Appendix A
alpha decay: The spontaneous emission of alpha particles by certain radioactive nuclei.
alpha particle: A bundle of two protons and two neutrons. Also, the nucleus of helium.
alpha ray: An alpha particle.
beta decay: The spontaneous emission of electrons and neutrinos from certain radioactive nuclei.
beta particle: An electron.
beta ray: A beta particle.
black body: An ideal object that absorbs all radiation. It’s also a perfect emitter of radiation.
black dwarf: A burned-out white dwarf star. The sun will end its life as a black dwarf.
black hole: An object with such strong gravity that not even light can escape.
coherent: Light beams vibrating in step.
conservation law: The physics law that specifies that a certain quantity cannot be destroyed.
conservation of energy: The law that states that the total amount of energy you start a process with has to equal what you end up with.
dark matter: An unknown invisible substance that can be detected only by its gravitational effects.
diffraction: The bending or spreading of a light beam as it passes through a narrow slit or hole.
Doppler effect: The change in the wavelength perceived by a listener who is in motion relative to a source of sound. It also applies to light and other electromagnetic waves.
electric current: The rate at which electric charges move through a conductor or through empty space.
electron: A fundamental particle that can’t be split and carries negative electric charge. As far as we know, electrons have no size.
energy: The capacity to perform work or the result of doing work.
entropy: A measure of the degree of disorder in a system.
epicycles: Circles that the planets were supposed to follow in their paths across the heavens, according to Ptolemy’s model of the universe. The centers of those circles moved around the Earth. Copernicus showed that the model was wrong.
escape velocity: The minimum upward speed that you must give an object so that it leaves the Earth and never falls back. This value is 11 kilometers (7 miles) per second.
ether: An invisible substance filling all space that physicists of the 19th century invented to explain the motion of light in space. Einstein’s theory of relativity did away with it.
event horizon: The surface of a sphere with a radius equal to the Schwarzschild radius.
field: The distortion of space due to the presence of a body that exerts a force on other bodies.
galaxy: A large island of billions of stars, held together by gravity.
gamma rays: Electromagnetic radiation emitted by certain radioactive nuclei.
geocentric: Earth-centered. Ptolemy’s model of the universe, with the sun and the stars revolving around the Earth, was geocentric.
Heisenberg’s uncertainty principle: You can’t determine with complete accuracy the position of a particle and at the same time measure where and how fast it’s moving.
inertia: The resistance of a body to an attempt to change its motion.
interference: The pattern that results from the overlap of two wave motions that run into each other. The overlap can be a reinforcement of the waves or a cancellation.
laser: Light amplification by stimulated emission of radiation produces a narrow beam of single-wavelength, coherent light.
mass: A measure of the resistance that you feel when you try to change the motion of an object; the measure of inertia.
neutrino: A subatomic particle that has no mass or a very tiny amount of mass — we don’t yet know which. It plays a role in radioactivity.
neutron: One of the subatomic particles that make up the nucleus of an atom.
neutron star: An object only tens of kilometers across but with a mass larger than that of the sun. Neutron stars are composed mainly of neutrons.
photon: A quantum of light.
proton: One of the subatomic particles that make up the nucleus of an atom. The proton has positive electric charge.
pulsar: A fast-spinning, small, and very dense astronomical body that emits light and radio waves, like a lighthouse.
quanta: Bundles or packets of energy that can’t be split. Light and all electromagnetic radiation are made up of quanta.
quasar: An extremely bright astronomical body, usually found very far away, that shines with the brightness of trillions of suns.
radioactivity: The spontaneous emission of particles or radiation from certain atoms.
red giant: A large, bright star with a low surface temperature. The sun will spend its late years as a red giant, eventually ending as a burned-out star called a black dwarf.
reference frame: A reference point for a moving object.
Schwarzschild radius: The largest radius of a star below which light will be trapped, as in a black hole.
singularity: Places where the laws of physics break down or don’t apply.
spacetime: The combination of the three dimensions of space and one of time. It’s needed in relativity because time and space are linked together.
spectrum: The array of the components of the emission of electromagnetic radiation arranged according to wavelength.
superstring theory: A new theory of the structure of matter that says that all elementary particles are represented by tiny vibrating strings. The theory promises to unify all the forces of nature.
thermodynamics: The study of heat and thermal effects.
uniform motion: Motion at a constant speed along a straight line.
virtual particle: A particle that exists only for the brief moment allowed by Heisenberg’s uncertainty principle.
wave: A mechanism for the transmission of energy.
wavelength: The length of an oscillation; the distance between two crests or two valleys in a wave.
white dwarf: A small, dense star. One of the final stages in the life of the sun.
worm hole: A tunnel through space that connects two black holes at different places in the universe. We don’t yet know if worm holes really exist.