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.
Elementary particle: A particle that, it is believed, cannot be subdivided.
Event: A point in space-time, specified by its time and place.
Event horizon: The boundary of a black hole.
Exclusion principle: The idea that two identical spin-1/2 particles cannot have (within the limits set by the
uncertainty principle) both the same position and the same velocity.
Field: Something that exists throughout space and time, as opposed to a particle that exists at only one point at
a time.
Frequency: For a wave, the number of complete cycles per second.
Gamma rays: Electromagnetic rays of very short wavelength, produced in radio-active decay or by collisions of
elementary particles.
General 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. It explains the force of gravity in terms of the curvature of a
four-dimensional space-time.
Geodesic: The shortest (or longest) path between two points.
Grand unification energy: The energy above which, it is believed, the electro-magnetic force, weak force, and
strong force become indistinguishable from each other.
Grand unified theory (GUT): A theory which unifies the electromagnetic, strong, and weak forces.
Imaginary time: Time measured using imaginary numbers.
Light cone: A surface in space-time that marks out the possible directions for light rays passing through a
given event.
Light-second (light-year): The distance traveled by light in one second (year).
Magnetic field: The field responsible for magnetic forces, now incorporated along with the electric field, into the
electromagnetic field.
Mass: The quantity of matter in a body; its inertia, or resistance to acceleration.
Microwave background radiation: The radiation from the glowing of the hot early universe, now so greatly
red-shifted that it appears not as light but as microwaves (radio waves with a wavelength of a few centimeters).
Also see COBE, on page 145.
Naked singularity: A space-time singularity not surrounded by a black hole.
Neutrino: An extremely light (possibly massless) particle that is affected only by the weak force and gravity.
Neutron: An uncharged particle, very similar to the proton, which accounts for roughly half the particles in an
atomic nucleus.
Neutron star: A cold star, supported by the exclusion principle repulsion between neutrons.
No boundary condition: The idea that the universe is finite but has no boundary (in imaginary time).
Nuclear fusion: The process by which two nuclei collide and coalesce to form a single, heavier nucleus.
Nucleus: The central part of an atom, consisting only of protons and neutrons, held together by the strong
force.
Particle accelerator: A machine that, using electromagnets, can accelerate moving charged particles, giving
them more energy.
Phase: For a wave, the position in its cycle at a specified time: a measure of whether it is at a crest, a trough,
or somewhere in between.
Photon: A quantum of light.
Planck’s quantum principle: The idea that light (or any other classical waves) can be emitted or absorbed
only in discrete quanta, whose energy is proportional to their wavelength.
Positron: The (positively charged) antiparticle of the electron.
Primordial black hole: A black hole created in the very early universe.
Proportional: ‘X is proportional to Y’ means that when Y is multiplied by any number, so is X. ‘X is inversely
proportional to Y’ means that when Y is multiplied by any number, X is divided by that number.
Proton: A positively charged particle, very similar to the neutron, that accounts for roughly half the particles in
the nucleus of most atoms.
Pulsar: A rotating neutron star that emits regular pulses of radio waves.
Quantum: The indivisible unit in which waves may be emitted or absorbed.
Quantum chromodynamics (QCD): The theory that describes the interactions of quarks and gluons.
Quantum mechanics: The theory developed from Planck’s quantum principle and Heisenberg’s uncertainty
principle.
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.
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ACKNOWLEDGMENTS
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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 laptop computer were mounted on my wheelchair
by David Mason, of Cambridge Adaptive Communication Ltd. With this system I can communicate better now
than before I lost my voice.
I have had a number of secretaries and assistants over the years in which I wrote and revised this book. On the
secretarial side, I’m very grateful to Judy Fella, Ann Ralph, Laura Gentry, Cheryl Billington, and Sue Masey. My
assistants have been Colin Williams, David Thomas, and Raymond Laflamme, Nick Phillips, Andrew Dunn,
Stuart Jamieson, Jonathan Brenchley, Tim Hunt, Simon Gill, Jon Rogers, and Tom Kendall. They, my nurses,
colleagues, friends, and family have enabled me to live a very full life and to pursue my research despite my
disability.
Stephen Hawking
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ABOUT THE AUTHOR
Stephen Hawking, who was born in 1942 on the anniversary of Galileo’s death, holds Isaac Newton’s chair as
Lucasian Professor of Mathematics at the University of Cambridge. Widely regarded as the most brilliant
theoretical physicist since Einstein, he is also the author of Black Holes and Baby Universes, published in 1993,
as well as numerous scientific papers and books.
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