The Elegant Universe - Brian Greene [223]
Electromagnetic force. One of the four fundamental forces, a union of the electric and magnetic forces.
Electromagnetic gauge symmetry. Gauge symmetry underlying quantum electrodynamics.
Electromagnetic radiation. The energy carried by an electromagnetic wave.
Electromagnetic wave. A wavelike disturbance in an electromagnetic field; all such waves travel at the speed of light. Visible light, X rays, microwaves, and infrared radiation are examples.
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.
Eleven-dimensional supergravity. Promising higher-dimensional supergravity theory developed in the 1970s, subsequently ignored, and more recently shown to be an important part of string theory.
Entropy. A measure of the disorder of a physical system; the number of rearrangements of the ingredients of a system that leave its overall appearance intact.
Equivalence principle. See principle of equivalence.
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.
Extended dimension. A space (and spacetime) dimension that is large and directly apparent; a dimension with which we are ordinarily familiar, as opposed to a curled-up dimension.
Extremal black holes. Black holes endowed with the maximal amount of force charge possible for a given total mass.
Families. Organization of matter particles into three groups, with each group being known as a family. The particles in each successive family differ from those in the previous by being heavier, but carry the same electric and nuclear force charges.
Fermion. A particle, or pattern of string vibration, with half a whole odd number amount of spin; typically a matter particle.
Feynman sum-over-paths. See sum-over-paths.
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.
Flat. Subject to the rules of geometry codified by Euclid; a shape, like the surface of a perfectly smooth tabletop, and its higher-dimensional generalizations.
Flop transition. Evolution of the Calabi-Yau portion of space in which its fabric rips and repairs itself, yet with mild and acceptable physical consequences in the context of string theory.
Foam. See spacetime foam.
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.
Frequency. The number of complete wave cycles a wave completes each second.
Gauge symmetry. Symmetry principle underlying the quantum-mechanical description of the three nongravitational forces; the symmetry involves the invariance of a physical system under various shifts in the values of force charges, shifts that can change from place to place and from moment to moment.
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.
Heterotic-E string theory (Heterotic E8 × E8 string theory). One of the five superstring theories; involves closed strings whose