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 onebrane is
a string, a twobrane is a membrane, a threebrane has three extended
dimensions, etc. More generally, a pbrane 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.
curledup 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 (leftright, backforth, updown) and the
familiar spacetime has four (the previous three axes plus the pastfuture
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 oneway 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.
Mtheory: theory emerging from the second superstring revolution that unites
the previous five superstring theories within a single overarching framework.
Mtheory 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 downquarks, one upquark).
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} gramscm/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 constantvelocity 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 upquarks and one downquark).
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 waveparticle 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: quantummechanical 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 onedimensional object that is the essential ingredient in
string theory.
string theory: unified theory of the universe postulating that fundamental
ingredients of nature are not zerodimensional point particles but tiny
onedimensional 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 quantummechanical 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, allencompassing framework.
velocity: the speed and the direction of an object's motion.
waveparticle duality: basic feature of quantum mechanics that objects manifest
both wavelike and particlelike properties.
weak force, weak nuclear force: one of the four fundamental forces, bestknown
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.
