Hiding in the Mirror (31 page)

GSO construction:
A
particular construction in string theory in ten dimensions,
associated with the names Gliozzi, Scherk, and Olive, which removed
the unwanted tachyon modes by introducing supersymmetry on the
strings.
Hadrons:
Elementary particles that
have strong interactions with other particles.

Heterotic string:
A
string theory involving closed string loops in ten dimensions in
which the different excitations of the string, moving in different
directions along the string, behave quite differently. In fact, the
left movers appear to live in a different number of dimensions than
the right movers. In this way, it turns out that one can have
consistent string theories in ten dimensions instead of twenty-six
dimensions. Moreover the gauge symmetries that one hopes might be
associated with the observed gauge symmetries in our world arise
naturally as a part of this construction.
Hierarchy problem:
Gravity is much weaker than all
of the other forces in nature. This extreme hierarchy of forces is
currently not understood, and is one form of what is often called
the hierarchy problem. Another example is that the length scale at
which the strength of all the nongravitational forces appears to
become the same—the length scale at which grand unification is
thought to occur—appears to be very much smaller than the scale
associated with the size of particles such as protons and neutrons,
and nuclei. It turns out to be very difficult mathematically to
devise theories in which this is the case, and trying to resolve
this difficulty is the hierarchy problem.

Hubble constant:
In a
uniformly expanding universe the recession velocity of distant
objects away from us is proportional to their distance from us. The
quantity determining the precise numerical relationship between
velocity and distance is named the Hubble constant, in honor of
Edwin Hubble, who first discovered this relation. Note that this
quantity is not in fact a constant over cosmological times for most
cosmological models.
Hypercube:
Another
name for a four-dimensional cube (tesseract).
Inflation:
The idea, based on notions coming from the
physics of elementary particles, that at very early times the
universe underwent a brief period of extremely rapid expansion,
during which distances increased by a factor greater than a
billion, billion, billion, billion, in a fraction of a second. Such
an expansion can naturally occur as the universe expanded and
cooled at early times if there was a phase transition associated
with a grand unified theory (see
Grand
unification
), and can moreover explain all of the observed
features of the universe today on the largest scales we can
measure.

Large hadron collider
(LHC):
The new large proton-proton collider being built at the
European Center for Nuclear Research (CERN) in Geneva. Planned to
come online by 2007–2008, it will achieve energies large enough to
explore for the mechanism underlying the origin of mass of
elementary particles, and may reveal other new phenomena such as
supersymmetry and possible large extra dimensions.
Local supersymmetry:
This involves the mathematical
formalism in which gravity and supersymmetry are combined together
in one framework. One consequence of this is that the graviton, the
fundamental quantum thought to convey the gravitational force, must
have a fermionic partner, called the gravitino.

Matrices:
Mathematical
objects which take the form of tables of numbers with separate
entries in the different rows and columns. Matrices can be
multiplied together, added together, etc. and thus have their own
kind of algebra that is more complex than the algebra of simple
real numbers. One of the eleven-dimensional limits of string
theories that form a part of M-theory involves a description of
nature in which matrices form the fundamental quantities akin to
the numbers that describe positions in our four-dimensional
space.

Metric:
The
mathematical quantity, called a tensor, that determines how
physical lengths are measured in terms of the coordinates one uses
to label the points in some space. For example, on a sphere, the
physical distance between neighboring lines of longitude decreases
as one moves to the poles. The metric tensor contains this
information of how the distance between lines of longitude changes
as you move around the surface of the sphere.

Moduli fields:
In
extra-dimensional theories such as string theory there are usually
dynamical “fields” observable in our three-dimensional world that
are associated with the actual radius of the presumably compactified
and unobservable extra dimensions. These fields are called moduli
fields, and their dynamics can either cause interesting new effects
that might be measurable in our space, or cause severe empirical
problems for model builders.

Momentum:
A force
acting on an object over some time imparts momentum to that object.
For objects moving slowly compared to the speed of light, the
momentum of the object is given by multiplying the mass of the
object by its speed.

M-theory:
The
eleven-dimensional theory that is thought to underlie all known
ten-dimensional string theories. Its existence was suggested once
it was recognized that D-branes must be included in string
descriptions, and these clarified the relationship between formerly
disparate string models, suggesting some evidence of a yet higher
dimensional theory. To date, no one has a clear understanding of
the precise nature of this theory, or even what its fundamental
variables are.

Muon:
An unstable
elementary particle, with a lifetime of one millionth of a second,
that appears to be identical to the electron, except that its mass
is about two hundred times greater. When it was first observed, the
physicist I. I. Rabi uttered, “Who ordered that?”

Naturalness:
In physics
formulas one often finds numbers comparable to unity, such as 2 or
pi. However, physicists call it “unnatural” when one finds in a
formula a very large or very small dimensionless number, like
0.00000000000000000000000001 or 35,000,000,000,000,000,000,000,
000,000. The ratio between the strength of gravity and
electromagnetism is one such very small number, for example, which
is why the hierarchy problem is one form of a naturalness
problem.

Neutrino:
A light
neutral particle produced in the radioactive decay of a neutron
(and various other particles). The neutrino has no electromagnetic
or strong interactions, and thus interacts so weakly with matter
that neutrinos produced in the decay of a neutron can, on average,
travel right through the Earth without a single collision or
interaction.

Neutron:
A neutral
elementary particle with a mass comparable to that of the proton,
and comprising, along with the proton, all atomic nuclei. Free
neutrons are unstable, decaying into protons, electrons, and
neutrinos with an average lifetime of about ten minutes.

Nonabelian gauge
theory:
A different name for Yang-Mills theories that reflects
the mathematical symmetry, called gauge invariance, that underlies
them.
Non-Euclidean geometry:
The specific
application of Riemannian geometry to spaces that are not flat.

Parallax:
The amount by
which nearby objects, when viewed from different vantage points,
will shift in comparison to distant background objects. The
magnitude of this shift can be used to determine the distance of
the nearby objects.

Parity:
A parity
transformation interchanges left and right. Certain interactions,
like the electromagnetic interaction, do not distinguish between
left and right. However, the weak interaction remarkably does
distinguish between left and right, so that neutrons rotating
around a certain axis will produce electrons that preferentially
head off in one hemisphere, as opposed to the other hemisphere.
Photon:
The elementary “quantum” of the
electromagnetic field, a.k.a. light. Because of quantum mechanics,
light has both wavelike and particlelike properties. In particular
light of a given frequency is transmitted via many individual
photons, so that for light of a low enough intensity, a detector
will be able to detect the individual packets of energy carried by
these particles, and never any smaller amounts.

Pions:
Elementary
particles produced in the collisions of energetic protons with
matter. These particles, about ten times lighter than the proton,
are made up of a quark and an antiquark, and are unstable with a
lifetime of less than a millionth of a billionth of a second.

Planck scale:
This is
the length scale (or equivalently the energy scale) at which
quantum mechanical effects relevant to gravity cannot be ignored.
Because gravity is so weak at normal scales, it turns out that one
must go to incredibly small scales before quantum effects become
important. The Planck length scale is about 10–33cm.

Precession:
If a
rotating or orbiting object returns to its initial position, and
repeats precisely the same motion again, there is no precession.
However, if upon returning to the same position, the next orbit, or
rotation, is shifted compared to the first, so that the motion does
not exactly repeat after one such cycle, one says that the orbit or
rotation is precessing.
Proton:
An
elementary particle with positive electric charge equal and
opposite to that found on the electron. The proton, which weighs
almost two thousand times as much as the electron, is located,
along with neutral particles called neutrons, within the dense
nucleus at the center of atoms. As far as we can measure, the
proton is absolutely stable, but most grand unified theories predict
the proton can decay with a lifetime too long to have yet been
measured.

Quantum electrodynamics
(QED):
The theory that successfully combines quantum mechanics,
relativity, and electromagnetism to correctly predict all phenomena
that have been observed associated with the interactions of matter
and electromagnetic radiation.

Quark confinement:
The
property that is associated with the fact that isolated quarks are
not observed in nature. While this property of the strong
interaction has not been mathematically proved yet, it appears to
arise naturally as a corollary to the fact that the force between
quarks gets weaker as they get closer together, and stronger as you
pull them apart. If this behavior continues indefinitely as you try
and pull them apart, it would take an infinite amount of energy to
produce a single quark, isolated from all its neighbors.

Riemannian geometry:
A
generalization of the flat two-dimensional geometric relations of
Euclid, applied instead to spaces that can also be curved and that
can also involve more than two dimensions.

Scattering:
When two
elementary particles collide together many different things can
happen, from a simple grazing collision in which the particles are
each deflected, to collisions in which the particles change their
identities, and in which new elementary particles are created. All
of these processes are called scattering processes.

Singularity:
Generally
describes the mathematical characteristic of any quantity that can
grow infinitely large. When referred to points in space, a
singularity refers to a region of space where the density of matter
and energy grows infinitely large, and where the classical laws of
general relativity appear to break down.
Spacetime supersymmetry:
A mathematical symmetry
that incorporates supersymmetry with the other known symmetries of
space and time, including the fact that the laws of physics are
unchanged from place to place, and from time to time.

Spacetime:
The
four-dimensional universe made up of three dimensions of space and
one dimension of time, unified together by Einstein in his special
theory of relativity, and first described by Hermann Minkowski.
Spectra:
The set of colors of
electromagnetic radiation emitted by different gases when you heat
them up. Each element has a unique set of such colors that
identifies it. The laws of quantum mechanics allow us to calculate
the spectrum of light emitted by atoms, in agreement with
observations.
Spontaneous symmetry
breaking:
This occurs when some symmetry of nature, such as
left-right symmetry, is violated by the particular circumstances in
which we find ourselves, but not by the underlying laws of physics
that govern that situation. So, for example, while electromagnetism
does not distinguish left from right, and electromagnetic
interactions are those that are chiefly responsible for the makeup
of material objects, I can nevertheless find myself standing next to
a mountain on one side of me and an ocean on another side of me. In
this case, I can clearly distinguish my left side from my right
side. Such an accident of my particular circumstances represents an
example of spontaneous symmetry breaking. Here is another one: Say
you are having dinner at a large round table. After everyone sits
down, every place-setting looks identical, and glasses are located
both to the left and right of each person. Nothing distinguishes
which glass is associated with which person until the first person
picks up a glass. After that, the original symmetry is broken, and
every glass is associated with a unique person.

Supergravity:
Another
name for local supersymmetry.
Supersymmetry:
A mathematical symmetry that relates
elementary particles having different spin angular momentum.
Specifically, supersymmetry implies that for all particles having
integer spin (bosons) there should exist particles of equal mass
having half-integer spin (fermions).
Tachyon:
A hypothetical elementary particle which
travels faster than the speed of light, and which can never be
slowed down to below the speed of light. Such a particle could
appear to an outside observer to be traveling backward in time.
Generally, if a theory appears to predict tachyonic states, it is a
sign that there is something unstable in the theory. Often such a
prediction is associated with a violation of unitarity in the
theory.
Tensor algebra:
Mathematical
relations that involve objects with multiple separate components,
each of which can have a different dependence on both space and
time.