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

Thermal dimension of quantum spacetime. (arXiv:1602.08020v1 [hep-th])

on 2016-2-27 8:55am GMT

Authors: Giovanni Amelino-Camelia, Francesco Brighenti, Giulia Gubitosi, Grasiele Santos

Recent results suggest that a crucial crossroad for quantum gravity is the characterization of the effective dimension of spacetime at short distances, where quantum properties of spacetime become significant. This is relevant in particular for various scenarios of “dynamical dimensional reduction” which have been discussed in the literature. We are here concerned with the fact that the related research effort has been based exclusively on analyses of the “spectral dimension”, which involves an unphysical Euclideanization of spacetime and is highly sensitive to the off-shell properties of a theory. As here shown, different formulations of the same physical theory can have wildly different spectral dimension. We propose that dynamical dimensional reduction should be described in terms of the “thermal dimension” which we here introduce, a notion that only depends on the physical content of the theory. We analyze a few models with dynamical reduction both of the spectral dimension and of our thermal dimension, finding in particular some cases where thermal and spectral dimension agree, but also some cases where the spectral dimension has puzzling properties while the thermal dimension gives a different and meaningful picture.

Turning on gravity with the Higgs mechanism. (arXiv:1602.07993v1 [gr-qc])

on 2016-2-27 8:55am GMT

Authors: Stephon Alexander, John D. Barrow, Joao Magueijo

We investigate how a Higgs mechanism could be responsible for the emergence of gravity in extensions of Einstein theory. In this scenario, at high energies, symmetry restoration could “turn off” gravity, with dramatic implications for cosmology and quantum gravity. The sense in which gravity is muted depends on the details of the implementation. In the most extreme case gravity’s dynamical degrees of freedom would only be unleashed after the Higgs field acquires a non-trivial vacuum expectation value, with gravity reduced to a topological field theory in the symmetric phase. We might also identify the Higgs and the Brans-Dicke fields in such a way that in the unbroken phase Newton’s constant vanishes, decoupling matter and gravity. We discuss the broad implications of these scenarios.

on 2016-2-27 8:55am GMT

Authors: Martine Chevrollier, Marcos Oriá

Experiments involving single or few elementary particles are completely described by Quantum Mechanics. Notwithstanding the success of that quantitative description, various aspects of observations, as nonlocality and the statistical randomness of results, remain as mysterious properties apart from the quantum theory, and they are attributed to the strangeness of the microscopic world. Here we restart from the fundamental relations of uncertainty to reformulate the probability law of Born including the temporal variable. Considering that both the spatial and the temporal variables play a symmetric role in the wave-function \Psi (x,t) , a temporal wavepacket is built and analysed. The probability density is written as p(x,t) = | \Psi (x,t) |^2, where the probabilistic interpretation for the temporal wavepacket is equivalent to Born’s law for the spatial variable, x. For the convenience of the discussion of the role of the temporal variable, we write p(x_0,t) = | \Psi (x_0,t) |^2 for a free particle, expressing only the temporal wavepacket, then we discuss its spread. In the light of the evolution of this temporal wavepacket we analyse basic processes of matter-wave interaction, involving single and entangled entities. Nonlocality appears then as a consequence of the spread of the temporal wavepacket; and the position of each detected event in two-slits interferometry as due to the independent phases of the spatial and temporal wavepackets.

on 2016-2-27 8:55am GMT

Authors: M. Abdi, P. Degenfeld-Schonburg, M. Sameti, C. Navarrete-Benlloch, M. J. Hartmann

The transition from quantum to classical physics remains an intensely debated question even though it has been investigated for more than a century. Further clarifications could be obtained by preparing macroscopic objects in spatial quantum superpositions and proposals for generating such states for nano-mechanical devices either in a transient or a probabilistic fashion have been put forward. Here we introduce a method to deterministically obtain spatial superpositions of arbitrary lifetime via dissipative state preparation. In our approach, we engineer a double-well potential for the motion of the mechanical element and drive it towards the ground state, which shows the desired spatial superposition, via optomechanical sideband cooling. We propose a specific implementation based on a superconducting circuit coupled to the mechanical motion of a lithium-decorated monolayer graphene sheet, introduce a method to verify the mechanical state by coupling it to a superconducting qubit, and discuss its prospects for testing collapse models for the quantum to classical transition.

Event ontology in quantum mechanics and the problem of emergence

PhilSci-Archive: No conditions. Results ordered -Date Deposited.

on 2016-2-26 7:54pm GMT

Gambini, Rodolfo and Pullin, Jorge (2016) Event ontology in quantum mechanics and the problem of emergence. [Preprint]

Leggett-Garg inequalities and no-signaling in time: A quasiprobability approach

on 2016-2-26 3:00pm GMT

Author(s): J. J. Halliwell

The Leggett-Garg (LG) inequalities were proposed in order to assess whether sets of pairs of sequential measurements on a single quantum system can be consistent with an underlying notion of macrorealism. Here, the LG inequalities are explored using a simple quasiprobability linear in the projection…

[Phys. Rev. A 93, 022123] Published Fri Feb 26, 2016

on 2016-2-23 8:55am GMT

Authors: Andrew Sid Lang, Caleb J Lutz

We discuss the role that intuitive theories of physics play in the interpretation of quantum mechanics. We compare and contrast na\”ive physics with quantum mechanics and argue that quantum mechanics is not just hard to understand but that it is difficult to believe, often appearing magical in nature. Quantum mechanics is often discussed in the context of “quantum weirdness” and quantum entanglement is known as “spooky action at a distance.” This spookiness is more than just because quantum mechanics doesn’t match everyday experience; it ruffles the feathers of our na\”ive physics cognitive module. In Everett’s many-worlds interpretation of quantum mechanics, we preserve a form of deterministic thinking that can alleviate some of the perceived weirdness inherent in other interpretations of quantum mechanics, at the cost of having the universe split into parallel worlds at every quantum measurement. By examining the role cognitive modules play in interpreting quantum mechanics, we conclude that the many-worlds interpretation of quantum mechanics involves a cognitive bias not seen in the Copenhagen interpretation.

The Hartle–Hawking wave function in 2D causal set quantum gravity

Classical and Quantum Gravity – latest papers

on 2016-2-22 12:00am GMT

We define the Hartle–Hawking no-boundary wave function for causal set theory (CST) over the discrete analogs of spacelike hypersurfaces. Using Markov Chain Monte Carlo and numerical integration methods we analyze the wave function in non-perturbative 2D CST. We find that in the low-temperature regime it is dominated by causal sets which have no continuum counterparts but possess physically interesting geometric properties. Not only do they exhibit a rapid spatial expansion with respect to the discrete proper time, but a high degree of spatial homogeneity. The latter is due to the extensive overlap of the causal pasts of the elements in the final discrete hypersurface and corresponds to high graph connectivity. Our results thus suggest new possibilities for the role of quantum gravity in the observable Universe.

Schrödinger Evolution for the Universe: Reparametrization

Classical and Quantum Gravity – latest papers

on 2016-2-22 12:00am GMT

We develop a new framework for constructing the quantum observables of theories invariant under global time reparametrizations. Our starting point is a generalised Hamilton–Jacobi formalism for totally constrained systems. Our end result is a quantum formalism that simultaneously preserves: (i) genuine evolution; and (ii) the constraint structure of the classical formalism in the semi-classical limit. We thus offer a novel resolution of the ‘problem of time’ for theories invariant under global time reparametrization. There is the potential for the application of our proposal to the quantization of gravity when understood in terms of the Shape Dynamics formalism.

Quantum mechanics on York slices

Classical and Quantum Gravity – latest papers

on 2016-2-22 12:00am GMT

For some time the York time parameter has been identified as a candidate for a physically meaningful time in cosmology. An associated Hamiltonian may be found by solving the Hamiltonian constraint for the momentum conjugate to the York time variable, although an explicit solution can only be found in highly symmetric cases. The Poisson structure of the remaining variables is not canonical. Here we quantise this dynamics in an anisotropic minisuperspace model via a natural extension of canonical quantisation. The resulting quantum theory has no momentum representation. Instead the position basis takes a fundamental role. We illustrate how the quantum theory and the modified representation of its momentum operators lead to a consistent theory in the presence of the constraints that arose during the Hamiltonian reduction. The quantised reduced Hamiltonian is Hermitian, although the momentum operators are not, the causes and implications of which we discuss. We are able to solve for…