Coming of Age in the Milky Way

by Timothy Ferris

  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.”

  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 × 1012 (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 indica
tes 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.

  Materialism. Belief that material objects and their interactions constitute the complete reality of all phenomena, including such seemingly insubstantial phenomena as thoughts and dreams. Compare spiritualism.

  Matter waves. Characteristic by virtue of which matter, like energy, displays the qualities of waves as well as of particles. See wave-particle duality.

  Mechanics. The study, in physics, of the influence of forces.

  Megaparsec. One million (106) parsecs.