Warped Passages (71 page)

down quark
One of the elementary quarks that compose the proton and the neutron.

dual theories
Two equivalent descriptions of a single theory that might be superficially quite different.

effective field theory
A quantum field theory defined at a particular energy that describes those particles and forces relevant to the energies to which it applies.

effective theory
A theory describing those elements and forces that are in principle observable at the distance or energy scales over which it applies.

Einstein’s equations
The equations of general relativity with which you determine the
metric
(and hence the gravitational field) from the distribution of matter and energy.

electromagnetism
One of the four known forces; electromagnetism describes both electricity and magnetism.

electron
A very light elementary particle with a negative charge.

electroweak theory
The theory incorporating both electromagnetism
and the weak force; an essential component of the Standard Model of particle physics.

equivalence principle
The principle that uniform acceleration and gravity are indistinguishable.

eV (electronvolt)
The energy required to move an electron against a potential difference of 1 volt.

external particles
Real physical particles that can enter and leave an interaction region.

family
See
generation
.

Fermi interaction
An interaction that is generated by the exchange of one of the massive weak gauge bosons.

Fermilab
A collider facility in Illinois; home of the
Tevatron
.

fermion
A particle with half-integer spin—½, 3/2, etc. (one of two categories of particle established by quantum mechanics, the other being the
boson
); quarks and electrons are examples of fermions.

Feynman diagram
A diagram that schematically illustrates allowed particle-physics interactions.

field
A physical quantity that exists and has a particular value for each point in space. Examples include the classical electric field and quantum fields.

fine-tuning
Fudging by adjusting a parameter to a very specific (and unlikely) value.

flavor
A label that distinguishes different types of quark or lepton (often used to distinguish quarks and leptons from different generations).

flavor problem (of supersymmetry)
The overly high prediction for flavor-changing processes (due to virtual squarks and sleptons) that plagues most models of supersymmetry breaking.

flavor symmetry
Symmetry that interchanges different flavors of a particular particle category.

frame of reference
An observational vantage point or a set of coordinates for describing events in space or spacetime.

gauge boson
A particle that communicates an elementary force.

gaugino
The superpartner of a force-carrying gauge boson.

gaugino mediation
Communication of supersymmetry breaking by gauginos.

general relativity
The theory of gravity that describes the gravitational field due to any source of matter and energy, including that stored by the gravitational field itself, in any frame of reference; general relativity encapsulates the gravitational field in the curvature of spacetime.

generation
Each of the three sets of the full complement of particle types (left-and right-handed charged lepton, up-type quark, down-type quark and left-handed neutrino).

geodesic
In space, the shortest path between two points; in spacetime, the path a free-falling observer (one with no forces acting on him) would follow.

GeV (gigaelectronvolt)
A unit of energy equal to one billion eV.

gluon
The elementary particle that communicates the strong force.

Grand Unified Theory (GUT)
A proposed theory in which the three known nongravitational forces fuse into a single force at high energy.

gravitational lensing
The splitting of light into multiple images as it bends around a massive object.

gravitino
The superpartner of the graviton.

graviton
The particle that communicates the force of gravity.

hadron
A strongly bound object with constituent quarks and/or gluons.

handedness
The direction of spin, to the left or to the right.

heterotic string theory
A version of string theory in which the oscillation modes that travel clockwise are different from the modes that travel counterclockwise.

hierarchy problem
The question of the weakness of gravity or, equivalently, of why the Planck scale mass that characterizes gravity’s strength is sixteen orders of magnitude greater than the weak scale mass associated with the weak force.

Higgs field
The field that participates in the Higgs mechanism, that is responsible for breaking the symmetry associated with the electroweak force.

Higgs mechanism
The spontaneous breaking of the electroweak symmetry that allows gauge bosons and other elementary particles to acquire mass.

Hořava-Witten theory
The strongly coupled heterotic string version of string theory, or equivalently (by duality) a version of string theory with two branes that are separated by an eleventh dimension in which the two branes house the forces of the heterotic string.

horizon
A region beyond which nothing can escape.

hypercube
A generalization of a cube to more than three dimensions.

inertial frame of reference
A reference frame that moves at fixed velocity with respect to a fixed reference frame, such as the one at rest.

intermediate (internal) particles
Virtual particles whose exchange mediates interactions among other particles.

internal symmetry
A symmetry in which physical laws do not change for a set of transformations that does not change the geometric position of particles, but only some internal properties or labels.

intrinsic spin (spin)
A number that characterizes how a particle behaves—as if it were spinning. Spin can take integer or half-integer values.

inverse square law
The rule that describes those forces whose strength decreases with separation as the square of the distance between them; classical gravitational and electric forces obey inverse square laws.

ion
A charged bound state of a nucleus and electrons; an atom with too few or too many electrons.

jet
An energetic cluster of strongly interacting particles surrounding an energetic quark or gluon that moves in a particular direction.

Kaluza-Klein (KK) mode
A four-dimensional particles with a higher-dimensional origin; KK modes are distinguished by their extra-dimensional momenta.

kinetic energy
Energy due to motion.

lepton
An elementary fermionic particle that does not experience the strong force.

LHC (Large Hadron Collider)
A high-energy particle collider that will bang together 7 TeV proton beams and produce particles with mass up to a few TeV.

local interaction
An interaction between adjacent or coincident objects.

localized gravity
A high concentration of the gravitational field in a
particular region of space; gravity appears to be lower-dimensional since it isn’t diluted into an extra dimension.

locally localized gravity
A theory in which four-dimensional gravity is not experienced everywhere, but only in the region of space where the probability function of the particle acting like a four-dimensional graviton is concentrated.

longitudinal polarization
Wave oscillation along the direction of motion.

M-theory
A hypothesized all-embracing theory that unifies all known versions of ten-dimensional string theory and eleven-dimensional supergravity.

matrix theory
A ten-dimensional quantum-mechanical theory that might be equivalent to string theory.

mediate
To communicate a particle’s influence (by an intermediate particle).

metric
A quantity or quantities that establish the measurement scale that determines physical distances and angles.

model
A candidate theory.

molecule
A bound state of two or more atoms in which electrons are shared between them.

multiverse
A hypothetical generalization of a universe containing regions that don’t interact or interact only extremely weakly.

muon
A short-lived, heavier version of the electron.

neutral object
An object that is immune to a force; neutral objects have net charge equal to zero.

neutrino
A fundamental elementary particle that interacts only via the weak force.

neutron
An ingredient of the atomic nucleus in which two down quarks and an up quark are tightly bound to each other.

Newton’s gravitational constant
The overall coefficient that determines the strength of gravitational attraction in Newton’s law of gravity; it is inversely proportional to the square of the Planck scale mass.

Newton’s gravitational force law
The classical law of gravity that says that the strength of gravity between two massive objects is proportional to their masses and inversely proportional to the square of their separation.

nucleon
A proton or neutron.

nucleus
The hard, dense central component of an atom.

old quantum theory
The predecessor to quantum mechanics that postulated quantization rules but didn’t systematically determine them or describe the evolution of a quantum state through time.

open string
A
string
with two ends.

p
-brane
A solution to Einstein’s equations that expands infinitely far in some spatial directions, but in the remaining dimensions acts as a black hole, trapping objects that come too close.

particle accelerator
A high-energy physics facility that accelerates particles to high energy.

particle collider
A high-energy accelerator that smashes together particles to create enormous amounts of energy.

particle physics
The study of the most elementary building blocks of matter.

Pauli exclusion principle
The statement that two identical fermions cannot occupy the same position.

perturbation
A small modification to a known theory.

perturbation theory
When the theory you are interested in is distinguished from a solvable (usually non-interacting) theory by a small parameter (which could be a small interaction strength, for example), perturbation theory allows you to extrapolate from the solvable theory to the theory of interest through a systematic expansion in that small parameter. The results are expressed as power expansions in the corresponding parameter, usually the coupling constant.

photino
The superpartner of the photon.

photon
The elementary particle that communicates the electromagnetic force; the quantum of light.

Planck scale energy
The energy at which gravity becomes a strong force and quantum-mechanical contributions need to be taken into account.

Planck scale length
The length scale at which gravity is strong and quantum effects must be included in gravitational predictions.

Planck’s constant
A quantum-mechanical quantity that relates energy to frequency and momentum to wavelength.

polarization
The direction of oscillation of a wave.

positron
The positively charged antiparticle of the electron.

potential energy
Stored energy that can be released as kinetic energy.

probability function
The square of the absolute value of the wavefunction that determines the probability of finding a particle at a given location.