This is a list of this week's papers on quantum foundations published in the various journals or uploaded to the preprint servers such as arxiv.org and PhilSci Archive.

on 2015-1-23 8:39am GMT

Authors: Marek Zukowski

Non-locality, or quantum-non-locality, are buzzwords in the community of quantum foundation and information scientists, which purportedly describe the implications of Bell's theorem. When such phrases are treated seriously, that is it is claimed that Bell's theorem reveals non-locality as an inherent trait of the quantum description of the micro-world, this leads to logical contradictions, which will be discussed here. In fact, Bell's theorem, understood as violation of Bell inequalities by quantum predictions, is consistent with Bohr's notion of complementarity. Thus, if it points to anything, then it is rather the significance of the principle of Bohr, but even this is not a clear implication. Non-locality is a necessary consequence of Bell's theorem only if we reject complementarity by adopting some form of realism, be it additional hidden variables, additional hidden causes, etc., or counterfactual definiteness.

The essay contains two largely independent parts. The first one is addressed to any reader interested in the topic. The second, discussing the notion of local causality, is addressed to people working in the field.

The thermodynamics of time. (arXiv:1501.05567v1 [quant-ph])

on 2015-1-23 8:39am GMT

Authors: Dries Sels, Michiel Wouters

The problem of time is a deep paradox in our physical description of the world. According to Aristotle's relational theory, time is a measure of change and does not exist on its own. In contrast, quantum mechanics, just like Newtonian mechanics, is equipped with a master clock that dictates the evolution of a system. This clock is infinitely precise and tacitly supplied free of charge from outside physics. Not only does this absolute time make it notoriously difficult to make a consistent theory of quantum gravity, it is also the underlying problem in establishing the second law. Indeed, contrary to our experience, the Wheeler-deWitt equation --a canonical quantization of general relativity-- predicts a static universe. Similarly, when simply concerned with the dynamics of a closed quantum system, there is no second law because the Von Neumann entropy is invariant under unitary transformations. Here we are mainly concerned with the latter problem and we show that it can be resolved by attributing a minimal amount of resources to the measurement of time. Although there is an absolute time in quantum mechanics, an observer can only establish a time by measuring a clock. For a local measurement, the minimal entropy production is equal to the number of ticks. This lower bound is attained by a black hole.

Quantum Logic and Quantum Reconstruction. (arXiv:1501.05492v1 [quant-ph])

on 2015-1-23 8:39am GMT

Authors: Allen Stairs

Quantum logic understood as a reconstruction program had real successes and genuine limitations. This paper offers a synopsis of both and suggests a way of seeing quantum logic in a larger, still thriving context.

Bekenstein-Hawking Entropy as Entanglement Entropy. (arXiv:1501.05402v1 [hep-th])

on 2015-1-23 8:39am GMT

Authors: Yu-Lei Feng, Yi-Xin Chen

We show that the Bekenstein-Hawking entropy $S_{BH}$ should be treated as an entanglement entropy, provided that the formation and evaporation of a black hole can be described by quantum unitary evolutions. To confirm this statement, we derive statistical mechanics from quantum mechanics effectively by means of open quantum systems. Then a new definition of Boltzmann entropy for a quantum closed system is given to count microstates in a way consistent with the superposition principle. In particular, this new Boltzmann entropy is a constant that depends only on the dimension of the system's relevant Hilbert subspace. Based on this new definition, some kind of "detailed balance" condition is obtained to stabilize the thermal equilibrium between two macroscopic subsystems within a larger closed system. However, the required "detailed balance" condition between black hole and matter would be broken, if the Bekenstein-Hawking entropy was treated as Boltzmann entropy together with the Hawking temperature as thermal temperature. This confirms that $S_{BH}$ is an entanglement entropy.

Virtual black holes, remnants and the information paradox

Classical and Quantum Gravity - latest papers

on 2015-1-21 12:00am GMT

We revisit the question of the contributions of Planckian quantum black holes in general and of remnants in particular to low-energy physics observables. As long as quantum gravity preserves the symmetries of the low-energy effective field theory, we find that the bounds on the number of quantum black holes or remnants are very weak. Typically, we rule out using data on the anomalous magnetic moment of the muon that there are more than 10 32 quantum black holes coupled to the standard model particles gravitationally. Remnants thus remain a viable option as a solution to the information paradox of black holes.

Discontinuities in quantum mechanical weak values. (arXiv:1501.04469v1 [quant-ph])

on 2015-1-20 1:41am GMT

Authors: B. E. Y. Svensson

The operational definition of a quantum mechanical weak value involves the limit of the weak measurement strength tending to zero. I study the continuity of this limit with respect to the case when there is no weak measurement and find that sometimes a discontinuity occurs. In these discontinuous cases, the weak value does not represent the undisturbed system. Conclusions drawn from such weak values regarding the properties of these systems can therefore not be upheld.

Quantum non-locality - It ain't necessarily so.... (arXiv:1501.04618v1 [quant-ph])

on 2015-1-20 1:41am GMT

Authors: Marek Zukowski, Caslav Brukner

Bell's theorem is 50 years old. Still there is a controversy about its implications. Much of it has its roots in confusion regarding the premises from which the theorem can be derived. Some claim that a derivation of Bell's inequalities requires just locality assumption, and nothing more. Violations of the inequalities are then interpreted as ``nonlocality'' or ``quantum nonlocality''. We show that such claims are unfounded and that every derivation of Bell's inequalities requires a premise -- in addition to locality and freedom of choice -- which is either assumed tacitly, or unconsciously, or is embedded in a single compound condition (like Bell's ``local causality''). The premise is equivalent to the assumption of existence of additional variables which do not appear in the quantum formalism (in form of determinism, or joint probability for outcomes of all conceivable measurements, or ``additional causes`'', or ``hidden variables'', ``complete description of the state'' or counterfactual definiteness, etc.). A certain irony is that perhaps the main message of violation of Bell's inequalities is that our notion of locality should be based on an operationally well-defined no-signalling condition, rather than on local causality.

on 2015-1-20 1:41am GMT

Authors: K. Piscicchia, C. Curceanu, S. Bartalucci, A. Bassi, S. Bertolucci, C. Berucci, A. M. Bragadireanu, M. Cargnelli, A. Clozza, L. De Paolis, S. Di Matteo, S. Donadi, A. d'Uffizi, J-P. Egger, C. Guaraldo, M. Iliescu, T. Ishiwatari, M. Laubenstein, J. Marton, E. Milotti, D. Pietreanu, T. Ponta, E. Sbardella, A. Scordo, H. Shi, D.L. Sirghi, F. Sirghi, L. Sperandio, O. Vazquez Doce, J. Zmeskal

The development of mathematically complete and consistent models solving the so-called "measurement problem", strongly renewed the interest of the scientific community for the foundations of quantum mechanics, among these the Dynamical Reduction Models posses the unique characteristic to be experimentally testable. In the first part of the paper an upper limit on the reduction rate parameter of such models will be obtained, based on the analysis of the X-ray spectrum emitted by an isolated slab of germanium and measured by the IGEX experiment.

The second part of the paper is devoted to present the results of the VIP (Violation of the Pauli exclusion principle) experiment and to describe its recent upgrade. The VIP experiment established a limit on the probability that the Pauli Exclusion Principle (PEP) is violated by electrons, using the very clean method of searching for PEP forbidden atomic transitions in copper.

Wheeler-DeWitt quantization and singularities. (arXiv:1501.04181v1 [gr-qc])

on 2015-1-20 1:41am GMT

Authors: Felipe Tovar Falciano, Nelson Pinto-Neto, Ward Struyve

We consider a Bohmian approach to the Wheeler-DeWitt quantization of the Friedmann-Lemaitre-Robertson-Walker model and investigate the question whether or not there are singularities, in the sense that the universe reaches zero volume. We find that for generic wave functions (i.e., non-classical wave functions), there is a non-zero probability for a trajectory to be non-singular. This should be contrasted to the consistent histories approach for which it was recently shown by Craig and Singh that there is always a singularity. This result illustrates that the question of singularities depends much on which version of quantum theory one adopts. This was already pointed out by Pinto-Neto et al., albeit with a different Bohmian approach. Our current Bohmian approach agrees with the consistent histories approach by Craig and Singh for single-time histories, unlike the one studied earlier by Pinto-Neto et al. Although the trajectories are usually different in the two Bohmian approach, their qualitative behavior is the same for generic wave functions.

The Assumptions of Bell's Proof. (arXiv:1501.04168v1 [quant-ph])

on 2015-1-20 1:41am GMT

Authors: Roderich Tumulka

While it is widely agreed that Bell's theorem is an important result in the foundations of quantum physics, there is much disagreement about what exactly Bell's theorem shows. It is agreed that Bell derived a contradiction with experimental facts from some list of assumptions, thus showing that at least one of the assumptions must be wrong; but there is disagreement about what the assumptions were that went into the argument. In this paper, I make a few points in order to help clarify the situation.

Is a description deeper than the quantum one possible?. (arXiv:1501.04127v1 [quant-ph])

on 2015-1-20 1:41am GMT

Authors: Giancarlo Ghirardi, Raffaele Romano

Recently, it has been argued that quantum mechanics is a complete theory, and that different quantum states do necessarily correspond to different elements of reality, under the assumptions that quantum mechanics is correct and that measurement settings can be freely chosen. In this work, we prove that this result is a consequence of an unnecessarily strong mathematical expression of the free choice assumption, which embodies more conditions than explicitly stated. The issues of the completeness of quantum mechanics, and of the interpretation of the state vector, are by no means resolved. Taking this perspective, we describe how the recently introduced class of crypto-nonlocal hidden variables theories can be used to characterize the maximal possible departure from quantum mechanics, when the system consists of a pair of qubits.

Emergence in Holographic Scenarios for Gravity. (arXiv:1501.04278v1 [hep-th])

on 2015-1-20 1:40am GMT

Authors: Dennis Dieks, Jeroen van Dongen, Sebastian de Haro

'Holographic' relations between theories have become an important theme in quantum gravity research. These relations entail that a theory without gravity is equivalent to a gravitational theory with an extra spatial dimension. The idea of holography was first proposed in 1993 by Gerard 't Hooft on the basis of his studies of evaporating black holes. Soon afterwards the holographic 'AdS/CFT' duality was introduced, which since has been intensively studied in the string theory community and beyond. Recently, Erik Verlinde has proposed that even Newton's law of gravitation can be related holographically to the 'thermodynamics of information' on screens. We discuss these scenarios, with special attention to the status of the holographic relation in them and to the question of whether they make gravity and spacetime emergent. We conclude that only Verlinde's scheme straightfowardly instantiates emergence. However, assuming a non-standard interpretation of AdS/CFT may create room for the emergence of spacetime and gravity there as well.

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