Page 100 - A Brief History of Time - Stephen Hawking

A Brief History of Time - Stephen Hawking... Glossary
Stationary state: One that is not changing with time: a sphere spinning at a constant rate is stationary because
it looks identical at any given instant.

String theory: A theory of physics in which particles are described as waves on strings. Strings have length but
no other dimension.

Strong force: The strongest of the four fundamental forces, with the shortest range of all. It holds the quarks
together within protons and neutrons, and holds the protons and neutrons together to form atoms.

Uncertainty principle: The principle, formulated by Heisenberg, that one can never be exactly sure of both the
position and the velocity of a particle; the more accurately one knows the one, the less accurately one can
know the other.

Virtual particle: In quantum mechanics, a particle that can never be directly detected, but whose existence
does have measurable effects.

Wave/particle duality: The concept in quantum mechanics that there is no distinction between waves and
particles; particles may sometimes behave like waves, and waves like particles.

Wavelength: For a wave, the distance between two adjacent troughs or two adjacent crests.

Weak force: The second weakest of the four fundamental forces, with a very short range. It affects all matter
particles, but not force-carrying particles.

Weight: The force exerted on a body by a gravitational field. It is proportional to, but not the same as, its mass.

White dwarf: A stable cold star, supported by the exclusion principle repulsion between electrons.

Wormhole: A thin tube of space-time connecting distant regions of the universe. Wormholes might also link to
parallel or baby universes and could provide the possibility of time travel.

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