Parallel Worlds (57 page)

hierarchy problem
The unwanted
mixing that takes place between low- energy physics and physics at the Planck
length in GUT theories, rendering them useless. The hierarchy problem can be
solved by adding supersymmetry.

Higgs field
The field that
breaks the symmetry of GUT theories when it makes the transition from the false
vacuum to the real vacuum. Higgs fields are the origin of mass in GUT theory
and also can be used to drive inflation. Physicists hope that the LHC will
finally discover the Higgs field.

horizon
The farthest
point you can see. Surrounding a black hole there is a magic sphere, at the
Schwarzschild radius, which is the point of no return.

horizon problem
The mystery of
why the universe is so uniform no matter where we look. Even regions of the
night sky on opposite sides of the horizon are uniform, which is strange
because they could not have been in thermal contact at the beginning of time
(since light has a finite velocity). This can be explained if the big bang took
a tiny uniform patch and then inflated it to the present-day universe.

Hubble's constant
The velocity of
a redshifted galaxy divided by its distance. Hubble's constant measures the
rate of expansion of the universe, and its inverse correlates roughly to the
age of the universe. The lower the Hubble constant, the older the universe. The
WMAP satellite has placed the Hubble constant at 71 km/s per million parsecs,
or 21.8 km/s per million light-years, ending decades of controversy.

Hubble's law
The farther a
galaxy is from Earth, the faster it moves. Discovered by Edwin Hubble in i929,
this observation agrees with Einstein's theory of an expanding universe.

hyperspace
Dimensions
higher than four. String theory (M-theory) predicts that there should be ten
(eleven) hyperspatial dimensions. At present, there is no experimental data
indicating the existence of these higher dimensions, which may be too small to
measure.

inflation
The theory
which states that the universe underwent an incredible amount of superliminal
expansion at the instant of its birth. Inflation can solve the flatness,
monopole, and horizon problems.

infrared radiation
Heat radiation,
or electromagnetic radiation, that is slightly below visible light in
frequency.

interference
The mixing of
two waves that are slightly different in phase or frequency, creating a
characteristic interference pattern. By analyzing this pattern, one may be
able to detect tiny differences between two waves which differ only by an
extremely small amount.

interferometry
The process of
using the interference of light waves to detect very small differences in the
waves from two different sources. Interferometry can be used to measure the
presence of gravity waves and other objects that are normally difficult to
detect.

isotope
A chemical that
has the same number of protons as an element but with a different number of
neutrons. Isotopes have the same chemical properties but have different weight.

Kaluza-Klein theory
The theory of
Einstein formulated in five dimensions. When reduced down to four dimensions,
we find Einstein's usual theory coupled to Maxwell's theory of light. Thus,
this was the first nontrivial unification of light with gravitation. Today,
Kaluza-Klein theory is incorporated within string theory.

Kerr black hole
An exact
solution of Einstein's equations which represents a spinning black hole. The
black hole collapses into a ring singularity. Objects falling into the ring
experience only a finite force of gravity and may, in principle, fall through
to a parallel universe. There are an infinite number of these parallel
universes for a Kerr black hole, but you cannot return once you enter one of
them. It is still not known how stable the wormhole is at the center of a Kerr
black hole. There are severe theoretical and practical problems trying to
navigate through a Kerr black hole.

Lambda
The
cosmological constant, which measures the amount of dark energy in the
universe. At present, the data supports Omega + Lambda = i, which fits the
prediction of inflation for a flat universe. Lambda, which was once thought to
be zero, is now known to determine the ultimate destiny of the universe.

laser
A device for
creating coherent light radiation. "Laser" stands for Light
Amplification through Stimulated Emission of Radiation. In principle, the only
limit to the energy contained on a laser beam is the stability of the lasing
material and the power source.

lepton
A weakly
interacting particle, such as the electron and neutrino, and its higher
generations, such as the muon. Physicists believe that all matter consists of
hadrons and leptons (strongly and weakly interacting particles).

LHC
The Large Hadron Collider, a particle accelerator for
creating energetic beams of protons, based in Geneva, Switzerland. When finally
completed, it will collide particles with energies not seen since the big bang.
It is hoped that the Higgs particle and sparticles will be found by the LHC
after it opens in 2007.

light-year
The distance
light travels in one year, or approximately 5.88 trillion miles (9.46 trillion
kilometers). The nearest star is about four light-years away, and the Milky Way
galaxy is about i00,000 light-years across.

LIGO
The Laser
Interferometry Gravitational-Wave Observatory, based in Washington state and
Louisiana, is the world's largest gravity wave detector. It went online in
2003.

LISA
The Laser
Interferometry Space Antenna is a series of three space satellites using laser
beams to measure gravity waves. It may be sensitive enough to confirm or
disprove the inflationary theory and possibly even string theory, when it is
launched in a few decades.

MACHO
Massive Compact
Halo Object. These are dark stars, planets, asteroids, and such which are hard
to detect by optical telescopes and may make up a portion of dark matter. The
latest data indicates that the bulk of dark matter is nonbaryonic and is not
made of MACHOs.

many-worlds theory
The quantum
theory which states that all possible quantum universes can exist simultaneously.
It solves the Schrodinger cat problem by stating that the universe splits at
each quantum juncture, and hence the cat is alive in one universe but dead in
another. Recently, an increasing number of physicists have voiced their support
for the many-worlds theory.

Maxwell's equation
The fundamental
equations for light, first written down by James Clerk Maxwell in the i860s.
These equations show that electric and magnetic fields can turn into each
other. Maxwell showed that these fields turn into each other in a wavelike
motion, creating an electromagnetic field that travels at the speed of light.
Maxwell then made the bold conjecture that this was light.

membrane
An extended
surface, in any dimensions. A zero-brane is a point particle. A one-brane is a
string. A two-brane is a membrane. Membranes are an essential feature of
M-theory. Strings can be viewed as membranes with one dimension compactified.

microwave background radiation
The remnant of the original radiation from the big bang,
with a temperature of about 2.7 degrees K. Tiny deviations in this background
radiation give scientists valuable data that can verify or rule out many
cosmological theories.

monopole
A single pole
of magnetism. Usually, magnets have an inseparable pair of north and south
poles, so monopoles have never been conclusively seen in the laboratory.
Monopoles should have been created in copious quantities at the big bang, but
we can find none today, probably because inflation diluted their number.

M-theory
The most
advanced version of string theory. M-theory exists in eleven-dimensional
hyperspace, where two-branes and five-branes can exist. There are five ways in
which M-theory can be reduced down to ten dimensions, thereby giving us the
five known superstring theories, which are now revealed to be the same theory.
The full equations governing M-theory are totally unknown.

multiply connected space
A space in which a lasso or loop cannot be continuously
shrunk down to a point. For example, a loop that winds around the surface of a
doughnut hole cannot be contracted to a point, hence a doughnut is multiply
connected. Wormholes are examples of multiply connected spaces, since a lasso
cannot be contracted around the throat of a wormhole.

multiverse
Multiple universes.
Once considered highly speculative, today the concept of the multiverse is
considered essential to understanding the early universe. There are several
forms of the multiverse which are all intimately related. Any quantum theory
has a multiverse of quantum states. Applied to the universe, it means that
there must be an infinite number of parallel universes which have decohered
from each other. Inflation theory introduces the multi- verse to explain the
process of how inflation started and then stopped. String theory introduces the
multiverse because of its large number of possible solutions. In M-theory,
these universes may actually collide with each other. On philosophical grounds,
one introduces the multiverse to explain the anthropic principle.

muon
A subatomic
particle identical to the electron but with a much larger mass. It belongs to
the second redundant generation of particles found in the Standard Model.

negative energy
Energy that is
less than zero. Matter has positive energy, gravity has negative energy, and
the two can cancel out in many cosmological models. The quantum theory allows
for a different kind of negative energy, due to the Casimir effect and other
effects, which can be used to drive a wormhole. Negative energy is useful in creating
and stabilizing wormholes.

neutrino
A ghostly,
almost massless subatomic particle. Neutrinos react very weakly with other
particles and may penetrate several light-years of lead without ever
interacting with anything. They are emitted in copious quantities from
supernovae. The number of neutrinos is so large that they heat up the gas surrounding
the collapsing star, thereby creating the explosion of the supernova.

neutron
A neutral
subatomic particle which, along with the proton, makes up the nuclei of atoms.

neutron star
A collapsed
star consisting of a solid mass of neutrons. It is usually about i0 to i5
miles across. When it spins, it releases energy in an irregular manner,
creating a pulsar. It is the remnant of a supernova. If the neutron star is
quite large, about 3 solar masses, it might collapse into a black hole.

nucleosynethesis
The creation of
higher nuclei from hydrogen, starting from the big bang. In this way, one can
obtain the relative abundance of all the elements found in nature. This is one
of the three "proofs" of the big bang. The higher elements are cooked
in the center of stars. The elements beyond iron are cooked in a supernova
explosion.

nucleus
The tiny core
of an atom, consisting of protons and neutrons, which is roughly i0
^-i3
cm across. The number of protons in a nucleus determines the number of
electrons in the shell surrounding the nucleus, which in turn determines the
chemical properties of the atom.

Olbers' paradox
The paradox
that asks why the night sky is black. If the universe is infinite and uniform,
then we must receive light from an infinite number of stars, and hence the sky
must be white, which violates observation. This paradox is explained by the big
bang and the finite lifetime of stars. The big bang gives a cutoff to the light
hitting our eyes from deep space.

Omega
The parameter
that measures the average density of matter in the universe. If Lambda = 0,
and Omega is less than 1, then the universe will expand forever into a big
freeze. If Omega is greater than 1, then there is enough matter to reverse the
expansion into a big crunch. If Omega equals 1, then the universe is flat.