Splitting light to make events invisible

It’s always about bending light so that an object becomes invisible when you hear about scientists working on invisibility cloaks. Dexter Johnson (Nanoclast blog on the IEEE [Institute of Electrical and Electronics Engineers] website) recently featured some of the newest work in this area in his July 7, 2011 posting about a graphene cloaking device (based on the concept of ‘mantle cloaking’) proposed by researchers at the University of Texas at Austin.

Ian Sample in his July 13, 2011 posting on The Guardian Science blogs describes an entirely different approach, one that focusses on cloaking events rather than objects. From Samples’s posting,

The theoretical prospect of a “space-time” cloak – or “history editor” – was raised by Martin McCall and Paul Kinsler at Imperial College in a paper published earlier this year. The physicists explained that when light passes through a material, such as a lens, the light waves slow down. But it is possible to make a lens that splits the light in two, so that half – say the shorter wavelengths – speed up, while the other half, the longer wavelengths, slow down. This opens a gap in the light in which an event can be hidden, because half the light arrives before it has happened, and the other half arrives after the event.

In their paper, McCall and Kinsler outline a scenario whereby a video camera would be unable to record a crime being committed because there was a means of splitting the light such that 1/2 of it reached the camera before the crime occurred and the other 1/2  reached the camera afterwards. Fascinating, non?

It seems researchers at Cornell University have developed a device that can in a rudimentary fashion cloak events (from Samples’s posting),

The latest device, which has been shown to work for the first time by Moti Fridman and Alexander Gaeta at Cornell University, goes beyond the more familiar invisibility cloak, which aims to hide objects from view, by making entire events invisible.

Fridman’s and Gaeta’s research is to be published in Nature magazine at some time in the future and I look forward to hearing more about how this ‘space/time invisibility cloak’ works and whether or not other scientists can replicate the effect.

One final comment, Samples mentioned a special July 2011 issue (freeish download)  of Physics World devoted to invisibility. Excerpted from Matin Durrani’s July 8, 2011 posting on the Physics World blog,

It is perhaps a little-known fact that Griffin – the main character in H G Wells’ classic novel The Invisible Man – was a physicist. In the 1897 book, Griffin explains how he quit medicine for physics and developed a technique that made himself invisible by reducing his body’s refractive index to match that of air.

While Wells’ novel is obviously a work of fiction, the quest for invisibility has made real progress in recent years – and is the inspiration for this month’s special issue of Physics World, which you can download for free via this link [they do  want your contact details].

Kicking off the issue is Sidney Perkowitz, who takes us on a whistle-stop tour of invisibility through the ages – from its appearance in Greek mythology to camouflaging tanks on the battlefield – before bringing us up to date with recent scientific developments.

While it’s not yet possible to hear more Fridman’s and Gaeta’s device until Nature publishes their research, Sample offers more details based on materials, Demonstration of temporal cloaking, the researchers submitted to the arvix database on Monday, July 11, 2011.

I wonder what would happen if you had both kinds of invisibility cloaks at work. It brings to mind a Zen koan (I’ve paraphrased it), If a tree falls in the forest and no one is there, does it make a sound?

Or in this case: If you can’t see the object (light bending cloak), and you never saw the event (temporal cloak), did it exist and did it happen?


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