A brief history of time - Stephen Hawking [89]
Quark: A (charged) elementary particle that feels the strong force. Protons and neutrons are each composed of three quarks.
Radar: A system using pulsed radio waves to detect the position of objects by measuring the time it takes a single pulse to reach the object and be reflected back.
Radioactivity: The spontaneous breakdown of one type of atomic nucleus into another.
Red shift: The reddening of light from a star that is moving away from us, due to the Doppler effect.
Singularity: A point in space-time at which the space-time curvature becomes infinite.
Singularity theorem: A theorem that shows that a singularity must exist under certain circumstances - in particular, that the universe must have started with a singularity.
Space-time: The four-dimensional space whose points are events.
Spatial dimension: Any of the three dimensions that are spacelike - that is, any except the time dimension.
Special relativity: Einstein’s theory based on the idea that the laws of science should be the same for all observers, no matter how they are moving, in the absence of gravitational phenomena.
Spectrum: The component frequencies that make up a wave. The visible part of the sun’s spectrum can be seen in a rainbow.
Spin: An internal property of elementary particles, related to, but not identical to, the everyday concept of spin.
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.
ACKNOWLEDGEMENTS
Many people have helped me in writing this book. My scientific colleagues have without exception been inspiring. Over the years my principal associates and collaborators were Roger Penrose, Robert Geroch, Brandon Carter, George Ellis, Gary Gibbons, Don Page, and Jim Hartle. I owe a lot to them, and to my research students, who have always given me help when needed.
One of my students, Brian Whitt, gave me a lot of help writing the first edition of this book. My editor at Bantam Books, Peter Guzzardi, made innumerable comments which improved the book considerably. In addition, for this edition, I would like to thank Andrew Dunn, who helped me revise the text.
I could not have written this book without my communication system. The software, called Equalizer, was donated by Walt Waltosz of Words Plus Inc., in Lancaster, California. My speech synthesizer was donated by Speech Plus, of Sunnyvale, California. The synthesizer and