Two weeks ago, the Wire reported on a new material, developed by scientists at the University of St Andrews, that might one day be used to create an invisibility cloak. Now comes the news that a different material, using the same scientific principles, could also be possible--except this material could render an object invisible in time as well as space.
Zeeya Merali at Nature explains:
The ideal space–time cloak — the theory behind which is published in the Journal of Optics today — would be an upgraded version of the 'invisibility cloak' that was first proposed in 2006... Such standard invisibility cloaks are made from metamaterials — substances with a complex internal structure that allows them to channel light around objects like water flowing around a rock in a river... "I realized that it may be possible to use metamaterials to bend light rays in both space and time, not just in space," says Martin McCall, an optical physicist at Imperial College London who led the team behind the new proposal. "This would add a new dimension to the invisibility cloak — literally." The key feature of the proposed space–time cloak is that its refractive index — the optical property that governs the speed of light within a material — is continually changed, pulling light rays apart in time.
So what does this mean? Essentially, Merali says, it means physicists have sketched out a design for a material that could bend light into the past. To borrow a metaphor from McCall: Throw an ordinary invisibility cloak over a safe, and it looks like the safe has vanished. Throw a time-warping invisibility cloak over the safe, and even if you break into the safe, what an observer will see is the safe as it existed in the past--ordinary and unmolested.
Panic! Chaos! Doc Brown is coming to steal our money! Well, no, says Merali. Most likely none of us will live long enough to see this get past the design stage:
McCall and his colleagues have calculated the precise properties of the metamaterial needed to build a space–time cloak that would be perfectly invisible, and there are fundamental problems that prevent it from being constructed. The theoretical calculations work only in a vacuum, and to create a space–time void of even a few minutes would require a cloak bigger than Earth because of the space required to recombine the accelerated leading edge and slowed trailing edge of the light wave. Worst of all, the theory requires the metamaterial to boost light rays beyond the fundamental speed of light.
So when we said "possible" up there in the first paragraph, obviously we meant "conceivable at the furthest edges of scientific speculation, in conditions of a perfect vacuum, assuming breakthroughs that haven't been made yet, and under suspension of some fundamental laws of physics." But! Let nothing you dismay, CNN--keep on writing about the "cloak" in the present tense, and talking about what it does "while currently only existing in mathematical theory."