Engage the cloaking device! Fire up the warp drive! Wait, how come the enemy ship can see us? It turns out that invisibility cloaks stop working completely when travelling near light speed, and even smaller velocities can give you away.
That reads a bit like a line from a Harry Potter/Star Trek mash-up, but researchers have been working on real invisibility cloaks for a while now. The devices draw on a theory called transformation optics, which allows for the design of materials that can bend the path of light around them, hiding anything inside from view. While full-blown cloaks don’t exist yet, there has been some progress in hiding small objects at certain frequencies.
“If you have an ideal cloak in front of you in a room, it’s hard to tell if there is an object with a cloak around it, or if there is nothing,” says Jad Halimeh at the Ludwig Maximilian University of Munich in Germany.
Although the cloak looks like a patch of empty space, Halimeh and his colleagues wondered if there was a way to distinguish between the cloak and nothingness.
It seems that the answer is yes: Albert Einstein’s theory of relativity spoils the fun. That’s because relativistic effects kick in as you approach the speed of light, preventing the cloak from working. “If I’m moving with respect to you, what I see as space and time is different from what you see,” he says.
