Cloaking devices are one of the inventions of science fiction that have made a few tentative steps towards the real world in recent years. Now, researchers have moved the concept into the fourth dimension, creating a setup that hides a specific point in time from being perceived by observers. But if you want to make an event disappear, you have to act fast: right now, we can only hide a few picoseconds worth of time.

The cloaking devices we’ve made all work based on a similar principle: light that enters the device is bent in such a way that when it exits, its location and direction make it appear that the device itself, and anything within it, were not present. In other words, while within the device, light travels as if it were present. It’s just that, once it exits the other side, there’s no evidence that anything unusual has taken place. The same general idea governs the action of a temporal cloaking device.

The basic idea is that, when it’s not in operation, a light beam can pass through the cloaking device unhindered. When it’s switched on, a short temporal gap is opened up in the beam, then sealed back up on its way out of the hardware. One way to think of this is to view the light beam as a bit of old-fashioned magnetic tape. You can cut the tape so that a single instant of a recording can be physically separated. While separated, you can pass anything you want through the gap, but when you glue the tape back together, the recording is seamless. There’s only a before and after while the tape is cut and separated.

It’s easy to do that with tape, but a bit harder to do it with a beam of light. The key to the process is what’s being termed a split time lens, which is matched with a dispersive medium. When activated, the lens takes the light that comes before the point of cloaking and shifts it to bluer wavelengths, which travel faster through the dispersive medium than the base speed of the light in the same medium. At the cloak point, the lens switches, shifting the light beam to longer, redder wavelengths. These travel through the dispersive medium more slowly.

(via Optical setup helps researchers hide an event from time)