Parallel Worlds (55 page)

GLOSSARY

anthropic principle
The principle
that the constants of nature are tuned to allow for life and intelligence. The
strong anthropic principle concludes that an intelligence of some sort was
required to tune the physical constants to allow for intelligence. The weak
anthropic principle merely states that the constants of nature must be tuned
to allow for intelligence (otherwise we would not be here), but it leaves open
the question of what or who did the tuning. Experimentally, we find that,
indeed, the constants of nature seem to be finely tuned to allow for life and
even consciousness. Some believe that this is the sign of a cosmic creator.
Others believe that this is a sign of the multiverse.

antigravity
The opposite of
gravity, which would be a repulsive rather than an attractive force. Today, we
realize that this antigravity force does exist, probably caused the universe
to inflate at the beginning of time, and is causing the universe to accelerate
today. This antigravity force, however, is much too small to be measured in the
laboratory, so it has no practical implications. Antigravity is also generated
by negative matter (which has never been seen in nature).

antimatter
The opposite of
matter. Antimatter, first predicted to exist by P. A. M. Dirac, has the
opposite charge of ordinary matter, so that antiprotons have negative charge
and antielectrons (positrons) have positive charge. When they come in contact,
they annihilate each other. So far, antihydrogen is the most complex antiatom
produced in the laboratory. It is a mystery why our universe is made mainly of
matter rather than antimatter. If the big bang had created equal quantities of
both, then they should have annihilated each other, and we would not exist.

atom smasher
The colloquial
term for a particle accelerator, a device used to create beams of subatomic
energy traveling near the speed of light. The largest particle accelerator is
the LHC, to be built near Geneva, Switzerland.

baryon
A particle like
the proton or neutron, which obeys the strong interactions. Baryons are a type
of hadron (a strongly interacting particle). Baryonic matter, we now realize,
makes up only a tiny fraction of the matter in the universe and is dwarfed by
dark matter.

big bang
The original
explosion that created the universe, sending the galaxies hurtling in all
directions. When the universe was created, the temperature was extremely hot,
and the density of material was enormous. The big bang took place i3.7 billion
years ago, according to the WMAP satellite. The afterglow of the big bang is
seen today as the background microwave radiation. There are three experimental
"proofs" of the big bang: the redshift of the galaxies, the cosmic
background microwave radiation, and nucleosynethsis of the elements.

big crunch
The final
collapse of the universe. If the density of matter is large enough (Omega being
larger than i), then there is enough matter in the universe to reverse the
original expansion and cause the universe to recollapse. Temperatures rise to
infinity at the instant of the big crunch.

big freeze
The end of the
universe when it reaches near absolute zero. The big freeze is probably the
final state of our universe, because the sum of Omega and Lambda is believed to
be i.0, and hence the universe is in a state of inflation. There is not enough
matter and energy to reverse the original expansion of the universe, so it will
probably expand forever.

black body radiation
The radiation
emitted by a hot object in thermal equilibrium with its environment. If we
take an object that is hollow (a black body), heat it up, wait for it to reach
thermal equilibrium, and drill a small hole in it, the radiation emitted
through the hole will be black body radiation. The Sun, a hot poker, and molten
magma all emit approximately a black body radiation. The radiation has a
specific frequency dependence that is easily measured by a spectrometer. The
microwave background radiation filling up the universe obeys this black body
radiation formula, giving concrete evidence for the big bang.

black hole
An object whose
escape velocity equals the speed of light. Because the speed of light is the
ultimate velocity in the universe, this means that nothing can escape a black
hole, once an object has crossed the event horizon. Black holes can be of
various sizes. Galactic black holes, lurking in the center of galaxies and
quasars, can weight millions to billions of solar masses. Stellar black holes
are the remnant of a dying star, perhaps originally up to forty times the mass
of our Sun. Both of these black holes have been identified with our
instruments. Mini-black holes may also exist, as predicted by theory, but they
have not yet been seen in the laboratory.

black hole evaporation
The radiation
that tunnels out of a black hole. There is a tiny but calculable probability
that radiation will gently seep out of a black hole, which is called
evaporation. Eventually, so much of a black hole's energy will leave via
quantum evaporation that it will cease to exist. This radiation is too weak to
be observed experimentally.

blueshift
The increase in
the frequency of starlight because of the Doppler shift. If a yellow star is
moving toward you, its light will look slightly bluish. In outer space,
blueshifted galaxies are rare. Blueshift can also be created by shrinking the
space between two points via gravity or space warps.

boson
A subatomic
particle with integral spin, such as the photon or the conjectured graviton.
Baryons are unified with fermions via supersymmetry.

brane
Abbreviation
for membrane. Branes can be in any dimension up to eleven. They are the basis
of M-theory, the leading candidate for a theory of everything. If we take a
cross-section of an eleven-dimensional membrane, we obtain a ten-dimensional
string. A string is therefore a one-brane.

Calabi-Yau manifold
A
six-dimensional space that is found when we take ten-dimensional string theory
and roll up or compactify six dimensions into a small ball, leaving a
four-dimensional supersymmetric space. Calabi-Yau spaces are multiply
connected—that is, they have holes in them, which can determine the number of
quark generations that exist in our four-dimensional space. They are important
in string theory because many of the features of these manifolds, such as the
number of holes they have, can determine the number of quarks there are in our
four-dimensional universe.

Casimir effect
Negative energy
created by two infinitely long parallel uncharged plates placed next to each
other. Virtual particles outside the plates exert more pressure than the
virtual particles inside the plates, and hence the plates are attracted to each
other. This tiny effect has been measured in the laboratory. The Casimir
effect may be used as the energy to drive a time machine or wormhole, if its
energy is large enough.

Cepheid variable
A star that
varies in brightness at a precise, calculable rate and hence serves as a
"standard candle" for distance measurements in astronomy. Cepheid
variables were decisive in helping Hubble calculate the distance to the
galaxies.

Chandrasekhar limit
1.4 solar masses. Beyond this mass, a white dwarf star's
gravity is so immense that it will overcome electron degeneracy pressure and
crush the star, creating a supernova. Thus, all white dwarf stars we observe in
the universe have mass less than 1.4 solar masses.

Chandra X-ray telescope
The X-ray telescope in outer space that can scan the heavens
for X-ray emissions, such as those emitted by a black hole or neutron star.

chaotic inflation
A version of
inflation, proposed by Andrei Linde, whereby inflation occurs at random. This
means that universes can bud off other universes in a continual, chaotic
fashion, creating a multiverse. Chaotic inflation is one way to solve the
problem of ending inflation, since we now have the random generation of
inflated universes of all types.

classical physics
Physics before
the coming of the quantum theory, based on the deterministic theory of Newton.
Relativity theory, because it does not incorporate the uncertainty principle,
is part of classical physics. Classical physics is deterministic—that is, we
can predict the future given the motions of all particles at present.

closed time-like curves
These are paths that go backward in time in Einstein's
theory. They are not allowed in special relativity but are allowed in general
relativity if we have a large enough concentration of positive or negative
energy.

COBE
The Cosmic
Observer Background Explorer satellite, which gave perhaps the most conclusive
proof of the big bang theory by measuring the black body radiation given off
by the original fireball. Its results have since been improved greatly by the
WMAP satellite.

coherent radiation
Radiation that
is in phase with itself. Coherent radiation, like that found in a laser beam,
can be made to interfere with itself, yielding interference patterns that can
detect small deviations in motion or position. This is useful in
interferometers and gravity wave detectors.

compactification
The process of
rolling up or wrapping up unwanted dimensions of space and time. Since string
theory exists in ten-dimensional hy- perspace, and we live in a
four-dimensional world, we must somehow wrap up six of the ten dimensions into
a ball so small that even atoms cannot escape into them.

conservation laws
The laws that
state that certain quantities never change with time. For example, the
conservation of matter and energy posits that the total amount of matter and
energy in the universe is a constant.

Copenhagen school
The school
founded by Niels Bohr, which states that an observation is necessary in order
to "collapse the wave function" to determine the state of an object.
Before an observation is made, an object exists in all possible states, even
absurd ones. Since we do not observe dead cats and live cats existing
simultaneously, Bohr had to assume that there is "wall" separating
the subatomic world from the everyday world we observe with our senses. This
interpretation has been challenged because it separates the quantum world from
the everyday, macroscopic world, while many physicists now believe that the
macroscopic world must also obey the quantum theory. Today, because of nan-
otechnology, scientists can manipulate individual atoms, so we realize that
there no "wall" separating the two worlds. Hence, the cat problem
resurfaces today.

cosmic microwave background radiation
The residual radiation left over from the big bang which is
still circulating around the universe, first predicted in i948 by George Gamow
and his group. Its temperature is 2.7 degrees above absolute zero. Its
discovery by Penzias and Wilson gave the most convincing "proof" of
the big bang. Today, scientists measure tiny deviations within this background
radiation to provide evidence for inflation or other theories.

cosmic string
A remnant of
the big bang. Some gauge theories predict that some relics of the original big
bang might still survive in the form of gigantic cosmic strings that are the
size of galaxies or larger. The collision of two cosmic strings may allow for a
certain form of time travel.

critical density
The density of
the universe where the expansion of the universe is poised between eternal
expansion and recollapse. The critical density, measured in certain units, is
Omega = i (where Lambda = 0), where the universe is precisely balanced between
two alternate futures, the big freeze and the big crunch. Today, the best data
from the WMAP satellite indicates that Omega + Lambda = i, which fits the
prediction of the inflation theory.

dark energy
The energy of
empty space. First introduced by Einstein in i9i7 and then discarded, this
energy of nothing is now known to be the dominant form of matter/energy in the
universe. Its origin is unknown, but it may eventually drive the universe into
a big freeze. The amount of dark energy is proportional to the volume of the
universe. The latest data shows that 73 percent of the matter/energy of the
universe is in the form of dark energy.

dark matter
Invisible
matter, which has weight but does not interact with light. Dark matter is
usually found in a huge halo around galaxies. It outweighs ordinary matter by a
factor of i0. Dark matter can be indirectly measured because it bends starlight
due to its gravity, somewhat similar to the way glass bends light. Dark matter,
according to the latest data, makes up 23 percent of the total matter/energy
content of the universe. According to string theory, dark matter may be made of
subatomic particles, such as the neutralino, which represent higher vibrations
of the superstring.