General Relativity and Gravitation: A Centennial Perspective. In relativistic classical field theories of gravitation, particularly general relativity, an energy condition is one of various alternative conditions which can be applied to the matter content of the theory, when it is either not possible or desirable to specify this content explicitly. The hope is then that any reasonable matter theory will satisfy this condition or at least will preserve the condition if it is satisfied by the starting conditions. In general relativity, energy conditions are often used (and required) in proofs of various important theorems about black holes, such as the no hair theorem or the laws of black hole thermodynamics. Explore spectacular advances in cosmology, relativistic astrophysics, gravitational wave science, mathematics, computational science, and the interface of gravitation and quantum physics with this unique celebration of the centennial of Einsteins discovery of general relativity. Twelve comprehensive and in-depth reviews, written by a team of world-leading international experts, together present an up-to-date overview of key topics at the frontiers of these areas, with particular emphasis on the significant developments of the last three decades. Interconnections with other fields of research are also highlighted, making this an invaluable resource for both new and experienced researchers. Commissioned by the International Society on General Relativity and Gravitation, and including accessible introductions to cutting-edge topics, ample references to original research papers, and informative colour figures, this is a definitive reference for researchers and graduate students in cosmology, relativity, and gravitational science. dec1.sinp.msu.ru/~panov/Lib/Papers/GR/1409.5823v1.pdf The discovery of general relativity by Albert Einstein 100 years ago was quickly recognized as a supreme triumph of the human intellect. To paraphrase Hermann Weyl, wider expanses and greater depths were suddenly exposed to the searching eye of knowledge, regions of which there was not even an inkling. For 8 years, Einstein had been consumed by the tension between Newtonian gravity and the spacetime structure of special relativity. At first no one had an appreciation for his passion. Indeed, “as an older friend,” Max Planck advised him against this pursuit, “for, in the first place you will not succeed, and even if you succeed, no one will believe you.” Fortunately Einstein persisted and discovered a theory that represents an unprecedented combination of mathematical elegance, conceptual depth and observational success. For over 25 centuries before this discovery, spacetime had been a stage on which the dynamics of matter unfolded. Suddenly the stage joined the troupe of actors. In subsequent decades new aspects of this revolutionary paradigm continued to emerge. It was found that the entire universe is undergoing an expansion. Spacetime regions can get so warped that even light can be trapped in them. Ripples of spacetime curvature can carry detailed imprints of cosmic explosions in the distant reaches of the universe. A century has now passed since Einstein’s discovery and yet every researcher r who studies general relativity in a serious manner continues to be enchanted by its magic. General Relativity and Gravitation: A Centennial Perspective Abhay Ashtekar , Beverly K. Berger , James Isenberg , and Malcolm A. H. MacCallum 1. Institute for Gravitation & the Cosmos, and Physics Department, Penn State, University Park, PA 16802, U.S.A. 2. 2131 Chateau PL, Livermore, CA 94550, USA 3. Department of Mathematics, University of Oregon, Eugene, OR 97403-1222, USA 4. School of Mathematical Sciences, Queen Mary University of London, Mile End Road, London E1 4NS, U.K. Image 1: Einsteins Zurich Notebook In the half century and more of Einsteins work in science, one discovery stands above all as his greatest achievement. It is his general theory of relativity. In it, Einstein found a new way to think of the gravity that pulls apples from their trees and keeps the moon in orbit around our earth. There are no forces pulling on them, he saw. They are merely responding to a curvature in the geometrical fabric of space and time. This discovery of this theory is somehow more than mere science. It is not the fitting of a formula to a set of data or the succumbing to the weight of unanswerable evidence. General relativity was an achievement of creative imagination. Through it, Einstein found the boundary of science and art. There he wrote equations linking space, time, matter and gravity every bit as beautiful as Shakespeares sonnets, but written in the universal language of mathematics. The evidence that favors general relativity is no where near as strong or thorough as that which speaks for quantum theory. Yet we favor general relativity simply because no conception this beautiful should be wrong. And it survives because no theorist in the many decades since 1915 has been imaginative enough to find a theory that does better than general relativity. Every time a new test is devised Einsteins theory wins. Image 1: Albert Einstein. Image 2: According to General Relativity, the wavelength of light (or any other form of electromagnetic radiation) passing through a gravitational field will be shifted towards redder regions of the spectrum. To understand this gravitational redshift, think of a baseball hit high into the air, slowing as it climbs. Einsteins theory says that as a photon fights its way out of a gravitational field, it loses energy and its color reddens. Gravitational redshifts have been observed in diverse settings. Earthbound Redshift In 1960, Robert V. Pound and Glen A. Rebka demonstrated that a beam of very high energy gamma rays was ever so slightly redshifted as it climbed out of Earths gravity and up an elevator shaft in the Jefferson Tower physics building at Harvard University. The redshift predicted by Einsteins Field Equations for the 74 ft. tall tower was but two parts in a thousand trillion. The gravitational redshift detected came within ten percent of the computed value. Quite a feat! Solar Redshift In the 1960s, a team at Princeton University measured the redshift of sunlight. Though small, given the Suns mass and density, the redshift matched Einsteins prediction very closely. White Dwarf Redshift Take a star like the white dwarf star, Sirius B that is 61,000 times denser than the Sun. Its gravitational field is correspondingly much stronger and so is the redshift for the light it emits: 30 times greater, ac cording to earlier observations from the Mount Wilson Observatory taken by W.S. Adams way back in 1924. Still larger redshifts have more recently been detected in studies of so-called neutron stars -- collapsed stars that are even denser. What about the redshift caused by a black hole? It can be thought of as infinite. In other words, photons inside the hole are so redshifted they can never get out!
Posted on: Fri, 26 Dec 2014 13:24:57 +0000
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