arXiv subjects

Steven B. Giddings

Publications and source records attributed to Steven B. Giddings.

At least 19 recordsLinked to original sources

arXiv2026

Deriving effective descriptions and signal predictions for dynamical gravitational systems

We investigate top-down derivations of effective descriptions of radiation from gravitational systems such as binaries. With a specified cutoff prescription, one can derive worldline effective field theories, although the associated cutoff dependence can complicate their formulation. We investigate a related effective approach in which the dynamics are parameterized by an action on the boundaries of cavities enclosing the individual bodies. We present examples of such cavity descriptions for black holes and for simple models of modified black hole behavior. We also show how cavity effective descriptions connect to observable quantities including Love numbers and aspects of wave profiles, such as the accumulated phase shifts of emitted signals. A primary motivation is to develop a systematic framework for inferring how modifications of classical black hole behavior affect gravitational wave signals. Such modifications may be motivated by the requirement that black hole evolution be consistent with quantum mechanics, or by other models of new black hole behavior. Accumulated phase shifts during the inspiral have, in particular, been argued to provide sensitivity to small new effects. To illustrate the basic principles clearly, we develop the formalism primarily for scalar radiation, and then outline its extension to gravitational perturbations.

gr-qc↗

arXiv2026

Gravitational dressing: from the crossed product to more general algebraic and mathematical structure

The crossed product, and consequent transition from von Neumann algebras of type III to II, is recovered from a truncation of more general gravitational dressing constructions, about certain spacetimes. This is done by extending "standard dressing" constructions previously used to give a perturbative definition of "gravitational splittings," defining approximate localization of information. This result appears to illustrate that this algebraic transition is a small piece of a more general algebraic, or other mathematical, structure associated with quantum gravity. The leading-order structure involves noncommutativity from separated regions, and at the nonperturbative level connects with a possible explanation of holographic behavior for gravity.

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arXiv2025

Visions in Quantum Gravity

To deepen our understanding of Quantum Gravity and its connections with black holes and cosmology, building a common language and exchanging ideas across different approaches is crucial. The Nordita Program "Quantum Gravity: from gravitational effective field theories to ultraviolet complete approaches" created a platform for extensive discussions, aimed at pinpointing both common grounds and sources of disagreements, with the hope of generating ideas and driving progress in the field. This contribution summarizes the twelve topical discussions held during the program and collects individual thoughts of speakers and panelists on the future of the field in light of these discussions.

hep-th↗

arXiv2025

Challenges for describing unitary evolution in nontrivial geometries: pictures and representations

Description of evolution between spatial slices in a general spacetime suffers from a significant difficulty: the states on the slices, in a given basis, are not related by a unitary transformation. This problem, which occurs in spacetime dimensions above two, is directly related to the infinite number of inequivalent representations of the canonical commutators, and in particular will arise for interacting theories in time-dependent spacetimes. We connect different facets of this issue, and discuss its possible resolution. It is directly related to discussions of failure of a standard Schrödinger picture of evolution, and of evolution via "many-fingered time." One requires a condition specifying a physical unitary equivalence class of states; in general this equivalence class evolves with time, and an important question is how it is determined. One approach to this in free theories is by imposing a Hadamard condition on the two point function. We explore a different approach, which also may be helpful for interacting theories, analyzing the structure of the state in a local limit, and relate these approaches. We also elucidate the non-Hadamard behavior of unphysical vacua, and discuss concrete examples of these approaches involving cosmological and black hole evolution. The issues are extended in the context of quantum dynamical geometry, and raise important questions for the proper description of the wavefunction of the universe and for the role of the Wheeler-DeWitt equation.

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arXiv2025

Quantum gravity observables: observation, algebras, and mathematical structure

The questions of describing observables and observation in quantum gravity appear to be centrally important to its physics. A relational approach holds significant promise, and a classification of different types of relational observables (gravitationally dressed, field relational, and more general) is outlined. Plausibly gravitationally dressed observables are particularly closely tied to the fundamental structure of the theory. These may be constructed in the quantum theory to leading order in Newton's constant, and raise important questions about localization of information. Approximate localization is given by a "standard dressing" construction of a "gravitational splitting." It is also argued that such gravitational dressings give a generalization of the crossed product construction, reducing to this and yielding type II von Neumann algebras in special cases. Gravity therefore introduces a significantly more general alteration of the algebraic structure of local quantum field theory, also with apparent connections to holography, but whose implications have not been fully understood. In particular, properties of the algebra of gravitationally dressed observables suggest a possible role for other non-algebraic structure on the Hilbert space for quantum gravity.

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arXiv2025

The unitarity crisis, nonviolent unitarization, and implications for quantum spacetime

This contribution overviews the information paradox, or perhaps more aptly "unitarity crisis," and a proposed resolution called nonviolent unitarization. It begins by examining the conflict of principles that yields the crisis, which can be phrased in terms of a "black hole theorem" summarizing how basic assumptions come into conflict. Proposed resolutions of the conflict, along with problems with them, are overviewed. The very important underlying question of localization of information and its role is discussed at some length, taking into account effects of perturbative gravity. The difficulty in finding a consistent scenario for black hole evolution strongly suggests new interactions on event horizon scales; a "minimal" set of assumptions about these are parameterized in nonviolent unitarization. Possible criticisms of this scenario, and some responses, are given. New interactions at event horizon scales potentially lead to observable effects, via gravitational wave or electromagnetic channels, which are briefly discussed. A possible origin of nonviolent unitarization effects from a more fundamental description of quantum spacetime, and possible implications for such a description, are also briefly discussed.

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arXiv2024

Perturbative quantum evolution of the gravitational state and dressing in general backgrounds

This paper sets up a perturbative treatment of the evolving quantum state of a gravitational system, in a Schrödinger-like picture, working about a general background. This connects gauge symmetry, the constraints, gravitational dressing, and evolution. Starting with a general time slicing, we give a simple derivation of the relation between the constraints, the hamiltonian, and its well-known boundary term. Among different approaches to quantization with constraints, we focus on a "gauge-invariant canonical quantization," which is developed perturbatively in the gravitational coupling. The leading-order solution of the constraints (including the Wheeler-DeWitt equation) for perturbations about the background is given in terms of an explicit construction of gravitational dressings built using certain generalized Green functions; different such dressings corresponding to adding propagating gravitational waves to a particular solution of the constraints. Dressed operators commute with the constraints, expressing their gauge invariance, and have an algebraic structure differing significantly from the undressed operators of the underlying field theory. These operators can act on the vacuum to create dressed states, and evolution of general such states is then generated by the boundary hamiltonian, and alternately may be characterized using other relational observables. This provides a concrete approach to studying perturbative time evolution, including the leading gravitational backreaction, of quantum states of black holes with flat or anti de Sitter asymptotics, for example on horizon-crossing slices. This description of evolution in turn provides a starting point for investigating possibly important corrections to quantum evolution, that go beyond quantized general relativity.

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arXiv2024

Gravitational wave signatures of departures from classical black hole scattering

We initiate a general investigation into gravitational wave signatures of modifications to scattering of gravitational radiation from black holes. Such modifications may be present due to the quantum dynamics that makes black holes consistent with quantum mechanics, or in other models for departures from classical black hole behavior. We propose a parameterization of the corrections to scattering as a physically meaningful, model-independent, and practical bridge between theoretical and observational aspects of the problem; this parameterization can incorporate different models in the literature. We then describe how these corrections influence the gravitational wave signal, e.g. of a body orbiting a much more massive black hole. In particular, they generically change the rate of energy emission; this effect can be leveraged over many orbits of inspiral to enhance the sensitivity to small corrections, as has been noticed in simple models. We provide preliminary estimates of the sensitivity of future gravitational wave observations to these corrections, and outline further work to be done to connect both to a more fundamental theory of quantum black holes, and to realistic observational situations.

gr-qc↗

arXiv2023

Comparing models for a unitary black hole S-matrix

This paper compares features, challenges, and puzzles of different models for a unitary black hole S-matrix, focussing on both recent nonisometric models, as well as "nonviolent unitarization," which is based on new quantum interactions of a black hole. As a foundation for comparison, the description of real-time Hawking evolution is first overviewed, including leading effects of gravitational dressing and backreaction. Connection is then made to qubit models for evolution, and some technology is outlined to facilitate their description. Important features of both nonisometric models and nonviolent unitarization are investigated in qubit models, which illustrate essential differences between the respective approaches. The nonisometric models present puzzles for understanding evolution of internal outgoing excitations, which can be excited by interactions such as particle decay. Qubit models for nonviolent unitarization are further developed, and nicely illustrate aspects of that approach. Some remaining questions in generalizing to more complete models for evolution are discussed.

hep-th↗

arXiv2023

A "black hole theorem," and its implications

A general formulation of the basic conflict of the information problem is given, encapsulated in a "black hole theorem." This is framed in a more general context than the usual one of quantum field theory on a background, and is based on describing a black hole as a quantum subsystem of a larger system, including its environment. This sharpens the limited set of possible consistent options; as with the Coleman-Mandula theorem, the most important point is probably the loophole in the "theorem," and what this tells us about the fundamental structure of quantum gravity. This "theorem" in particular involves the general question of how to define quantum subsystems in quantum gravity. If black holes do behave as quantum subsystems, at least to a good approximation, evolve unitarily, and do not leave remnants, the "theorem" implies the presence of interactions between a black hole and its environment that go beyond a description based on local quantum fields. This provides further motivation for and connects to previous work giving a principled parameterization of these interactions, and investigating their possible observational signatures via electromagnetic or gravitational wave observations of black holes.

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arXiv2022

Frontiers of Quantum Gravity: shared challenges, converging directions

Understanding the quantum nature of spacetime and gravity remains one of the most ambitious goals of theoretical physics. It promises to provide key new insights into fundamental particle theory, astrophysics, cosmology and the foundations of physics. Despite this common goal, the community of quantum gravity researchers is sometimes seen as divided into sub-communities working on different, mutually exclusive approaches. In practice however, recent years have shown the emergence of common techniques, results and physical ideas arising from different sub-communities, suggesting exciting new prospects for collaboration and interaction between traditionally distinct approaches. In this White Paper we discuss some of the common themes which have seen a growing interest from various directions, and argue that focusing on them will help the quantum gravity community as a whole towards shared objectives.

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arXiv2022

Quantum evolution of the Hawking state for black holes

We give a general description of the evolving quantum state of a Schwarzschild black hole, in the quantum field theory approximation. Such a time-dependent description is based on introducing a choice of time slices. We in particular consider slices that smoothly cross the horizon, and introduction of "stationary" such slices simplifies the analysis. This analysis goes beyond standard derivations of Hawking radiation that focus on asymptotic excitations, and in particular gives an evolving state that is regular at the horizon, with no explicit transplanckian dependence, and that can in principle be generalized to incorporate interacting fields. It is also expected to be useful in connecting to information-theoretic investigation of black hole evolution. The description of the evolving state depends on the choice of slices as well as coordinates on the slices and mode bases; these choices give different "pictures" analogous to that of Schrödinger. Evolution does have a simpler appearance in an energy eigenbasis, but such a basis is also singular at the horizon; evolution of regular modes has a more complicated appearance, whose properties may be inferred by comparing with the energy eigenbasis. In a regular description, Hawking quanta are produced in a black hole atmosphere, at scales comparable to the horizon size. This approach is also argued to extend to more general asymptotics, such as that of anti de Sitter space. In the latter context, this analysis provides a description of the hamiltonian and evolution of a black hole that may be compared to the large-$N$ dynamics of the proposed dual CFT.

hep-th↗

arXiv2022

The deepest problem: some perspectives on quantum gravity

Quantum gravity is likely the deepest problem facing current physics. While traditionally associated with short distance nonrenormalizability, it is evident that the long distance problem of unitarity, arising at high energies with black hole formation, is more profound. This reveals a conflict between foundational principles of quantum field theory: those of quantum mechanics, relativity, and locality. Difficulties modifying quantum mechanics suggest a "quantum-first" approach, with other principles as mathematical properties of a quantum space of states. A challenge is how to describe locality, in terms of Hilbert space structure. Perturbative gravity gives clues, with structure apparently different than in field theory. The mathematical structure of subsystems plausibly supplants conventional locality and plays a foundational role in the theory. This view differs from one of spacetime "emerging" from another quantum system. If a black hole behaves as a subsystem, a "black hole theorem" says that unitarity requires interactions with its environment depending on its state, or more drastic phenomena. Minimal interactions can be parameterized, in an effective approach; they could arise from wormholes or other fundamental dynamics. These or other near-horizon modifications potentially alter electromagnetic or gravitational signatures of this strong gravity region, now being probed in a new era of observation; it is important to seek observational clues for or constraints on such scenarios. One may also investigate quantum gravity via its S-matrix. New perturbative structure has been discovered there, but the harder question again goes beyond to the nonperturbative regime. Long-distance behavior of amplitudes indicates novel analytic behavior; further exploration may provide important clues. Other key questions regard quantum description of cosmologies, and of associated observables.

hep-th↗

arXiv2022

On the questions of asymptotic recoverability of information and subsystems in quantum gravity

A longstanding question in quantum gravity regards the localization of quantum information; one way to formulate this question is to ask how subsystems can be defined in quantum-gravitational systems. The gauge symmetry and necessity of solving the constraints appear to imply that the answers to this question here are different than in finite quantum systems, or in local quantum field theory. Specifically, the constraints can be solved by providing a "gravitational dressing" for the underlying field-theory operators, but this modifies their locality properties. It has been argued that holography itself may be explained through this role of the gauge symmetry and constraints, at the nonperturbative level, but there are also subtleties in constructing a holographic map in this approach. There are also claims that holography is implied even by perturbative solution of the constraints. This short note provides further examination of these questions, and in particular investigates to what extent perturbative or nonperturbative solution of the constraints implies that information naively thought to be localized can be recovered by asymptotic measurements, and the relevance of this in defining subsystems. In the leading perturbative case, the relevant effects are seen to be exponentially suppressed. These questions are, for example, important in sharply characterizing the unitarity problem for black holes.

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arXiv2021

Schrödinger evolution of two-dimensional black holes

This paper systematically treats the evolving quantum state for two-dimensional black holes, with particular focus on the CGHS model, but also elucidating features generalizing to higher dimensions. This is done in Schrödinger picture(s), to exhibit the dynamic evolution of the state at intermediate times. After a review of classical solutions, also connecting to descriptions of higher-dimensional black holes, it overviews the canonical quantum treatment of the full evolution, including gravitational dynamics. Derived in an approximation to this, following conversion to "perturbation picture," is the evolution of the quantum matter on the background geometry. Features of the evolving matter state are described, based on choice of a time slicing to put the evolution into ADM form. The choices of slicing as well as coordinates on the slices result in different quantum "pictures" for treating the evolution. If such a description is based on smooth trans-horizon slices, that avoids explicit reference to ultra-planckian modes familiar from traditional treatments, and exhibits the Hawking excitations as emerging from a "quantum atmosphere" with thickness comparable to the inverse temperature. Detailed study of the state exhibits the entanglement structure between Hawking quanta and the partner excitations inside the black hole, and the corresponding "missing information." This explicit description also allows direct study of the evolution and features, e.g. as seen by infalling observers, of these partner excitations, helping to address various puzzles with them. Explicit treatment of the evolving state, and its extension to higher dimensions, provides further connections to information theory and a starting point for study of corrections that can unitarize evolution, arising from new quantum gravity effects -- whether wormholes or something entirely different.

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arXiv2021

Black holes and other clues to the quantum structure of gravity

Bringing gravity into a quantum-mechanical framework is likely the most profound remaining problem in fundamental physics. The "unitarity crisis" for black hole evolution appears to be a key facet of this problem, whose resolution will provide important clues. Investigating this raises the important structural question of how to think about subsystems and localization of information in quantum gravity. Paralleling field theory, the answer to this is expected to be an important ingredient in the mathematical structure of the theory. Perturbative gravity results indicate a structure different from that of quantum field theory, but suggest an avenue to defining subsystems. If black holes do behave similarly to familiar subsystems, unitarity demands new interactions that transfer entanglement from them. Such interactions can be parameterized in an effective approach, without directly addressing the question of the fundamental dynamics, whether that is associated with quantum spacetime, wormholes, or something else. Since such interactions need to extend outside the horizon, that raises the question of whether they can be constrained, or might be observed, by new electromagnetic or gravitational wave observations of strong gravity regions. This note overviews and provides connections between these developments.

gr-qc↗

arXiv2020

Schrödinger evolution of the Hawking state

A Schrödinger-picture description of the evolving quantum state of Hawking radiation is given, based on an ADM decomposition using time slicings that smoothly cross the horizon. This treatment avoids requiring a role for trans-planckian modes, which can be viewed as artifacts of Hawking's original calculation, and also supports arguments that radiation from black holes is produced in a "quantum atmosphere" with thickness comparable to the horizon size, rather than microscopically far from it. Particularly explicit formulas are given for the two-dimensional analog of the Schwarzschild geometry. This analysis is expected to generalize to other black holes, and to cosmology. The resulting quantum evolution also provides important background for investigating corrections to the Hawking process, as are necessary for restoring unitary evolution of black hole decay.

hep-th↗

arXiv2020

Wormhole calculus, replicas, and entropies

We investigate contributions of spacetime wormholes, describing baby universe emission and absorption, to calculations of entropies and correlation functions, for example those based on the replica method. We find that the rules of the "wormhole calculus," developed in the 1980s, together with standard quantum mechanical prescriptions for computing entropies and correlators, imply definite rules for $\textit{limited}$ patterns of connection between replica factors in simple calculations. These results stand in contrast with assumptions that all topologies connecting replicas should be summed over, and call into question the explanation for the latter. In a "free" approximation baby universes introduce probability distributions for coupling constants, and we review and extend arguments that successive experiments in a "parent" universe increasingly precisely fix such couplings, resulting in ultimately pure evolution. Once this has happened, the nontrivial question remains of how topology-changing effects can modify the standard description of black hole information loss.

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