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鈥檛 exist yet, there has been some progress in hiding small objects at certain frequencies.
鈥淚f you have an ideal cloak in front of you in a room, it鈥檚 hard to tell if there is an object with a cloak around it, or if there is nothing,鈥� says 聽at the Ludwig Maximilian University of Munich in Germany.
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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鈥檚 theory of relativity spoils the fun. That鈥檚 because relativistic effects kick in as you approach the speed of light, preventing the cloak from working. 鈥淚f I鈥檓 moving with respect to you, what I see as space and time is different from what you see,鈥� he says.
Speed of light
The team studied a simulated cloak capable of hiding a single frequency of green light, and modelled it moving at various speeds in a laboratory illuminated by that frequency. They found that just 1 per cent of the speed of light (around 10 million kilometres per hour) was enough to start ruining the cloaking effect. The light鈥檚 frequency decreases if the cloak is moving away from you or increases if it鈥檚 moving towards you, similar to the way an ambulance siren鈥檚 pitch changes as it drives past you. That means the light is no longer at the cloak鈥檚 operational frequency, revealing its presence. The effect is even more extreme at higher speeds.
鈥淵ou start seeing something fishy,鈥� says Halimeh. 鈥淚t鈥檚 as if you see some ghost emerging in front of you, popping out of nowhere into existence.鈥� In a , Halimeh studied cloaks that operate at multiple frequencies, and found that a slightly different effect also prevents them from working at relativistic speeds.
of the Weizmann Institute of Science in Rehovot, Israel, says he always expected invisibility cloaks would struggle to keep up at high speeds. 鈥淲hat I found surprising was that one needs rather larger velocities, a per cent of the speed of light in a vacuum, to see significant adverse effects,鈥� he says. 鈥淚t reminds me of the driver who claimed to have seen the red traffic light shifted to green, and was fined for relativistic speeding.鈥�
We won鈥檛 need to worry about near-light speed travel any time soon, but Halimeh points out that GPS satellites already have to account for minute relativistic effects, so it鈥檚 possible that future applications of transformation optics will have to pay attention to Einstein鈥檚 theory. 鈥淧hysicists have to gauge the problem from every angle,鈥� he says.
Journal reference: Physical Review A,
