Harry Potter and the Invisibility Cloak: Part 2. Hidden in time

Scientists have demonstrated an upgrade of the invisibility cloak. Rather than hiding objects from view, it hides events
Harry Potter
Where did we put that invisibility cloak? Harry Potter and the Deathly Hallows: Part 2. Photograph: Jaap Buitendijk/AP

The timing could hardly have been better. As the final curtain closes on the Harry Potter franchise, scientists have demonstrated a newfangled upgrade of the invisibility cloak.

A nod to Harry Potter has become the Wilhelm scream of journalists' stories about invisibility cloaks. The main reason, of course, is to help out the picture editor, who understandably struggles to illustrate the invisible. But the boy wizard serves another purpose: a gentle reminder that as factual as the stories are, invisibility cloaks are still in the realms of fiction.


Fridman declined to discuss the cloak, details of which were posted on the arxiv database on Tuesday, because the paper has been submitted to Nature, which has strict rules about what can and cannot be said before an article is published. There is enough in the paper to draw out the basic principle though.

The first generation of invisibility cloaks remain a work in progress. They fool the eye by bending light around an object, much as water flows around a pebble in a stream. So far as an observer is concerned, the object simply isn't there. That, at least, is the idea. So far, few invisibility cloaks work with visible light, and those that do hide only small objects, such as paperclips, in polarised light.

The next generation of cloaks demonstrated by Fridman's group work in a different way. Instead of bending light around an object, they create a blindspot in time, during which an event can happen without being noticed.


Writing in July in a special issue of Physics World devoted to invisibility (you can download it here), McCall and Kinsler describe the ultimate bank heist, where a robbery takes place under the watchful gaze of CCTV cameras that completely miss the crime because it is hidden by a space-time cloak. Switch the cloak on, and half the light scattering off the bank vault into the CCTV camera arrives before the break-in begins, while the second half arrives after the robber has tidied up and fled. The camera sees nothing but an unchanging scene.

Fridman's demonstration is not quite so dramatic. He used one set of lenses to prise open a gap in a beam of light, by slowing down long wavelengths, such as red, and speeding up short wavelengths, such as blue. With a second set of lenses, he then closed the gap, so at the end of the experiment, the light beam looked exactly as it did at the start.

Fridman's cloak is not about to aid the perfect crime. The longest event it could hide would last only around 1.25 microseconds. A test described in the paper hid an event – some interference caused by another light beam – that was even faster.

It is worth remembering these are early days for invisibility cloaks. The first rudimentary device came out of Duke University only five years ago. But already scientists are making great progress. The space-time cloak has a long way to go, but make it work and who knows what it will be used for? Even Harry Potter had to start somewhere.