Entropic gravity
Entropic gravity is a hypothesis in modern physics that describes gravity as an entropic force; not a fundamental interaction mediated by a quantum field theory and a gauge particle (like photons for the electromagnetic force, and gluons for the strong nuclear force), but a probabilistic consequence of physical systems' tendency to increase their entropy. The proposal has been intensely contested in the physics community but it has also sparked a new line of research into thermodynamic properties of gravity. Origin[edit] The probabilistic description of gravity has a history that goes back at least to research on black hole thermodynamics by Bekenstein and Hawking in the mid-1970s. These studies suggest a deep connection between gravity and thermodynamics, which describes the behavior of heat. Erik Verlinde's theory[edit] Criticism and experimental tests[edit] Even so, entropic gravity in its current form has been severely challenged on formal grounds. See also[edit] References[edit]
The Post-Detection SETI Protocol
FOREWORD This open document is a proposal to begin serious international consultation on the question of future attempts deliberately to transmit electromagnetic signals from Earth to extraterrestrial civilizations. It was prepared over a number of years in the SETI Committee of the International Academy of Astronautics by a special subcommittee under the leadership of Michael Michaud. It has been endorsed by the Board of Trustees of the Academy, which decided to make it a formal Academy Position Paper. It has also been endorsed by the Board of Directors of the International Institute of Space Law.
Dark energy
Adding the cosmological constant to cosmology's standard FLRW metric leads to the Lambda-CDM model, which has been referred to as the "standard model" of cosmology because of its precise agreement with observations. Dark energy has been used as a crucial ingredient in a recent attempt to formulate a cyclic model for the universe.[8] Nature of dark energy[edit] Many things about the nature of dark energy remain matters of speculation. The evidence for dark energy is indirect but comes from three independent sources: Distance measurements and their relation to redshift, which suggest the universe has expanded more in the last half of its life.[9]The theoretical need for a type of additional energy that is not matter or dark matter to form our observationally flat universe (absence of any detectable global curvature).It can be inferred from measures of large scale wave-patterns of mass density in the universe. Effect of dark energy: a small constant negative pressure of vacuum[edit] .
Event horizon
In general relativity, an event horizon is a boundary in spacetime beyond which events cannot affect an outside observer. In layman's terms, it is defined as "the point of no return", i.e., the point at which the gravitational pull becomes so great as to make escape impossible. The most common case of an event horizon is that surrounding a black hole. Light emitted from beyond the event horizon can never reach the outside observer. More specific types of horizon include the related but distinct absolute and apparent horizons found around a black hole. Existence and evolution of the particle horizon[edit] Note: This section is based mainly on two relatively new articles (2012 and 2013) and is not yet accepted as part of the standard discussion of Black Holes. A simple example of event horizon emerges from some cases of the FLRW cosmological model. , partial pressure and state equation , such that they add up to the total density and total pressure (or equivalently ). where . is the lowest ). by
Loop quantum gravity
More precisely, space can be viewed as an extremely fine fabric or network "woven" of finite loops. These networks of loops are called spin networks. The evolution of a spin network over time is called a spin foam. The predicted size of this structure is the Planck length, which is approximately 10−35 meters. According to the theory, there is no meaning to distance at scales smaller than the Planck scale. Therefore, LQG predicts that not just matter, but also space itself has an atomic structure. Today LQG is a vast area of research, developing in several directions, which involves about 50 research groups worldwide.[1] They all share the basic physical assumptions and the mathematical description of quantum space. Research into the physical consequences of the theory is proceeding in several directions. History[edit] General covariance and background independence[edit] In mathematics, a diffeomorphism is an isomorphism in the category of smooth manifolds. and . . , we have where 2. 3. . .
Black hole
A black hole is defined as a region of spacetime from which gravity prevents anything, including light, from escaping.[1] The theory of general relativity predicts that a sufficiently compact mass will deform spacetime to form a black hole.[2] Around a black hole, there is a mathematically defined surface called an event horizon that marks the point of no return. The hole is called "black" because it absorbs all the light that hits the horizon, reflecting nothing, just like a perfect black body in thermodynamics.[3][4] Quantum field theory in curved spacetime predicts that event horizons emit radiation like a black body with a finite temperature. This temperature is inversely proportional to the mass of the black hole, making it difficult to observe this radiation for black holes of stellar mass or greater. Objects whose gravity fields are too strong for light to escape were first considered in the 18th century by John Michell and Pierre-Simon Laplace. History General relativity
Quantum triviality
In a quantum field theory, charge screening can restrict the value of the observable "renormalized" charge of a classical theory. If the only allowed value of the renormalized charge is zero, the theory is said to be "trivial" or noninteracting. Thus, surprisingly, a classical theory that appears to describe interacting particles can, when realized as a quantum field theory, become a "trivial" theory of noninteracting free particles. This phenomenon is referred to as quantum triviality. Strong evidence supports the idea that a field theory involving only a scalar Higgs boson is trivial in four spacetime dimensions,[1] but the situation for realistic models including other particles in addition to the Higgs boson is not known in general. The situation becomes more complex in theories that involve other particles however. Triviality and the renormalization group[edit] with the “bare” charge where is the mass of the particle, and is the momentum cut-off. is finite, then . The growth of with for
#26: How Matter Defeated Antimatter | Subatomic Particles
The Big Bang theory has a Big Problem. The leading models of cosmology imply that the universe should have begun with equal quantities of matter and antimatter. But when the two meet, they annihilate each other, so an equal balance would have yielded an empty cosmos. In May, physicists at the Tevatron particle accelerator in Illinois singled out a strange particle that could help explain the conundrum. Studying nearly eight years’ worth of high-speed smashups between protons and antiprotons, Guennadi Borissov of Lancaster University in the U.K. and other members of the Tevatron team focused on the B meson, a short-lived particle that emerges from the collisions. During its brief life, this particle rapidly oscillates between matter and antimatter: One moment it’s a B meson, the next it’s an anti-B meson. Follow-up experiments planned for this year at both the Tevatron and the Large Hadron Collider will test the team’s findings.
