Summarizing General Relativity and Quantum Gravity

General Relativity and Quantum Gravity Summary

• Einstein’s theory of general relativity includes accelerated frames and, thus, encompasses special relativity and gravity. Created by use of careful thought experiments, it has been repeatedly verified by real experiments.
• One direct result of this behavior of nature is the gravitational lensing of light by massive objects, such as galaxies, also seen in the microlensing of light by smaller bodies in our galaxy.
• Another prediction is the existence of black holes, objects for which the escape velocity is greater than the speed of light and from which nothing can escape.
• The event horizon is the distance from the object at which the escape velocity equals the speed of light \(c\). It is called the Schwarzschild radius \({R}_{\text{S}}\) and is given by

  \({R}_{\text{S}}=\cfrac{2\text{GM}}{{c}^{2}}\text{,}\)

  where \(G\) is the universal gravitational constant, and \(M\) is the mass of the body.

• Physics is unknown inside the event horizon, and the possibility of wormholes and time travel are being studied.
• Candidates for black holes may power the extremely energetic emissions of quasars, distant objects that seem to be early stages of galactic evolution.
• Neutron stars are stellar remnants, having the density of a nucleus, that hint that black holes could form from supernovas, too.
• Gravitational waves are wrinkles in space, predicted by general relativity but not yet observed, caused by changes in very massive objects.
• Quantum gravity is an incompletely developed theory that strives to include general relativity, quantum mechanics, and unification of forces (thus, a TOE).
• One unconfirmed connection between general relativity and quantum mechanics is the prediction of characteristic radiation from just outside black holes.

Glossary

black holes

objects having such large gravitational fields that things can fall in, but nothing, not even light, can escape

general relativity

Einstein’s theory thatdescribes all types of relative motion including accelerated motion and the effects of gravity

gravitational waves

mass-created distortions in space that propagate at the speed of light and that are predicted by general relativity

escape velocity

takeoff velocity when kinetic energy just cancels gravitational potential energy

event horizon

the distance from the object at which the escape velocity is exactly the speed of light

neutron stars

literally a star composed of neutrons

Schwarzschild radius

the radius of the event horizon

thought experiment

mental analysis of certain carefully and clearly defined situations to develop an idea

quasars

the moderately distant galaxies that emit as much or more energy than a normal galaxy

Quantum gravity

the theory that deals with particle exchange of gravitons as the mechanism for the force