q_{0}
 The cosmological deceleration parameter.

QCD
 Quantum Chromodynamics

QED
 Quantum Electrodynamics

QEH
 Quantum Efficiency Hysteresis An increase
in QE after exposure to light. [McL97]

QSO
 QuasiStellar Object (Quasar)

QBranch
 A set of lines in the spectra of molecules
corresponding to
changes in vibrational energy with none in rotational energy. [H76]

QE Pinned
 The quantum efficiency of certain CCDs can be
driven to
their maximum by UV flooding and pinned there by immediate cooling. [McL97]

Quadrant
 An instrument, based on a quarter of a circle,
employed to
measure the altitude above the horizon of astronomical
bodies. Eventually replaced by the sextant. [F88]

Quadrature
 (a) Elongation of a planet when it makes a
90° angle with the Sun as seen from Earth.
(b) A configuration in which two celestial bodies have apparent
longitudes (see Longitude, Celestial) that differ by 90°
as viewed
from a third body. Quadratures are usually tabulated with respect to
the Sun as viewed from the center of the Earth. [S92]
(c) A means of combining errors or noise by summing the
"square" of the values and then taking the "square root". [McL97]
(d) A calculation involving a definite integral. [H76]

Quadrupole
 When referred to a system containing charges, a
quadrupole is
equivalent to the presence of two equal dipoles parallel to
each other, but with their corresponding charges reversed; or
more generally, that component of the charge distribution which
has axial or triaxial symmetry. Similarly, when referred to mass
distributions, it arises from unequal components of the
momentofinertia tensor along three principal directions. [H76]

Quanta
 (a) Fundamental units of energy. [F88]
(b) Light can carry energy only in specific amounts,
proportional
to the frequency, as though it came in packets. The term quanta was
given to these discrete packets of electromagnetic energy by Max
Planck. [McL97]
(c) The smallest physical units into which something can be
partitioned, according to the laws of quantum mechanics. For instance,
photons are the quanta of the electromagnetic field. [G99]
(d) Each particle is surrounded by a field for each of the kinds
of charges it
carries, such as an electromagnetic field if it has electric charge. In the
quantum theory, the field is described as made up of particles that are the
quanta of the field. More loosely, the smallest amount of something that
can exist. [K2000]

Quantization
 The restriction of various quantities to certain
discrete
values; or, more generally, to deriving the quantummechanical
laws of a system from its corresponding classical laws. [H76]

Quantum
 A discrete quantity of energy hv
associated with a wave of
frequency v. It is the smallest amount of energy that can be
absorbed or radiated by matter at that frequency. [H76]

Quantum Chromodynamics
 QCD (a) The quantum field theory
describing the interactions of quarks
through the strong "color" field (whose quanta are gluons). QCD is a
gauge theory with the nonAbelian gauge symmetry group
SU(3)_{C}. [CD99]
(b) This is the accepted theory
of the forces that bind quarks together to form protons, neutrons, and
other strongly interacting particles. see YangMills Theories [G97]
(c) The quantum theory of the strong nuclear
force, which it envisions as being conveyed by quanta called gluons.
The name derives from the assignment of a quantum number called color
to designate how quarks function in response to the strong
force. [F88]
(d) Relativistic quantum field theory of the
strong force and quarks, incorporating special relativity. [G99]
(e) The modern theory of the strong forces
between quarks, and hence of the forces between hadrons. It is a
generalisation of quantum electrodynamics, with color charge replacing
electric charge and gluons replacing photons. [D89]

Quantum Cosmology
 (a) The study of the Planck era. [c97]
(b) The subfield of cosmology that deals with the
niverse during its first 10^{43} seconds, when quantum mechanical
effects and gravity were both extremely important. [LB90]

Quantum Defect
 The principal quantum number responsible for a
spectral series,
minus the Rydberg denominator for any actual spectral term of the
series. (also
called Rydberg correction) [H76]

Quantum Determinism
 Property of quantum mechanics that knowledge of the
quantum state of a system at one moment completely determines its
quantum state at future and past moments. Knowledge of the quantum
state, however, determines only the probability that one or another
future will actually ensue. [G99]

Quantum Efficiency
 QE (a) The efficiency of a counter in
detecting photons;
the probability that a photon will liberate an electron and thus
be detected. [H76]
(b) The ratio of the number of photoelectrons
released for each incident photon of light absorbed by a detector. This
ratio cannot exceed unity. [McL97]
(c) The fraction of the incoming photon stream that is converted into
signal. [R04]

Quantum Efficiency Hysteresis
 QEH An increase in QE (quantum
efficiency) after exposure to light. [McL97]

Quantum Electrodynamics
 QED
(a)The quantum field theory describing the
interactions between
electrically charged particles through the electromagnetic field
(whose quantum is the photon). QED is a gauge theory with the Abelian
gauge symmetry group U(1). [CD99]
(b) The theory of photons and electrons
(or other electrically charged particles) and their interactions. It
is called `quantum' when the electromagnetic radiation (i.e. light
etc.) is being treated by quantum theory, so that its discrete photon
nature is important. [D89]
(c) The quantum theory of the electromagnetic
force, which it envisions as being carried by quanta called
photons. [F88]
(d) This is the accepted theory
of electromagnetic interactions, including all the effects of
relativity and quantum theory. The photon acts as the carrier of the
electromagnetic force. [G97]
(e) Relativistic quantum field theory of the
electromagnetic force and electrons, incorporating special
relativity. [G99]

Quantum Electronics
 this is the name used for those parts of quantum
optics which have practical device applications.[D89]

Quantum Field
 A distribution of energy that is constantly
creating and
destroying particles, according to the probabilities of quantum
mechanics, and transmitting the forces of nature. see Field Theory;
Quantum Mechanics [LB90]

Quantum Field Theory
 (a) The relativistically invariant version of quantum
mechanics. [H76]
(b) The theory used to describe the physics of
elementary particles.
According to this theory, quantum fields are the ultimate reality and
particles are merely the localized quanta of these fields. [CD99]
(c) The theory that describes the quantum effects
of a classical system of fields defined on spacetime and satisfying
various partial differential equations. [D89]
(d) When interactions among particles are described as transmitted
via the exchange of bosons, the methods of quantum field theory are used.
[K2000]

Quantum Fluctuations
 (a) The spontaneous fluctuation of energy in a volume
of space. A
consequence of the Heisenberg uncertainty principle. [c97]
(b) Turbulent behavior of a system on microscopic
scales due to the uncertainty principle. [G99]
(c) Continuous variations in the properties of a
physical system, caused by the probabilistic character of nature as
dictated by quantum mechanics. For example, the number of photons in a
box with perfectly reflecting walls is constantly varying because of
quantum fluctuations. Quantum fluctuations can cause particles to
appear and disappear. Some theories hold that the entire Universe was
created out of nothing, in a quantum fluctuation. [LB90]

Quantum Genesis
 (a) Hypothesis that the origin of the Universe may be
understood in terms of a quantum chance. [F88]
(b) A quantum theory of gravity in which gravitons
transmit the force
between particles, rather than the curvature of the spacetime continuum,
as in the general theory of relativity. [c97]

Quantum Geometry
 Modification of Riemannian geometry required to
describe accurately the physics of space on ultramicroscopic scales,
where quantum effects become important. [G99]

Quantum Gravity
 (a) A theory of gravity that would properly include
quantum mechanics. To date, there is no complete and selfconsistent
theory of quantum gravity, although successful quantum theories have
been found for all the forces of nature except gravity. see Quantum
Mechanics [LB90]
(b) A general term used to describe attempts to quantize
gravity. The elementary particle of the gravitational field is the
graviton. [P88]
(c) A theory that successfully mergers quantum mechanics
and general relativity, possibly involving modifications of one or
both. String theory is an example of a theory of quantum gravity. [G99]

Quantum Hall Effect
 in a twodimensional electron system at
sufficiently low temperature and in sufficiently high magnetic field
the ratio of the current to the voltage applied in a direction
perpendicular to the current is very accurately a multiple (integer or
fraction with small odd denominator) of
e^{2}/, where e is
the electron charge and is
Planck's constant. [D89]

Quantum Leap
 The disappearance of a subatomic particle  e.g., an
electron  at one location and its simultaneous reappearance at
another. The counterintuitive weirdness of the concept results in
part from the limitations of the particle metaphor in describing a
phenomenon that is also in many respects a wave. [F88]

Quantum Liquid
 a system of particles which are both sufficiently
mobile and at sufficiently low temperature to display the effects of
quantummechanical indistinguishability. Examples include the
electrons in superconducting metals and the atoms in liquid helium. [D89]

Quantum Mechanical Amplitude
 A mathematical quantity in quantum mechanics
whose absolute square
determines the probability of a particular process occurring. [CD99]

Quantum Mechanics
 (a) The theory that explains the dual wavelike and
particlelike behavior of matter and the probabilistic character of
nature. According to quantum mechanics, it is impossible to have
complete and certain information about the state of a physical system,
just as a wave cannot be localized to a single point in space but
spreads out over many points. This uncertainty is an intrinsic aspect
of the system or particle, not a reflection of our inaccuracy of
measurement. Consequently, physical systems must be described in terms
of probabilities. For example, in a large collection of uranium
atoms, it is possible to accurately predict what fraction of the atoms
will radioactively disintegrate over the next hour, but it is
impossible to predict which atoms will do so. As another example, an
electron with a wellknown speed cannot be localized to a small region
of space but behaves as if it occupied many different places at the
same time. Any physical system, such as an atom, may be viewed as
existing as a combination of its possible states, each of which has a
certain probability. Quantum theory has been extremely successful at
explaining the behavior of nature at the subatomic level, although
many of its results violate our commonsense intuition. see
Copenhagen Interpretation of Quantum Mechanics; ManyWorlds
Interpretation of Quantum Mechanics; Uncertainty Principle; Wave
Function [LB90]
(b) The theory of the interaction of matter and
radiation, which rests on the original idea of Planck that radiating
bodies emit energy in discrete units or quanta of radiation whose
energy is proportional to the frequency of the light. [Silk90]
(c) Framework of laws governing the Universe whose
unfamiliar features such as uncertainty, quantum fluctuations, and
waveparticle duality become most apparent on the microscopic scales
of atoms and subnuclear particles. [G99]

Quantum Physics
 Physics based upon the quantum principle, that energy
is emitted not as a continuum but in discrete units. [F88]

Quantum Solid
 A degenerate gas in which the densities are so great
that the nuclei are fixed with respect to each other so that they
resemble a crystalline lattice. [H76]

Quantum Space
 Vacuum with the potential to produce virtual
particles. [F88]

Quantum Theory
 (a) A theory which seeks to explain that the action
of forces is a result of
the exchange of subatomic particles. [c97]
(b) The theory used to describe physical systems
which are very small, of
atomic dimensions or less. A feature of the theory is that certain
quantities (e.g. energy, angular momentum, light) can only exist in
certain discrete amounts, called quanta. [CD99]
(c) Initially, the theory developed by Planck that
radiating bodies
emit energy not in a continuous stream, but in discrete units called
quanta, the energy of which is directly proportional to the
frequency. Now, all aspects of quantum mechanics. [H76]
(d) The quantum theory provides the rules with which to calculate how
matter
behaves. Once scientists specify what system they want to describe and what
the interactions among the particles of the system are, then the
equations of the quantum theory are solved to learn the properties of
the system. [K2000]

Quantum Tunneling
 A quantum leap through a barrier. [F88]
(b) A process by which a quantum system can suddenly
and discontinuously make a transition from an initial configuration to
a final one, even if the system does not have enough energy to
classically attain the configurations between the two. [G97]
(c) Feature of quantum mechanics showing that objects can
pass through barriers that should be impenetrable according to Newton's
classical laws of physics. [G99]

Quantum Yield
 (In photochemistry, the number of molecules decomposed
per photon absorbed. [H76]

Quark
 (a) Fundamental particle of which protons, neutrons
and electrons
are now thought perhaps to be made up. There are possibly three or
four types of quark. It is even possible that quarks themselves may be
made up of still more fundamental particles. [A84]
(b) A subatomic particle which is a fundamental
building block of the hadrons. [c97]
(c) A, spin1/2 particle with fractional electric
charge (+2/3 or 1/3). Baryons
are composed of three (valence) quarks which are bound together by the
strong color forces, and mesons consist of a bound quark and
antiquark. Quarks come in six flavors (u, d, s, c, b, t) and three
colors (r, g, b). [CD99]
(d) 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). [G99]
(e) One of the fundamental, indestructible particles
of nature, out
of which many other subatomic particles are made. The neutron, for
example, is built of three quarks. Five types of quarks have been
discovered, and it is believed that a sixth also exists. Quarks
interact mainly via the strong nuclear force and the electromagnetic
force. [LB90]
(f) The hypothetical constituent of the elementary
particles that
interacts via glue forces. Originally only three quarks were
hypothesized; today it appears that six are required. For a variety of
theoretical reasons, free quarks can never be seen. [P88]
(g) Fundamental particles from which all hadrons are made.
According to the theory of quantum chromodynamics, protons, neutrons,
and their higherenergy cousins are composed of trios of quarks, while
the mesons are each made of one quark and one antiquark. Held together
by the strong nuclear force, quarks are not found in isolation in
nature today. see Asymptotic Freedom [F88]
(h) Fundamental particle of all hadrons. There are six
known varieties (or flavors') of quarks, and they combine in twos or threes
to make up particles such as protons, neutrons and mesons. [D89]
(i) A fundamental particle. Quarks are very much like electrons, but
they also carry strong charge and thus have another interaction, one
that can bind
them into protons and neutrons. There are six quarks, called up (u), down
(d), charmed (c), strange (s), top (t), and bottom (b).[K2000]

QuarkHadron Phase Transition
 A phase transition in the early Universe
when freely roaming quarks combined to form neutrons, protons, and
other strongly interacting particles called hadrons. see Neutron;
Phase Transition; Quark [LB90]

Quark Jet
 Because quarks must end up in hadrons, quarks that
are produced in collisions
actually appear in detectors as a narrow jet of hadrons, mostly pions.
see also Gluon Jet [K2000]

Quasar
 QSO
(a) The brightest objects in the Universe, quasars
can generate
over a trillion times as much light as the Sun from a region little
larger than the Solar System. Most are extremely distant, which means
that they existed long ago. [C95]
(b) An intensely bright extragalactic object which
superficially resembles a
star. Most exist at very high redshifts and are therefore thought to be
the nuclei of active galaxies. [c97]
(c) Compactlooking objects, often radio sources, with
emission lines in their spectrum which are displaced by very large
amounts towards the red. These redshifts correspond to velocities which
are a large fraction of the speed of light, and hence these objects are
believed to lie at great distances. [McL97]
(d) Extremely distant and luminous astronomical
objects that are
much smaller than a galaxy and much more luminous. Quasars may be the
central regions of certain very energetic galaxies at an early stage
of their evolution. It is believed that the power of a quasar derives
from a massive black hole at its center. [LB90]
An object with a
dominant starlike (i.e., diameter less than 1") component, with
an emission line spectrum showing a large redshift  up to z =
3.53 (0.91c) for OQ 172. (The largest redshift known for a normal
radio galaxy is z = 0.637 for 3C 123.) Many have multiple
absorption redshifts; a few have multiple emission redshifts.
(Bahcall system: class I, z_{abs} z_{em};
class II, z_{abs} significantly less
than z_{em}.) The light of most if not all quasars is
variable over time intervals between a few days and several years, so their
diameters must not be much larger than the diameter of the
Solar System; yet they are the intrinsically brightest objects known
(for 3C 273 (z = 0.158), M_{v} = 27.5 if its
redshift is cosmological). The energy output of a typical quasar at
"cosmological" distance is of the order of 10^{47} ergs per
second  which would require a mass of 10^{10} M_{} if it derives its
energy solely from nuclear fusion. (Energy requirement under the
"local" hypothesis is on the order of 10^{42} ergs per
second.) The basic problem of quasars is that they emit too much
radiation in too short a time from too small an area. [H76]
(e) An extreme form of active galactic nucleus in
which the
luminosity of the nucleus far exceeds the luminosity of the underlying
galaxy. As a result, these objects have a stellar appearance on
photographic plates. The first members of this class were discovered
through the optical identification of extragalactic radio sources and
hence the origin of the name `quasistellar radio source' abbreviated
to quasar. The quasars are the most luminous objects known in the
Universe. [D89]

QuasiStellar Radio Source
 QSRS A quasar with detectable radio
emission. [H76]

Quiet Sun
 The Sun when the 11year cycle of activity is at
a minimum. [H76]
