Parallel Worlds (56 page)

decoherence
When waves are
no longer in phase with each other. Decoherence can be used to explain the
Schrodinger cat paradox. In the many worlds interpretation, the wave function
of the dead cat and live cat have decohered from each other and hence no
longer interact, thus solving the problem of how a cat be simultaneously dead
and live. The wave function of the dead cat and the wave function of the live
cat both exist simultaneously, but they no longer interact because they have decohered.
Decoherence simply explains the cat paradox without any additional assumptions,
such as the collapse of the wave function.

de Sitter universe
A cosmological
solution of Einstein's equations that expands exponentially. The dominant term
is a cosmological constant that creates this exponential expansion. It is
believed that the universe was in a de Sitter phase during inflation, and that
it has slowly returned to a de Sitter phase within the last 7 billion years,
creating an accelerating universe. The origin of this de Sitter expansion is
not known.

determinism
The philosophy
that everything is predetermined, including the future. According to Newtonian
mechanics, if we know the velocity and position of all the particles in the
universe, then we can in principle calculate the evolution of the entire
universe. The uncertainty principle, however, has proved that determinism is
incorrect.

deuterium
The nucleus of
heavy hydrogen, consisting of a proton and a neutron. Deuterium in outer space
was mainly created by the big bang, not by stars, and its relative abundance
allows for the calculation of the early conditions of the big bang. The
abundance of deuterium can also be used to disprove the steady state theory.

dimension
A coordinate or
parameter by which we measure space and time. Our familiar universe has three
dimensions of space (length, width, and depth) and one dimension of time. In
string and M-theory, we need ten (eleven) dimensions in which to describe the
universe, only four of which can be observed in the laboratory. Perhaps the
reason why we don't see these other dimensions is either that they are curled
up or that our vibrations are confined to the surface of a membrane.

Doppler effect
The change in
frequency of a wave, as an object approaches or moves away from you. If a star
moves toward you, the frequency of light increases, so a yellow star appears
slightly bluish. If a star moves away from you, the frequency of its light
decreases, so a yellow star appears slightly reddish. This change in light
frequency can also be created by expanding space itself between two points, as
in the expanding universe. By measuring the amount of shift in the frequency,
you can calculate the velocity with which a star is moving away from you.

Einstein lenses and rings
The optical distortions of starlight as it passes through
intergalactic space due to gravity. Distant galactic clusters often have a
ringlike appearance. Einstein lenses can be used to calculate many key measurements,
including the presence of dark matter and even the value of Lambda and the
Hubble constant.

Einstein-Podolsky-Rosen (EPR) experiment
An experiment devised to disprove the quantum theory but
which actually showed that the universe is nonlocal. If an explosion sends two
coherent photons in opposite directions, and if spin is conserved, then the
spin of one photon is the opposite of the other's spin. Hence, by measuring one
spin, you automatically know the other, even though the other particle may be
on the other side of the universe. Information has hence spread faster than
light. (However, no usable information, such as a message, can be sent in this
fashion.)

Einstein-Rosen bridge
A wormhole
formed by joining two black hole solutions together. Originally, the solution
was meant to represent a subatomic particle, such as the electron, in
Einstein's unified field theory. Since then, it has been used to describe
space-time near the center of a black hole.

electromagnetic force
The force of
electricity and magnetism. When they vibrate in unison, they create a wave
that can describe ultraviolet radiation, radio, gamma rays, and so on, which
obeys Maxwell's equations. The electromagnetic force is one of the four forces
governing the universe.

electron
A negatively
charged subatomic particle that surrounds the nucleus of an atom. The number of
electrons surrounding the nucleus determines the chemical properties of the
atom.

electron degeneracy pressure
In a dying star, this is the repulsive force that prevents
electrons or neutrons from completely collapsing. For a white dwarf star, this
means that its gravity can overcome this force if its mass is greater than 1.4
solar masses. This force is due to the Pauli exclusion principle, which states
that no two electrons can occupy precisely the same quantum state. If gravity
is sufficiently large to overcome this force in a white dwarf star, it will
collapse and then explode.

electron volt
The energy that
an electron accumulates by falling through a potential of one volt. By
comparison, chemical reactions normally involve energies measured in electron
volts or less, while nuclear reactions may involve hundreds of millions of
electron volts. Ordinary chemical reactions involve rearranging the electron
shells. Nuclear reactions involve rearranging the shells of the nucleus. Today,
our particle accelerators can generate particles with energies in the billions
to trillions of electron volts.

entropy
The measure of
disorder or chaos. According to the second law of thermodynamics, the total
entropy in the universe always increases, which means that everything must
eventually run down. Applied to the universe, it means that the universe will
tend toward a state of maximum entropy, such as a uniform gas near absolute
zero. To reverse the entropy in a small region (such as a refrigerator), the
addition of mechanical energy is required. But even for a refrigerator, the
total entropy increases (which is why the back of a refrigerator is warm).
Some believe that the second law ultimately predicts the death of the universe.

event horizon
The point of no
return surrounding a black hole, often called the horizon. It was once believed
to be a singularity of infinite gravity, but this was shown to be an artifact
of the coordinates used to describe it.

exotic matter
A new form of
matter with negative energy. It is different from antimatter, which has
positive energy. Negative matter would have anti- gravity, so it would fall up
instead of down. If it exists, it could be used to drive a time machine.
However, none has ever been found.

extrasolar planet
A planet
orbiting a star other than our own. Over a hundred such planets have now been
detected, at a rate of about two a month. Most of them, unfortunately, are
Jupiter-like and are not favorable to the creation of life. Within a few
decades, satellites will be sent into outer space that will identify
Earth-like extrasolar planets.

false vacuum
A vacuum state
that does not have the lowest energy. The false vacuum state can be one of
perfect symmetry, perhaps at the instant of the big bang, so this symmetry
breaks when we descend to a state of lower energy. A state of false vacuum is
inherently unstable, and inevitably a transition is made to a true vacuum,
which has lower energy. The false vacuum idea is essential to inflationary
theory, where the universe began in a state of de Sitter expansion.

fermion
A subatomic
particle with half-integral spin, such as the proton, electron, neutron, and
quark. Fermions can be unified with bosons via super- symmetry.

fine-tuning
The adjustment
of a certain parameter to incredible accuracy. Physicists dislike fine-tuning,
considering it artificial and contrived, and try to impose physical principles
to eliminate the necessity for fine-tuning. For example, the fine-tuning
necessary to explain a flat universe can be explained by inflation, and the
fine-tuning necessary to solve the hierarchy problem in GUT theory can be
solved using supersymmetry.

flatness problem
The fine-tuning
necessary to have a flat universe. In order for Omega to be roughly equal to i,
it must have been fine-tuned to incredible accuracy at the instant of the big
bang. Current experiments show that the universe is flat, so either it was
fine-tuned at the big bang, or perhaps the universe inflated, which flattened
it out.

Friedmann universe
The most
general cosmological solution of Einstein's equations based on a uniform,
isotropic, homogeneous universe. This is a dynamic solution, where the
universe can expand into a big freeze, collapse into a big crunch, or inflate
forever, depending on the value of Omega and Lambda.

fusion
The process of
combining protons or other light nuclei so they form higher nuclei, releasing
energy in the process. The fusion of hydrogen to helium creates the energy of a
main sequence star, like our Sun. The fusion of the light elements in the big
bang gives us the relative abundance of light elements, like helium.

galaxy
A huge
collection of stars, usually containing on the order of i00 billion stars.
They come in several varieties, including elliptical, spiral (normal and barred
spirals), and irregular. Our galaxy is called the Milky Way galaxy.

general relativity
Einstein's
theory of gravity. Instead of being a force, gravity is reduced in Einstein's
theory to a byproduct of geometry, so that the curvature of space-time gives
the illusion that there is a force of attraction called gravity. It has been
verified experimentally to better than 99.7 percent accuracy and predicts the
existence of black holes and the expanding universe. The theory, however, must
break down at the center of a black hole or the instant of creation, where the
theory predicts nonsense. To explain these phenomena, one must resort to a
quantum theory.

Goldilocks zone
The narrow band
of parameters in which intelligent life is possible. In this band, Earth and
the universe are "just right" to create the chemicals that are
responsible for intelligent life. Scores of Goldilocks zones have been
discovered for the physical constants of the universe, as well as for the
properties of Earth.

Grand Unified Theory (GUT)
A theory that unifies the weak, strong, and electromagnetic
interactions (without gravity). The symmetry of GUT theories, such as SU(5),
mixes the quarks and leptons together. The proton is not stable in these
theories and can decay into positrons. GUT theories are inherently unstable
(unless one adds supersymmetry). GUT theories also lack gravity. (Adding
gravity to GUT theories makes them diverge with infinities.)

grandfather paradox
In time travel
stories, this is the paradox that emerges when you alter the past, making the
present impossible. If you go back in time and kill your parents before you are
born, then your very existence is impossible. This paradox can be resolved
either by imposing self-consistency, so you can journey to the past but cannot
change it arbitrarily, or by assuming parallel universes.

graviton
A conjectured
subatomic particle that is the quanta of gravity. The graviton has spin 2. It
is too small to be seen in the laboratory.

gravity wave
A wave of
gravity, predicted by Einstein's general relativity theory. This wave has been
indirectly measured by looking at the aging of pulsars rotating around each
other.

gravity wave detector
A new
generation of devices that measure tiny disturbances due to gravity waves via
laser beams. Gravity wave detectors like LIGO may soon discover them. Gravity
wave detectors can be used to analyze radiation emitted within a trillionth of
a second of the big bang. The space-based LISA gravity wave detector may even
give the first experimental evidence of string theory or some other theory.

Hawking radiation
The radiation
that slowly evaporates from a black hole. This radiation is in the form of
black body radiation, with a specific temperature, and is due to the fact that
quantum particles can penetrate the gravitational field surrounding a black
hole.

heterotic string theory
The most physically realistic string theory. Its symmetry
group is E(8) X E(8), which is large enough to incorporate the symmetry of the
Standard Model. Via M-theory, the heterotic string can be shown to be equivalent
to the other four string theories.