Gravity and strings

TL;DR

This review discusses how string theory offers solutions to key issues in gravitational physics, including non-renormalizability, extra dimensions, and the cosmological constant, highlighting its successes and ongoing challenges.

Contribution

It provides a comprehensive overview of string theory's approach to gravity, emphasizing recent progress in understanding extra dimensions and their implications for fundamental physics.

Findings

String theory is an ultraviolet-finite quantum gravity theory.

Extra dimensions can explain fermion generations and gauge symmetry breaking.

Recent work may illuminate the origin of the cosmological constant and the weakness of gravity.

Abstract

This is a broad-brush review of how string theory addresses several important questions of gravitational physics. The problem of non-renormalizability is first reviewed, followed by introduction of string theory as an ultraviolet-finite theory of gravity. String theory's successes also include predicting both gauge theory and fermions. The difficulty of extra dimensions becomes a possible virtue, when one notes that these lead to mechanisms to explain fermion generations, as well as a means to break the large gauge symmetries of string theory. Finally, a long standing problem of string theory, that of fixing the size and shape of the extra dimensions, has recently been addressed and may shed light on the origin of the cosmological constant, the ultimate fate of our universe, as well as the question of why gravity_is_ so weak.