Spacetime foam: a review

Carlip, S.

doi:10.48550/arxiv.2209.14282

Cited by 5 publications

(8 citation statements)

References 240 publications

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“…The Planck constant has the same dimension as the Planck length, [ ] = [l P ] = [L]. Whether this "Planck constant length" is related to the "Planck length scale", is an open question [26]. Anyway, this suggests the close connection between gravity and quantum mechanics.…”

Section: De Sitter Spacetime and Planck Constantmentioning

confidence: 99%

Dimensionless Physics: Planck Constant as an Element of the Minkowski Metric

Volovik

2023

Jetp Lett.

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Diakonov theory of quantum gravity, in which tetrads emerge as the bilinear combinations of the fermionic fields, suggests that in general relativity the metric may have dimension 2, i.e. [gμν] = 1/[L]2. Several other approaches to quantum gravity, including the model of superplastic vacuum and BF-theories of gravity support this suggesuion. The important consequence of such metric dimension is that all the diffeomorphism invariant quantities are dimensionless for any dimension of spacetime. These include the action S, interval s, cosmological constant Λ, scalar curvature R, scalar field Φ, etc. Here we are trying to further exploit the Diakonov idea, and consider the dimension of the Planck constant. The application of the Diakonov theory suggests that the Planck constant $$\hbar $$ is the parameter of the Minkowski metric. The Minkowski parameter $$\hbar $$ is invariant only under Lorentz transformations, and is not diffeomorphism invariant. As a result the Planck constant $$\hbar $$ has nonzero dimension – the dimension of length [L]. Whether this Planck constant length is related to the Planck length scale, is an open question. In principle there can be different Minkowski vacua with their own values of the parameter $$\hbar $$. Then in the thermal contact between the two vacua their temperatures obey the analog of the Tolman law: $${{\hbar }_{1}}$$/T1 = $${{\hbar }_{2}}$$/T2.

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Section: De Sitter Spacetime and Planck Constantmentioning

confidence: 99%

Dimensionless Physics: Planck Constant as an Element of the Minkowski Metric

Volovik

2023

Jetp Lett.

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“…Perhaps the most compelling case for our spatial manifold having a non-trivial topology is 'why not?' We expect that a theory of quantum gravity may contain topology-changing processes, e.g., quantum foam [2,3]. If so, there would be no particular reason that, when the Universe emerges from the very early quantum-gravity era, the space would be the infinitevolume covering space of one of the eight possible homogeneous 3-geometries.…”

Section: Introductionmentioning

confidence: 99%

Cosmic topology. Part I. Limits on orientable Euclidean manifolds from circle searches

Pip

Akrami

Copi

et al. 2023

J. Cosmol. Astropart. Phys.

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The Einstein field equations of general relativity constrain the local curvature at every point in spacetime, but say nothing about the global topology of the Universe. Cosmic microwave background anisotropies have proven to be the most powerful probe of non-trivial topology since, within ΛCDM, these anisotropies have well-characterized statistical properties, the signal is principally from a thin spherical shell centered on the observer (the last scattering surface), and space-based observations nearly cover the full sky. The most generic signature of cosmic topology in the microwave background is pairs of circles with matching temperature and polarization patterns. No such circle pairs have been seen above noise in the WMAP or Planck temperature data, implying that the shortest non-contractible loop around the Universe through our location is longer than 98.5% of the comoving diameter of the last scattering surface. We translate this generic constraint into limits on the parameters that characterize manifolds with each of the nine possible non-trivial orientable Euclidean topologies, and provide a code which computes these constraints. In all but the simplest cases, the shortest non-contractible loop in the space can avoid us, and be shorter than the diameter of the last scattering surface by a factor ranging from 2 to at least 6. This result implies that a broader range of manifolds is observationally allowed than widely appreciated. Probing these manifolds will require more subtle statistical signatures than matched circles, such as off-diagonal correlations of harmonic coefficients.

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“…Quantum gravity effects start at the Planck length l P = G N h/c 3 ≈ 1.6 × 10 −35 m because of quantum fluctuations. It is worth mentioning that the smooth metric fields g µν of the classical gravity can not be used at r ≤ l P but the idea of "space-time foam" [65] is fruitful for describing space-time microscopic degrees of freedom.…”

Section: Heat Capacitymentioning

confidence: 99%

Magnetic black holes in AdS space with nonlinear electrodynamics, extended phase space thermodynamics and Joule–Thomson expansion

Kruglov

2022

Int. J. Geom. Methods Mod. Phys.

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This paper studies thermodynamics of magnetically charged black holes in Anti-de Sitter space in an extended phase space. The cosmological constant is considered as a pressure and the black hole mass is treated as the chemical enthalpy. The black hole thermodynamics is similar to the Van der Walls liquid–gas thermodynamics. Quantities conjugated to the nonlinear electrodynamics parameter and a magnetic charge are obtained. The first law of thermodynamics and the generalized Smarr relation take place. We investigate critical behavior of black holes and Joule–Thomson expansion. The Gibbs free energy, the Joule–Thomson coefficient and the inversion temperature are calculated.

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Spacetime foam: a review

Cited by 5 publications

References 240 publications

Dimensionless Physics: Planck Constant as an Element of the Minkowski Metric

Dimensionless Physics: Planck Constant as an Element of the Minkowski Metric

Cosmic topology. Part I. Limits on orientable Euclidean manifolds from circle searches

Magnetic black holes in AdS space with nonlinear electrodynamics, extended phase space thermodynamics and Joule–Thomson expansion

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