The revolutionary concept of string theory is a bold realization of Einstein's dream of an ultimate explanation for everything from the tiniest quanta of particle physics to the cosmos itself. String theory unifies physics by producing all known forces and particles as different vibrations of a single substance called superstrings. String theory brings quantum consistency to physics with an elegant mathematical construct that appears to be unique.
Do superstrings exist? The strings themselves are probably too tiny to observe directly, but string theory makes a number of testable predictions. It implies supersymmetry and predicts seven undiscovered dimensions of space, dimensions that would give rise to much of the mysterious complexity of particle physics. Testing the validity of string theory requires searching for the extra dimensions and exploring their properties. How many are there? What are their shapes and sizes? How and why are they hidden? And what are the new particles associated with the extra dimensions?
The physical effects of extra dimensions depend on their sizes and shapes, and on what kinds of matter or forces can penetrate them. The sizes of the extra dimensions are unknown, but they should be related to fundamental energy scales of particle physics: the cosmological scale; the density of dark energy; the TeV electroweak scale; or the scale of ultimate unification. It may be possible to infer extra dimensions of macroscopic size from inconsistencies in cosmological observations, or from precision tests of short-range gravitational forces. More likely, the extra dimensions are microscopic, in which case high-energy particle accelerators and cosmic-ray experiments are the only ways to detect their physical effects.
The ATLAS and CMS experiments at the LHC will address many questions about extra dimensions: How many extra dimensions are there? What are their shapes and sizes? How are they hidden? What are the new particles associated with extra dimensions? Through the production of new particles that move in the extra space, the LHC experiments will have direct sensitivity to extra dimensions 10 billion times smaller than the size of an atom. There is also a chance that, due to the existence of extra dimensions, microscopic black holes may be detected at the LHC or in the highest energy cosmic rays.
Ultimately particle physics seeks to know if dark energy, dark matter and cosmic inflation are affected by the physics of extra dimensions. Collider data will provide insight into the exploration of these deep connections.