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antimatter: matter that has the same gravitational properties as ordinary matter, but that has an opposite electric charge as well as opposite nuclear force charges.

atom: fundamental building block of matter, consisting of a nucleus (comprising protons and neutrons and an orbiting swarm of electrons.

big bang: currently accepted theory that the expanding universe began some 15 billion years ago from a state of enormous energy, density, and compression.

black hole: an object whose immense gravitational field entraps anything, even light, that gets too close (closer than the black hole's event horizon).

boson: a particle, or pattern of string vibration, with a whole number amount of spin; typically a messenger particle.

brane: any of the extended objects that arise in string theory. A one-brane is a string, a two-brane is a membrane, a three-brane has three extended dimensions, etc. More generally, a p-brane has p spatial dimensions.

cosmic microwave background radiation: microwave radiation suffusing the universe, produced during the big bang and subsequently thinned and cooled as the universe expanded.

cosmological constant: a modification of general relativity's original equations, allowing for a static universe; interpretable as a constant energy density of the vacuum.

curled-up dimension: a spatial dimension that does not have an observably large spatial extent; a spatial dimension that is crumpled, wrapped, or curled up into a tiny size, thereby evading direct detection.

curvature: the deviation of an object or of space or of spacetime from a flat form and therefore from the rules of geometry codified by Euclid.

dimension: an independent axis or direction in space or spacetime. The familiar space around us has three dimensions (left-right, back-forth, up-down) and the familiar spacetime has four (the previous three axes plus the past-future axis). Superstring theory requires the universe to have additional spatial dimensions.

electromagnetic force: one of the four fundamental forces, a union of the electric and magnetic forces.

electron: negatively charged particle, typically found orbiting the nucleus of an atom.

electroweak theory: relativistic quantum field theory describing the weak force and the electromagnetic force in one unified framework.

event horizon: the one-way surface of a black hole; once penetrated, the laws of gravity ensure that there is no turning back, no escaping the powerful gravitational grip of the black hole.

fermion: a particle, or patter of string vibration, with half a whole odd number amount of spin; typically a matter particle.

field, force field: from a macroscopic perspective, the means by which a force communicates its influence; described by a collection of numbers at each point in space that reflect the strength and direction of the force at that point.

force charge: a property of a particle that determines how it responds to a particular force. For instance, the electric charge of a particle determines how it responds to the electromagnetic force.

general relativity: Einstein's formulation of gravity, which shows that space and time communicate the gravitational force through their curvature.

gluon: smallest bundle of the strong force field; messenger particle of the strong force.

grand unification: class of theories that merge all three nongravitational forces into a single theoretical framework.

gravitational force: the weakest of the four fundamental forces of nature. Described by Newton's universal theory of gravity, and subsequently by Einstein's general relativity.

graviton: smallest bundle of the gravitational force field; messenger particle for the gravitational force.

macroscopic: refers to scales typically encountered in the everyday world and larger; roughly the opposite of microscopic.

messenger particle: smallest bundle of a force field; microscopic conveyer of a force.

M-theory: theory emerging from the second superstring revolution that unites the previous five superstring theories within a single overarching framework. M-theory appears to be a theory involving eleven spacetime dimensions, although many of its detailed properties have yet to be understood.

nucleus: The core of an atom, consisting of protons and neutrons.

neutrino: chargeless species of particle, subject only to the weak force.

neutron: chargeless particle, typically found in the nucleus of an atom, consisting of three quarks (two down-quarks, one up-quark).

Newton's universal theory of gravity: theory of gravity declaring that the force of attraction between two bodies is proportional to the product of their masses and inversely proportional to the square of the distance between them. Subsequently supplanted by Einstein's general relativity.

nonperturbative: feature of a theory whose validity is not dependent on approximate, perturbative calculations; an exact feature of a theory.

particle accelerator: machine for boosting particles to nearly light speed and slamming them together in order to probe the structure of matter.

perturbation theory: framework for simplifying a difficult problem by finding an approximate solution that is subsequently refined as more details, initially ignored, are systematically included.

Planck length: about 10^-33 centimeters. The scale below which quantum fluctuations in the fabric of spacetime would become enormous. The size of a typical string in string theory.

Planck's constant: Planck's constant is a fundamental parameter in quantum mechanics. It determines the size of the discrete units or energy, mass, spin, etc. into which the microscopic world is partitioned. Its value is 1.05 x 10^-27 grams-cm/sec.

principle of equivalence: core principle of general relativity declaring the indistinguishability of accelerated motion and immersion in a gravitational field (over small enough regions of observation). Generalizes the principle of relativity by showing that all observers, regardless of their state of motion, can claim to be at rest, so long as they acknowledge the presence of a suitable gravitational field.

principle of relativity: core principle of special relativity declaring that all constant velocity observers are subject to an identical set of physical laws and that, therefore, every constant-velocity observer is justified in claiming that he or she is at rest. This principle is generalized by the principle of equivalence.

proton: positively charged particle, typically found in the nucleus of an atom, consisting of three quarks (two up-quarks and one down-quark).

quantum chromodynamics (QCD): relativistic quantum field theory of the strong force and quarks, incorporating special relativity.

quantum fluctuation: turbulent behavior of a system on microscopic scales due to the uncertainty principle.

quantum gravity: a theory that successfully merges quantum mechanics and general relativity, possibly involving modifications of one or both. String theory is an example of a theory of quantum gravity.

quantum mechanics: framework of laws governing the universe whose unfamiliar features such as uncertainty, quantum fluctuations, and wave-particle duality become most apparent on microscopic scales of atoms and subnuclear particles.

quark: a particle that is acted upon by the strong force. Quarks exist in six varieties (up, down, charm, strange, top, bottom) and three "colors" (red, green, blue).

relativistic quantum field theory: quantum-mechanical theory of fields, such as the electromagnetic field, that incorporates special relativity.

Riemannian geometry: mathematical framework for describing curved shapes of any dimension. Plays a central role in Einstein's description of spacetime in general relativity.

Schroedinger equation: equation governing the evolution of probability waves in quantum mechanics.

second superstring revolution: period in the development of string theory beginning around 1995 in which some nonperturbative aspects of the theory began to be understood.

singularity: location where the fabric of space or spacetime suffers a devastating rupture.

spacetime: a union of space and time originally emerging from special relativity. Can be viewed as the "fabric" out of which the universe is fashioned; it constitutes the dynamical arena within which the events of the universe take place.

special relativity: Einstein's laws of space and time in the absence of gravity (see also general relativity).

spin: a quantum mechanical version of the familiar notion of the same name; particles have an intrinsic amount of spin that is either a whole number or half a whole number (in multiples of Planck's constant), and which never changes.

standard model of particle physics, standard model, standard theory: an enormously successful theory of the three nongravitational forces and their action on matter. Effectively the union of quantum chromodynamics and the elecroweak theory.

string: fundamental one-dimensional object that is the essential ingredient in string theory.

string theory: unified theory of the universe postulating that fundamental ingredients of nature are not zero-dimensional point particles but tiny one-dimensional filaments called strings. String theory harmoniously unites quantum mechanics and general relativity, the previously known laws of the small and the large, that are otherwise incompatible. Often short for superstring theory.

strong force, strong nuclear force: strongest of the four fundamental forces, responsible for keeping quarks locked inside protons and neutrons and for keeping protons and neutrons crammed inside of atomic nuclei.

superpartners: particles whose spins differ by 1/2 unit and that are paired by supersymmetry.

superstring theory: string theory that incorporates supersymmetry.

supersymmetric standard model: generalization of the standard model of particle physics to incorporate supersymmetry. Entails a doubling of the known elementary particle species.

supersymmetry: a symmetry principle that relates the properties of particles with a whole number amount of spin (bosons) to those with half a whole (odd) number of spin (fermions).

theory of everything: a quantum-mechanical theory that encompasses all forces and all matter.

topology: classification of shapes into groups that can be deformed into one another without ripping or tearing their structure in any way.

uncertainty principle: principle of quantum mechanics, discovered by Heisenberg, that there are features of the universe, like the position and velocity of a particle, that cannot be known with complete precision. Such uncertain aspects of the microscopic world become ever more severe, as the distance and time scales on which they are considered become ever smaller. Particles and fields undulate and jump between all possible values consistent with the quantum uncertainty. This implies that the microscopic realm is a rolling frenzy, awash in a violent sea of quantum fluctuations.

unified theory, unified field theory: any theory that describes all four forces and all of matter within a single, all-encompassing framework.

velocity: the speed and the direction of an object's motion.

wave-particle duality: basic feature of quantum mechanics that objects manifest both wavelike and particle-like properties.

weak force, weak nuclear force: one of the four fundamental forces, best-known for mediating radioactive decay.

weak gauge boson: smallest bundle of the weak force field; messenger particle of the weak force; called W or Z boson.