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Glossary
quantum : The indivisible unit in which waves may be emitted or absorbed.
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quantum mechanics: The theory developed from Planck's quantum principle and Heisenberg's uncertainty principle (page 41).
quark: A (charged) elementary particle that feels the strong force. Protons and neutrons are each believed to be composed of three quarks (page 44).
radioactivity: The spontaneous breakdown of one type of atomic nucleus into another (page 17).
space-time-continuum: According to the special theory of relativity, our reality is four-dimensional, in which space has three dimensions and the fourth dimension is time. Thus, there is no such thing as space and time; but there is only space-time. A continuum is something that cannot be broken down into parts. It flows continuously.
spatial dimensions: Any of the three dimensions of space-time that are spacelike - that is, any except the time dimension.
spectrum: The splitting of, say, an electromagnetic wave into its component frequencies/wave-lengths (page 15).
spin: An internal property of elementary particles, related to, but not identical to, the everyday concept of spin (page 39).
standing wave: Standing waves are patterns of waves confined to a finite region like the waves in a vibrating string such as a clothes-line. Electrons orbiting in an atom are not spherical objects, but patterns of standing waves (page 26).
stationary state: One that is not changing with time: a sphere spinning at a constant rate is stationary because it looks identical at any instant, even though it is not static.
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 (page 31).
theory of relativity: Einstein's theory based on the idea that the laws of science should be the same for all observers, no matter
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