Coming of Age in the Milky Way (62 page)

Grand unified theories.
Class of theories that purport to reveal identities linking the
strong
and
electroweak
forces. The differences between these forces in nature today is attributed to the breaking of symmetrical relationships among force-carrying particles as the very early universe expanded and cooled.

Gravitational force (or interaction).
Fundamental force of nature, generated by all particles that possess
mass
. Interpreted by means of Newtonian mechanics or by the general theory of
relativity
.

Gravitinos.
Hypothetical force-carrying particles predicted by
supersymmetry
theories. The gravitino’s spin would be
Vi
. Its mass is unknown.

Gravitons.
The quanta thought to convey
gravitational force;
analogous to the photons, gluons, and intermediate vector bosons of electromagnetism and the strong and weak nuclear forces. Predicted by quantum theory of gravity, gravitons have not yet been detected.

Gravity.
(1) In Aristotelian physics, an innate tendency of the elements earth and water to fall. (2) In Newtonian physics, the universal, mutual, attraction of all massive objects for one another; its force is directly proportional to the mass of each object, and decreases by the square of the distance separating the objects involved. (3) In Einstein’s general
relativity
, gravity is viewed as a consequence of the curvature of space induced by the presence of a massive object. In quantum mechanics the gravitational field is said to be conveyed by quanta called
gravitons
.

Great year.
Ancient concept of a celestial and historical cycle, its duration roughly a thousand or ten thousand years, at the end of which there is universal destruction and a new great year begins.

GUT.
Acronym for
grand unified theory
.

Hadrons.
Elementary particles that are influenced by the
strong nuclear force. There are two sorts of hadrons—mesons, which have integral spin, and baryons, which have spin ½ or
³
/2.

Half-life.
The time it takes for half of a given quantity of radioactive material to decay. Halo, galactic. See
galactic halo
.

Heliocentric cosmology.
School of models in which the sun was portrayed as standing at the center of the universe.

Hermetic.
Of or relating to Hermes Trismegistus, a mythical philosopher beloved of the Neoplatonists and usually identified with ancient Egypt.

Hertz.
A unit of frequency equal to one cycle (or wave) per second.

Hertzsprung-Russell diagram.
Plot that reveals a relationship between the colors and absolute
magnitudes
of stars.

Higgs field.
Mechanism operating in symmetry-breaking events; in
electroweak theory
, the Higgs field is said to have imparted mass to the
W and Z particles
.

High-energy physics.
See
particle physics
.

Horizon problem.
A quandary in standard
big bang theory
, which indicates that few of the particles of the early universe would have had time to be in causal contact with one another at the outset of cosmic expansion. It appears to have been resolved in the
inflationary universe theory
.

Hubble constant.
The rate at which the universe expands, equal to approximately fifty kilometers of velocity per
megaparsec
of distance.

Hubble diagram.
Plot of galaxy
redshifts
against their distances. This was the first evidence of the
expansion of the universe
.

Hubble law.
That distant galaxies are found to be receding from one another at velocities directly correlated to their distances apart.

Hyperbolic space.
See geometry
.

Hyperdimensional.
Involving more than the customary four dimensions (three of space plus one of time) of relativistic space-time.

Hypothesis.
A scientific proposition that purports to explain a given set of phenomena; less comprehensive and less well established than a
theory
.

Indeterminacy principle.
Quantum precept indicating that the position and trajectory of a particle cannot both be known with perfect exactitude. Indeterminacy thus indicates the existence of a basic quantum of knowledge of the particle world. And, since information about one quantity can be extracted at the expense of another, it demonstrates that the answers we obtain about natural events result to some extent from the questions we choose to ask about them.

Induction.
System of reasoning in which the conclusion, though implied by the premises and consistent with them, does not necessarily follow from them.

Inertia.
Quality of
mass
, such that any massive particle tends to remain at rest relative to a given reference frame, and to remain in constant motion once in motion, unless acted upon by a
force
.

Inflationary universe.
Theory that the expansion of the very early universe proceeded much more rapidly than it does today—at an exponential rather than a linear rate.

Infrared light.
Electromagnetic radiation of a slightly longer wavelength than that of visible light.

Infrared slavery.
Inability of
quarks
to escape the bonds of the strong force that confines them to the company of other quarks.

Initial condition.
(1) In physics, the state of a system at the time at which a given interaction begins—e.g., the approach of two electrons that are about to undergo an
electromagnetic
interaction. (2) In cosmology, a quantity inserted as a given in cosmogonic equations describing the early universe.

Intelligence.
Defined in
SETI
as the ability and willingness to transmit electromagnetic signals across interstellar space.

Interaction.
Event involving an exchange between two or more
particles
. Since the fundamental forces are portrayed by quantum theory as involving the exchange of force-carrying particles (the
bosons)
, the forces are more correctly described as interactions.

Interferometer.
A device for observing the interference of waves of light or similar emanations caused by a shift in the phase or wavelength of some of the waves.

Intermediate vector bosons.
See
W, Z particles
.

Inverse-square law.
In Newtonian mechanics, the rule that the measured intensity of light diminishes by the square of the distance of its source—so that, e.g., if stars A and B are of equal absolute
magnitude
but star B is twice as distant, it will appear to be one quarter as bright as star A.

Invisible astronomy.
The study of celestial objects by observing their radiation at wavelengths other than those of visible light.

Island universe hypothesis.
Assertion that the sun belongs to a
galaxy
and that the
spiral nebulae
are other galaxies of stars, which in turn are separated from one another by vast voids of space. Compare
nebular hypothesis
.

Isotopes.
Atoms having the same number of
protons
in their nuclei but different numbers of
neutrons
, with the result that their mass differs though they may have the same number of
electrons
.

Isotropy.
Quality of being the same in all directions. Compare
anisotropy
.

Jet Propulsion Laboratory.
NASA installation near Pasadena, operated by
Caltech
and specializing in unmanned space exploration.

Jovian.
Giant planets that have a gaseous surface; the sun’s known Jovian planets are Jupiter, Saturn, Uranus, and Neptune.

Kaluza-Klein theory.
Five-dimensional relativity theory that played a role in the development of
unified theory
.

Latitude.
On Earth, distance north or south of the equator along a line connecting the poles.

Lattice.
See gluon
.

Law.
A theory of such wide and invariable application that its violation is thought to be impossible.

Leptons.
Elementary
particles
that have no measurable size and are not influenced
by the
strong nuclear force
. Electrons, muons, and neutrinos are leptons. Light. Electromagnetic radiation with wavelengths of or close to those detectable by the eye.

Light-year.
The distance light travels in one year, equal to 5.8 × 10
¹²
(about six trillion) miles.

Local Group.
The association of galaxies to which the
Milky Way galaxy
belongs. Longitude. On Earth, distance east or west of Greenwich, England, measured along lines drawn parallel to the equator. Lookback time. Phenomenon that, owing to the finite velocity of light, the more distant an object being observed, the older is the information received from it. A galaxy one billion light-years away, for instance, is seen as it looked one billion years ago.

Lorentz contraction.
Diminution in the observed length of an object along the axis of its motion, as perceived by an external observer who does not share its velocity.

Luminosity.
The intrinsic brightness of a star. Usually defined in terms of absolute
magnitude
.

M.
Designation of objects in the Messier catalog of nebulae, star clusters, and galaxies, published in the eighteenth century.

Mach’s principle.
Precept that the inertia of objects results not from their relationship to Newtonian
absolute space
, but to the rest of the mass and energy distributed throughout the universe. Though unproved and perhaps unprovable, Mach’s principle inspired Einstein, who sought with partial success to incorporate it into the general theory of
relativity
.

Magellanic Clouds.
Two galaxies that lie close to the
Milky Way galaxy
. They are visible in the southern skies of Earth.

Magnetic monopole.
A massive particle with but one magnetic pole, the production of which is indicated in some theories of the early universe.

Magnitude.
The brightness of a star or planet, expressed on a scale in which lower numbers mean greater brightness.
Apparent magnitude
indicates the brightness of objects as we see them from Earth, regardless of their distance.
Absolute magnitude
is defined as the apparent magnitude a star would have if viewed from a distance of ten
parsecs
, or 32.6 light-years. Each step in magnitude equals a difference of 2.5 times in brightness: The brightest stars in the sky are apparent magnitude 1; the dimmest, 6. The magnitudes of extremely bright objects are expressed in negative values—e.g., the apparent magnitude of the sun is about •26.

Main sequence.
The curving path in the
Hertzsprung-Russell diagram
along which most stars lie.

Many body problem.
The difficulty of calculating the interactions—e.g., the Newtonian gravitational interactions—of three or more objects.

Mass.
Measure of the amount of matter in an object. Inertial mass indicates the object’s resistance to changes in its state of motion. Gravitational mass indicates its response to the
gravitational force
. In the general theory of
relativity
, gravitational and inertial mass are revealed to be aspects of the same quantity.