Spacetime invisibility cloaks can hide entire events inside temporal voids

Invisibility cloaks are taking baby steps from science fiction to science fact. But there's an even stranger and more incredible possibility - an invisibility cloak that works in both space and time, shielding whole events from history itself.

The basic idea is surprisingly simple. We see things because our eyes are able to interpret information from visible light. That light travels at, obviously enough, the speed of light, which means we see things once light has traveled from them to our eyes. Technically speaking, we never see anything the exact instant happens - there's always a time lag between the object and when we see it.

This is fairly well-known for anyone with an interest in astronomy. The Moon is one light-second away, the Sun a little over eight light-minutes away, the nearest star 4.3 light-years away, the nearest galaxy , and so on. There are time lags on even everyday distances, however impossibly slight. For something a meter away, we see it as it was about 1/300,000,000th of a second ago.

Here's how you use that information to build a spacetime cloak. All those times are for the speed of light in a vacuum, but light can slow down depending on what material it passes through. Researchers have slowed down light to as little as 38 miles per hour by sending it through a special, super-dense form of matter. And it's through carefully slowing down and speeding up light that we can create a temporal void.

Let's say you're standing a mile away from an observer. You start slowly decreasing the speed of the light traveling towards the observer so that it's only traveling at 60 miles per hour, or a mile per minute. Since you're slowing the light down gradually, the observer won't be able to perceive the change. Once the light has reached 60 miles per hour, the observer is now seeing you as you were one minute ago. You've now created a one minute spacetime corridor. You've now got a minute to do whatever you want without the observer having any idea what you're up to.

Spacetime invisibility cloaks can hide entire events inside temporal voids

Once your minute is up, you switch off the machine slowing down the light, it speeds back up, and the observer now sees you again as you are right now - or at least as you were a nanosecond ago. That minute is a temporally compressed blip that the observer cannot perceive. That a basic version of the idea put forward by researchers at Imperial College London. Here's their somewhat more technical explanation:

This process is thus utterly distinct from a spatial 'object' cloak, which instead achieves concealment by bending light around an object. The STC opens a temporary
corridor through which energy, information and matter can be manipulated or transported undetected. Once the concealed passage has been used, the STC closes by slowing the leading part of the light, whilst speeding up the trailing part, leaving no trace of the cloak, or the concealed events, on the field profile. To a distant observer, any non-emitting object whose presence persisted throughout the lifetime of the STC will have had a finite interval excised from its history. Any object which emits light during STC operation will have that part of its history temporally compressed so that, if not absorbed, it will appear to the observer as if occurring in a single instant.

This is quite possibly either the most awesome or the most dangerous idea ever, something the authors themselves admit when their first example of a potential application is a safe-cracker using such a cloak to rob a bank undetected. But can such a cloak be built? The researchers certainly think so, even if there are a mountain of technical challenges to navigate, and they foresee a race to create the first spacetime cloak. For now, the idea can just be an awesome new thing for science fiction writers to add to their repertoire...assuming any sane reader would ever be able to believe such a device could be possible.

[Journal of Optics]