Session F05: Quantum Gravity (Thought) Experiments

Asymptotic Charge Induced Decoherence in QED and Quantum Gravity

In QED and (linearized) quantum gravity, we show that any localized charge will eventually decohere in the momentum basis in an asymptotically flat spacetime. This places an upper bound on the size of any coherent quantum superposition in space, and also generates an enhanced rate of wavepacket spreading. We estimate the size of these effects, which arise because any massive (or charged) particle necessarily radiates soft, entangling gravitons/photons to null infinity as it evolves. In the limit of infinite time—such as in QED scattering theory—this soft radiation gives rise to superselection in the electron momentum basis, with the result that almost all scattering states exhibit total delocalization of the charges. It is an experimental fact that this does not obstruct accurate predictions for collider experiments, where the central-momentum dependence of scattering cross sections can still be calculated. Nevertheless, in regimes where quantum coherence of charged particles becomes important, this total loss of coherence in traditional scattering theory is a fundamental obstacle to realistic predictions. In QED scattering, realistic physics only survives within a small class of carefully dressed states. In (nonlinear) quantum gravity, the conclusion is different, and suggests that valid physical states in quantum-gravitational scattering theory can only be described in terms of relational observables, e.g. by the introduction of extended objects.   

Bell meets Cavendish: a quantum signature of gravity?

The inclusion of gravitation within the framework of quantum theory remains one of the most prominent open problem in physics. To date, the absence of empirical evidence hampers conclusions regarding the fundamental nature of gravity -- whether it adheres to quantum principles or remains a classical field manifests solely in the macroscopic domain. This presentation discusses a thought experiment aimed at discerning the quantum signature of gravity through the lens of macroscopic nonlocality. The experiment integrates a standard Bell test with a classical Cavendish experiment. I will illustrate that the measurement apparatuses employed in a Bell experiment, despite lacking entanglement, defy classical descriptions; their statistical behaviors resist explanations through local hidden variable models. Extending this argument to encompass the massive objects in the Cavendish experiment allows for further disputing classical models of the gravitational field. Under favorable conditions and in light of corroborating evidence from the recent loophole-free Bell experiments, the quantum character of gravity is essentially substantiated.   

Violations of Einstein’s Equivalence Principle for Macroscopic Quantum Bodies

Different macroscopic ensembles of the hydrogen atoms are considered in an external gravitational field using the so-called weak field approximation. We demonstrate that these ensembles can be subdivided into two big groups with completely different gravitational properties. For the majority of them the Einstein’s Equivalence Principle is fulfilled, whereas for some special ensembles [1] (which we call “Gravitational demons”) the Equivalence Principle is broken for the both passive and active gravitational masses. We discuss [2] if it is possible or not to create the latter ensembles in some laboratories in the Earth and how to discover the violations of the Einstein’s Equivalence Principle. [1] A.G. Lebed, Editor, Breakdown of Einstein’s Equivalence Principle (World Scientific, Singapore, 2023). [2] A.G. Lebed, Amer. Inst. of Phys., Conf. Proc. 2872, 120048 (2023).

    

A thermodynamic analysis on gravitational redshift

Gravitational redshift is derived from General Relativity: when a photon moves upwards in gravity, its wavelength increases, and vice versa. However, is it compatible with the second law of thermodynamics? Particularly, Maxwell’s model of double-column engine demands that in any substance, the equilibrium temperature distribution must be uniform. The main concern is that photon-photon interaction is rare and therefore, photon gas is not a thermodynamic system. In classical mechanics, it is well known that certain nonchaotic particle movements are non-thermodynamic, but they tend to be small-scale, and their energy properties are “trivial”. Yet, as a photon gas is beyond the boundary of the second law of thermodynamics, it could be macroscopic and the consequence is nontrivial: without any other effect, in a gravitational field, with a thermal bath, useful work may be produced through heat absorption from a single thermal reservoir; in an isolated setup, entropy may decrease. Such phenomena, while counterintuitive, can be analyzed on the basis of the principle of maximum entropy, with the additional constraints of nonchaoticity.   

500 Galaxies Rotations Powered by a Baryonic Dark Matter.

This paper proposes a conceptual framework that predicts the rotational velocity of spiral galaxies as a function of their radial distances from their center of mass and the flattening of these curves at large distances. The model is grounded in an emergent modified gravity paradigm derived from Einstein’s general relativity and relies on an ERFC potential metric, whose constant offset can be assimilated to a baryonic dark matter reservoir. A short recall of the modified gravity model is presented and the equation describing a galaxy velocity profile is established, exploiting a single emergent parameter, the galaxy's proper length σ_Gal. Levenberg-Marquard curve fitting optimizations on the 551 galaxies of Sofue’s (2018) database are reported. The model fits the global velocity behavior all over the radial distances. It predicts very good results (SNR ≧ 20dB) in 78% of the galaxies. The whole description is consistent with the Thully-Fischer relationship that can be derived from it. According to this new paradigm, the constant component of the ERFC potential associated with a given mass provides a huge baryonic energy reservoir playing the role of dark matter in galaxy rotations, fixing for each galaxy, a constant velocity upper limit. In this perspective, taking the other side of the coin, the velocity profile of galaxies, with their tendencies to become constant at large distances, can be interpreted as a direct manifestation of the emergent ERFC potential metrics and its corresponding modified Newton’s law of gravitation.   

An experiment to measure electromagnetic memory

We describe an experiment to measure the electromagnetic analog of gravitational wave memory, the so-called electromagnetic memory.  Whereas gravitational wave memory is a residual displacement of test masses, electromagnetic memory is a residual velocity (i.e. kick) of test charges.

The source of gravitational wave memory is energy that is not confined to any bounded spatial region: in the case of binary black hole mergers the emitted energy of gravitational radiation as well as the recoil energy of the final black hole.  Similarly, electromagnetic memory requires a source whose charges are not confined to any bounded spatial region. 

While particle beams can provide unbounded charges, their currents are too small to be practical for such an experiment.  Instead we propose a short microwave pulse applied to the center of a long dipole antenna.  In this way the measurement of the kick can be done quickly enough that the finite size of the antenna does not come into play and it acts for our purposes the same as if it were an infinite antenna.