Page 97 - A Brief History of Time - Stephen Hawking

A Brief History of Time - Stephen Hawking... Glossary
GLOSSARY

Absolute zero: The lowest possible temperature, at which substances contain no heat energy.

Acceleration: The rate at which the speed of an object is changing.

Anthropic principle: We see the universe the way it is because if it were different we would not be here to
observe it.

Antiparticle: Each type of matter particle has a corresponding antiparticle. When a particle collides with its
antiparticle, they annihilate, leaving only energy.

Atom: The basic unit of ordinary matter, made up of a tiny nucleus (consisting of protons and neutrons)
surrounded by orbiting electrons.

Big bang: The singularity at the beginning of the universe.

Big crunch: The singularity at the end of the universe.

Black hole: A region of space-time from which nothing, not even light, can escape, because gravity is so
strong.

Casimir effect: The attractive pressure between two flat, parallel metal plates placed very near to each other in
a vacuum. The pressure is due to a reduction in the usual number of virtual particles in the space between the
plates.

Chandrasekhar limit: The maximum possible mass of a stable cold star, above which it must collapse into a
black hole.

Conservation of energy: The law of science that states that energy (or its equivalent in mass) can neither be
created nor destroyed.

Coordinates: Numbers that specify the position of a point in space and time.

Cosmological constant: A mathematical device used by Einstein to give space-time an inbuilt tendency to
expand.

Cosmology: The study of the universe as a whole.

Dark matter: Matter in galaxies, clusters, and possibly between clusters, that can not be observed directly but
can be detected by its gravitational effect. As much as 90 percent of the mass of the universe may be in the
form of dark matter.

Duality: A correspondence between apparently different theories that lead to the same physical results.

Einstein-Rosen bridge: A thin tube of space-time linking two black holes. Also see Wormhole.

Electric charge: A property of a particle by which it may repel (or attract) other particles that have a charge of
similar (or opposite) sign.

Electromagnetic force: The force that arises between particles with electric charge; the second strongest of
the four fundamental forces.

Electron: A particle with negative electric charge that orbits the nucleus of an atom.

Electroweak unification energy: The energy (around 100 GeV) above which the distinction between the
electromagnetic force and the weak force disappears.

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