s rule could be in principle postulated, but as Everett noted fifty years ago this should not be necessary. I show that both (i) einselection and (ii) Born’s rule follow from symmetries of entangled quantum states. Entanglement represents information transfer between the to-be-classical quantum systems and their environments. Information transfer in course of decoherence produces multiple copies of the state of the system: its redundant imprints in the environment. This multiplicity of records can account for the objective existence of preferred pointer states: (iii) Quantum Darwinism singles out the “fittest observable” of the system (the observable that produces the most information-theoretic “offspring” of its state, i.e. the most copies in the environment). These fittest observables exist objectively: information about them can be found out indirectly, from the environment, without perturbing the underlying state of the system. The objective existence of pointer states is the foundation of the existential interpretation. The existential interpretation recognizes with Everett the relative nature of quantum states, but accounts for the effectively classical states (which unlike quantum states of isolated systems – can be found out without getting disrupted in the process) through quantum Darwinism

I shall present an overview of quantum mechanics in the Everett interpretation, that emphasises its structural characteristics, as a theory of what exists. In this respect it shares common ground with other fundamental theories in physics. As such its appeal is conservative; it makes do with the purely unitary equations of quantum mechanics as exceptionless and universal. It also makes do with standard methods for extracting \'high level\' or \'emergent\' ontology, the furniture of macroscopic worlds, from largish molecules on up. It would appeal all the more if it made do with standard epistemological principles too - for example, in the context of inductive statistical confirmation, with standard Bayesian epistemology. But this links to the question of the interpretation of probability in the Everett interpretation, and here the theory seems anything but conservative. It is a common complaint that the approach leaves no room at all for talk of uncertainty. I shall argue, again on conservative interpretative practises, that this claim is incorrect. Chance events are, indeed, revealed in a surprising light - as quantum branchings - but they are the more perspicuous, and their properties and quantitative measure better explained, in light of that.

The k-essence theories admit the superluminal propagation of the perturbations on classical nontrivial backgrounds. In this talk I will review our arguments from arXiv:0708.0561v1 and show that in spite of the superluminal propagation the causal paradoxes do not arise in these theories and in this respect they are not less safe than General Relativity.

We investigate the effect of evaporating primordial black holes on the ionization history of the universe, with emphasis on limits derivable from the CMB and future 21-cm observations of high-redshift neutral hydrogen.