The Quest for Unification

The Standard Model partially unifies the strong, electromagnetic and weak interactions, suggesting a common origin for them. A more fundamental theory, a Grand Unified theory or a string theory, can complete this unification and explain many of the features which in the Standard Model are put in by hand. However many possible implementations of such a theory have been suggested. We emphasize the importance of the prediction for gauge coupling unification in distinguishing between these implementations and how it can select a particular string profile. We discuss how the superstring can extend the unification to include gravity leading to a testable relation between the gravitational and Standard Model interactions. A calculation of the threshold effects to be expected from the superheavy modes shows that the relation given by the weakly coupled heterotic string is in good agreement with experiment. We discuss the resulting profile of the supersymmetric extension of the Standard Model which is consistent with these unification predictions. Its phenomenological implications should be testable by future precision experiments looking for rare flavor changing processes and also more directly by direct supersymmetric particle searches at the Large Hadron collider.