Star. A celestial object that generates energy by means of nuclear fusion at its core. To do this it must have more than about 0.08 the sun’s mass. If, for instance, the planet Jupiter were some fifty to one hundred times more massive than it is, fusion reactions would transpire in its core and it would be a star. See planet.
Star clusters. Gravitationally bound aggregations of stars, smaller and less massive than galaxies. “Globular” clusters are the largest category; they are old, and may harbor hundreds of thousands to millions of stars, and are found both within and well away from the galactic disk. “Open” clusters are smaller, have a wide range of ages, and reside within the disk. Statute mile. See mile.
Steady state. Theory that the expanding universe was never in a state of appreciably higher density—i.e., that there was no “big bang”—and that matter is constantly being created out of empty space in order to maintain the cosmic matter density.
Stellar evolution. The building of complex atomic nuclei from simpler nuclei in stars, with the result that succeeding generations of stars and planets contain a greater variety of chemical elements than did their predecessors. See evolution.
Stochastic cooling. The gathering (i.e., focusing) of clouds of subatomic particles in an accelerator by monitoring their scattering vectors and altering the magnetic environment in an accelerator storage ring to keep them close together. First employed in storing particles of antimatter, which are expensive to manufacture and ought not to be wasted.
Storage ring. A ring in which particles are kept in a circular motion, suspended in a magnetic field, until they can be injected into the larger ring of an accelerator.
String theory. Theory that subatomic particles actually have extension along one axis, and that their properties are determined by the arrangement and vibration of the strings.
Strong nuclear force (or interaction). Fundamental force of nature that binds quarks together, and holds nucleons (which are comprised of quarks) together as the nuclei of atoms. Portrayed in quantum chromodynamics as conveyed by quanta called gluons.
Subatomic. Of a scale smaller than that of an atom. Subatomic particles. See particles.
Sum over histories. Probabilistic interpretation of a system’s past, in which quantum indeterminacy is taken into account and the history is reconstructed in terms of each possible path and its relative likelihood.
Sun. The star orbited by the earth.
Supercluster. A cluster of clusters of galaxies. Superclusters are typically about one hundred million (108) light-years in diameter and contain tens of thousands of galaxies.
Superconducting super collider. A proposed accelerator of great size and high energy.
Supergiants. The largest and brightest class of stars. Supernovae. The explosions of giant stars. Superstring theory. Alternate name for string theory.
Supersymmetry. Class of theories that seek to identify symmetrical relationships linking fermions and bosons—i.e., particles of half-integral spin, like electrons, protons, and neutrinos, with those of integral spin, like photons and gluons. If attainable, a fully realized supersymmetry theory would provide a unified account of all four fundamental forces, and might well shed light on the very early evolution of the universe as well.
Superunified theory. Hypothetical theory that presumably would show how all four fundamental forces of nature functioned as a single force in the extremely early universe. The best current candidates for such a potential achievement are thought to be supersymmetry and string theory. Symmetry. State of a system such that it has a significant quantity that remains invariant after a transformation. More generally, an apt or pleasing proportion based upon such a state. Symmetry breaking. The loss of symmetry in a transformation. See broken symmetry.
Symmetry group. A mathematical group with a common property that unites its members and evinces a symmetry.
Telescope. A device for gathering and amplifying light or other energy. Refracting telescopes gather light by means of a lens, reflecting telescopes by means of a mirror. Radiotelescopes gather radio energy, typically by using a metallic dish antenna. Telescopes have also been built that can gather X rays, gamma rays, and other forms of energy.
TeV. Equal to one teraelectron volt, or 1,000 GeV.
Tevatron. A particle accelerator capable of attaining an energy of 1 TeV.
Theory. A rationally coherent account of a wider range of phenomena than is customarily accounted for by a hypothesis.
Thermodynamics. The study of the behavior of heat (and, by implication, other forms of energy) in changing systems.
Thought experiment. An experiment that cannot be or is not carried out in practice, but can—given sufficient imagination and rigor—be reasoned through by thought and intuition alone.
Time. A dimension distinguishing past, present, and future. In relativity, time is portrayed as a geometrical dimension, analogous to the dimensions of space.
Transit. The passage of a smaller, nearer astronomical object across the face of a larger object in the background, as in a transit of Venus across the sun.
Triangulation. Measurement of the distance of a planet or nearby star by sighting its apparent position against background stars from two or more separate locations. See parallax.
Trillion. A thousand billion (1012).
Ultraviolet light. Electromagnetic radiation of a wavelength slightly shorter than that of visible light.
Unified theory. In particle physics, any theory exposing relationships between seemingly disparate classes of particles. More generally, a theory that gathers a wide range of fundamentally different phenomena under a single precept, as in Maxwell’s discovery that light and magnetism are aspects of a single, electromagnetic force.
Uniformitarianism. The hypothesis that the extensive changes in the earth, as evinced in the geological record, have resulted, not from massive catastrophes, but from the slow operation of wind, weather, volcanism, and the like over many millions of years. Compare catastrophism.
Vacuum genesis. Hypothesis that the universe began as nothingness, from which matter and energy arose by a process analogous to the appearance of virtual particles from a vacuum.
Variable star. A star that changes in brightness periodically.
Virgo Cluster. A nearby cluster of galaxies.
Virgo Supercluster. An aggregation of galaxies—roughly ten thousand of them—to which the Virgo Cluster and our own galaxy belong. Virtual particles. Short-lived particles that arise from a vacuum. Their existence is permitted by the indeterminacy principle. Voyager. Pair of unmanned American spacecraft launched in 1977 on missions to Jupiter, Saturn, and beyond. Wave function. A quantum mechanical expression that describes all the relevant properties of a particle. Wave-particle duality. Quantum realization that particles of matter and energy also exhibit many of the characteristics of waves. Waves. Propagation of energy by means of coherent vibration. Weak nuclear force (or interaction). Fundamental force of nature that governs the process of radioactivity. It is currently accounted for by the electroweak theory. Weinberg-Salam theory. See electroweak theory.
World line. In relativity, the path traced out in four-dimensional space-time by a given object or particle. W particles. Massive bosons thought to have been abundant in the early universe, when the unified electroweak force was manifest. X rays. Short-wavelength electromagnetic energy. The X ray portion of the electromagnetic spectrum lies between the realms of gamma rays and that of ultraviolet light. Yang-Mills theory. See gauge theory.
Zoo hypothesis. Hypothesis that life on Earth has been detected by intelligent extraterrestrials who scruple not to visit us because they do not wish to interfere with our development.
Z particles. Massive bosons thought to have been abundant in the early universe, when the unified electroweak force was manifest.
A BRIEF HISTORY OF THE UNIVERSE
Time: 0
Noteworthy Events*: Origin of time, space, and energy—of the universe as we know it.
> Time: 10−41 second ABT†
Noteworthy Events: End of Planck epoch; gravitational radiation comes out of thermal equilibrium with the rest of universe.
Time: 10−34 second
Noteworthy Events: Universe, in vacuum state, begins “inflating”—i.e., expanding at an exponential rate, some 1050 times present expansion rate.
Time: 10−30 second
Noteworthy Events: Inflationary epoch ends; particles precipitate out of the vacuum.
Time: 10−11 second
Noteworthy Events: Symmetry-breaking phase transition shatters the electroweak force into the electromagnetic and weak nuclear forces.
Time: 10−6-10−5 second
Noteworthy Events: Quarks and antiquarks cease mutual annihilation. The survivors link up in trios as protons and neutrons, the components of all future atomic nuclei.
Time: 10−4 second
Noteworthy Events: Universe 1/10,000 second old. Constant capture of electrons and positrons turns neutrons into protons and vice versa. As slightly more energy is required to make neutrons than protons, the process leaves the universe with five times as many protons as neutrons.
Time: 102 second
Noteworthy Events: Particles of matter and energy interact in thermal equilibrium.
Time: 1 second
Noteworthy Events: Neutrinos, previously embroiled with other particles, decouple and go their own way.
Time: 3 minutes 42 seconds
Noteworthy Events: Protons and neutrons have linked up, forming nuclei of helium. Universe now composed of about 20 percent helium nuclei, 80 percent hydrogen.
Time: 1 hour
Noteworthy Events: Universe has cooled to the point that most nuclear processes have stopped.
Time: 1 year
Noteworthy Events: Ambient temperature of universe about that of the center of a star.
Time: < 106 years
Noteworthy Events: Origin of cosmic background radiation. Photons decouple, leaving electrons free to combine with nuclei, forming stable atoms. Hereafter, matter can begin to congeal into galaxies and stars.
Time: - 109 years ABT
(≈ 13 billion years BP*)
Noteworthy Events: Protogalaxies, globular clusters forming. Epoch of quasars begins.
Time: 4.5 billion years BP
Noteworthy Events: Sun and planets congeal from a cloud of gas and dust in a spiral arm of the Milky Way galaxy.
Time: 3.8 billion years BP
Noteworthy Events: Earth has cooled sufficiently for solid crust to form; age of oldest dated terrestrial rocks.
Time: 3.5–3.2 billion years
Noteworthy Events: Microscopic living cells evolve on Earth.
Time: 1.8–1.3 billion years
Noteworthy Events: Plants appear. Oxygen poisons Earth’s atmosphere, and aerobic (“oxygen-loving”) organisms proliferate.
Time: 900–700 million years
Noteworthy Events: Advent of sex accelerates the pace of biological evolution.
Time: 700 million years
Noteworthy Events: Animals—mostly flatworms and jellyfish—appear.
Time: 600 million years
Noteworthy Events: First crustaceans.
Time: 500 million years
Noteworthy Events: First vertebrates.
Time: 425 million years
Noteworthy Events: Life migrates to dry land.
Time: 395 million years
Noteworthy Events: First insects.
Time: 325 million years
Noteworthy Events: First land vertebrates.
Time: 200 million years
Noteworthy Events: First mammals.
Time: 180 million years
Noteworthy Events: North America separates from Africa; genesis of the Atlantic.
Time: 100 million years
Noteworthy Events: Half a galactic year ago; Earth looks out on the other side of the universe.
Time: 70 million years
Noteworthy Events: Preprimates evolve.
Time: 55 million years
Noteworthy Events: Early horses appear.
Time: 35 million years
Noteworthy Events: Early cats, dogs.
Time: 24 million years
Noteworthy Events: Appearance of grass.
Time: 21 million years
Noteworthy Events: Apes, monkeys depart along separate evolutionary pathways.
Time: 20 million years
Noteworthy Events: Atmosphere approaches modern composition.
Time: 15 million years
Noteworthy Events: Antarctica freezes over.
Time: 11 million years
Noteworthy Events: Grazing animals proliferate.
Time: 5 million years
Noteworthy Events: Apeman diverges from chimpanzee family.
Time: 3.7 million years
Noteworthy Events: Apemen walk upright.
Time: 3.5 million years
Noteworthy Events: Onset of latest series of ice ages.
Time: 1.8–1.7 million years
Noteworthy Events: Homo erectas, “first true man,” in China.
Time: 600,000 years
Noteworthy Events: Homo sapiens emerges.
Time: 360,000 years
Noteworthy Events: Controlled use of fire common among genus Homo.
Time: 150,000 years
Noteworthy Events: Woolly mammoth roam.
Time: 100,000 years
Noteworthy Events: Stars take on the forms of the recognizable modern constellations.
Time: 40,000 years
Noteworthy Events: Invention of complex language; modern humans flourish.
Time: 35,000 years
Noteworthy Events: Neanderthal man disappears. First musical instruments are crafted.
Time: 20,000–15,000 years
Noteworthy Events: Agriculture invented.
Time: 19,000 years
Noteworthy Events: Peopling of the Americas begins.
Time: 18,000 years
Noteworthy Events: Animals are herded by humans.
Time: 14,000 years
Noteworthy Events: Invention of fishhooks.
Time: 13,000 years
Noteworthy Events: Development of ceramic pottery.
Time: 10,000 years
Noteworthy Events: Cultivation of wheat, rice begins.
Time: 6,700 years
Noteworthy Events: Early Babylonian calendar in use.
Time: 6,200 years
Noteworthy Events: Refined solar calendar employed.
Time: 6,500 years
Noteworthy Events: Copper is smelted.
Time: 5,600 years
Noteworthy Events: First taxes.
Time: 5,500 years
BP (= 3,500 BC)
Noteworthy Events: Development of writing.
Time: 3,600–3,400 BC
Noteworthy Events: Cotton cultivated in Peru, Mexico.
Time: 2,500 years
Noteworthy Events: Stonehenge built.
Time: 2,200 years
Noteworthy Events: Systematic astronomy in Egypt, Babylonia, India, China.
Time: 1,500 years
Noteworthy Events: Sundial invented, in Egypt.
Time: 1,000 years
Noteworthy Events: Homer declaims the Odyssey.
Time: 800 years
Noteworthy Events: Olmec culture in Mexico.
Time: 700 years
Noteworthy Events: Hesiod, Works and Days.
Time: 650 years
Noteworthy Events: Mayan culture in Guatemala.
Time: 600 years
Noteworthy Events: Lao-tzu, Confucius, Buddha, Zoroaster; Old Testament in Hebrew.
Time: 540 years
Noteworthy Events: Pythagoras teaches that “all is number” and that nature is harmonious.
Time: 450 years
Noteworthy Events: Leucippus and Democritus propose that matter is made of indivisible entities, the atoms. Paradoxes of Zeno raise do
ubts about the concept of the infinitesimal.
Time: 400 years
Noteworthy Events: Plato teaches that the material world is but a shadow of a geometrically perfect reality. Aristotle, Eudoxus, theorize that universe is composed of crystalline spheres centered on Earth.
Time: 300 years
Noteworthy Events: Euclid’s geometry marries mathematical perfection to the world of experience.
Time: 260 years
Noteworthy Events: Aristarchus of Samos hypothesizes that the earth orbits the sun in a gigantic universe.
Time: 100 years
Noteworthy Events: Chinese seafarers reach the east coast of India.
Time: 60 BC
Noteworthy Events: Lucretius writes De Rerum Natura (On the Nature of Things), espousing Epicurean cosmology.
Time: AD 100
Noteworthy Events: Claudius Ptolemy constructs a complex geocentric cosmological model that “saves the appearances”—i.e., makes reasonably accurate predictions at the expense of claims to represent physical reality.
Time: 325
Noteworthy Events: Eusebius, chairman of the Council of Nicaea convened by the emperor Constantine, estimates that the world was created 3,184 years prior to the birth of Abraham.
Coming of Age in the Milky Way Page 45
