rationalskepticism.org

It is worthwhile to note that the E8 x E8 Heterotic string has historically been considered to be the most promising string theory for describing the physics beyond the Standard Model. It was discovered in 1987 by Gross, Harvey, Martinec, and Rohm and for a long time it was thought to be the only string theory relevant for describing our universe. This is because the SU(3) x SU(2) x U(1) gauge group of the standard model can fit quite nicely within one of the E8 gauge groups. The matter under the other E8 would not interact except through gravity, and might provide a answer to the Dark Matter problem in astrophysics. Due to our lack of a full understanding of string theory, answers to questions such as how is supersymmetry broken and why are there only 3 generations of particles in the Standard Model have remained unanswered. Most of these questions are related to the issue of compactification (discussed on the next page). What we have learned is that string theory contains all the essential elements to be a successful unified theory of particle interactions, and it is virtually the only candidate which does so. However, we don't yet know how these elements specifically come together to describe the physics that we currently observe.