Beta-decay: radioactive decay, or the emission of a beta particle or electron from an atom
Black-body radiation: thermal radiation (that is, heat radiating) from a closed system heated to a particular temperature
Born's probability interpretation: a reconciliation of wave mechanics with quantum mechanics that explains waves as containing the probable location of an electron
Bose-Einstein statistics: first proposed by S. N. Bose and championed by Einstein, these statistics explain the behavior of bosons.
Bosons: one of two general classes of elementary particles (along with fermions) determined by how they spin
Bright-line spectra: the unique spectral lines formed when an element is heated and its atoms emit light; every atom has its own signature of bright lines.
Brownian motion: in fluids, the seemingly random movements of tiny particles as they are struck by the molecules of the fluid; Einstein applied statistical mechanics to explain the movements.
Copenhagen interpretation: under Niels Bohr, the most persuasive, complete framework for understanding quantum mechanics
Double-slit experiment: a method for analyzing light by diffracting light beams through two slits and observing the resulting patterns on a screen
Exclusion principle: Pauli's discovery, expressed as a law, that no two particles occupy the same space at the same time
Fermions: see Bosons
Field: the extension of a physical quality throughout space (as the electromagnetic field); in classical or quantum physics, field theory describes the dynamics and effects within the field.
General Relativity: Einstein's generalization of his theory of special relativity to include gravity. It reconceived Newton by showing that apples fall to the ground because the earth's mass curves the adjacent space-time, forcing apples to move in a special way, that is, towards the surface of the earth. It has proven extremely difficult to unify general relativity with quantum mechanics. String theory is currently the best hope.
Gödel numbering: the assignment of numbers to mathematical symbols and formulas, to allow for mathematical statements about mathematics (that is, metamathematical statements)
Incompleteness theorems: Gödel's demonstrations that in any formal system, there are statements that are true, but are not provable using the axioms of that system. The paper containing the theorems was entitled “On Formally Undecidable Propositions of Principia Mathematica and Related Systems,” a reference to the work by Russell and Whitehead.
Induction: reasoning that begins with information from particular instances and leads to general propositions
Locality: in classical physics, the idea that two objects in separate places are independent and cannot interact, a restriction seemingly disproved by recent experiments demonstrating quantum particles to be “entangled” no matter what the distance between them
Logicism: an approach to philosophy in which mathematics is subsumed into logic
Matrix mechanics: the first complete definition of the laws and properties of subatomic particles using matrices to describe their properties
Maxwell's equations: four equations that describe electric and magnetic fields and their interaction with matter
Neutrino: an extremely light particle, the existence of which was hypothesized by Pauli
Paradox: in logic, reasoning that leads to contradiction, revealing false assumptions or faulty processes
Particle physics: the study of the basic subatomic elements and the forces acting upon and among them
Photoelectric effect: the emission of electrons when exposed to electromagnetic radiation and the subject of Einstein's first 1905 paper, in which he proposed that rather than waves, light was made of quanta (later called photons)
Photon: the elementary particle that forms light
Planck's constant: the ratio of a photon’s energy to its frequency
Quantum mechanics: a theory of subatomic systems that derives information about particles through an application of statistics, conceives of electrons and protons as both particle and wave in their behavior, and acknowledges uncertainty in measuring both movement and position simultaneously
Quantum physics: the entire body of modern, postclassical physics incorporating quantum mechanics and treating both large and small scale forces
Russell's paradox: a conundrum discovered by Russell in 1901. The problem occurs when, in attempting to account for and classify all sets, one imagines a “set of all sets that are not members of themselves.” A set can be a member of itself: Imagine the set of all objects that are not cars. Since the set of all noncars is not a car, it can be a member of itself. Now, we must move up a rung in abstraction, because set theory conceives of “sets of sets.” The set of all sets that are members of themselves is, indeed, possible: e.g., the set of all sets of cars can be a member of itself. But the set of all sets that are not members of themselves is a paradox. Is it a member of itself? It must be, since it is a set that includes just that: sets that are not members of themselves; and yet it cannot be, since by being a member of the set of all sets that are not members of themselves, it would become a member of itself.
Special relativity: Einstein's explanation of the relationships among light, energy, and matter, introducing the concept of space-time and defining the speed of light as the one constant not dependent on the observer
Spin: in quantum mechanics, a particle's angular momentum—either half-integer (fermions) or integer (bosons)
String theory: a theory (as yet unproven) that attempts to unify all of the known forces (weak, strong, electromagnetic, and gravitational). In this theory, matter is made not of particles that behave like waves, but of strings that vibrate.
Strong and weak nuclear forces: within the atom, the strong force holds neutrons and protons in the nucleus; the weak force allows beta decay, or radiation, to occur.
Ultraviolet catastrophe: a false prediction, based on classical physics, that when black-body radiation reaches equilibrium, it will emit infinite heat
Uncertainty principle: a cornerstone of quantum mechanics; Heisenberg's assertion that it is impossible to measure and determine both the position and the momentum of a particle simultaneously
Unified theory: a theory of everything, that is, all of the elemental forces (weak, strong, electromagnetic, and gravitational)
Wave mechanics: Erwin Schrödinger's explanation of quantum behavior, in which electrons move like waves around the nucleus
Wave/particle duality: in quantum mechanics, the idea that all objects exhibit the properties of both waves and particles