General relativity typically deals with situations involving large mass objects in fairly large regions of spacetime whereas quantum mechanics
is generally reserved for scenarios at the atomic scale (small
spacetime regions).
The two are very rarely used together, and the most
common case in which they are combined is in the study of black holes.
Having "peak density", or the maximum amount of matter possible in a
space, and very small area, the two must be used in synchrony in order
to predict conditions in such places; yet, when used together, the
equations fall apart, spitting out impossible answers, such as imaginary
distances and less than one dimension.
The major
problem with their congruence is that, at sub-Planck (an extremely small
unit of length) lengths, general relativity predicts a smooth, flowing
surface, while quantum mechanics predicts a random, warped surface,
neither of which are anywhere near compatible.
Superstring theory
resolves this issue, replacing the classical idea of point particles
with loops. These loops have an average diameter of the Planck length,
with extremely small variances, which completely ignores the quantum
mechanical predictions of sub-Planck length dimensional warping, there
being no matter that is of sub-Planck length.
The Five Superstring Interactions
The five superstring interactions
There are five ways open and closed strings can interact. An interaction in superstring theory is a topology changing event. Since superstring theory has to be a local theory to obey causality
the topology change must only occur at a single point. If C represents a
closed string and O an open string, then the five interactions are,
symbollically:
OOO + CCC + OOOO + CO + COO
All
open superstring theories also contain closed superstrings since closed
superstrings can be seen from the fifth interaction, they are
unavoidable. Although all these interactions are possible, in practice
the most used superstring model is the closed heterotic E8xE8
superstring which only has closed strings and so only the second
interaction (CCC) is needed.
Sources:
Wikipedia
