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arises when someone travels at close to light speed to, say, Alpha Centauri and back while their twin stays at home. According to special relativity, the space-travelling twin ages less. However, from another point of view, it is Earth that receded from the space-travelling twin at close to the speed of light and therefore the stay-at-home-twin who ages less. The paradox is resolved by realising that the two situations are not equivalent. The space-travelling twin must undergo a deceleration and an acceleration at the turnaround at Alpha Centauri, and accelerations require general relativity not special relativity.

ULTRAVIOLET Type of invisible light that is given out by very hot bodies which is responsible for sunburn.

UNCERTAINTY PRINCIPLE See Heisenberg Uncertainty Principle.

UNIFICATION The idea that at extremely high energy the four fundamental forces of nature are one, united in a single theoretical framework.

UNIVERSE All there is. This is a flexible term once used for what we now call the solar system. Later, it was used for what we call the MilkyWay. Now it is used for the sum total of all the galaxies, of which there appear to be about 100 billion within the observable Universe.

UNIVERSE, EXPANSION OF The fleeing of the galaxies from each other in the aftermath of the Big Bang.

UNIVERSE, OBSERVABLE All we can see out to the Universe’s horizon.

URANIUM The heaviest naturally occurring element.

VIRTUAL PARTICLE Subatomic particle that has a fleeting existence, popping into being and popping out again according to the constraint imposed by the Heisenberg uncertainty principle.

VISCOSITY The internal friction of a liquid. Treacle has high viscosity and water has low viscosity.

WAVE FUNCTION A mathematical entity that contains all that is knowable about a quantum object such as an atom. The wave function changes in time and space according to the Schrödinger equation.

WAVELENGTH The distance for a wave to go through a complete oscillation cycle.

WAVE-PARTICLE DUALITY The ability of a subatomic particle to behave as a localised billiard ball-like particle or a spread-out wave.

WEAK NUCLEAR FORCE The second force experienced by protons and neutrons in an atomic nucleus, the other being the strong nuclear force. The weak nuclear force can convert a neutron into a proton and so is involved in beta decay.

WHITE DWARF A star that has run out of fuel and that gravity has compressed until it is about the size of Earth. A white dwarf is supported against further shrinkage by electron degeneracy pressure. A sugar cube of white dwarf material weighs about as much as a family car.

WORMHOLE A tunnel through space-time that connects widely spaced regions and so provides a shortcut.

X-RAYS A high-energy form of light.

FURTHER READING

ATOMS AND QUANTUM THEORY

Quantum: A Guide for the Perplexed, by Jim Al-Khalili (Weidenfeld & Nicolson, London, 2003).

Taming the Atom, by Hans Christian von Baeyer (Penguin, London, 1994).

Minds, Machines, and the Multiverse, by Julian Brown (Little Brown, New York, 2000).

The Magic Furnace, by Marcus Chown (Vintage, London, 2000).

The Fabric of Reality, by David Deutsch (Penguin, London, 1997).

Thirty Years That Shook Physics, by George Gamow (Dover, New York, 1985).

The Great Physicists from Galileo to Einstein, by George Gamow (Dover, New York, 1988).

The New Quantum Universe, by Tony Hey and Patrick Walters, 2nd edition (Cambridge University Press, Cambridge, England, 2004).

The Feynman Lectures on Physics, edited by Robert Leighton et al. (Addison-Wesley, New York, 1989).

RELATIVITY AND COSMOLOGY

Afterglow of Creation, by Marcus Chown (University Science Books, Sausalito, California, 1994).

The Universe Next Door, by Marcus Chown (Headline, London, 2002).

Cosmology, by Edward Harrison (Cambridge University Press, Cambridge, England, 1991).

The River of Time, by Igor Novikov (Cambridge University Press, Cambridge, England, 1998).

Einstein’s Legacy, by Julian Schwinger (Scientific American Library, New York, 1986).

The Physical Universe, by Frank Shu (University