The Science Book

312 GABRIELE VENEZIANO

even before this, some scientists
had been murmuring that the
theory was conceptually flawed.
The more work they did, the more it
seemed as though strings were not
describing the strong force at all.

The rise of superstrings
Groups of physicists continued
to work on string theory, but they
needed to find solutions to some
of its problems before the wider
scientific community would take
it seriously again. A breakthrough
came in the early 1980s with the
idea of supersymmetry. This
is the suggestion that each of
the known particles found in the
standard model of particle physics
(pp.302–05) has an undiscovered
“superpartner”—a fermion to
match every boson, and a boson
to match every fermion. If this
were the case, then many of the
outstanding problems with strings
would promptly vanish, and the
number of dimensions required to

describe them would be reduced to

ten. The fact that these additional

particles remain undetected might

be due to the fact that they are only

capable of independent existence

at energies far above those

produced in even the most powerful

modern particle accelerators.

This revised “supersymmetric

string theory” soon became known

more simply as “superstring

theory.” However, major issues

remained—particularly the fact

that five rival interpretations

of superstrings emerged.

Evidence also began to mount

that superstrings should give rise

not only to 2-dimensional strings

and 1-dimensional points, but also

to multidimensional structures,

collectively known as “branes.”

Branes can be thought of as

analogous to 2-dimensional

membranes moving in our

3-dimensional world: similarly,

a 3-dimensional brane could

move in a 4-dimensional space.

M-theory

In 1995, US physicist Edward

Witten presented a new model

known as M-theory, which offered

a solution to the problem of

competing superstring theories.

He added a single additional

dimension, bringing the total

up to 11, and this allowed all five

superstring approaches to be

described as aspects of a single

theory. The 11 dimensions of

space-time required by M-theory

mirrored the 11 dimensions

required by then-popular models

of “supergravity” (supersymmetric

gravity). According to Witten’s

theory, the seven additional

dimensions of space required

would be “compactified”—curled

up into tiny structures analogous to

spheres that would effectively act

and appear as points on all but the

most microscopic of scales.

The major problem of M-theory,

however, is that the detail of the

theory itself is currently unknown.

Rather, it is a prediction of the

existence of a theory with certain

characteristics that would neatly

fulfill a number of observed or

predicted criteria.

String theory envisions a

multiverse in which our

universe is one slice of bread

in a big cosmic loaf. The

other slices would be

displaced from ours in some

extra dimension of space.

Brian Greene

Superstring theory predicts
the existence of multidimensional
branes. Our universe might be one
such brane. It is suggested that
a Big Bang event occurs when
two branes collide, producing
a “cyclic universe” model.

1. Branes collide
   producing a
   Big Bang.

2. One brane develops into

our universe today.

4. Ripples form in the branes.

3. The branes

expand to

become flat

and empty.