A laser beam in an experiment propagates along the two-dimensional surface of
a glass object shaped like an hourglass, curling once around the middle of the object.
This is an example of an object with negative surface curvature (like a saddle,
for example), in contrast to an object with positive surface curvature, such as a sphere.
(Image: Vincent Schultheiß)
(January 15, 2016) To investigate the influence of gravity on the propagation of light, researchers usually have to examine astronomical length scales and huge masses. However, physicists at Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU) and Friedrich Schiller University Jena have shown that there is another way. In a recent issue of the journal Nature Photonics they find the answers to astronomical questions in the laboratory, shifting the focus to a previously underappreciated material property – surface curvature
According to Einstein’s general theory of relativity, gravity can be described as the curvature of four-dimensional spacetime. In this curved space, celestial bodies and light move along geodesics, the shortest paths between two points, which often look anything but straight when viewed from the outside.
The team of researchers led by Prof. Dr. Ulf Peschel from Friedrich Schiller University Jena used a special trick to examine the propagation of light in such curved spaces in the laboratory. Instead of changing all four dimensions of spacetime, they reduced the problem to two dimensions and studied the propagation of light along curved surfaces. However, not all curved surfaces are the same. ‘For example, while you can easily unfold a cylinder or a cone into a flat sheet of paper, it is impossible to lay the surface of a sphere out flat on a table without tearing or at least distorting it,’ says Vincent Schultheiß, a doctoral candidate at FAU and lead author of the study. ‘A well known example of this is world maps that always show the surface in a distorted way. The curvature of the surface of a sphere is an intrinsic property that can’t be changed and has an effect on geometry and physics inside this two-dimensional surface.’