Coming of Age in the Milky Way (61 page)

Critical density.
The cosmic density of matter required to “close” the universe and so eventually to halt cosmic expansion. Its value amounts to about ten hydrogen atoms per cubic meter of space. The observed density is so close to the critical value that the question of whether the universe is open or closed has not yet been resolved by observation. See
open universe, closed universe
.

Dark matter.
Matter whose existence is inferred on the basis of dynamical studies—e.g., the orbits of stars in galaxies—but which does not show up as bright objects such as stars and nebulae. Its composition is unknown: It might consist of subatomic particles, or of dim dwarf stars or black holes, or a combination of various sorts of objects.

Darwinism.
Theory that species arise through the
natural selection
of random mutations that better fit changing conditions in a generally
uniformitarian
Earth.

Dead reckoning.
Navigation by recording one’s heading, velocity, and elapsed time, with little or no reference to the stars.

Deceleration parameter.
Quantity designating the rate at which the
expansion of the universe
is slowing down, owing to the braking effect of the galaxies’ gravitational tug on one another. It is a function of the
cosmic matter density
.

Declination.
Location on the sky in a north-south direction. Lines of declination are the celestial equivalent of latitude on Earth. Compare
right ascension
.

Decoupling.
Separation of classes of particles from regular interaction with one another, as in the decoupling of photons from particles of matter that produced the
cosmic background radiation
.

Deduction.
Process of reasoning in which a conclusion is derived from a given premise or premises, without a need for additional information. Compare
induction
.

Degree.
(1) A measure of temperature: Unless otherwise specified, all temperatures in this book are in degrees Kelvin. (To convert to Celsius, subtract 273.) (2) An angle subtended in the sky: From the zenith to the horizon is 90 degrees; the distance between the pointer stars of the Big Dipper is 5 degrees.

Detector.
Device for recording the presence of
subatomic particles
. A typical modern detector consists of an array of electronic sensors connected to a computer, capable of recording the paths of the particles as they fly out from the collision site in a particle
accelerator
.

Determinism.
The doctrine that all events are the predictable effects of prior causes. See
causation
.

Deuterium.
An isotope of hydrogen, the nucleus of which comprises one neutron plus one proton.

Dimension.
A geometrical axis.

Dirac equation.
Mathematical description of the electron, derived by Paul Dirac, that incorporates both
quantum mechanics
and
special relativity
.

DNA.
Deoxyribonucleic acid, the macromolecule that carries the genetic information requisite to life on Earth.

Doppler shift.
Change in the apparent wavelength of radiation (e.g., light or sound) emitted by a moving body. A star moving away from the observer will appear to be radiating light at a lower frequency than if at rest; consequently,
lines in the star’s spectrum will be shifted toward the red (lower frequency) end of the spectrum. The existence of a direct relationship between the
redshift
of light from galaxies and their distances is the fundamental evidence for the
expansion of the universe
. Double star. See
binary star
.

Dwarf stars.
Main-sequence stars with masses equal to or less than that of the sun. More generally, any star on or below the
main sequence
in the
Hertzsprung-Russell diagram
.

Dynamics.
Study, in physics, of the motion and equilibrium of systems under the influence of
force
.

Dynamo.
An electric generator that employs a spinning magnetic field to produce electricity.

Eccentrics.
In Ptolemaic cosmology, displacement of the center of a rotating celestial sphere from the center of the universe.

Eclipse.
Obscuration of one astronomical object (such as the sun) by another such object (such as the moon).

Electrodynamics.
Study of the behavior of
electromagnetic force
in motion.

Electromagnetic force (or interaction).
Fundamental force of nature that acts on all electrically charged
particles
. Classical electromagnetics is based on Maxwell’s and Faraday’s equations, quantum electromagnetics on the theory of quantum electrodynamics
(QED)
.

Electrons.
Light elementary particles with a negative electrical charge. Electrons are found in shells surrounding the nuclei of
atoms;
their interactions with the electrons of neighboring atoms create the chemical bonds that link atoms together as
molecules
.

Electron shells.
Zones in which the electrons in
atoms
reside. Their radius is determined by the quantum principle, their population by the
exclusion principle
.

Electronuclear force.
Single fundamental force thought to have functioned in the very early universe and to have combined the attributes thereafter parceled out to the
electromagnetic
and the
strong
and
weak nuclear
forces.
See grand unified theory
.

Electron volt.
Measure of energy, equal to 1.6 × 10
⁻¹²
erg.

Electroweak theory.
Theory demonstrating links between the
electromagnetic
and the
weak nuclear
forces. Indicates that in the high energies that characterized the very early universe, electromagnetism and the weak force functioned as a single, electroweak force. Also known as the Weinberg-Salam theory. Ellipse. A plane curve in which the sum of the distances of each point along its periphery from two points—its “foci”—are equal. Emission lines. Bright lines produced in a
spectrum
by a luminous source, such as a star or a bright nebula. Compare
absorption lines
. Empiricism. An emphasis on sense data as a source of knowledge, in opposition to the rationalist belief that reasoning is superior to experience. Energy. (1) The capacity to do work. (2) Manifestation of a particular variety of
force
.

Epicycles.
In Ptolemaic cosmology, a circular orbit around a point that itself orbits another point. Escape velocity. The speed at which an object can leave another object behind,
without being recalled by its gravitational force. The escape velocity of Earth —which must, for instance, be attained by a spacecraft if it is to reach another planet—is 25,000 miles per hour. Euclidean geometry. See
geometry
.

Evolution.
(1) In biology, the theory that complex and multifarious living things developed from generally simpler and less various organisms. (2) In astronomy, the theory that more complex and varied atoms develop from simpler ones, as through the synthesis of heavy atomic nuclei in stars.

Exclusion principle.
The rule that no two
fermions
can occupy the same quantum state.

Expansion of universe.
Constant increase, with time, in the distance separating distant galaxies from one another. Expansion does not take place within individual galaxies or clusters of galaxies, which are bound together gravitationally, but evidences itself on the supercluster level.

Fermilab.
The Fermi National Accelerator Laboratory, in Batavia, Illinois.

Fermions.
Particles with half-integral spin. Fermions obey the
exclusion principle
, which says that no two fermions can exist in an atom in the same quantum state; in practice this restricts the number of electrons, which are fermions, permitted in each electron shell.

Fermi’s question.
The question of why, if spacefaring extraterrestrial civilizations exist, their representatives haven’t visited Earth.

Feynman diagram.
Schematic representation of an
interaction
between
particles
.

Field.
Domain or environment in which the real or potential action of a
force
can be described mathematically at each point in space.

Fission, nuclear.
Interaction in which
nucleons
previously united in an atomic nuclei are disjoined, releasing energy. Fission powers “atomic” bombs. Compare
fusion
.

Flatness problem.
The riddle of why the universe is neither dramatically
open
nor
closed
, but appears to be almost perfectly balanced between these states. Flavor. Designation of quark types—up, down, strange, charmed, top, and bottom. Flavor determines how the
weak nuclear force
influences
quarks
. Force. Agency responsible for a change in a system. In Newtonian mechanics, gravitational force bends the moon away from the straight trajectory it would otherwise pursue. Fossils. Geological remains of what was once a living thing. Fraunhofer lines. Dark lines in a
spectrum
.

Fusion, nuclear.
Interaction in which
nucleons
are forged together, creating new atomic nuclei and releasing energy. Fusion powers “hydrogen” bombs.

Galactic disk.
The plate-shaped component of a spiral
galaxy
, in which the spiral arms are found.

Galactic halo.
A spherical aggregation of stars, globular star clusters, and thin gas clouds, centered on the nucleus of the galaxy and extending beyond the known extremities of the galactic disk.

Galaxy.
A large aggregation of stars, bound together gravitationally. There are three major classifications of galaxies—spiral, elliptical, and irregular—and several subclassifications. The sun belongs to a spiral galaxy, the
Milky Way galaxy
.

Gamma rays.
Extremely short-wavelength electromagnetic energy. Gauge theory. Account of forces that views them as arising from broken
symmetries
.

Geocentric cosmology.
School of ancient theories that depicted the earth as standing, immobile, at the center of the universe.

Geology.
Scientific study of the dynamics and history of the earth, as evidenced in its rocks, chemicals, and fossils.

Geometry.
The mathematics of lines drawn through space. In euclidean geometry, space is postulated to be “flat,” i.e., to be the three-dimensional analog of a plane. In noneuclidean geometry, space is “curved,” i.e., is the three-dimensional analog of a sphere or a hyperbola.

GeV.
One billion (10’)
electron volts
. Sometimes written as one BeV.

Giant stars.
High-luminosity stars that lie above the
main sequence
on the
Hertz-sprung-Russell diagram
.

Globular clusters.
See
star clusters
.

Glueballs.
Theoretical particles made exclusively of
gluons
. Tentative evidence of the existence of glueballs had been found in accelerator experiments by the mid-1980s.

Gluon lattice.
Force field generated by the
strong nuclear force
that holds quarks together.
See gluons
.

Gluons.
Quanta that carry the
strong nuclear force
. Like photons, vector bosons, and gravitons—the carriers respectively of electromagnetism, the weak force, and gravitation—gluons are massless bosons. Consequently, for simplicity’s sake, some physicists lump together all the force-carrying quanta under the term “gluons.”