The Cosmic Landscape (60 page)

Graviton
—The quantum of the gravitational field. Its exchange accounts for the gravitational force.

Harmonics
—The patterns of vibration of a string, such as a guitar string.

Heisenberg Uncertainty Principle
—The principle that says that it is impossible to determine both the position and momentum of any object.

Higgs boson
—The quantum of the Higgs field.

Higgs field
—The field in the Standard Model whose value controls the masses of elementary particles such as the electron and quark.

Holographic Principle
—The principle that says that a region of space can be completely described by degrees of freedom on its boundary, with no more than one degree of freedom per Planck area—an area equal to one square Planck length.

Homogeneous
—Everywhere the same; completely smooth and without variation from point to point.

Horizon
—The point of no return at which an observer would be receding with the speed of light. It applies to both black holes and to rapidly inflating cosmic space.

Hubble constant
—The constant appearing in Hubble’s Law.

Hubble’s Law
—The law stating that the recessional velocity of galaxies is proportional to their distance. It can be expressed as an equation: V = HD, where V is velocity, D is distance, and H is the Hubble constant.

Inflation
—The rapid exponential expansion of space that ironed out all the wrinkles and created a large, smooth universe. Inflation has become the standard theory of the early universe.

Isotropic
—The same in every direction.

Joule
—An ordinary unit of energy. The quantity of energy needed to heat one gram of water 1 degree centigrade.

Landscape
—The space of possible vacuums (environments) allowed by fundamental theory. In practice, the space of vacuums of String Theory.

Long range
—Refers to forces that reach out over long distances to pull or push objects. Gravity, electric, and magnetic forces are long range.

Magnetic field
—The relative of the electric field which is created by charges in motion (currents).

Matrix Theory
—The underlying mathematical framework for M-theory.

Megaverse
—The huge vastness of pocket universes.

Mode of oscillation
—Same as harmonic.

Moduli
—The parameters that determine the size and shape of the compact directions of space, particularly in String Theory.

MRI machine
—Medical imaging machine utilizing a space with a large magnetic field in it.

M-theory
—The eleven-dimensional theory that unifies many of the diverse String Theories. M-theory has membranes but no strings.

Neutrino
—The “ghostly” particle emitted by a neutron, along with an electron, when the neutron decays and becomes a proton.

Neutron
—One of two particles composing the nucleus. The neutron is electrically neutral.

Non-abelian gauge theory
—A class of quantum field theories that form the basis for the Standard Model of particle physics.

Nucleon
—Proton or neutron.

Pauli exclusion principle
—The principle that says that no two fermions can occupy the same quantum state.

Photon
—Quantum of the electromagnetic field. The basis for Einstein’s particle theory of light.

Planck length or Planck distance
—The natural unit of length determined by Planck’s constant, Newton’s gravitational constant, and the speed of light. It is about 10-
³³
centimeters.

Planck mass
—The natural unit of mass determined by Planck’s constant, Newton’s gravitational constant, and the speed of light. It is about 10-
⁵
grams.

Planck’s constant
—Very small numerical constant that determines the limit in the simultaneous determination of position and momentum (Heisenberg Uncertainty Principle).

Planck time
—The natural unit of time determined by Planck’s constant, Newton’s gravitational constant, and the speed of light. It is about 10-
⁴²
seconds.

Plasma
—Gas that has been heated to the point where some or all of the electrons have been torn free of the atoms and are free to move through the material. Plasmas are good electrical conductors and are opaque to light.

Pocket universe
—A portion of the universe in which the Laws of Physics take a particular form.

Positron
—The electron’s antiparticle.

Principle of Black Hole Complementarity
—The principle that allows two apparently contradictory descriptions of matter that falls into a black hole.

Propagator
—The component of Feynman diagrams that represent the motion of a particle from one space-time point to another; also the mathematical expression that controls the probability for such a process.

Proton
—The positively charged nucleon.

Quantum Chromodynamics (QCD)
—The theory of quarks and gluons that explains the existence and properties of nucleons and nuclei. The modern nuclear physics.

Quantum Electrodynamics (QED)
—The theory of electrons and photons. The basis for all atomic physics and chemistry.

Quantum field theory
—The mathematical theory of elementary particles that originated by combining quantum mechanics with the Special Theory of Relativity.

Quantum jitters
—The unpredictable fluctuating motion of particles or fields that derive from the principles of quantum mechanics.

Quark
—The elementary particles that combine together, three at a time, to make up nucleons.

Reductionism
—The philosophy that says that nature can be understood by reducing all phenomena to ultimately simple microscopic events.

Rube Goldberg machine
—An overly complicated, inelegant solution to an engineering problem. Named after the cartoonist Rube Goldberg, whose cartoons depicted fantastic and silly Rube Goldberg machines.

Scalar field
—A field that has a magnitude (strength) but no sense of direction. The Higgs field is a scalar; the electric and magnetic fields are not.

Short range
—Refers to forces that do not reach out over long distances, i.e., forces between objects that act only when the objects are in contact or almost in contact.

Space-time
—The four-dimensional world including time in which all phenomena take place.

Spectral lines
—The discrete sharp lines in the spectrum of light that arise from atomic transitions in which an electron makes a quantum jump from one energy level to another and in the process emits a photon.

Standard Model
—The currently accepted quantum field theory that describes elementary particles. It includes QED, QCD, and the weak interactions as well as phenomena involving the Higgs boson.

Supercooled water
—Water that has been cooled below the freezing temperature but that has remained liquid.

Supernova
—The final event in the life of certain stars that results in a collapse to a neutron star. At the same time an explosion sprays chemical elements into the surrounding space.

Supersymmetry
—A mathematical symmetry relating fermions and bosons.

Symmetry
—An operation that leaves the laws of nature unchanged.

Vacuum
—A background or environment in which the Laws of Physics take on a certain form.

Vacuum energy
—Energy stored in the quantum fluctuations of empty space.

Vacuum fluctuation
—The jittery fluctuation of quantum fields in empty space.

Vacuum selection principle
—A mathematical principle that would select a single unique String Theory vacuum out of all the diverse vacuums that the theory describes. Thus far, no such principle has ever been found.

Vector field
—A field that has, in addition to a strength, a direction in space. The electric and magnetic fields are vectors.

Vertex diagram
—The Feynman diagram depicting the elementary event in which a particle is emitted by another particle.

Virtual particle
—A particle in the interior of a Feynman diagram. Not one of the particles that enters or leaves at the beginning or end of the process.

W-boson
—One of the particles whose exchange gives rise to the weak interactions.

Weak interactions
—Phenomena that are similar to the decay of the neutron.

Weinberg’s bound
—The bound on the size of the cosmological constant that derives from the condition that galaxies could form in the early universe.

Yang Mills theory
—Same as non-abelian gauge theory.

Z-boson
—A close relative of the W-boson, also involved in weak interactions.

About the Author

Leonard Susskind grew up in the South Bronx, where he worked as a plumber and steam fitter during his early adult years. As an engineering student in CCNY, he discovered that physics was more to his liking than either plumbing or engineering. He later earned a PhD in theoretical physics at Cornell University.

Susskind has been a professor of physics at the Belfer Graduate School in New York City, Tel Aviv University in Israel, and since 1978 at Stanford University, where he is the Felix Bloch Professor of Physics. During the past forty years he has made contributions to every area of theoretical physics, including quantum optics, elementary-particle physics, condensed-matter physics, cosmology, and gravitation. In 1969 Susskind and Yoichiro Nambu independently discovered string theory. Later he developed the theory of quark confinement (why quarks are stuck inside the nucleus and can never escape), the theory of baryogenesis (why the universe is full of matter but no antimatter), the Principle of Black Hole Complementarity, the Holographic Principle, and numerous other concepts of modern physics.

^*
A binary star system just means two stars orbiting around their center of mass.

^*
Samuel Wilberforce, an Anglican bishop, was called Soapy because of his slipperiness in ecclesiastical debate. Thomas Huxley, Darwin’s chief disciple, was called Darwin’s Bulldog for obvious reasons. The two squared off in 1860 to debate Darwin’s
On the Origin of Species by Means of Natural Selection
. Soapy Sam gleefully asked Huxley if it was his grandmother or grandfather who had been the ape? Huxley turned on him and said, “I’d rather be descended from a monkey than from someone who would so prostitute the truth.”

^*
I don’t know the religious beliefs of Davies or Greenstein, but I would be wary of too literal an interpretation. Physicists often use terms like
design, agency,
and even
God
as metaphors for that which is not known—period. I have used the term
agent
in print and have been sorry ever since. Einstein often spoke of God: “God is cunning but He is not malicious.” “God does not play dice.” “I want to know how God created the world.” Most commentators believe Einstein was using the term
God
as a metaphor for an orderly set of laws of nature.

^†
Will this sentence appear out of context on a religious Internet site? I hope not.

^*
For the experts, the weakness of gravity is equivalent to the lightness of the usual elementary particles. The smallness of particle masses is called the gauge hierarchy problem. Although interesting ideas have been posited, there is no consensus on its solution.

^*
See chapter 3 for the definition of a Rube Goldberg machine.

^*
Also known as dark energy.

¹
Alfred Russel Wallace (1823-1913), a contemporary of Darwin’s, was the codiscoverer of natural selection as the mechanism driving the evolution of species. It was reading a short note of Wallace’s that finally induced Darwin to publish his own work.

²
Richard Dawkins,
The Blind Watchmaker: Why the Evidence of Evolution Reveals a Universe without Design
(New York: Norton, 1996). Dawkins invokes the metaphor of a blind watchmaker to describe how evolution blindly created the universe of biology. The meta-phor could easily be extended to the creation of the cosmos.

³
One light-year is, of course, the distance light travels in a year. It comes to about ten trillion kilometers.

⁴
The term
multiverse
has been widely used instead of
megaverse.
Personally, I prefer the sound of
megaverse.
My apologies to the multiverse enthusiasts.

⁵
In our world, atomic nuclei are thousands of times heavier than electrons.