**T**he
holy grail of physicists has been to develop a “theory of everything,”
a theory
that would combine general relativity and quantum mechanics into one.
Perhaps
the most promising current idea for such a theory is called superstring theory.
In it, vibrating strings, not point-like particles, are the fundamental
constituents.

The different resonances of the strings create the different
particles we see.
Each string is unimaginably small, about 10^{20}, or
100 billion billion times smaller
than a proton, and can vibrate only in a
space-time consisting of 10 dimensions.
Physicists and cosmologists will have to
wait to see whether superstring theory
ends up being the theory of
everything—its predictions have to be verified, after al,
or just another
beautiful theory that falls by the wayside. But at least they have found
a
theory that seems worth pursuing.

Superstring theory resolves the most enigmatic problem of twentieth century theoretical physics: the mathematical incompatibility of the foundational pillars of quantum mechanics and the General Theory of Relativity. In doing so, string theory modifies our understanding of spacetime and the gravitational force. One recently discovered consequence of this modification is that spacetime can undergo remarkable rearrangements of its basic structure requiring the fabric of spacetime to tear apart and subsequently reconnect. Such processes are at best unlikely and probably impossible in pre-string theories as they would be accompanied by violent physical effects. In string theory, on the contrary, these processes are physically sensible and thoroughly common.

The usual domains of general relativity and quantum
mechanics are quite different. General relativity describes the force of gravity
and hence is usually applied to the largest and most massive structures
including stars, galaxies, black holes and even, in cosmology, the universe
itself. Quantum mechanics is most relevant in describing the smallest structures
in the universe such as electrons and quarks. In most ordinary physical
situations, therefore, either general relativity *or* quantum mechanics
is required for a theoretical understanding, but not both. There are, however,
extreme physical circumstances which require both of these fundamental theories
for a proper theoretical treatment.

Prime examples of such situations are spacetime *singularities* such
as the central point of a black hole or the state of the universe just before
the big bang. These exotic physical structures involve enormous mass scales
(thus requiring general relativity) and extremely small distance scales (thus
requiring quantum mechanics). Unfortunately, general relativity and quantum
mechanics are mutually incompatible: any calculation which simultaneously uses
both of these tools yields nonsensical answers. The origin of this problem can
be traced to equations which become badly behaved when particles interact with
each other across minute distance scales on the order of 10^{-33} cm (
10^{-34} in): the *Planck length*.

String theory solves the deep problem of the incompatibility of these two
fundamental theories by modifying the properties of general relativity when it
is applied to scales on the order of the Planck length. String theory is based
on the premise that the elementary constituents of matter are not described
correctly when we model them as point-like objects. Rather, according to this
theory, the elementary ``particles'' are actually tiny closed loops of string
with radii approximately given by the Planck length. Modern accelerators can
only probe down to distance scales around 10^{–16} cm (10^{-17}
in) and hence these loops of string *appear* to be point objects.
However, the string theoretic hypothesis that they are actually tiny loops,
changes drastically the way in which these objects interact on the shortest of
distance scales. This modification is what allows gravity and quantum mechanics
to form a harmonious union.

There is a price to be paid for this solution, however. It turns out that the
equations of string theory are self consistent only if the universe contains, in
addition to time, *nine* spatial dimensions. As this is in gross conflict
with the perception of three spatial dimensions, it might seem that string
theory must be discarded. This is not true.

The idea that our universe might have more than the three
familiar spatial dimensions is one which was introduced more than half a century
before the advent of string theory by T. Kaluza and by O. Klein. The basic
premise of such Kaluza-Klein theories is that a dimension can be either large
and directly observable or small and essentially invisible. An analogy with a
garden hose can be helpful. From a distance, a garden hose looks like a long one
dimensional object. From a closer vantage point (or from a long distance using a
visual aid) an additional dimension, the circular dimension winding around the
hose, becomes evident. Thus, depending on the scale of sensitivity of the
observer, the hose will either appear as one or two dimensional. Kaluza-Klein
theories state that the same thing can be true of the universe. No experiment
rules out the possible existence of additional spatial dimensions curled up
(like the circular dimension of the hose) on scales smaller than 10^{-16}
cm ( 10^{-17} in), the limit of present day accessibility. Although
originally introduced in the context of point particle theories, this notion can
be applied to strings. String theory, therefore, is physically sensible if the
six extra dimensions which it requires curl up in this fashion.

A universe with both extended dimensions (two shown) and curled up dimensions (two shown).

A remarkable property of these theories is that the precise size, shape,
number of holes, etc. of these extra dimensions *determines* properties
such as the masses and electric charges of the elementary `particles'.

A number of issues, unresolved at present, prevent the
application of string theory to the analysis of the kind of spacetime
singularities described above. The theory can be successfully applied, though,
to another class of singularities which control the *topology* of the
universe.

Topology is a mathematical concept that embodies those properties of a geometrical space which do not change if the space is stretched, twisted or bent but not torn. A doughnut and a sphere are distinct from the topological viewpoint because there is no way to deform one into the other smoothly, that is, without tearing either object. A doughnut and a teacup, both of which have one hole, can be continuously deformed into each other and hence have the same topology.

General relativity predicts that the fabric of spacetime will smoothly deform
its size and shape in response to the presence of matter and energy. A familiar
manifestation of this spacetime stretching is the expansion of the universe. The
*topology* of the universe, however, remains fixed. A long standing
question is whether there might be physical processes which, unlike those
familiar from general relativity, cause the topology of the universe to change.
There is a heuristic reason for suspecting this possibility based on a naive
application of quantum mechanics. Namely, a universal feature of quantum
mechanics is that on the smallest distance scales even the most quiescent
systems undergo `quantum jitter': the value of quantities characterizing the
system fluctuate, sometimes violently, averaging out to their measured values on
larger distance scales. This notion, applied the fabric of spacetime, yields the
image of a frothing, undulating structure on small distance scales which
averages out on larger scales to the smooth geometrical description of general
relativity. It is conceivable that, behind the veil of quantum jitter, the
fabric of spacetime could momentarily tear and subsequently reconnect in a
manner resulting in a change of the topology of the universe. Prior to the
advent of string theory, the incompatibility of general relativity and quantum
mechanics made it impossible to address this possibility in a quantitative
manner.

Due to the above reasoning, the possibility of spacetime topology change was suggested as a novel characteristic of the union of gravity and quantum mechanics. String theory, which achieves this union, has recently been shown to permit physical processes which do result in a particular kind topology change, at least in the extra six dimensional component of spacetime.

There is a well studied mathematical operation called a *flop* which
is a systematic procedure for changing the topology of a geometrical space in a
“minimal” manner. It involves singling out a sphere in the space,
continuously shrinking its volume down to zero (leaving the rest of the space
fully intact) and then blowing its volume back up, but in an orthogonal
direction. The point at which the volume is zero is the singularity which may be
considered as a minimal tear. The result of this operation is a new geometrical
space whose topology is different from the original. The change in topology is
not as drastic as that between a doughnut and a sphere, but nonetheless it is
different.

Mathematically, this is a rigorously defined and well studied operation. It
can, for instance, be applied to the curled up six dimensional part of spacetime
in a theory based on strings. The crucial question is whether this operation is *physically*
realizable. The criterion for determining this is simple: can this operation be
achieved in a manner which does not result in any catastrophic physical
consequences? In general relativity the answer to this question is no as the
physical model ceases to make sense at the singular point - the point at which
the chosen sphere has zero volume. Since string theory differs from general
relativity on short distance scales, it is conceivable that a different answer
might emerge. At first sight, however, even the equations of string theory
appear difficult to analyze in this context. Only with the tool of mirror
manifolds can this question be addressed.

Four years ago it was shown that the interpretation of string theory using the Kaluza-Klein idea of curled up dimensions comes with a remarkable twist. Two completely different possibilities for the curled up space (different sizes, shapes and number of holes) can, if properly chosen, give rise to identical observable physics. This is completely unexpected from a point particle viewpoint. The reason for this is that in point particle theories, the physical and mathematical descriptions of a geometric space are both based on considering it to be a collection of an infinite number of points grouped together in a particular manner. In string theory, the physical model is based on tiny loops and hence differs markedly from the mathematical description. This, in turn, allows two mathematically distinct curled up spaces to yield physically identical string models. This is a purely string theoretic phenomenon which relies profoundly on the extended nature of a string.

Although either member of a mirror pair gives rise to the same physical theory, the technical description of a given physical process very often differs drastically between the two constructions. In fact, certain processes which have an extremely complicated, and difficult to analyze, description when one curled up space is used, have a transparent, and easy to analyze, description when the mirror is used.

Recently, the mirror description of the topology changing flop operation discussed above has been analyzed. This results in a remarkable simplification of the string equations governing this process. An analysis of these simplified equations has revealed that there are no catastrophic physical consequences of this topology changing process. In fact, the mirror description makes it clear that such topology changing events are not only physically realizable, but commonplace as well. Thus, using the tool of mirror manifolds, it has been shown that the long suspected possibility of topology changing processes can be explicitly realized in string theory.