Possible Low-Energy Manifestations of Strings and Gravity

Despite the important experimental success of general relativity, there are several theoretical reasons indicating that gravitational phenomena may change radically from the predictions of Einstein’s theory at very short distances. A main motivation comes from studies of unifying all fundamental forces in the framework of a consistent quantum theory, called string theory. In the first part of my lectures (Sects. 1–6), I discuss the main motivations and directions for physics beyond the Standard Model of elementary particles and I give a short introduction on perturbative string theory. This theory introduces a new physical constant, the string length, under which a new fundamental structure appears, changing drastically the laws of nature. In particular, lowering the string scale in the TeV region provides a theoretical framework for solving the so-called mass hierarchy problem: the apparent weakness of gravity can be accounted for by the existence of large internal dimensions, in the submillimeter region, and transverse to a braneworld where our observed universe is confined. In the second part of my lectures (Sects. 7–11), I describe the main properties of this scenario and its implications for new gravitational phenomena that can be observed at both particle colliders, and in non-accelerator experiments searching for new short-range forces.