General Relativity as a Collection of Collections of Models

“…Following Manchak (2021), consider that Misner space-time is not globally hyperbolic and so, if only globally hyperbolic space-times are physically reasonable, Misner space-time would not be considered physically reasonable. But there is a region of Misner space-time that is hyperbolic.…”

Section: Identifying the Universe's Past Boundarymentioning

confidence: 99%

“…Roughly, space-time (M, g µν ) is observationally indistinguishable from some numerically distinct space-time (M ′ , g ′ µν ) just in case any observer at any arbitrarily chosen point in (M, g µν ) cannot determine, from any of the data in their past light cone, which of the two spacetimes they inhabit (Manchak, 2011, 412). As the Manchak-Malament theorem has established (Malament (1977); Manchak (2009); also see Beisbart (2009); Butterfield (2014); Manchak (2021); Norton (2011)), the members of a fairly broad class of space-times are observationally indistinguishable from numerically distinct space-times.…”

Section: Identifying the Universe's Past Boundarymentioning

confidence: 99%

Neo-Lorentzian Relativity and the Beginning of the Universe

Linford¹

2021

Preprint

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Many physicists have thought that absolute time became otiose with the introduction of Special Relativity. William Lane Craig disagrees. Craig argues that although relativity is empirically adequate within a domain of application, relativity is literally false and should be supplanted by a Neo-Lorentzian alternative that allows for absolute time. Meanwhile, Craig and co-author James Sinclair have argued that physical cosmology supports the conclusion that physical reality began to exist at a finite time in the past. However, on their view, the beginning of physical reality requires the objective passage of absolute time, so that the beginning of physical reality stands or falls with Craig's Neo-Lorentzian metaphysics. Here, I raise doubts about whether, given Craig's NeoLorentzian metaphysics, physical cosmology could adequately support a beginning of physical reality within the finite past. Craig and Sinclair's conception of the beginning of the universe requires a past boundary to the universe. A past boundary to the universe cannot be directly observed and so must be inferred from the observed matter-energy distribution in conjunction with auxilary hypotheses drawn from a substantive physical theory. Craig's brand of Neo Lorentzianism has not been sufficiently well specified so as to infer either that there is a past boundary or that the boundary is located in the finite past. Consequently, Neo Lorentzianism implicitly introduces a form of skepticism that removes the ability that we might have otherwise had to infer a beginning of the universe. Furthermore, in analyzing traditional big bang models, I develop criteria that Neo-Lorentzians should deploy in thinking about the direction and duration of time in cosmological models generally. For my last task, I apply the same criteria to bounce cosmologies and show that Craig and Sinclair have been wrong to interpret bounce cosmologies as including a beginning of physical reality.

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“…or the validity of various kinds of energy conditions. These notions, however, are not all compatible with each other [58][59][60][61][62][63]. One might find oneself in the uncomfortable situation that a dearly held and important property (stability, completeness, fulfilled energy conditions, .…”

Section: Introductionmentioning

confidence: 99%

Emergent Time and Time Travel in Quantum Physics

Alonso-Serrano,

Schuster,

Visser

2024

Universe

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Entertaining the possibility of time travel will invariably challenge dearly-held concepts in fundamental physics. It becomes relatively easy to construct multiple logical contradictions using differing starting points from various well-established fields of physics. Sometimes, the interpretation is that only a full theory of quantum gravity will be able to settle these logical contradictions. Even then, it remains unclear if the multitude of problems could be overcome. Yet as definitive as this seems to the notion of time travel in physics, such recourse to quantum gravity comes with its own, long-standing challenge to most of these counter-arguments to time travel: These arguments rely on time, while quantum gravity is (in)famously stuck with the problem of time. One attempt to answer this problem within the canonical framework resulted in the Page–Wootters formalism, and its recent gauge-theoretic reinterpretation as an emergent notion of time. Herein, we will begin a program to study toy models implementing the Hamiltonian constraint in quantum theory, with an aim toward understanding what an emergent notion of time can tell us about the (im)possibility of time travel.

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General Relativity as a Collection of Collections of Models

Cited by 6 publications

References 29 publications

On the (In?)Stability of Spacetime Inextendibility

On the (In?)Stability of Spacetime Inextendibility

Neo-Lorentzian Relativity and the Beginning of the Universe

Emergent Time and Time Travel in Quantum Physics

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