"A spacetime cloak, or event cloak, is a means of manipulating electromagnetic radiation in space and time in such a way that a certain collection of happenings, or events, is concealed from distant observers," writes Miguel Lerma. This is his introduction to a simple device, mainly built from mirrors, that has the ability to cloak time. Or, as Technology Review writes, create holes in time. Which sounds both awesome and incredibly dangerous.
Isn't this the kind of thing Back to the Future's Doc Brown warned us about? Can Lerma's device create some kind of world-rending time paradox? Well, not really. As dramatic as the term "spacetime cloak" may be, the actual effect of a spacetime cloak is more like an optical illusion, though it's an illusion with some rad science behind it.
Lerma's paper explains how a spacetime cloak typically functions: "An event cloak design using metamaterials...works by using a medium in which diﬀerent parts of the light illuminating a certain region can be either slowed down or speed up. A leading portion of the light is speeded up so that it arrives before the events occur, whilst a trailing part is slowed down and arrives too late. After their occurrence, the light is reformed by slowing down the leading part and speeding up the trailing part. The distant observer therefore only sees a continuous illumination, whilst the events that occurred during the dark period of the cloak’s operation remain undetected."
His device creates the same effect, but using a different method. The metamaterials mentioned in the excerpt above are expensive, but mirrors are cheap. Lerma writes "rather than slowing down and speeding up light, we manipulate an obscurity gap by diverting the light through paths of appropriate length with an arrangement of switchable transflective mirrors."
By inserting a series of reflective mirrors between a light source and an object--like, say a clock--and then another series of mirrors between that object and a camera (or observer), it's possible to extend the length of time it takes light to travel from one point to another. Lerma's setup would use switchable transflective mirrors that can alternate between reflective and nonreflective states.
By precisely controlling those mirror states, the spacetime cloak could completely obscure the object from the observer. Lerma writes that "Conceptually, a safecracker can enter a scene, steal the cash and exit, whilst a surveillance camera records the safe door locked and undisturbed all the time." However, as we understand the system, the obscurity gap only lasts as long as it takes the light to travel through the first series of mirrors. And given how fast light travels, you're going to need some serious distance to create a substantial time gap.