Parallel Worlds (58 page)

perturbation theory
The process by
which physicists solve quantum theories by summing over an infinite number of
small corrections. Almost all the work in string theory is done via string
perturbation theory, but some of the most interesting problems lie beyond the
reach of perturbation theory, such as super- symmetry breaking. Thus, we need
nonperturbative methods to solve string theory, which at the present time do
not really exist in any systematic fashion.

photon
A particle or
quantum of light. The photon was first proposed by Einstein to explain the
photoelectric effect—that is, the fact that shining light on a metal results in
the ejection of electrons.

Planck energy
i0
ⁱ⁹
billion electron volts. This might be the
energy scale of the big bang, where all the forces were unified into a single
superforce.

Planck length
i0
^-33
cm. This is the scale found at the big bang in which the gravitational force
was as strong as the other forces. At this scale, space-time becomes
"foamy," with tiny bubbles and wormholes appearing and disappearing
into the vacuum.

powers of ten
Shorthand
notation used by scientists to denote very large or very small numbers. Thus,
i0
ⁿ
means i followed by
n
zeros. A thousand is therefore i0
³
. Also, i0
^-n
means the inverse of i0
ⁿ
—that is, 000 . . . 00i, where there are
n
— i zeros. A
thousandth is therefore i0
^-3
or 0.00i.

proton
A positively
charged subatomic particle which, along with neutrons, makes up the nuclei of
atoms. They are stable, but GUT theory predicts that they may decay over a long
period of time.

pulsar
A rotating
neutron star. Because it is irregular, it resembles a rotating lighthouse
beacon, giving the appearance of a blinking star.

quantum fluctuation
Tiny variations
from the classical theory of Newton or Einstein, due to the uncertainty
principle. The universe itself may have started out as a quantum fluctuation in
nothing (hyperspace). Quantum fluctuations in the big bang give us the galactic
clusters of today. The problem with quantum gravity, which has prevented a
unified field theory for many decades, is that the quantum fluctuations of
gravity theory are infinite, which is nonsense. So far, only string theory can
banish these infinite quantum fluctuations of gravity.

quantum foam
Tiny, foamlike
distortions of space-time at the level of the Planck length. If we could peer into
the fabric of space-time at the Planck length, we would see tiny bubbles and
wormholes, with a foam-like appearance.

quantum gravity
A form of
gravity that obeys the quantum principle. When gravity is quantized, we find a
packet of gravity, which is called the graviton. Usually, when gravity is
quantized, we find its quantum fluctuations are infinite, which renders the
theory useless. At present, string theory is the only candidate which can
remove these infinities.

quantum leap
A sudden change
in the state of an object that is not allowed classically. Electrons inside an
atom make quantum leaps between orbits, releasing or absorbing light in the
process. The universe might have made a quantum leap from nothing to our
present-day universe.

quantum mechanics
The complete
quantum theory proposed in i925, which replaced the "old quantum
theory" of Planck and Einstein. Unlike the old quantum theory, which was
a hybrid of old classical concepts and newer quantum ideas, quantum mechanics
is based on wave equations and the uncertainty principle and represents a
significant break from classical physics. No deviation from quantum mechanics
has ever been found in the laboratory. Its most advanced version today is
called quantum field theory, which combines special relativity and quantum
mechanics. A fully quantum mechanical theory of gravity, however, is
exceedingly difficult.

quantum theory
The theory of
subatomic physics. It is one of the most successful theories of all time.
Quantum theory plus relativity together make up the sum total of all physical
knowledge at a fundamental level. Roughly speaking, the quantum theory is based
on three principles: (i) energy is found in discrete packets called quanta; (2)
matter is based on point particles but the probability of finding them is given
by a wave, which obeys the Schrodinger wave equation; (3) a measurement is
necessary to collapse the wave and determine the final state of an object. The
postulates of the quantum theory are the reverse of the postulates of general
relativity, which is deterministic and based on smooth surfaces. Combining
relativity and the quantum theory is one of the greatest problems facing
physics today.

quark
A subatomic
particle that makes up the proton and neutron. Three quarks make up a proton or
neutron, and a quark and antiquark pair make up a meson. Quarks in turn are
part of the Standard Model.

quasar
Quasi-stellar
object. They are huge galaxies that were formed shortly after the big bang.
They have huge black holes at their center. The fact that we do not see quasars
today was one way to disprove the steady state theory, which says that the
universe today is similar to the universe billions of years ago.

red giant
A star that
burns helium. After a star like our Sun exhausts its hydrogen fuel, it begins
to expand and form a helium-burning red giant star. This means that Earth will
ultimately die in fire when our Sun becomes a red giant, about 5 billion years
from now.

redshift
The reddening
or decrease in frequency of light from distant galaxies due to the Doppler
effect, indicating that they are moving away from us. The redshift can also
take place via the expansion of empty space, as in the expanding universe.

relativity
The theory of
Einstein, both special and general. The first theory is concerned with light
and flat, four-dimensional space-time. It is based on the principle that the
speed of light is constant in all inertial frames. The second theory deals with
gravity and curved space. It is based on the principle that gravitating and
accelerating frames are indistinguishable. The combination of relativity with
the quantum theory represents the sum total of all physical knowledge.

Schrodinger's cat paradox
The paradox that asks if a cat can be dead and alive at the
same time. According to the quantum theory, a cat in a box may be dead and
alive simultaneously, at least until we make an observation, which sounds
absurd. We must add the wave function of a cat in all possible states (dead,
alive, running, sleeping, eating, and so forth) until a measurement is made.
There are two main ways to resolve the paradox, either assuming that consciousness
determines existence or assuming an infinite number of parallel worlds.

Schwarzschild radius
The radius of
the event horizon, or the point of no return for a black hole. For the Sun,
the Schwarzschild radius is roughly two miles. Once a star is compressed to
within its event horizon, it collapses into a black hole.

simply connected space
A space in
which any lasso can be continuously shunk to a point. Flat space is simply
connected, while the surface of a doughnut or a wormhole is not.

singularity
A state of
infinite gravity. In general relativity, singularities are predicted to exist
at the center of black holes and at the instant of creation, under very
general conditions. They are thought to represent a breakdown of general
relativity, forcing the introduction of a quantum theory of gravity.

special relativity
Einstein's i905
theory, based on the constancy of the speed of light. Consequences include:
time slows down, mass increases, and distances shrink the faster you move.
Also, matter and energy are related via
E
= mc
²
.
One consequence of special relativity is the atomic bomb.

spectrum
The different
colors or frequencies found within light. By analyzing the spectrum of
starlight, one can determine that stars are mainly made of hydrogen and helium.

standard candle
A source of
light that is standardized and the same throughout the universe, which allows
scientists to calculate astronomical distances. The fainter a standard candle
is, the farther away it is. Once we know the luminosity of a standard candle,
we can calculate its distance. The standard candles used today are type Ia
supernovae and Cepheid variables.

Standard Model
The most
successful quantum theory of the weak, electromagnetic, and strong
interactions. It is based on the SU(3) symmetry of quarks, the SU(2) symmetry
of electrons and neutrinos, and the U(i) symmetry of light. It contains a large
collection of particles: quarks, gluons, leptons, W- and Z-bosons, and Higgs
particles. It cannot be the theory of everything because (a) it lacks any
mention of gravity; (b) it has nineteen free parameters which have to be fixed
by hand; and (c) it has three identical generations of quarks and leptons,
which is redundant. The Standard Model can be absorbed into a GUT theory and
eventually into string theory, but at present there is no experimental evidence
for either.

steady state theory
The theory
which states that the universe had no beginning but constantly generates new
matter as it expands, keeping the same density. This theory has been
discredited for various reasons, one being when the microwave background
radiation was discovered. Also, it was found that quasars and galaxies have
distinct evolutionary phases.

string theory
The theory
based on tiny vibrating strings, such that each mode of vibration corresponds
to a subatomic particle. It is the only theory that can combine gravity with
the quantum theory, making it the leading candidate for a theory of everything.
It is only mathematically self-consistent in ten dimensions. Its latest
version is called M-theory, which is defined in eleven dimensions.

strong nuclear force
The force that
binds the nucleus together. It is one of the four fundamental forces.
Physicists use Quantum Chromodynamics to describe the strong interactions,
based on quarks and gluons with SU(3) symmetry.

supernova
An exploding
star. They are so energetic that they can sometimes outshine a galaxy. There
are several types of supernovae, the most interesting being the type Ia
supernova. They all can be used as standard candles to measure galactic
distances. Type Ia supernovae are caused when an aging white dwarf star steals
matter from its companion and is pushed beyond the Chandrasekhar limit, causing
it to suddenly collapse and then blow up.

supersymmetry
The symmetry
that interchanges fermions and bosons. This symmetry solves the hierarchy
problem, and it also helps to eliminate any remaining divergences within
superstring theory. It means that all the particles in the Standard Model must
have partners, called sparticles, which have so far never been seen in the
laboratory. Supersymmetry in principle can unify all the particles of the
universe into a single object.

symmetry
A reshuffling or rearrangement of an object that leaves it
invariant, or the same. Snowflakes are invariant under a rotation of a
multiple of 60 degrees. Circles are invariant under a rotation of any angle. The
quark model remains invariant under a reshuffling of the three quarks, giving
SU(3) symmetry.

Strings are invariant under supersymmetry and also under
conformal deformations of its surface. Symmetry is crucial in physics because
it helps to eliminate many of the divergences found in quantum theory.

symmetry breaking
The breaking of
a symmetry found in the quantum theory. It is thought that the universe was in
perfect symmetry before the big bang. Since then, the universe has cooled and
aged, and hence the four fundamental forces and their symmetries have broken
down. Today, the universe is horribly broken, with all the forces split off
from each other.

thermodynamics
The physics of
heat. There are three laws of thermodynamics: (i) the total amount of matter
and energy is conserved; (2) total entropy always increases; and (3) you cannot
reach absolute zero. Thermodynamics is essential to understanding how the
universe might die.