Coming of Age in the Milky Way (60 page)

Astrolabe.
Sighting instrument employed since antiquity to determine the elevation above the horizon of celestial objects. Eventually replaced by the
sextant
.

Astrology.
The belief that human affairs and people’s personalities and characters are influenced by (or encoded in) the positions of the planets.

Astronomical unit.
The mean distance from the earth to the sun, equal to 92.81 million miles or 499.012 light-seconds.

Astronomy.
The science that studies the natural world beyond the earth.

Astrophysics.
The science that studies the physics and chemistry of extraterrestrial objects. The alliance of physics and astronomy, which began with the advent of
spectroscopy
, made it possible to investigate
what
celestial objects are and not just
where
they are.

Asymmetry.
A violation of
symmetry
.

Asymptotic freedom.
Orwellian liberty enjoyed by
quarks
, which move freely when close together but are reined in by an increasingly powerful
strong nuclear force
whenever they begin to drift apart.

Atoms.
The fundamental units of a chemical element. An atom consists of a
nucleus
, which may contain
protons
and
neutrons
, and
electrons
, which occupy shells that surround the nucleus and are centered on it.

Avoidance.
The fact that
galaxies
appear to “avoid” the
Milky Way
, and are most numerous in other parts of the sky. When galaxies were known as spiral
nebulae
and their nature was not yet understood, avoidance was thought by some researchers to indicate a connection between them and the Milky Way. Now the effect is understood to be due to dark clouds of dust and gas in our galaxy, which obscure our view of the universe beyond in those quarters of the sky.

Background radiation
. See
cosmic background radiation
.

Baryon number.
The total number of
baryons
in the universe, minus the total number of antibaryons. An index, therefore, of the cosmic matter-antimatter
asymmetry
.

Baryons.
Massive elementary particles with half-integral spin that experience the strong nuclear force. Protons and neutrons are baryons. See
hadrons
.

GeV. One billion (10’)
electron volts
. See
GeV
.

Big bang theory.
Model of cosmic history in which the universe begins in a state of high density and temperature, both of which decrease as the universe expands. Less a theory than a school of theories that attempt to trace how the universe evolved.

Billion.
This book employs the American billion, equal to one thousand million or 10
⁹
.

Binary star.
A double star system, in which the two stars are bound together by their mutual
gravitational force
.s

Biology.
The scientific study of life and living matter.

Black-body curve.
Plot of energy level against wavelength for heat or other radiation emitted by an object capable of absorbing all the energy that strikes it. The curve has a pronounced hump that moves toward shorter wavelengths as the temperature increases. The
cosmic background radiation
, thought to consist of photons emitted during the
big bang
, conforms to a black-body curve.

Black holes.
Objects with a gravitational field so intense that their
escape velocity
exceeds that of light. No macroscopic object inside the black hole, therefore, can escape it. In terms of general relativity, space surrounding a black hole is said to reach infinite curvature, making it a
singularity
.

Bosons.
Elementary particles with integer spin that do not obey the
Pauli exclusion principle
. They include the
photons
and the
W and Z particles
, carriers of the
electromagnetic
and the
electroweak
forces respectively.

Boundary condition.
Restriction on the limits of applicability of an equation. Examples include the definition of a “closed system” in thermodynamics, and the theater within which one collapses the
wave function
in quantum mechanics. Every equation in physics may in principle be reduced to two fundamentals—the
initial conditions
and the boundary conditions.

Broken symmetry.
In cosmology and particle physics, a state in which traces
of an earlier
symmetry
may be discerned. A broken symmetry condition differs from chaos in that its parts, can in theory be united in a symmetrical whole, like the pieces of a jigsaw puzzle.

Caltech.
The California Institute of Technology, in Pasadena.

Carbon reaction.
An important
nuclear fusion
process that occurs in stars. Carbon-12 both initiates it and, following interactions with nuclei of nitrogen, hydrogen, oxygen, and other elements, reappears at its conclusion.

Catastrophism.
Nineteenth-century hypothesis that depicted the many changes evinced by the geological record as having resulted from cataclysms occurring during a relatively brief period of history. Compare
uniformitarianism
.

Causation, causality.
The doctrine that every new situation must have resulted from a previous state. Causation underlay the original atomic hypothesis of the Greeks, and was popular in
classical physics
. It is eroded in
quantum mechanics
and has, in any case, never been proved essential to the scientific world view. See
chance
, determinism.

Centauras A.
Giant elliptical galaxy, located between the
Local Group
and the center of the
Virgo Supercluster
.

Cepheid variable.
A pulsating
variable star
whose periodicity—i.e., the time it takes to vary in brightness—is directly related to its absolute
magnitude
. This correlation between brightness and period makes Cepheids useful in measuring intergalactic distances. CERN. The Center for European Nuclear Research, located outside Geneva, Switzerland.

Chance.
Characteristic of a regime in which predictions cannot be made exactly, but only in terms of probabilities. In
classical physics
, chance was thought to pertain only where ignorance limited our understanding of an underlying mechanism of strict
causation
. But in the Copenhagen interpretation of
quantum mechanics
, chance is portrayed as inherent to all observations of nature.

Charm.
The fourth
flavor
of quarks. Predicted by theory, charmed quarks were discovered in 1974.

Chromatic aberration.
Introduction of spurious colors by a lens. This defect flawed the performance of refracting
telescopes
for centuries, until attenuated by the introduction of corrective elements into a compound lens.

Chronometer.
A highly accurate timepiece.

Circle.
An
ellipse
possessing but one focus.

Classical physics.
Physics prior to the introduction of the quantum principle. Classical physics incorporates Newtonian mechanics, views energy as a continuum, and is strictly causal.

Closed universe.
Cosmological model in which the universe eventually stops expanding and begins to collapse, presumably to end in a fireball like that of the big bang. Compare
open universe
.

Cloud chamber.
A glass-walled enclosure containing a vapor in which
particles
can be detected by photographing the tracks of water droplets they leave behind when they pass through the chamber.

Collider.
See
accelerator
.

Color.
Property of
quarks
that expresses their behavior under the
strong force
. Analogous to the concept of charge in electromagnetism, except that, whereas
there are two electrical charges (plus and minus), the strong force involves three color charges—red, green, and blue. The term is whimsical, and has nothing to do with color in the conventional sense, any more than quark
“flavor,”
which determines the weak force behavior of quarks, has anything to do with taste.

Comets.
Minor members of the solar system, thought to be lumps of dirt and ice left over from the formation of the solar system. Millions of comets are believed to reside in the
Oort Cloud
, a spherical region with a radius of some thirty to one hundred thousand
astronomical units
centered on the sun. Comets falling in from the Oort Cloud are heated by the sun and grow glowing tails, which can make them conspicuous in the skies of Earth.

Confinement.
The inability of
quarks
to escape the bonds that hold them in pairs and triplets at the energy levels found in the universe today. See
gluon lattice, asymptotic freedom
.

Conservation laws.
Laws that identify a quantity, such as
energy
, that remains unchanged throughout a transformation. All conservation laws are thought to involve
symmetries
.

Copernicanism.
Broadly, the hypothesis that the earth and the other planets orbit the sun.

Cosmic background radiation.
Microwave radio emission coming from all directions and corresponding to a
black-body curve;
its properties coincide with those predicted by the
big bang theory
as having been generated by
photons
released from the big bang when the universe was less than one million years old. The big bang theory suggests the existence of
neutrino
and
gravitational
background radiations as well, though the means to detect such do not yet exist.

Cosmic matter density.
The average number of
fermions
per unit volume of space throughout the universe. Since matter is depicted in general relativity as bending space, the value of the cosmic matter density, if known, could reveal the overall curvature of cosmic space. See
critical density
, omega.

Cosmic rays.
Subatomic particles, primarily
protons
, that speed through space and strike the earth. The fact that they are massive, combined with their high velocities, means that they pack considerable energy—from 10
⁸
to more than 10
²²
electron volts
.

Cosmogony.
The study of the origin of the universe.

Cosmology.
(1) The science concerned with discerning the structure and composition of the universe as a whole. Combines astronomy, astrophysics, particle physics, and a variety of mathematical approaches including geometry and topology. (2) A particular cosmological theory.

Cosmological constant.
A term sometimes employed in cosmology to express a force of “cosmic repulsion,” such as the energy released by the false vacuum thought to power exponential expansion of the universe in the
inflationary universe
models. Whether any such thing as cosmic repulsion exists or ever played a role in cosmic history remains an open question.

Coulomb barrier.
Electromagnetic zone of resistance surrounding protons (or other electrically charged particles) that tends to repel other protons (or other particles of like charge).

Creationism.
Belief that the universe was created by God in the relatively recent past, as implied by literal interpretations of biblical chronology, and that the
species of terrestrial life did not arise through Darwinian evolution but, rather, all came into existence at once.