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.