An ordinary mirror swaps left and right. Many laws of physics don't notice the swap: they are mirror-symmetric. But some phenomena, like the weak interaction, break this symmetry. Until now, it was thought that gravity and light each break mirror symmetry in their own independent way.
The authors of the new work thought differently: perhaps this is just an illusion caused by viewing the world from four dimensions — three spatial and one temporal. Add a fifth dimension, and gravity and light might turn out to be parts of a single geometry. Then their mirror-symmetry violations should be shared. The researchers built such a five-dimensional model in two geometric variants. The first is Riemannian geometry, where space is curved like the surface of a sphere. The second is teleparallel, where there is no curvature and interactions are transmitted through torsion.
The most striking conclusion concerns waves. The model predicts that the frequency shift for an electromagnetic wave is exactly six times larger than for a gravitational wave. This ratio does not depend on the background, i.e., the surrounding environment. It can be tested by comparing data on the polarization of the cosmic microwave background, the ancient light left over from the Big Bang, and gravitational waves.
The exact factor of six between the frequency shifts of light and gravity was not fine-tuned: it fell out of the equations on its own, as an inevitable consequence of the five-dimensional geometry.
The idea of extra dimensions where forces unify evokes Michio Kaku's 'Hyperspace' or the Tesseract world from the movie 'Interstellar'.