Cameron McBride with Glenn Starkman

The Propagation of Light and Gravity through Matter-filled Spacetime with Stabilized Compactified Extra Dimensions

The idea of extra dimensions beyond the usual four is far from new.  In the 1920s, Kaluza and Klein postulated an extra dimension in an attempt to unify the theory of gravity with electromagnetism.  While not completely successful, the idea continues. Modern string theory requires 10 spatial dimensions to maintain mathematical consistency.   There are also attempts to use large extra dimensions to solve the hierarchy problem  — the large ratio between the electroweak and Planck  energy scales,  which is manifested by the weakness of gravity in relation to the other forces.

      In the standard approach to string theory, the extra dimensions exist as finite curled up (“compactified”) spatial dimensions with sizes on the order of the Planck length (10^-35 meters).  In 1998, Arkani-Hamed, Dimopoulos and Dvali proposed that these could be as large as 100 microns to a millimeter, if ordinary matter (standard model particles) is confined to a three-dimensional  membrane (or “brane”) within the multi-dimensional space (or “bulk”). The spreading out of gravitational field lines in the bulk would then explain the apparent weakness of gravity. 

     Whether large or small, one can ask why the extra dimensions  are the size that they are — why don’t they expand or contract like the ordinary  dimensions?  The usual answer is that there is some dynamical mechanism (vaguely specified) which stabilizes them.  Whatever that mechanism, we should expect the size of the extra dimensions to respond, even if only slightly, to the presence of mass.  Gravity and light (unless it is confined to a brane) propagating through space should ‘sense’ the changing size of the extra dimensions. This should cause phenomena such as bending or retardation of light or gravity waves. 

      The behavior of waves traveling through inhomogeneous extra dimensions will be investigated.  An attempt will be made to use the results to constrain theories of extra dimensions.