scholarly journals Quantum gravity as a metaphysical problem

2017 ◽  
Vol 1 (5) ◽  
pp. 163-170
Author(s):  
Ilja Schmelzer
Keyword(s):  
Field Theory ◽  
Quantum Gravity ◽  
Degrees Of Freedom ◽  
Einstein Equations ◽  
Absolute Space ◽  
Quantum Theories ◽  
Approximation Techniques ◽  
Metaphysical Problem ◽  
Space And Time ◽  
Important Argument

The problem with quantum gravity is usually presented as if it would be difficult to construct even a single quantum theory of relativistic gravity. This is shown to be wrong. A straightforward approach using standard, well-studied methods allows to construct mathematically well-defined quantum theories which give, in a certain classical limit, the Einstein equations of GR: GR may be transformed into a field theory on a fixed background by breaking diffeomorphism symmetry using harmonic coordinates. The resulting field theory may be regularized using standard lattice approximation techniques. The result is a well-defined canonical theory with a finite number of degrees of freedom, which can be quantized without problems in a canonical way. Why such a straightforward way to quantize gravity is simply ignored? We identify missing explanation of relativistic symmetry as an important argument, and propose a solution. evaluate possible explanations why this simple possibility to construct a theory of quantum gravity is ignored. While a lot of different metaphysical and sociological reasons play a role, we identify as a main point a preference of the scientific community for the relational philosophy behind the spacetime interpretation of GR, in opposition to the Newtonian concept of absolute space and time (substantivalism). We conclude that the quantization of gravity is not a problem of physics, but a metaphysical problem. It is a problem of the relational philosophy of space and time in the tradition of Descartes and Leibniz, which is the base of the spacetime interpretation of GR, because this philosophy is incompatible with the known examples of theories of quantum gravity.

scholarly journals Nonlocality in quantum gravity and the breakdown of effective field theory

2021 ◽  
Author(s):  
Nicolás Valdés-Meller
Keyword(s):  
Field Theory ◽  
Black Holes ◽  
Quantum Gravity ◽  
Effective Field Theory ◽  
Degrees Of Freedom ◽  
Effective Field ◽  
Length Scales ◽  
Effective Metric

We argue that quantum gravity is nonlocal, first by recalling well-known arguments that support this idea and then by focusing on a point not usually emphasized: that making a conventional effective field theory (EFT) for quantum gravity is particularly difficult, and perhaps impossible in principle. This inability to realize an EFT comes down to the fact that gravity itself sets length scales for a problem: when integrating out degrees of freedom above some cutoff, the effective metric one uses will be different, which will itself re-define the cutoff. We also point out that even if the previous problem is fixed, naïvely applying EFT in gravity can lead to problems — we give a particular example in the case of black holes.

scholarly journals The Fröhlich-Morchio-Strocchi mechanism and quantum gravity

2020 ◽  
Vol 8 (4) ◽  
Author(s):  
Axel Maas
Keyword(s):  
Field Theory ◽  
Quantum Gravity ◽  
Degrees Of Freedom ◽  
Quantum Field ◽  
Yang Mills ◽  
Guiding Principle ◽  
Quantum Numbers ◽  
Gravity Fields ◽  
Physical Spectrum ◽  
Physical Objects

Taking manifest invariance under both gauge symmetry and diffeomorphisms as a guiding principle physical objects are constructed for Yang-Mills-Higgs theory coupled to quantum gravity. These objects are entirely classified by quantum numbers defined in the tangent space. Applying the Fröhlich-Morchio-Strocchi mechanism to these objects reveals that they coincide with ordinary correlation functions in quantum-field theory, if quantum fluctuations of gravity and curvature become small. Taking these descriptions literally exhibits how quantum gravity fields need to dress quantum fields to create physical objects, i. e. giving a graviton component to ordinary observed particles. The same mechanism provides access to the physical spectrum of pure gravitational degrees of freedom.

Newton To Maxwell: The Principia and British Physics

1988 ◽  
Vol 42 (1) ◽  
pp. 75-96 ◽  
Keyword(s):  
Quantum Mechanics ◽  
Absolute Space ◽  
Theory Of Relativity ◽  
Quantum Theories ◽  
Classical Physics ◽  
Newtonian Physics ◽  
World Picture ◽  
Space And Time ◽  
The World ◽  
Modern Physics

It is conventional to denote the physics of the period 1700-1900, from A the Principia to the advent of the relativity and quantum theories, as ‘classical’ or ‘Newtonian’ physics. These terms are not, however, very satisfactory as historical categories. The contrast between classical and ‘modern’ physics is perceived in terms that highlight the innovatory features of physics after 1900: the abandonment of the concepts of absolute space and time in Einstein’s theory of relativity, and of causality and determinism in quantum mechanics. ‘ Classical ’ physics is thus defined by ‘non-classical’ physics. The definitions and axioms of Principia , Newton’s exposition of the concepts of absolute space and time, and his statement of the Newtonian laws of motion, are rightly seen as fundamental to the 17th-century mechanization of the world picture.

The Development of Machian Themes in the Twentieth Century

2006 ◽  
Author(s):  
Julian Barbour
Keyword(s):  
Twentieth Century ◽  
General Relativity ◽  
Quantum Gravity ◽  
Space Time ◽  
Field Theories ◽  
Absolute Space ◽  
Rates Of Change ◽  
Space And Time

This chapter charts the complicated legacy of Mach's critique of absolute space and time. In 1902, Poincaré achieved a clear formulation of what a truly Machian mechanics should accomplish: it should permit a unique prediction of future motion on the basis of just the relative separations of bodies, and these separations' rates of change. However, this work made no impact on Einstein, despite his admiration for Mach. The discussion explains how several independent ideas that dominated Einstein's thinking about space, time and matter led him to a quite different interpretation (or misinterpretation) of Mach. This chapter also argues that, despite the misinterpretation, general relativity is perfectly Machian (in a sense that is the analogue for field theories of Poincaré's criterion), and that this shows general relativity to be ‘timeless’ in a certain sense, which is suggestive of quantum gravity.

Philosophy Beyond Spacetime

2021 ◽  
Keyword(s):  
Quantum Gravity ◽  
Philosophy Of Mind ◽  
Degrees Of Freedom ◽  
Loop Quantum Gravity ◽  
Quantum Theories ◽  
Philosophical Foundations ◽  
Naturalized Metaphysics ◽  
Theories Of Gravity ◽  
Methodological Aspects ◽  
Emergence Of Spacetime

The present volume collects essays on the philosophical foundations of quantum theories of gravity, such as loop quantum gravity and string theory. Central for philosophical concerns is quantum gravity's suggestion that space and time, or spacetime, may not exist fundamentally, but instead be a derivative entity emerging from non-spatiotemporal degrees of freedom. In the spirit of naturalized metaphysics, contributions to this volume consider the philosophical implications of this suggestion. In turn, philosophical methods and insights are brought to bear on the foundations of quantum gravity itself. For instance, the idea of functionalism, borrowed from the philosophy of mind and discussed by several chapters, exemplifies this mutual interaction the collection seeks to foster. The chapters of this collection cover three main subjects: first, the potential emergence of spacetime in various approaches to quantum gravity; second, metaphysical and epistemological considerations concerning the nature of this relation of emergence; and third, broader methodological aspects of the philosophy of quantum gravity.

scholarly journals QUANTUM GRAVITY AND HIGHER CURVATURE ACTIONS

2007 ◽  
Vol 04 (01) ◽  
pp. 25-52 ◽  
Author(s):  
MARTIN BOJOWALD ◽  
AURELIANO SKIRZEWSKI
Keyword(s):  
Quantum Gravity ◽  
Degrees Of Freedom ◽  
General Scheme ◽  
Hamiltonian Formalism ◽  
Classical Type ◽  
Physical Situation ◽  
Quantum Theories ◽  
Higher Derivative ◽  
Physical Information

Effective equations are often useful to extract physical information from quantum theories without having to face all technical and conceptual difficulties. One can then describe aspects of the quantum system by equations of classical type, which correct the classical equations by modified coefficients and higher derivative terms. In gravity, for instance, one expects terms with higher powers of curvature. Such higher derivative formulations are discussed here with an emphasis on the role of degrees of freedom and on differences between Lagrangian and Hamiltonian treatments. A general scheme is then provided which allows one to compute effective equations perturbatively in a Hamiltonian formalism. Here, one can expand effective equations around any quantum state and not just a perturbative vacuum. This is particularly useful in situations of quantum gravity or cosmology where perturbations only around vacuum states would be too restrictive. The discussion also demonstrates the number of free parameters expected in effective equations, used to determine the physical situation being approximated, as well as the role of classical symmetries such as Lorentz transformation properties in effective equations. An appendix collects information on effective correction terms expected from loop quantum gravity and string theory.

scholarly journals Chern-Weil global symmetries and how quantum gravity avoids them

2021 ◽  
Vol 2021 (11) ◽  
Author(s):  
Ben Heidenreich ◽  
Jacob McNamara ◽  
Miguel Montero ◽  
Matthew Reece ◽  
Tom Rudelius ◽  
...  
Keyword(s):  
Field Theory ◽  
String Theory ◽  
Quantum Gravity ◽  
Global Symmetries ◽  
Effective Field Theory ◽  
Degrees Of Freedom ◽  
Effective Field ◽  
Global Symmetry ◽  
Unified Framework ◽  
Particle Phenomenology

Abstract We draw attention to a class of generalized global symmetries, which we call “Chern-Weil global symmetries,” that arise ubiquitously in gauge theories. The Noether currents of these Chern-Weil global symmetries are given by wedge products of gauge field strengths, such as F2 ∧ H3 and tr($$ {F}_2^2 $$ F 2 2 ), and their conservation follows from Bianchi identities. As a result, they are not easy to break. However, it is widely believed that exact global symmetries are not allowed in a consistent theory of quantum gravity. As a result, any Chern-Weil global symmetry in a low-energy effective field theory must be either broken or gauged when the theory is coupled to gravity. In this paper, we explore the processes by which Chern-Weil symmetries may be broken or gauged in effective field theory and string theory. We will see that many familiar phenomena in string theory, such as axions, Chern-Simons terms, worldvolume degrees of freedom, and branes ending on or dissolving in other branes, can be interpreted as consequences of the absence of Chern-Weil symmetries in quantum gravity, suggesting that they might be general features of quantum gravity. We further discuss implications of breaking and gauging Chern-Weil symmetries for particle phenomenology and for boundary CFTs of AdS bulk theories. Chern-Weil global symmetries thus offer a unified framework for understanding many familiar aspects of quantum field theory and quantum gravity.

scholarly journals Phase transitions in tensorial group field theories: Landau-Ginzburg analysis of models with both local and non-local degrees of freedom

2021 ◽  
Vol 2021 (12) ◽  
Author(s):  
Luca Marchetti ◽  
Daniele Oriti ◽  
Andreas G. A. Pithis ◽  
Johannes Thürigen
Keyword(s):  
Field Theory ◽  
Phase Transition ◽  
Phase Transitions ◽  
Quantum Gravity ◽  
Compact Group ◽  
Large Volume ◽  
Degrees Of Freedom ◽  
Scalar Fields ◽  
Theory Approach ◽  
Non Local

Abstract In the tensorial group field theory approach to quantum gravity, the theory is based on discrete building blocks and continuum spacetime is expected to emerge from their collective dynamics, possibly at criticality, via a phase transition. On a compact group of fixed volume this can be expected to be only possible in a large-volume or thermodynamic limit. Here we show how phase transitions are possible in TGFTs in two cases: a) considering the non-local group degrees of freedom on a non-compact Lie group instead of a compact one (or taking a large-volume limit of a compact group); b) in models including ℝ-valued local degrees of freedom (that can be interpreted as discrete scalar fields, often used in this context to provide a matter reference frame). After adapting the Landau-Ginzburg approach to this setting of mixed local/non-local degrees of freedom, we determine the critical dimension beyond which there is a Gaussian fixed point and a continuous phase transition which can be described by mean-field theory. This is an important step towards the realization of a phase transition to continuum spacetime in realistic TGFT models for quantum gravity.

scholarly journals Limiting fluctuations in quantum gravity to diffeomorphisms

2021 ◽  
pp. 2150197
Author(s):  
Brian Slovick
Keyword(s):  
Field Theory ◽  
Quantum Gravity ◽  
Degrees Of Freedom ◽  
Background Field ◽  
Quantum Corrections ◽  
Quantum Field ◽  
Classical Action ◽  
Gauge Transformations ◽  
Background Independence ◽  
Field Formalism

Within the background field formalism of quantum gravity, I show that if the quantum fluctuations are limited to diffeomorphic gauge transformations rather than the physical degrees of freedom, as in conventional quantum field theory, all the quantum corrections vanish on shell and the effective action is equivalent to the classical action. In principle, the resulting theory is finite and unitary, and requires no renormalization. I also show that this is the unique parameterization that renders the path integral independent of the on-shell condition for the background field, a form of background independence. Thus, a connection is established between background independence and renormalizability and unitarity.

scholarly journals Ghost problems from Pauli–Villars to fourth-order quantum gravity and their resolution

2020 ◽  
Vol 29 (14) ◽  
pp. 2043009
Author(s):  
Philip D. Mannheim
Keyword(s):  
Field Theory ◽  
Quantum Field Theory ◽  
Hilbert Space ◽  
Quantum Gravity ◽  
Fourth Order ◽  
Quantum Field ◽  
Quantum Theories ◽  
Inner Products ◽  
History Of ◽  
Theories Of Gravity

We review the history of the ghost problem in quantum field theory from the Pauli–Villars regulator theory to currently popular fourth-order derivative quantum gravity theories. While these theories all appear to have unitarity-violating ghost states with negative norm, we show that in fact these ghost states only appear because the theories are being formulated in the wrong Hilbert space. In these theories, the Hamiltonians are not Hermitian but instead possess an antilinear symmetry. Consequently, one cannot use inner products that are built out of states and their Hermitian conjugates. Rather, one must use inner products built out of states and their conjugates with respect to the antilinear symmetry, and these latter inner products are positive. In this way, one can build quantum theories of gravity in four spacetime dimensions that are unitary.

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