Aug. 15-19 2022, Vancouver, Canada

Quantum Mechanics & Gravity: Marrying Theory & Experiment


Name & Title

Rana Adhikari –
Opportunities in Gravitational Experiments

The experimental signatures of QG are often parameterized into two buckets: placing upper limits on parameterized models of alt gravity, or making a dramatically improved measurement of some physical variable hoping for a serendipitous discovery. In this talk I will describe how some adaptation/extension of the LIGO technology can be utilised to make a search for the underlying micro-physics of spacetime in the latter bucket. In addition, I will give a brief overview of the gravitational wave detectors, their results so far, and some plans for the next decades.

Abhay Ashtekar –
Gravity, Geometry, and the Quantum

I will begin with a broad perspective on the conceptual issues that must be faced to unify general relativity and quantum physics, and explain why viewpoints, preferences and aversions play a significant role at this forefront of theoretical physics. I will then introduce one of the leading approaches, loop quantum gravity, where Riemannian geometry underlying general relativity emerges from a background independent gauge theory. As a result, there is a new syntax to formulate and answer fundamental questions. The resulting interplay between gravity, geometry and the quantum has been used to address several long-standing questions of physics beyond Einstein. I will conclude with an example -the very early universe- to bring out the interplay between theory and observations. Following guidelines provided by the organizers, this talk will be addressed to a general audience rather than experts.

Markus Aspelmeyer –
Solving the gravity-quantum dilemma in experiments

Modern physics is facing a philosophical dilemma: its two main pillars, quantum theory and the theory of gravity, are rooted in world views that mutually exclude each other. If quantum physics is correct, we need to radically rethink our notions of space and time. If gravity theory is correct, quantum physics requires a dramatic revision. This is an experimental problem. However, up to this date there is no evidence that gravity requires a quantum description. Quantum experiments with increasingly massive particles may change that situation – by directly probing how gravity reacts to a quantum object.

Steve Carlip –
From discrete causal sets to a spacetime continuum

Causal set theory offers an interesting discrete model for spacetime, in which the causal structure is fundamental. But while we know how to approximate a spacetime manifold by a causal set, the vast majority of causal sets are not at all continuum-like. I will talk about progress in showing that large classes of these “bad” sets are very strongly suppressed in the gravitational path integral, perhaps allowing us to recover the observed continuum structure of spacetime from the quantum theory of discrete structures.

Daniel Carney –
Comments on graviton detection

A major open question at the heart of quantum gravity is whether the field is quantized in the first place. Recent proposals for tabletop gravitational entanglement experiments may help answer this question; I will review some arguments for and against this. Another approach is to try to directly detect single gravitational quanta – gravitons. While Dyson has argued that this is likely to be impossible, I will sketch some recent ideas for single graviton detection similar in spirit to the photoelectric effect, providing counterexamples to Dyson. Finally I’ll comment on whether detecting “single gravitons” in this manner really demonstrates that the gravitational field is quantized.

Yanbei Chen –
Towards Observational Signatures of Quantum Gravity

Experimental progress in quantum optomechanics has allowed preparing, manipulating, and probing the mechanical motions of macroscopic objects in the quantum regime. For example, laser interferometer gravitational-wave detectors are now sensitive to displacements of kg-scale test masses near the free mass Standard Quantum Limit, which arises from applying the Heisenberg Uncertainty Principle to kg scale test masses. Quantum optomechanics provides new opportunities for testing quantum measurement theory and studying the quantum nature of gravity. Recently, considerable interests arose in confirming quantum nature of gravity via lab-scale experiments. I will discuss possible experimental signatures of alternative theories in which gravity is classical, and explore in which regimes they can be best ruled out. I will also discuss the possibility of detecting space-time fluctuations in gravitational-wave-detector-like experiments.

Clifford Cheung –
Hidden Structures in Gravitational Scattering

In the modern S-matrix program, dynamics are bootstrapped directly from physical principles without reference to a Lagrangian or equations of motion. Aspects of the real world, including Einstein’s general relativity, Yang-Mills theory, and the theory of pions are then derived from the properties of amplitudes rather than vice versa. Remarkably, the expressions gleaned from this line of attack are marvelously simple, revealing new structures, long hidden in plain sight, which inextricably unify and connect these naively disparate theories. As an example, I describe how gravity is the “mother of all theories” whose S-matrix secretly encodes all scattering amplitudes of gluons, pions, and other particles. I also discuss a mysterious duality relating kinematics and color—a fact with far-reaching theoretical and phenomenological applications, including recent progress that has yielded now state-of-the-art results for gravitational wave theory describing the orbital dynamics of binary black holes in the post Minkowskian expansion.

John F. Donoghue –
Quantum General Relativity at Low Curvature

I will review the logic of making quantum predictions in general relativity using effective field theory and will mention some of the lessons. A new result related to the cosmological constant will be described. Finally I discuss some of the limitations of the effective field theory.

Peter Galison –
Imaging the Black Hole: The Past Propelling Us into the Future

Over the course of the development of modern science, atlases depicting the working objects of inquiry—from bodies, clouds, plants, to crystals and insects-, scientists worked out what counted as scientific objectivity. This long-term history, with its various takes on what a reliable image should be, converged in the years-long struggle of the Event Horizon Telescope (EHT) to produce a robust picture of a black hole. April 10, 2019 (of the supermassive black hole M87*) and then again May 12, 2022 (of the supermassive black hole at the center of the Milky Way), the team released the first images of a black hole, images viewed within a very few days of each release by more than a billion people. This presentation, back and forth between science, history, and documentary footage, about how the EHT team of some 200 scientists came to judge the glowing, crescent-like ring as objective. Here is a history propelling us into future of imaging black holes.

Steven B. Giddings –
A “quantum-first” approach to gravity and black holes

Traditional approaches to gravity attempt to quantize classical theories, and encounter vexing difficulties. An alternative is to investigate what properties a quantum theory needs to describe gravity. Matching known and expected physics supplies significant clues, complementing the tight structure of quantum mechanics and raising interesting questions of mathematical structure. Applied to black holes, the requirement of unitarity together with a key assumption that black holes behave as quantum subsystems (and do not leave problematic microscopic remnants) implies interactions going beyond a field theory description, and that may lead to measurable effects on black hole observations.

Gary Hinshaw –
Taking the Measure of the Universe

I will discuss recent progress and promising near-term prospects in observational cosmology. I will emphasize those aspects that I see as having the greatest promise for informing fundamental physics – perhaps even quantum gravity. This includes probes of accelerating expansion: early (hypothesized) and late, and prospects for measuring non-gaussianity in the primordial matter fluctuations.

Mark A. Kasevich –
Observation of a gravitational Aharonov-Bohm effect and its implications for quantum superpositions of Newtonian gravitational fields

The gravitational interaction of a tungsten source mass with atomic wavepackets has been observed in an atom deBroglie wave interferometer, in a regime where the separation distance between the interfering wavepackets is comparable to their distance to the source mass. We will discuss this experiment in the context of Aharonov-Bohm effects. We will describe the relevance of these results to observation of quantum superpositions of Newtonian gravitational fields.

Renate Loll –
Quantum Gravity: Getting There

New observational windows on strongly gravitating systems raise the exciting prospect to yield new insights into the quantum foundations of general relativity. This puts pressure on us theorists to deliver predictions, and in turn directs the focus on two properties any successful quantum gravity theory must have: uniqueness (no or few free parameters) and the ability to produce “numbers”, by using effective computational tools beyond perturbation theory. Our current best bet are no-frills quantum field theoretic approaches, which emulate the nonperturbative toolbox and successes of a theory like QCD. However, the symmetry structure of gravity is completely different from that of a gauge field theory, and it has taken many years to adapt lattice and renormalization group methods to a situation where geometry is dynamical and there is no pre-existing background metric or notion of scale. The good news is that sufficient progress has been made in some of these approaches to produce numbers, in the form of quantitative results on the spectrum of certain invariant quantum observables at or near the Planck scale. Most results on observables have been obtained in Causal Dynamical Triangulations or CDT quantum gravity, which uses a simplicial lattice regularization based on Regge’s idea of “General relativity without coordinates” and builds on the rich, exact mathematics of models of random geometry in two dimensions. The biggest breakthrough of CDT quantum gravity in four dimensions is the emergence, from first principles, of a nonperturbative ground state with properties of a de Sitter universe. I will summarize these results and explain what nonperturbative quantum gravity can and cannot do for you, highlighting the nonlocal character of observables and the structural challenges of relating Planckian and (semi-)classical gravity.

Viatcheslav Mukhanov –
How predictive are cosmological theories

I will discuss the advances in theoretical and experimental cosmology during the past 40 years. In particular, I will show how the precision measurements of the Cosmic Microwave Background radiation have allowed us to prove General Relativity in a fully nonperturbative regime and “marry” gravity with quantum mechanics in a theory of the quantum origin of the structure of the universe.

Julio Parra-Martinez –
Causality and Unitarity in Classical and Quantum Gravity

I will explain how the basic requirements of causality and unitarity put strong constraints on low energy theories of gravity.

P.J.E. Peebles –
How will the next big advance in gravity physics be found?

The last three big ideas in physical science go by the names Maxwell’s equations, Einstein’s field equation, and the Schrödinger equation. Einstein hit on general relativity by close to pure thought, with just a few hints from phenomenology. Maxwell put together results from many developments in the laboratory. Quantum physics grew in an intermediate way: several brilliant ideas driven by anomalies in the phenomena. The next big idea about gravity might be found by a stroke of genius, like Einstein, or grow out of examinations of phenomenology. Gravity figures large in cosmology, and there are curious anomalies in this subject. I’ll mention a few that just possibly hint at something interesting.

Roger Penrose –
Time-asymmetric GR singularity Puzzle as Addressed by CCC

The “singularity theorems” of the 1960s, demonstrated the generic status of space-time singularities in black holes, but also in the Big Bang, the latter being puzzlingly enormously constrained by the suppression of gravitational degrees of freedom at the Big Bang. The conventional viewpoint has been to try to resolve the space-time singularity problem through some yet-to-be discovered theory of quantum gravity, aided by the introduction of an additional physical field to provide an initial “inflationary” phase immediately following the Big Bang. Yet, the problem of the gross suppression of initial gravitational degrees of freedom, fundamental to the second law of thermodynamics, remains unresolved by such means. The proposal of conformal cyclic cosmology (CCC) was originally introduced in around 2005 as a highly speculative proposal to replace inflation by an earlier exponentially expanding “cosmic aeon”, where CCC’s history consists of an unending succession of such aeons, each following from the preceding one through a smooth conformal space-time geometry—the geometry of a physics without massive particles, such a geometry being argued as being the relevant one both at the Big Bang and at each aeon’s remote future. Two distinct families of observational predictions have been confirmed in recent years to high confidence levels.

Roger Penrose –
Wave-function Collapse from a conflict of Principles of QM and GR

The foundational principle underlying Einstein’s general theory of relativity (GR), as a development from special relativity, was through Galileo’s “principle of equivalence”, according to which the local gravitational force can be eliminated by falling freely under gravity. However, when we try to combine this principle with Schrödinger’s fundamental equation for quantum mechanics (QM) we find a conflict when a body is put into a quantum superposition of two locations at once. This has the nature of a mass-energy uncertainty, implying a time-limitation of such superpositions. Various experiments have been proposed to test this predicted effect but, so far, no such experiment has reached the required level.

Martin J. Rees –
Cosmology: what we know, what we don’t know, and what we may never know

Abstract TBA

Suzanne T. Staggs –
Primordial Gravitational Waves and the Cosmic Microwave Background

The cosmic microwave background (CMB) has yielded surprisingly detailed and precise information about the form, content and dynamics of the early universe. High angular resolution maps, and polarization data at all angular scales, are the focus of current and next-generation instruments. Primordial gravitational waves would induce odd-parity polarization patterns at degree angular scales in the CMB. Galactic foregrounds and other systematic effects must be well-understood to prevent them swamping the subtle gravitational wave signatures. I will elaborate on experimental methods to overcome such difficulties, and give a quick description of a new suite of CMB instruments comprising the Simons Observatory, including context to the emerging CMB-S4 program.

Philip Stamp –
A CWL Primer

A short intro to the Correlated Worldline (CWL) theory is provided. I will stress the underlying physical reasoning and rationale for the theory, and explain in simple terms the structure of the theory. A key feature of the theory is that quantum mechanics is violated for objects exceeding a mass ~ 10^{-5} Planck mass, because of gravitational interactions between paths in a path integral. I will also explain why CWL theory is now believed to be a consistent quantum field theory. In the remainder of the talk I will focus on the phenomenon of “path-bunching”. Because paths in a path integral for any quantum object attract each other gravitationally, they will bunch together if they are able to get rid of the relevant gravitational energy. This can happen in various ways and I will analyze these, and discuss the implications for lab experiments.

Anatoly Svidzinsky –
Unruh, Cherenkov and Hawking radiation from a negative frequency perspective and generation of entangled photon pairs

A ground-state atom uniformly accelerated through the Minkowski vacuum can become excited by emitting an Unruh-Minkowski photon. From the perspective of an accelerated atom, the sign of the frequency of the Unruh-Minkowski photons can be positive or negative depending on the acceleration direction and the accelerated atom becomes excited by emitting a negative frequency photon, and decays by emitting a positive frequency photon. This process yields generation of entangled photon pairs in a squeezed state which mimics entanglement of the Minkowski vacuum. Similar effects take place for the Cherenkov and Hawking radiation. I will also discuss radiation of atoms freely falling into a black hole through a Boulware vacuum, which is analogous to atom excitation by a uniformly accelerated mirror. Such radiation looks to a distant observer much like (but is different from) Hawking radiation.

Kip S. Thorne –
Is Backward Time Travel Compatible with Quantum Physics? 

The laws of classical physics, particularly Einstein’s general relativity, permit backward time travel. However, there is evidence but no firm proof, that quantum physics effects will cause any machine for backward time travel to self destruct at the moment it is first activated. The laws of quantum mechanics as usually formulated seem unable to deal with backward time travel. However, an alternative formulation of the quantum laws, due to Richard Feynman and extended by James Hartle and Murray Gell-Mann, appears to be fully compatible with time travel, but gives some strange predictions that I shall describe. What does all this mean for the nature of time in our universe?

Hendrik Ulbricht –
Testing quantum mechanics and gravity by levitated mechanical systems 

I will discuss our experiments on Levitated mechanics based on optical, Meissner and Paul trapping nano- and micro-particles in vacuum. Experiments hold promise for testing the quantum superposition principle interferometrically and non-interferometrically, will explain how that works and where we are with experiments, and emphasize the role of noise in this context. Experiments are pushed in a parameter regime where both quantum states, such as superpositions, and gravity effects are possible to be generated and detected. That low energy regime is expected to provide a new test area for experiments into the overlap of quantum mechanics and gravity in the non-relativistic regime.

William G. Unruh –
Measurement of acceleration temperature in a BEC

In 1976, I predicted that an accelerated detector (eg, of the electromagnetic field) would respond, in the vacuum, as though immersed in a thermal bath of that field with temperature proportional to the the acceleration. A group of us has made a proposal to use a laser probe as a type of microphone (converting sound waves to electromagnetic signals) to measure this effect in a Bose Einstein Condensate (BEC). This proposal thus combines BECs, and quantum optics, and uses lessons learned from the LIGO laser interferometric gravitational wave detector (including noise reduction by squeezing of the vacuum) to measure a novel quantum field theory effect.

Alexander Vilenkin –
Quantum cosmology and the beginning of the universe  

The spacetime of an expanding universe cannot be indefinitely extended to the past; it must have a beginning. The question is then: what determines the initial state of the universe? This question is addressed in quantum cosmology, where the entire universe is treated quantum mechanically and is described by a wave function. I will review the ideas of quantum cosmology and some of its conceptual problems.

Robert M. Wald –
Quantum Superpositions of Massive Bodies and Gravitationally Mediated Entanglement

In order to avoid contradictions with complementarity and causality in a gedankenexperiment involving a quantum superposition of a massive body, it was previously shown (in arXiv:1807.07015) that it is necessary for there to be both quantized gravitational radiation and local vacuum fluctuations of the spacetime metric We review this gedankenexperiment and the previously given “back of the envelope” arguments that resolve it. We then improve upon this analysis by providing a precise and rigorous description of the entanglement and decoherence effects (given in arXiv:2112.10798). As a by-product of our analysis, we show that under the protocols of the gedankenexperiment, there is no clear distinction between entanglement mediated by the Newtonian gravitational field of a body and entanglement mediated by on-shell gravitons emitted by the body. This suggests that Newtonian entanglement implies the existence of graviton entanglement and supports the view that the experimental discovery of Newtonian entanglement—as envisioned in proposed experiments—may be viewed as evidence for the existence of the graviton.

K. Birgitta Whaley –
Using quantum states of trapped nanoparticles to probe the effect of gravity on quantum mechanics of large systems

The question of how gravity may affect quantum mechanics of large systems presents an essential challenge for our fundamental understanding of the world in terms of quantum principles. We shall describe how these issues can be addressed by studying the effects of the Correlated Worldline (CWL) theory on the dynamics of non-classical states of trapped nanoparticles and discuss prospects for near-term experiments that may be able to look for signatures of CWL theory or other gravity-based deviations from quantum mechanics at large mass scale.

Jordan Wilson-Gerow –
Low energy signatures of a Correlated Worldline theory of quantum gravity

Is gravity necessarily quantized like the other fields? Prominent physicists including Feynman, Penrose, Leggett, and others, have suggested that the standard structure of quantum mechanics might break down for macroscopic objects. One might try to phenomenologically model such a break down but nature already provides us with a very natural mechanism which becomes dominant for large objects, gravitation. In this talk I will discuss one such construction, the Correlated Worldline theory, which is an apparently self-consistent theory wherein a modification to the quantum gravity path-integral leads to a violation of the superposition principle for high-energies/large masses. I will introduce the central ideas, recent results, and prospects for near-term tests via quantum optomechanical experiments on “macroscopic quantum objects”.

Michael Wright –
The Archive For Mathematical Sciences (aka The Michael Wright Collection) 1973- 2022 Some recollections and insights from the last 50 years 

In the Summer of 1973, I read what Physicists almost always refer to as “The Big Black Book” (Misner Thorne and Wheeler’s “General Relativity”) shortly after its publication. It was the impact of that which more than anything else prompted the formation of the Archive which is shortly to become a centerpiece of the planned QGS Institute. I will describe the growth of this project over the subsequent 50 years with an account of the way it has diversified into other key areas in addition to its original central focus on Gravity and Cosmology. I will defend the merits and motives for the project and present a short film recording the current appearance and activities of the archive. I will end by inviting questions and a discussion of how the collection can serve as a resource for teachers and researchers and provide the raw material for further projects.