A brief history of time - Stephen Hawking [88]
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