Page 99 - A Brief History of Time - Stephen Hawking
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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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