Lorentzian length spaces

We study the properties of a causal quantum theory in phase space for which phase space classical mechanics is obtained as a limit. The causal quantum theory is obtained from a generalized coherent state representation. The behavior for the one particle case and the manyparticle case are illustrated for the harmonic oscillator. We also answer to the arguments against the possibility of constructing causal theories in phase space.

Abstract

<i>Leibniz and Kant</i>

This book is an introduction to constructive mathematics with an emphasis on techniques and results that have been obtained in the last twenty years. The text covers fundamental theory of the real line and metric spaces, focusing on locatedness in normed spaces and with associated results about operators and their adjoints on a Hilbert space. Some of the other areas that are discussed in this book are the Ishihara's tricks, Separation theorems, and Locally convex spaces. There are two appendices to the book. The first gathers together some basic notions about sets and orders, the second gives the axioms for intuitionistic logic. The intended readership of the book consists of postgraduate or senior undergraduate students, and professional research mathematicians. No background in intuitionistic logic or constructive analysis is needed in order to read the book, but some familiarity with the classical theories of metric, normed and Hilbert spaces is recommended.

We develop causality theory for upper semi-continuous distributions of cones over manifolds generalizing results from mathematical relativity in two directions: non-round cones and non-regular differentiability assumptions. We prove the validity of most results of the regular Lorentzian causality theory including: causal ladder, Fermat’s principle, notable singularity theorems in their causal formulation, Avez–Seifert theorem, characterizations of stable causality and global hyperbolicity by means of (smooth) time functions. For instance, we give the first proof for these structures of the equivalence between stable causality, [Formula: see text]-causality and existence of a time function. The result implies that closed cone structures that admit continuous increasing functions also admit smooth ones. We also study proper cone structures, the fiber bundle analog of proper cones. For them, we obtain most results on domains of dependence. Moreover, we prove that horismos and Cauchy horizons are generated by lightlike geodesics, the latter being defined through the achronality property. Causal geodesics and steep temporal functions are obtained with a powerful product trick. The paper also contains a study of Lorentz–Minkowski spaces under very weak regularity conditions. Finally, we introduce the concepts of stable distance and stable spacetime solving two well-known problems (a) the characterization of Lorentzian manifolds embeddable in Minkowski spacetime, they turn out to be the stable spacetimes, (b) the proof that topology, order and distance (with a formula à la Connes) can be represented by the smooth steep temporal functions. The paper is self-contained, in fact we do not use any advanced result from mathematical relativity.

Based on bipolar dynamic logic and bipolar quantum linear algebra, a causal theory of YinYang bipolar atom is intro-duced in a completely background independent geometry that transcends spacetime. The causal theory leads to an equilibrium-based super symmetrical quantum cosmology of negative-positive energies. It is contended that the new theory has opened an Eastern road toward quantum gravity with bipolar logical unifications of particle and wave, matter and antimatter, relativity and quantum entanglement. Information recovery after a black hole is discussed. It is shown that not only can the new theory be applied in physical worlds but also in logical, mental, social and biological worlds. Falsifiability of the theory is discussed.

This chapter formally introduces the causal metric hypothesis, and describes in detail its motivations and justifications. Foremost among these are the metric recovery theorems of Hawking and Malament, which state, roughly, that “the causal structure of relativistic spacetime determines its metric structure up to scale.” As understood in causal set theory, the novel assumption that spacetime is discrete provides a natural notion of scale, given by the “sizes of fundamental elements and relations.” This suggests that causal structure alone can account for emergent geometry in the discrete context. Section 2.1 introduces a general version of the causal metric hypothesis, which states, very broadly, that “the properties of the physical universe are manifestations of causal structure.” This basic idea may be modified and/or interpreted in various ways; in particular, the strong interpretation of the causal metric hypothesis ascribes all of physics, including “nongravitational matter,” to causal structure at the fundamental scale. Section 2.2 introduces a classical version of the causal metric hypothesis, which states that classical spacetime may be modeled in terms of mathematical objects called directed sets, or, more conventionally, directed graphs. The term “directed set” has a different conventional meaning, but I prefer to re-purpose the term than to use awkward graph-theoretic terminology. Section 2.3 begins the study of metric recovery by introducing five types of structure on relativistic spacetime; namely, metric, conformal, causal, smooth, and topological structure, in decreasing order of detail. Section 2.4 discusses metric structure in the relativistic context, i.e. pseudo-Riemannian geometry. Section 2.5 covers conformal structure, which defines “geometry up to scale.” Section 2.6 discusses causal structure, which generalizes the “null cone structure” on Minkowski spacetime. The metric recovery theorems state that “causal structure determines conformal structure” under suitable assumptions. Section 2.7 introduces causality conditions on relativistic spacetime, which play a technical role in the metric recovery theorems. Section 2.8 gives a formal statement of metric recovery, sketches its proof, and describes how it motivates the causal metric hypothesis in the discrete setting. Section 2.9 explains why continuum-based theories are inherently awkward for modeling fundamental physics. Section 2.10 outlines some of the basic principles underlying the technical developments of subsequent chapters.

We explore the idea of asymptotic silence in causal set theory and find that causal sets approximated by continuum spacetimes exhibit behavior akin to asymptotic silence. We make use of an intrinsic definition of spatial distance between causal set elements in the discrete analogue of a spatial hypersurface. Using numerical simulations for causal sets approximated by and 4 dimensional Minkowski spacetime, we show that while the discrete distance rapidly converges to the continuum distance at a scale roughly an order of magnitude larger than the discreteness scale, it is significantly larger on small scales. This allows us to define an effective dimension which exhibits dimensional reduction in the ultraviolet, while monotonically increasing to the continuum dimension with increasing continuum distance. We interpret these findings as manifestations of asymptotic silence in causal set theory.

The holy grail of theoretical physics is to find the theory of everything that combines all the forces of nature, including gravity. This book addresses the question: how far are we from such discovery? Over the last few decades, multiple roads to finding a quantum theory of gravity have been proposed but no obvious description of nature has emerged in this domain. What is to be made of this situation? This volume probes the state-of-the art in this daunting quest of theoretical physics by collecting critical interviews with nearly forty leading theorists in this field. These broad-ranging conversations give important insights and candid opinions on the various approaches to quantum gravity, including string theory, loop quantum gravity, causal set theory and asymptotic safety. This unique, readable overview provides a gateway into cutting edge research for students and others who wish to engage with the open problem of quantum gravity.

The two fundamental pillars of physics for over 100 years have been quantum theory and general relativity, but their unification at short distances remains elusive, both technically and conceptually. This work is a philosophical investigation of the second kind of problem, and in particular of the striking fact that in many approaches to ‘quantum gravity’ classical spacetime structures are not merely quantized, but arguably absent—so that spacetime is not merely a classical limit, but ‘emergent’. This issue is not only central to the problem of quantum gravity, but of deep significance for our philosophical understanding of physical reality, promising a conceptual revolution at least as profound as Einstein’s. We give an introduction to the question of spacetime emergence in general, for philosophers of metaphysics and science, and argue that spacetime functionalism explains how something non-spatiotemporal could ever appear as space and time. More technical chapters investigate the issue in detail for causal set theory, loop quantum gravity, and string theory, and the book also serves as a philosophical introduction to those theories for philosophers of physics. Our results help physicists clarify what new conceptual framework—not resting on space and time—may be necessary to achieve a theory of quantum gravity; and show philosophers how the world may not be spatiotemporal at root, and what kind of a world we might then live in.

A result on the existence of timelike periodic trajectories in a general class of Lorentzian manifolds R × M \mathbb R{}\times M , with compact M M , is obtained. The proof is based on arguments concerning closed geodesics and causality theory.

I take non-locality to be the Michelson–Morley experiment of the early 21st century, assume its universal validity, and try to derive its consequences. Spacetime, with its locality, cannot be fundamental, but must somehow be emergent from entangled coherent quantum variables and their behaviors. There are, then, two immediate consequences: (i). if we start with non-locality, we need not explain non-locality. We must instead explain an emergence of locality and spacetime. (ii). There can be no emergence of spacetime without matter. These propositions flatly contradict General Relativity, which is foundationally local, can be formulated without matter, and in which there is no “emergence” of spacetime. If these be true, then quantum gravity cannot be a minor alteration of General Relativity but must demand its deep reformulation. This will almost inevitably lead to: matter not only curves spacetime, but “creates” spacetime. We will see independent grounds for the assertion that matter both curves and creates spacetime that may invite a new union of quantum gravity and General Relativity. This quantum creation of spacetime consists of: (i) fully non-local entangled coherent quantum variables. (ii) The onset of locality via decoherence. (iii) A metric in Hilbert space among entangled quantum variables by the sub-additive von Neumann entropy between pairs of variables. (iv) Mapping from metric distances in Hilbert space to metric distances in classical spacetime by episodic actualization events. (v) Discrete spacetime is the relations among these discrete actualization events. (vi) “Now” is the shared moment of actualization of one among the entangled variables when the amplitudes of the remaining entangled variables change instantaneously. (vii) The discrete, successive, episodic, irreversible actualization events constitute a quantum arrow of time. (viii) The arrow of time history of these events is recorded in the very structure of the spacetime constructed. (ix) Actual Time is a succession of two or more actual events. The theory inevitably yields a UV cutoff of a new type. The cutoff is a phase transition between continuous spacetime before the transition and discontinuous spacetime beyond the phase transition. This quantum creation of spacetime modifies General Relativity and may account for Dark Energy, Dark Matter, and the possible elimination of the singularities of General Relativity. Relations to Causal Set Theory, faithful Lorentzian manifolds, and past and future light cones joined at “Actual Now” are discussed. Possible observational and experimental tests based on: (i). the existence of Sub- Planckian photons, (ii). knee and ankle discontinuities in the high-energy gamma ray spectrum, and (iii). possible experiments to detect a creation of spacetime in the Casimir system are discussed. A quantum actualization enhancement of repulsive Casimir effect would be anti-gravitational and of possible practical use. The ideas and concepts discussed here are not yet a theory, but at most the start of a framework that may be useful.

We study quantum gravity signatures emerging from phenomenologically motivated multiscale models, spectral actions, and Causal Set Theory within the detector approach to the Unruh effect. We show that while the Unruh temperature is unaffected, Lorentz-invariant corrections to the two-point function leave a characteristic fingerprint in the induced emission rate of the accelerated detector. Generically, quantum gravity models exhibiting dynamical dimensional reduction exhibit a suppression of the Unruh rate at high energy while the rate is enhanced in Kaluza-Klein theories with compact extra dimensions. We quantify this behavior by introducing the "Unruh dimension" as the effective spacetime dimension seen by the Unruh effect and show that it is related, though not identical, to the spectral dimension used to characterize spacetime in quantum gravity. We comment on the physical origins of these effects and their relevance for black hole evaporation.

Copula Theory and Its Applications

The concluding chapter returns to the spacetime functionalism that we proposed in the first two chapters, and highlights the ways in which the book has concretely implemented that program for causal set theory, loop quantum gravity, and string theory: it reviews the arguments that these theories are fundamentally non-spatiotemporal in important ways, and the derivations of spacetime structures from each. We focus especially on Maudlin’s challenge regarding the physical salience of those derivations, identifying the principles that must be accepted in order to conclude that the elements of the various theories of quantum gravity in fact play various spacetime roles.