34.2 General Relativity and Quantum Gravity

• Einstein’s theory of general relativity describes all types of relative motion including accelerated motion and the effects of gravity. General relativity encompasses special relativity and classical relativity in situations where acceleration is zero and relative velocity is small compared with the speed of light.
• Quantum gravity is the theory that deals with particle exchange of gravitons as the mechanism for the force, and with extreme conditions where quantum mechanics and general relativity must both be used.
• Much of what Einstein did to develop his ideas was to mentally analyze certain carefully and clearly defined situations—doing this is to perform a thought experiment.
• Black holes are objects having such large gravitational fields that things can fall in, but nothing, not even light, can escape.
• Bodies, like the Earth or the Sun, have what is called an escape velocity.
• In fact, no light comes from inside the event horizon, which is defined to be at a distance from the object at which the escape velocity is exactly the speed of light.
• The radius of the event horizon is known as the Schwarzschild radius 𝑅s
• Neutron stars are literally a star composed of neutrons.
• Some of the moderately distant galaxies, and hence among the younger, are known as quasars and emit as much or more energy than a normal galaxy but from a region less than a light year across.
• Gravitational waves are mass-created distortions in space that propagate at the speed of light and are predicted by general relativity.