Light, which in everyday experience travels in straight beams, has been trapped on complex curved surfaces. The feat is not just a parlour trick - it could help people visualise how light travels in the curved fabric of space.
According to Einstein's general theory of relativity, gravity is the result of an object's mass deforming space itself, like a bowling ball on a trampoline. To model how light's path would change in space curved by gravity, Ulf Peschel of the University of Erlangen-Nuremberg in Germany and colleagues constructed smooth 3D objects and sent laser beams shooting along their surfaces (Physical Review Letters, in press).
They took advantage of the fact that light bends, or refracts, when it moves from one medium to another. In their simplest experiment, they shot laser light at the edge of a solid glass sphere. The angle of the beam was chosen so that the light - initially travelling in air - would be bent just enough when it entered the glass that it would keep reflecting off the inside surface of the sphere, and so travel along it. When the light inside the sphere reflected off its inner surface, some was also transmitted through the glass, creating a glowing ring on the outside surface (see image).
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