Advanced general relativity

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About John Stewart. John Stewart. Books by John Stewart. Trivia About Advanced General No trivia or quizzes yet. Welcome back. Just a moment while we sign you in to your Goodreads account. Due to this loss, the distance between the two orbiting bodies decreases, and so does their orbital period. Within the Solar System or for ordinary double stars , the effect is too small to be observable.

This is not the case for a close binary pulsar, a system of two orbiting neutron stars , one of which is a pulsar: from the pulsar, observers on Earth receive a regular series of radio pulses that can serve as a highly accurate clock, which allows precise measurements of the orbital period. Because neutron stars are immensely compact, significant amounts of energy are emitted in the form of gravitational radiation.

This was the first detection of gravitational waves, albeit indirect, for which they were awarded the Nobel Prize in physics. Several relativistic effects are directly related to the relativity of direction. Near a rotating mass, there are gravitomagnetic or frame-dragging effects. A distant observer will determine that objects close to the mass get "dragged around". This is most extreme for rotating black holes where, for any object entering a zone known as the ergosphere , rotation is inevitable.

The deflection of light by gravity is responsible for a new class of astronomical phenomena. If a massive object is situated between the astronomer and a distant target object with appropriate mass and relative distances, the astronomer will see multiple distorted images of the target. Such effects are known as gravitational lensing. Gravitational lensing has developed into a tool of observational astronomy.

It is used to detect the presence and distribution of dark matter , provide a "natural telescope" for observing distant galaxies, and to obtain an independent estimate of the Hubble constant. Statistical evaluations of lensing data provide valuable insight into the structural evolution of galaxies. Observations of binary pulsars provide strong indirect evidence for the existence of gravitational waves see Orbital decay , above. Detection of these waves is a major goal of current relativity-related research. Observations of gravitational waves promise to complement observations in the electromagnetic spectrum.

Whenever the ratio of an object's mass to its radius becomes sufficiently large, general relativity predicts the formation of a black hole, a region of space from which nothing, not even light, can escape.

In the currently accepted models of stellar evolution , neutron stars of around 1. Astronomically, the most important property of compact objects is that they provide a supremely efficient mechanism for converting gravitational energy into electromagnetic radiation. Black holes are also sought-after targets in the search for gravitational waves cf. Gravitational waves , above. Merging black hole binaries should lead to some of the strongest gravitational wave signals reaching detectors here on Earth, and the phase directly before the merger "chirp" could be used as a " standard candle " to deduce the distance to the merger events—and hence serve as a probe of cosmic expansion at large distances.

Astronomical observations of the cosmological expansion rate allow the total amount of matter in the universe to be estimated, although the nature of that matter remains mysterious in part. An authoritative answer would require a complete theory of quantum gravity, which has not yet been developed [] cf. The solutions require extreme physical conditions unlikely ever to occur in practice, and it remains an open question whether further laws of physics will eliminate them completely.

Since then, other—similarly impractical—GR solutions containing CTCs have been found, such as the Tipler cylinder and traversable wormholes.

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In general relativity, no material body can catch up with or overtake a light pulse. No influence from an event A can reach any other location X before light sent out at A to X. In consequence, an exploration of all light worldlines null geodesics yields key information about the spacetime's causal structure. This structure can be displayed using Penrose—Carter diagrams in which infinitely large regions of space and infinite time intervals are shrunk " compactified " so as to fit onto a finite map, while light still travels along diagonals as in standard spacetime diagrams.

Aware of the importance of causal structure, Roger Penrose and others developed what is known as global geometry. In global geometry, the object of study is not one particular solution or family of solutions to Einstein's equations. Rather, relations that hold true for all geodesics, such as the Raychaudhuri equation , and additional non-specific assumptions about the nature of matter usually in the form of energy conditions are used to derive general results.

Advanced General Relativity

Using global geometry, some spacetimes can be shown to contain boundaries called horizons , which demarcate one region from the rest of spacetime. The best-known examples are black holes: if mass is compressed into a sufficiently compact region of space as specified in the hoop conjecture , the relevant length scale is the Schwarzschild radius [] , no light from inside can escape to the outside. Since no object can overtake a light pulse, all interior matter is imprisoned as well. Passage from the exterior to the interior is still possible, showing that the boundary, the black hole's horizon , is not a physical barrier.

Early studies of black holes relied on explicit solutions of Einstein's equations, notably the spherically symmetric Schwarzschild solution used to describe a static black hole and the axisymmetric Kerr solution used to describe a rotating, stationary black hole, and introducing interesting features such as the ergosphere. Using global geometry, later studies have revealed more general properties of black holes. With time they become rather simple objects characterized by eleven parameters specifying: electric charge, mass-energy, linear momentum , angular momentum , and location at a specified time.

This is stated by the black hole uniqueness theorem : "black holes have no hair", that is, no distinguishing marks like the hairstyles of humans. Irrespective of the complexity of a gravitating object collapsing to form a black hole, the object that results having emitted gravitational waves is very simple. Even more remarkably, there is a general set of laws known as black hole mechanics , which is analogous to the laws of thermodynamics.

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For instance, by the second law of black hole mechanics, the area of the event horizon of a general black hole will never decrease with time, analogous to the entropy of a thermodynamic system. This limits the energy that can be extracted by classical means from a rotating black hole e. As thermodynamical objects with non-zero temperature, black holes should emit thermal radiation.

Semi-classical calculations indicate that indeed they do, with the surface gravity playing the role of temperature in Planck's law. This radiation is known as Hawking radiation cf. There are other types of horizons. In an expanding universe, an observer may find that some regions of the past cannot be observed " particle horizon " , and some regions of the future cannot be influenced event horizon. Another general feature of general relativity is the appearance of spacetime boundaries known as singularities. Spacetime can be explored by following up on timelike and lightlike geodesics—all possible ways that light and particles in free fall can travel.

But some solutions of Einstein's equations have "ragged edges"—regions known as spacetime singularities , where the paths of light and falling particles come to an abrupt end, and geometry becomes ill-defined. In the more interesting cases, these are "curvature singularities", where geometrical quantities characterizing spacetime curvature, such as the Ricci scalar , take on infinite values.

Given that these examples are all highly symmetric—and thus simplified—it is tempting to conclude that the occurrence of singularities is an artifact of idealization. While no formal proof yet exists, numerical simulations offer supporting evidence of its validity. It describes the state of matter and geometry everywhere and at every moment in that particular universe.

Due to its general covariance, Einstein's theory is not sufficient by itself to determine the time evolution of the metric tensor. It must be combined with a coordinate condition , which is analogous to gauge fixing in other field theories. To understand Einstein's equations as partial differential equations, it is helpful to formulate them in a way that describes the evolution of the universe over time.

The best-known example is the ADM formalism. The notion of evolution equations is intimately tied in with another aspect of general relativistic physics. In Einstein's theory, it turns out to be impossible to find a general definition for a seemingly simple property such as a system's total mass or energy. The main reason is that the gravitational field—like any physical field—must be ascribed a certain energy, but that it proves to be fundamentally impossible to localize that energy.

Nevertheless, there are possibilities to define a system's total mass, either using a hypothetical "infinitely distant observer" ADM mass [] or suitable symmetries Komar mass. The hope is to obtain a quantity useful for general statements about isolated systems , such as a more precise formulation of the hoop conjecture.

If general relativity were considered to be one of the two pillars of modern physics, then quantum theory, the basis of understanding matter from elementary particles to solid state physics , would be the other. Ordinary quantum field theories , which form the basis of modern elementary particle physics, are defined in flat Minkowski space, which is an excellent approximation when it comes to describing the behavior of microscopic particles in weak gravitational fields like those found on Earth.

These theories rely on general relativity to describe a curved background spacetime, and define a generalized quantum field theory to describe the behavior of quantum matter within that spacetime. The demand for consistency between a quantum description of matter and a geometric description of spacetime, [] as well as the appearance of singularities where curvature length scales become microscopic , indicate the need for a full theory of quantum gravity: for an adequate description of the interior of black holes, and of the very early universe, a theory is required in which gravity and the associated geometry of spacetime are described in the language of quantum physics.

Attempts to generalize ordinary quantum field theories, used in elementary particle physics to describe fundamental interactions, so as to include gravity have led to serious problems. One attempt to overcome these limitations is string theory , a quantum theory not of point particles , but of minute one-dimensional extended objects. Another approach starts with the canonical quantization procedures of quantum theory. Using the initial-value-formulation of general relativity cf.

Space is represented by a web-like structure called a spin network , evolving over time in discrete steps. Depending on which features of general relativity and quantum theory are accepted unchanged, and on what level changes are introduced, [] there are numerous other attempts to arrive at a viable theory of quantum gravity, some examples being the lattice theory of gravity based on the Feynman Path Integral approach and Regge Calculus , [] dynamical triangulations , [] causal sets , [] twistor models [] or the path integral based models of quantum cosmology.

All candidate theories still have major formal and conceptual problems to overcome. They also face the common problem that, as yet, there is no way to put quantum gravity predictions to experimental tests and thus to decide between the candidates where their predictions vary , although there is hope for this to change as future data from cosmological observations and particle physics experiments becomes available.

General relativity has emerged as a highly successful model of gravitation and cosmology, which has so far passed many unambiguous observational and experimental tests. However, there are strong indications the theory is incomplete. Mathematical relativists seek to understand the nature of singularities and the fundamental properties of Einstein's equations, [] while numerical relativists run increasingly powerful computer simulations such as those describing merging black holes.

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(PDF) Notes on Advanced General Relativity | Dr. J. M. Ashfaque (AMIMA, MInstP) -

For the graduate textbook by Robert Wald, see General Relativity book. Einstein's theory of gravitation as curved spacetime. For a more accessible and less technical introduction to this topic, see Introduction to general relativity. Introduction History. Fundamental concepts. Principle of relativity Theory of relativity Frame of reference Inertial frame of reference Rest frame Center-of-momentum frame Equivalence principle Mass—energy equivalence Special relativity Doubly special relativity de Sitter invariant special relativity World line Riemannian geometry.

Equations Formalisms. Play media. Main articles: History of general relativity and Classical theories of gravitation. Main articles: Einstein field equations and Mathematics of general relativity. Main article: Alternatives to general relativity. See also: Mathematics of general relativity and Physical theories modified by general relativity. Main article: Gravitational time dilation. Main articles: Schwarzschild geodesics , Kepler problem in general relativity , Gravitational lens , and Shapiro delay. Main article: Gravitational wave. Main article: Kepler problem in general relativity.

Main article: Apsidal precession. Main articles: Geodetic precession and Frame dragging. Main article: Gravitational lensing. Main articles: Gravitational wave and Gravitational wave astronomy. Main article: Black hole. Main article: Physical cosmology. Main article: Causal structure. Main articles: Horizon general relativity , No hair theorem , and Black hole mechanics. Main article: Spacetime singularity.

Main article: Initial value formulation general relativity. Main article: Mass in general relativity. Main article: Quantum field theory in curved spacetime. Main article: Quantum gravity. Science portal Physics portal. Retrieved 18 April Andrews , Scotland. Retrieved Einstein's original papers are found in Digital Einstein , volumes 4 and 6.

An early key article is Einstein , cf. Pais , ch. The publication featuring the field equations is Einstein , cf. Einstein's condemnation would prove to be premature, cf. The future of theoretical physics and cosmology: celebrating Stephen Hawking's 60th birthday. Cambridge University Press. It is more. For the experimental evidence, cf. As the ball moves through spacetime, its position in spacetime is given by appropriate functions of this parameter.

We can rewrite things slightly, to relate its position in space to its position in time. Then, when we look at this trajectory, it appears that the object is accelerating towards the earth, giving rise to the idea that gravity is acting as a force. What is really happening, however, is that the object's motion in our coordinate system is described by the geodesic equation. If you want some maths, this equation looks like the following:. Here, x with superscript Greek indices describes the position of the ball in our coordinate system. The indices indicate whether we're talking about the x,y,z or time coordinate.

The parameter t that the derivatives are being taken with respect to is the affine parameter; in this case, it is known as the "proper time" of the object for slowly moving objects, we can think of t as the time coordinate in our coordinate system. The first term in this equation is the acceleration of the object in our coordinate system.

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  • The second term describes the effect of gravity. The thing that looks like part of a hangman's game is called a connection symbol. It encodes all of the effects of the bending of space time as well as information about our choice of coordinate system.

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    There are actually sixteen terms here: it's written in a convention called Einstein summation convention. This shows that the effects of the bending of spacetime change the acceleration of an object, based on its velocity through not only space but also through time. If there is no curvature to spacetime, then the connection symbols are all zero, and we see that an object moves with zero acceleration constant velocity unless acted upon by an external force which would replace the zero on the right-hand side of this equation.

    Again, there are some technicalities: this is only true in a Cartesian coordinate system; in something like polar coordinates, the connection symbols may not be vanishing, but they're just describing the vagaries of the coordinate system in that case. If there is some bending to spacetime, then the connection symbols are not zero, and all of a sudden, there appears to be an acceleration. It is this curvature of spacetime that gives rise to what we interpret as gravitational acceleration.

    Note that there is no mass in this equation - it doesn't matter what the mass of the object is, they all follow the same geodesic so long as it's not massless, in which case things are a little different. So, what good is this geodesic description of the force of gravity? Can't we just think of gravity as a force and be done with it? It turns out that there are two cases where this description of the effect of gravity gives vastly different results compared to the concept of gravity as a force.

    The first is for objects moving very very fast, close to the speed of light. Newtonian gravity doesn't correctly account for the effect of the energy of the object in this case. A particularly important example is for exactly massless particles, such as photons light. One of the first experimental confirmations of general relativity was that light can be deflected by a mass, such as the sun. Another effect related to light is that as light travels up through the earth's gravitational field, it loses energy.

    This was actually predicted before general relativity, by considering conservation of energy with a radioactive particle in the earth's gravitational field. However, although the effect was discovered, it had no description in terms of Newtonian gravity. The second case in which the effect of gravity vastly differs is in the realm of extremely strong gravitational fields, such as those around black holes. Here, the effect of gravity is so severe that not even light can escape from the gravitational pull of such an object.

    Again, this effect was calculated in Newtonian gravity by thinking about the escape velocity of a body, and contemplating what happens when it gets larger than the speed of light. Surprisingly, the answer you arrive at is exactly the same as in general relativity. However, as light is massless, you once again do not have a good description of this effect in terms of Newtonian gravity, which tells you that there has to be a more complete theory.

    So, to summarize, general relativity says that matter bends spacetime, and the effect of that bending of spacetime is to create a generalized kind of force that acts on objects. However, it isn't a force as such that acts on the object, but rather just the object following its geodesic path through spacetime. Home Physics The Theory of Relativity. Similar Questions that might Interest You If gravity is a "curvature of space" rather than a force, why do a ball and bullet follow different paths?

    Intermediate If light has no mass, then what draws it into a black hole?