Page 99 - A Brief History of Time - Stephen Hawking

A Brief History of Time - Stephen Hawking... Glossary
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.

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