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Search for quantum gravity in space

R. Bingham, José Tito Mendonça, Charles H.-T. Wang

Published Mar 29, 2006• 0 citations
OpenAlex

Abstract

One of the biggest unsolved problems in fundamental physics is the unification of quantum mechanics and general relativity. The lack of experimental guidance makes the issue particularly evasive. Recent models of decoherence demonstrated that conformal fluctuations of spacetime can change the correlation function of matter-waves. Latest progress on conformal decomposition in canonical gravity makes it possible for these decoherence models to be developed without freezing any degrees of freedom for general relativity. The new approach allows the roles of the conformal field and (spin-2) gravitational waves to be identified in the Brownian motion principle based on which Planck scale physics could be accessed. A spacecraft mission called Hyper was proposed to conduct matter-wave experiments in space. Orders of magnitude improvement may be gained by going into orbit reducing the effects of environmental interactions. Such experiments could put constraints on the upper bounds of measurements of decoherence of a matter-wave interferometer due to quantum fluctuations of spacetime.

Related papers

The Search for Quantum Gravity Using Matter Interferometers

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One of the biggest unsolved problems in fundamental physics is the unification of quantum mechanics and gravity. One of the consequences of unification could be the existence of "incoherent conformal waves" in gravitational fields due to quantum mechanical zero-point fluctuations. Recent theories have demonstrated that these fluctuations change the first order correlation function of matterwaves. A spacecraft mission called Hyper is being designed to conduct matterwave experiments in space. An order of magnitude improvement may be gained by going into orbit reducing the effects of environmental interactions. Such experiments could put constraints on the upper bands of measurements of decoherence of a matterwave interferometer due to quantum fluctuations.

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One of the biggest unsolved problems in fundamental physics is the unification of quantum mechanics and general relativity. The lack of experimental guidance makes the issue particularly evasive. Recent models of decoherence demonstrated that conformal fluctuations of spacetime can change the correlation function of matter-waves. Latest progress on conformal decomposition in canonical gravity makes it possible for these decoherence models to be developed without freezing any degrees of freedom for general relativity. The new approach allows the roles of the conformal field and (spin-2) gravitational waves to be identified in the Brownian motion principle based on which Planck scale physics could be accessed. A spacecraft mission called Hyper was proposed to conduct matter-wave experiments in space. Orders of magnitude improvement may be gained by going into orbit reducing the effects of environmental interactions. Such experiments could put constraints on the upper bounds of measurements of decoherence of a matter-wave interferometer due to quantum fluctuations of spacetime.
One of the biggest unsolved problems in fundamental physics is the unification of quantum mechanics and gravity. One of the consequences of unification could be the existence of "incoherent conformal waves" in gravitational fields due to quantum mechanical zero-point fluctuations. Recent theories have demonstrated that these fluctuations change the first order correlation function of matterwaves. A spacecraft mission called Hyper is being designed to conduct matterwave experiments in space. An order of magnitude improvement may be gained by going into orbit reducing the effects of environmental interactions. Such experiments could put constraints on the upper bands of measurements of decoherence of a matterwave interferometer due to quantum fluctuations.
One of the biggest unsolved problems in physics is the unification of quantum mechanics and general relativity. The lack of experimental guidance has made the issue extremely evasive, though various attempts have been made to relate the loss of matter wave coherence to quantum spacetime fluctuations. We present a new approach to the gravitational decoherence near the Planck scale, made possible by recently discovered conformal structure of canonical gravity. This leads to a gravitational analogue of the Brownian motion whose correlation length is given by the Planck length up to a scaling factor. With input from recent matter wave experiments, we show that the minimum value of this factor to be well within the expected range for quantum gravity theories. This suggests that the sensitivities of advanced matter wave interferometers may be approaching the fundamental level due to quantum spacetime fluctuations and that investigating Planck scale physics using matter wave interferometry may become a reality in the near future. Physics on the large scale is based on Einstein’s theory of general relativity (GR), which interprets gravity as the curvature of spacetime. Despite its tremendous success as an isolated
One of the biggest unsolved problems in physics is the unification of quantum mechanics and general relativity. The lack of experimental guidance has made the issue extremely evasive, though various attempts have been made to relate the loss of matter wave coherence to quantum spacetime fluctuations. We present a new approach to the gravitational decoherence near the Planck scale, made possible by the recently discovered conformal structure of canonical gravity. This leads to a gravitational analogue of Brownian motion whose correlation length is given by the Planck length up to a scaling factor. With input from recent matter wave experiments, we show the minimum value of this factor to be well within the expected range for quantum gravity theories. This suggests that the sensitivities of advanced matter wave interferometers may be approaching the fundamental level due to quantum spacetime fluctuations, and that investigating Planck scale physics using matter wave interferometry may become a reality in the near future.
Real world quantum systems are open to perpetual influence from the wider environment. Quantum gravitational fluctuations provide a most fundamental source of the environmental influence through their universal interactions with all forms of energy and matter causing decoherence. This may have subtle implications on precision laboratory experiments and astronomical observations and could limit the ultimate capacities for quantum technologies prone to decoherence. To establish the essential physical mechanism of decoherence under weak spacetime fluctuations, we carry out a sequence of analytical steps utilizing the Dirac constraint quantization and gauge invariant influence functional techniques resulting in a general master equation of a compact form that describes an open quantum gravitational system with arbitrary bosonic fields. An initial application of the theory is illustrated by the implied quantum gravitational dissipation of light as well as (non)relativistic massive or massless scalar particles. Related effects could eventually lead to important physical consequences including those on a cosmological scale and for a large number of correlated particles.
One of the biggest problems faced by those attempting to combine quantum theory and general relativity is the experimental inaccessibility of the unification scale. In this paper, we show how incoherent conformal waves in the gravitational field, which may be produced by quantum mechanical zero–point fluctuations, interact with the wave packets of massive particles. The result of this interaction is to produce decoherence within the wave packets which could be accessible in experiments at the atomic scale.Using a simple nonlinear model for the coherence properties of the gravitational field, we derive an equation for the evolution of the density matrix of such a wave packet. Following the primary state diffusion programme, the most promising source of space–time fluctuations for experimental detection is these zero–point energy fluctuations. According to our model, the absence of intrinsic irremovable decoherence in matter interferometry experiments puts bounds on some of the parameters of quantum gravity theories. Current experiments give λ >18, where λtPlanck is an effective cut–off for the validity of low–energy quantum gravity theories.
Several phenomenological approaches to quantum gravity predict the existence of a minimal measurable length and/or a maximum measurable momentum near the Planck scale. When embedded into the framework of quantum mechanics, such constraints induce a modification of the canonical commutation relations and thus a generalization of the Heisenberg uncertainty relations, commonly referred to as generalized uncertainty principle (GUP). Different models of quantum gravity imply different forms of the GUP. For instance, in the framework of string theory the GUP is quadratic in the momentum operator, while in the context of doubly special relativity it includes an additional linear dependence. Among the possible physical consequences, it was recently shown that the quadratic GUP induces a universal decoherence mechanism, provided one assumes a foamy structure of quantum spacetime close to the Planck length. Along this line, in the present work we investigate the gravitational decoherence associated to the linear-quadratic GUP and we compare it with the one associated to the quadratic GUP. We find that, despite their similarities, the two generalizations of the Heisenberg uncertainty principle yield decoherence times that are completely uncorrelated and significantly distinct. Motivated by this result, we introduce a theoretical and experimental scheme based on cavity optomechanics to measure the different time evolution of nonlocal quantum correlations corresponding to the two aforementioned decoherence mechanisms. We find that the deviation between the two predictions occurs on time scales that are macroscopic and thus potentially amenable to experimental verification. This scenario provides a possible setting to discriminate between different forms of the GUP and therefore different models of quantum gravity.