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The thermal time hypothesis is a possible solution to the problem of time in classical and quantum theory as has been put forward by Carlo Rovelli and Alain Connes. Physical time flow is modeled as a fundamental property of the theory, a macroscopic feature of thermodynamical origin. [1] [2]
Generally covariant theories do not have a notion of a distinguished physical time with respect to which everything evolves. [3] However, it is not needed for the full formulation and interpretation of the theory. The dynamical laws are determined by correlations which are sufficient to make predictions. But then a mechanism is needed which explains how the familiar notion of time eventually emerges from the timeless structure to become such an important ingredient of the macroscopic world we live in as well as of our conscious experience.
Quantum gravity (QG) is a field of theoretical physics that seeks to describe gravity according to the principles of quantum mechanics, and where quantum effects cannot be ignored, such as in the vicinity of black holes or similar compact astrophysical objects, and where the effects of gravity are strong, such as neutron stars.
T-symmetry or time reversal symmetry is the theoretical symmetry of physical laws under the transformation of time reversal,
Loop quantum gravity (LQG) is a theory of quantum gravity, which aims to merge quantum mechanics and general relativity, incorporating matter of the Standard Model into the framework established for the pure quantum gravity case. It is an attempt to develop a quantum theory of gravity based directly on Einstein's geometric formulation rather than the treatment of gravity as a force. As a theory LQG postulates that the structure of space and time is composed of finite loops woven into an extremely fine fabric or network. These networks of loops are called spin networks. The evolution of a spin network, or spin foam, has a scale above the order of a Planck length, approximately 10−35 meters, and smaller scales are meaningless. Consequently, not just matter, but space itself, prefers an atomic structure.
In physics, black hole thermodynamics is the area of study that seeks to reconcile the laws of thermodynamics with the existence of black-hole event horizons. As the study of the statistical mechanics of black-body radiation led to the development of the theory of quantum mechanics, the effort to understand the statistical mechanics of black holes has had a deep impact upon the understanding of quantum gravity, leading to the formulation of the holographic principle.
Laurent Freidel is a French theoretical physicist and mathematical physicist known mainly for his contributions to quantum gravity, including loop quantum gravity, spin foam models, doubly special relativity, group field theory, relative locality and most recently metastring theory. He is currently a faculty member at Perimeter Institute for Theoretical Physics in Waterloo, Ontario, Canada.
Carlo Rovelli is an Italian theoretical physicist and writer who has worked in Italy, the United States and, since 2000, in France. He is also currently a Distinguished Visiting Research Chair at the Perimeter Institute. He works mainly in the field of quantum gravity and is a founder of loop quantum gravity theory. He has also worked in the history and philosophy of science. He collaborates with several Italian newspapers, including the cultural supplements of the Corriere della Sera, Il Sole 24 Ore and La Repubblica.
In particle physics, the hypothetical dilaton particle is a particle of a scalar field that appears in theories with extra dimensions when the volume of the compactified dimensions varies. It appears as a radion in Kaluza–Klein theory's compactifications of extra dimensions. In Brans–Dicke theory of gravity, Newton's constant is not presumed to be constant but instead 1/G is replaced by a scalar field and the associated particle is the dilaton.
In physics, the topological structure of spinfoam or spin foam consists of two-dimensional faces representing a configuration required by functional integration to obtain a Feynman's path integral description of quantum gravity. These structures are employed in loop quantum gravity as a version of quantum foam.
The Immirzi parameter is a numerical coefficient appearing in loop quantum gravity (LQG), a nonperturbative theory of quantum gravity. The Immirzi parameter measures the size of the quantum of area in Planck units. As a result, its value is currently fixed by matching the semiclassical black hole entropy, as calculated by Stephen Hawking, and the counting of microstates in loop quantum gravity.
The history of loop quantum gravity spans more than three decades of intense research.
In theoretical physics, quantum field theory in curved spacetime (QFTCS) is an extension of quantum field theory from Minkowski spacetime to a general curved spacetime. This theory treats spacetime as a fixed, classical background, while giving a quantum-mechanical description of the matter and energy propagating through that spacetime. A general prediction of this theory is that particles can be created by time-dependent gravitational fields, or by time-independent gravitational fields that contain horizons. The most famous example of the latter is the phenomenon of Hawking radiation emitted by black holes.
In physics, the Bekenstein bound is an upper limit on the thermodynamic entropy S, or Shannon entropy H, that can be contained within a given finite region of space which has a finite amount of energy—or conversely, the maximal amount of information required to perfectly describe a given physical system down to the quantum level. It implies that the information of a physical system, or the information necessary to perfectly describe that system, must be finite if the region of space and the energy are finite. In computer science, this implies that there is a maximal information-processing rate for a physical system that has a finite size and energy, and that a Turing machine with finite physical dimensions and unbounded memory is not physically possible.
In physics the Einstein aether theory, also called aetheory, is a generally covariant modification of general relativity which describes a spacetime endowed with both a metric and a unit timelike vector field named the aether. The theory has a preferred reference frame and hence violates Lorentz invariance.
In physical cosmology, fractal cosmology is a set of minority cosmological theories which state that the distribution of matter in the Universe, or the structure of the universe itself, is a fractal across a wide range of scales. More generally, it relates to the usage or appearance of fractals in the study of the universe and matter. A central issue in this field is the fractal dimension of the universe or of matter distribution within it, when measured at very large or very small scales.
In mathematical physics, the concept of quantum spacetime is a generalization of the usual concept of spacetime in which some variables that ordinarily commute are assumed not to commute and form a different Lie algebra. The choice of that algebra still varies from theory to theory. As a result of this change some variables that are usually continuous may become discrete. Often only such discrete variables are called "quantized"; usage varies.
The Minisuperspace in physics, for quantum gravity, the phase space is infinite dimensional as we are dealing with a field theory. An approximation which is sometimes taken is to consider the largest wavelength modes of the order of the size of the universe when studying cosmological models. This is the minisuperspace approximation. The validity of this approximation holds as long as the adiabatic approximation holds.
In mathematical physics, vanishing scalar invariant (VSI) spacetimes are Lorentzian manifolds with all polynomial curvature invariants of all orders vanishing. Although the only Riemannian manifold with VSI property is flat space, the Lorentzian case admits nontrivial spacetimes with this property. Distinguishing these VSI spacetimes from Minkowski spacetime requires comparing non-polynomial invariants or carrying out the full Cartan–Karlhede algorithm on non-scalar quantities.
In general relativity, the Ehrenfest–Tolman effect, created by Richard C. Tolman and Paul Ehrenfest, argues that temperature is not constant in space at thermal equilibrium, but varies with the spacetime curvature. Specifically, it depends on the spacetime metric. In a stationary spacetime with timelike Killing vector field , the temperature satisfies instead the Tolman-Ehrenfest relation: , where is the norm of the timelike Killing vector field.
In theoretical physics, shape dynamics is a theory of gravity that implements Mach's principle, developed with the specific goal to obviate the problem of time and thereby open a new path toward the resolution of incompatibilities between general relativity and quantum mechanics.
In theoretical physics, the problem of time is a conceptual conflict between general relativity and quantum mechanics in that quantum mechanics regards the flow of time as universal and absolute, whereas general relativity regards the flow of time as malleable and relative. This problem raises the question of what time really is in a physical sense and whether it is truly a real, distinct phenomenon. It also involves the related question of why time seems to flow in a single direction, despite the fact that no known physical laws at the microscopic level seem to require a single direction. For macroscopic systems the directionality of time is directly linked to first principles such as the second law of thermodynamics.
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