There are five (and only five) types of superstring theory which are

free of mathematical inconsistencies. The adjective "super" means

that the string theory entails both fermions (matter, like quarks and

electrons) and bosons (force carriers, such as the photon and gluon),

and is thus capable of describing everything in the Universe.

Type I - the first to be discovered, this is the only superstring

theory which contains both open and closed strings (think of broken

and unbroken rubber bands) which can link up with one another or break

apart. All the other types deal exclusively with closed strings. The

open strings can carry charges, labels which identify the particle, on

their free ends.

Type IIA and Type IIB - the first is "non-chiral" and the second is

"chiral". Non-chiral means that a certain particle in the theory can

spin along its direction of motion or opposite to its direction of

motion. In the chiral theory, this particle can only spin one way.

Another word for chirality is "handedness". For example, most screws

are right-handed because they advance when rotated clockwise. If all

the screws in your theory were right-handed, then your theory would be

chiral. If your theory contains both right- and left-handed screws,

then your theory is non-chiral. Of the five superstring theories,

only Type IIA is non-chiral.

Type II theories have two "supersymmetries", while the other three

theories have only one supersymmetry. Briefly, a supersymmetry is a

relation between fermions and bosons.

SO(32) Heterotic and E8 x E8 Heterotic - "heterotic" means that

vibrations propagating around the closed string in one direction are

distinct from vibrations propagating around in the opposite direction;

that is, they correspond to completely different particles.

The first is based on a symmetry called SO(32), which is the

32-dimensional analog of rotations. For example, a two-dimensional

circular disk can be rotated in the plane and still appears the same

-- it has symmetry SO(2), for rotations in two dimensions. A sphere

in three dimensions has rotational symmetry SO(3).

The second is based on two copies of the E8 symmetry. This theory

describes two parallel universes which can only interact with each

other through the gravitational force. We inhabit one copy; the other

copy is referred to as the "shadow" universe. A shadow planet could

in theory exist very close to Earth, but we would not be able to see

it because its shadow photons would not interact with our eyes. In

fact, shadow matter could pass right through ordinary matter because

the shadow quarks and shadow electrons do not "feel" ordinary quarks

and electrons. We could, however, feel the shadow planet's gravity.

These five superstring theories and eleven dimensional supergravity

(which is not a string theory) are related by duality transformations.

That is, one theory in a certain limit looks like another theory in a

different limit. Because it is possible to transform any of these six

theories into any other, they all appear to be limiting cases of one

all-encompassing theory dubbed "M-theory".

Taken from Randall Scalise, Faculty, Physics, Southern Methodist University

Source is linked.

Resources

Top Categories

Product

Company