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
cm ( 10
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
cm ( 10
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.
Information provided by: http://www.lassp.cornell.edu