Key Ideas

The Special Theory of Relativity: This theory, published by Einstein in 1905, describes how measurements of lengths and durations of time depend on the motion of an observer and can appear different to different observers.

• The speed of light is the same to all observers, no matter how fast they are moving.
• An observer will note a slowing of clocks and a shortening of rulers that are moving with respect to the observer. This effect becomes significant only if the clock or ruler is moving at a substantial fraction of the speed of light.
• Space and time are not wholly independent of each other, but are aspects of a single entity called spacetime.

The General Theory of Relativity: Published by Einstein in 1915, this is a theory of gravity. Any massive object causes space to curve and time to slow down, and these effects manifest themselves as a gravitational force. These distortions of space and time are most noticeable in the vicinity of large masses or compact objects.

• The general theory of relativity is our most accurate description of gravitation. It predicts a number of phenomena, including the bending of light by gravity and the gravitational redshift, whose existence has been confirmed by observation and experiment.
• The general theory of relativity also predicts the existence of gravitational waves, which are ripples in the overall geometry of space and time produced by moving masses. Gravitational waves have been detected indirectly, and specialized antennas are under construction to make direct measurement of the gravitational waves from cosmic cataclysms.

Black Holes: If a stellar corpse has a mass greater than about 2 to 3 M_⊙, gravitational compression will overwhelm any and all forms of internal pressure.

• A black hole has an escape speed greater than the speed of light.
• Nothing, not even light, can escape a black hole once it crosses into a black hole’s event horizon.

Observing Black Holes: Black holes have been detected using indirect methods.

• Some binary star systems contain a black hole. In such a system, gases captured from the companion star by the black hole emit detectable X-rays. The emission comes from outside the event horizon.
• Many galaxies have supermassive black holes at their centers. These are detected by observing the motions of material around the black hole.

Gamma-ray Bursts: Short, intense bursts of gamma rays are observed at random times coming from random parts of the sky.

• By observing the afterglow of long-duration gamma-ray bursts, astronomers find that these objects have very large redshifts and appear to be located within distant galaxies. The bursts are correlated with supernovae and may be due to an exotic type of supernova called a collapsar.
• The origin of short–duration gamma–ray bursts is unknown.

Properties of Black Holes: The entire mass of a black hole gets quickly concentrated in a nearly infinitely dense singularity.

• A black hole has only three physical properties: mass, electric charge, and angular momentum.
• A rotating black hole (one with angular momentum) has an ergoregion around the outside of the event horizon. In the ergoregion, space and time themselves are dragged along with the rotation of the black hole.
• Black holes emit Hawking radiation from a quantum physics process that occurs just outside of their event horizons.
• Black holes have temperatures and their Hawking radiation is emitted like the blackbody radiation that is emitted by any object with a temperature.
• Black holes can evaporate, but they do so at extremely slow rates unless the black holes are very small.