Does gravity attract light?

An electromagnetic "black hole" that sucks in surrounding light has been built for the first time. The device, which works at microwave frequencies, may soon be extended to trap visible light, leading to an entirely new way of harvesting solar energy to generate electricity. A theoretical design for a table-top black hole to trap light was proposed in a paper published earlier this year by Evgenii Narimanov and Alexander Kildishev of Purdue University in West Lafayette, Indiana.

Their idea was to mimic the properties of a cosmological black hole, whose intense gravity bends the surrounding space-time, causing any nearby matter or radiation to follow the warped space-time and spiral inwards. Narimanov and Kildishev reasoned that it should be possible to build a device that makes light curve inwards towards its centre in a similar way. They calculated that this could be done by a cylindrical structure consisting of a central core surrounded by a shell of concentric rings.

The key to making light curve inwards is to make the shell's permittivity – which affects the electric component of an electromagnetic wave – increase smoothly from the outer to the inner surface. This is analogous to the curvature of space-time near a black hole. At the point where the shell meets the core, the permittivity of the ring must match that of the core, so that light is absorbed rather than reflected.

Now Tie Jun Cui and Qiang Cheng at the Southeast University in Nanjing, China, have turned Narimanov and Kildishev's theory into practice, and built a "black hole" for microwave frequencies. It is made of 60 annular strips of so-called "meta-materials", which have previously been used to make invisibility cloaks.