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Canonical quantization of gravity in general relativity is greatly simplified by the artificial decomposition of space and time into a 3+1 formalism. Such a simplification may appear to come at the cost of general covariance. This requires tangential and perpendicular infinitesimal diffeomorphisms generated by the symmetry group under the Legendre transformation of the given action. This gauge generator, along with the fact that Weyl curvature scalars may act as ``intrinsic coordinates" (or a dynamical reference frame) which depend only on the spatial metric $g_{ab}$ and the conjugate momenta $p^{cd}$, allow for an alternative approach to canonical quantization of gravity. In this paper we present the tensorial solution of the set of Weyl scalars in terms of canonical phase-space variables.

The vanishing phase space generator of the full four-dimensional diffeomorphism-related symmetry group in the context of the Barbero-Immirzi-Holst Lagrangian is derived directly for the first time from Noether's second theorem. Its applicability in the construction of classical diffeomorphism invariants is reviewed., Comment: 11 pages, invited contribution to "Recent Advances in Gravity: A Themed Issue in Honor of Prof. Jorge Pullin on His 60th Anniversary"

The conventional Rosenfeld-Bergmann-Dirac constrained Hamiltonian algorithm applied to Einstein-Yang-Mills theory is shown to be equivalent to a local gauge theoretic extension of Cartan's invariant integral approach to classical mechanics. In addition, the Hamiltonian generators of Legendre-projectable spacetime diffeomorphism and gauge symmetries are derived directly as vanishing Noether charges. This leads directly to their interpretation as delivering the correct symmetry variations of both configuration and momentum feld variables., Comment: To be published in Particles, Fields and Topology: Celebrating A.P. Balachandran, a Festschrift volume for A.P. Balachandran

The main focus is on the Hamilton-Jacobi techniques in classical general relativity that were pursued by Peter Bergmann and Arthur Komar in the 1960's and 1970's. They placed special emphasis on the ability to construct the factor group of canonical transformations, where the four-dimensional diffeomorphism phase space transformations were factored out. Equivalence classes were identified by a set of phase space functions that were invariant under the action of the four-dimensional diffeomorphism group. This is contrasted and compared with approaches of Paul Weiss, Julian Schwinger, Richard Arnowitt, Stanley Deser, Charles Misner, Karel Kuchar - and especially the geometrodynamical program of John Wheeler and Bryce DeWitt where diffeomorphism symmetry is replaced by a notion of multifingered time. The origins of all of these approaches are traced to Elie Cartan's invariant integral formulation of classical dynamics. A related correspondence concerning the thin sandwich dispute is also documented., Comment: 53 pages

This is a short memoriam celebrating the life and work of the general relativist Joshua N. Goldberg, who passed away in October, 2020., Comment: 5 pages, to appear in the Spring 2021 Newsletter of the APS Forum on the History and Philosophy of Physics

In a series of papers published in the course of his dissertation work in the mid 1950's, Andrzej Trautman drew upon the slow motion approximation developed by his advisor Infeld, the general covariance based strong conservation laws enunciated by Bergmann and Goldberg, the Riemann tensor attributes explored by Goldberg and related geodesic deviation exploited by Pirani, the permissible metric discontinuities identified by Lichnerowicz, O'Brien and Synge, and finally Petrov's classification of vacuum spacetimes. With several significant additions he produced a comprehensive overview of the state of research in equations of motion and gravitational waves that was presented in a widely cited series of lectures at King's College, London, in 1958. Fundamental new contributions were the formulation of boundary conditions representing outgoing gravitational radiation the deduction of its Petrov type, a covariant expression for null wave fronts, and a derivation of the correct mass loss formula due to radiation emission. Ivor Robinson had already in 1956 developed a bi-vector based technique that had resulted in his rediscovery of exact plane gravitational wave solutions of Einstein's equations. He was the first to characterize shear-free null geodesic congruences. He and Trautman met in London in 1958, and there resulted a long-term collaboration whose initial fruits were the Robinson-Trautman metric, examples of which were exact spherical gravitational waves., Comment: 45 pages. Daniel Kennefick has joined this update as a co-author

The text presented here and revised by the authors is based on the original oral history interview by Donald Salisbury with Andrzej Trautman recorded in Warsaw on June 24 and June 28, 2016., Comment: 25 pages, 5 figures, to appear in European Physical Journal H

Peter Bergmann and his students embarked in 1949 on a mainly canonical quantization program whose aim was to take into account the underlying four-dimensional diffeomorphism symmetry in the transition from a Lagrangian to a Hamiltonian formulation of Einstein's theory. They argued that even though one seemed to destroy the full covariance through the focus on a temporal foliation of spacetime, this loss was illusory. Early on they convinced themselves that only the construction of classical invariants could adequately reflect the fully relativistic absence of physical meaning of spacetime coordinates. Efforts were made by Bergmann students Newman and Janis to construct these classical invariants. Then in the late 1950's Bergmann and Komar proposed a comprehensive program in which classical invariants could be constructed using the spacetime geometry itself to fix intrinsic spacetime landmarks. At roughly the same time Dirac formulated a new criterion for identifying initial phase space variables, one of whose consequences was that Bergmann himself abandoned the gravitational lapse and shift as canonical variables. Furthermore, Bergmann in 1962 interpreted the Dirac formalism as altering the very nature of diffeomorphism symmetry. One class of infinitesimal diffeomorphism was to be understood as depending on the perpendicular to the given temporal foliation. Thus even within the Bergmann school program the preservation of the full four-dimensional symmetry in the Hamiltonian program became problematic. Indeed, the ADM and associated Wheeler-DeWitt program that gained and has retained prominence since this time abandoned the full symmetry. There do remain dissenters - raising the question whether the field of quantum gravity has witnessed a Renaissance in the ensuing decades - or might the full four-dimensional symmetry yet be reborn?, Comment: 34 pages, minor typos corrected. Version appearing in 2020 Einstein Studies volume entitled The Renaissance of General Relativity edited by Alexander Blum, Roberto Lalli, and J\"urgen Renn, pp. 221 - 255, ISSN 978-3-030-50754-1

This commentary reflects on the 1930 discoveries of L\'eon Rosenfeld in the domain of phase-space constraints. We start with a short biography of Rosenfeld and his motivation for this article in the context of ideas pursued by W. Pauli, F. Klein, E. Noether. We then comment on Rosenfeld's General Theory dealing with symmetries and constraints, symmetry generators, conservation laws and the construction of a Hamiltonian in the case of phase-space constraints. It is remarkable that he was able to derive expressions for all phase space symmetry generators without making explicit reference to the generator of time evolution. In his Applications, Rosenfeld treated the general relativistic example of Einstein-Maxwell-Dirac theory. We show, that although Rosenfeld refrained from fully applying his general findings to this example, he could have obtained the Hamiltonian. Many of Rosenfeld's discoveries were re-developed or re-discovered by others two decades later, yet as we show there remain additional firsts that are still not recognized in the community., Comment: 61 pages. Commentary on Rosenfeld's "Zur Quantelung der Wellenfelder", Ann. Phys. 5, 113 (1930). A translation into English by us will appear in European Physical Journal H. This commentary will also appear in EPJH. Revision contains more elementary examples, an appendix with an overview of constrained Hamiltonian dynamics, additional references, and corrections of typos

Classical background independence is reflected in Lagrangian general relativity through covariance under the full diffeomorphism group. We show how this independence can be maintained in a Hamilton-Jacobi approach that does not accord special privilege to any geometric structure. Intrinsic spacetime curvature based coordinates grant equal status to all geometric backgrounds. They play an essential role as a starting point for inequivalent semi-classical quantizations. The scheme calls into question Wheeler's geometrodynamical approach and the associated Wheeler-DeWitt equation in which three-metrics are featured geometrical objects. The formalism deals with variables that are manifestly invariant under the full diffeomorphism group. Yet, perhaps paradoxically, the liberty in selecting intrinsic coordinates is precisely as broad as is the original diffeomorphism freedom. We show how various ideas from the past five decades concerning the true degrees of freedom of general relativity can be interpreted in light of this new constrained Hamiltonian description. In particular, we show how the Kucha\v{r} multi-fingered time approach can be understood as a means of introducing full four-dimensional diffeomorphism invariants. Every choice of new phase space variables yields new Einstein-Hamilton-Jacobi constraining relations, and corresponding intrinsic Schr\"odinger equations. We show how to implement this freedom by canonical transformation of the intrinsic Hamiltonian. We also reinterpret and rectify significant work by B. Dittrich on the construction of `Dirac observables'., Comment: 28 pages, some errors in the published version corrected

Leon Rosenfeld published in 1930 the first systematic Hamiltonian approach to Lagrangian models that possess a local gauge symmetry. The application of this formalism to theories with local internal symmetries, such as electromagnetism in interaction with charged matter fields, is valid and complete, and predates by two decades the work by Dirac and Bergmann. Although he provided a group-theoretical justification for gauge fixing procedures that had just been implemented in the first expositions of quantum electrodynamics by Heisenberg and Pauli, and also by Fermi, his contribution went largely unnoticed. This lack of impact seems to be related to a generalized disenchantment with second quantization in the 1930's and 1940's., Comment: 32 pages, submitted to Proceedings of the HQ2 Conference on the History of Quantum Physics, Utrecht, 14-17 July, 2008

This is a review of the four-volume set entitled The Genesis of General Relativity edited by Juergen Renn, Springer, Dordrecht (2007), Comment: 11 pages, to appear in General Relativity and Gravitation

A quantum physical projector is proposed for generally covariant theories which are derivable from a Lagrangian. The projector is the quantum analogue of the integral over the generators of finite one-parameter subgroups of the gauge symmetry transformations which are connected to the identity. Gauge variables are retained in this formalism, thus permitting the construction of spacetime area and volume operators in a tentative spacetime loop formulation of quantum general relativity., Comment: To appear in Foundations of Physics