Coming of Age in the Milky Way (65 page)

Sun.
The star orbited by the earth.

Supercluster.
A cluster of clusters of galaxies. Superclusters are typically about one hundred million (10
⁸
) 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 (10
¹²
).

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.

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 10
⁵⁰
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
:
10
²
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
: < 10
⁶
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
: - 10
⁹
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.