Invisibility comes into view

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They won't help you sneak around Hogwarts unobserved. Nor will they help a Klingon spaceship attack the USS Enterprise without being detected.

But scientists are getting ever closer to fabricating practical Harry Potter-style invisibility cloaks.

In the past, they have experimented with materials that can direct and control the propagation and transmission of specified parts of the light spectrum - and so could be used to render an object seemingly invisible.

Last month, a team at the University of St Andrews in Scotland, led by Dr Andrea Di Falco, reported the creation of a flexible cloaking material they call Metaflex, which may bring commercial and industrial applications significantly closer.

Now the journal Nature has revealed that two groups - one in Singapore and the other at the University of Birmingham and Imperial College London - have made objects each a few centimetres in diameter invisible.

They credit the properties of calcite crystals - and calcite is a cheap and common mineral made of calcium carbonate.

"Carpet cloaks" - the term the scientists prefer to use - make covered objects invisible by bending light rays as they enter the cloak and then when they leave it.

Calcite has unique optical properties and in this instance, light is bent so that the rays seem to have been reflected directly from the ground below the object - as though it was not there.

The team at the Singapore-MIT Alliance for Research and Technology (Smart) has built a calcite carpet cloak that can shield a small steel wedge measuring 38mm by 2mm from red, green and blue visible light. It is designed to work under water.

"I think governments could make a lot of use out of a cloak that can hide objects on the seabed," George Barbastathis, a mechanical engineer who helped develop the Smart cloak, told Nature.

Cloaking devices are keenly awaited by the military. Defence chiefs believe such devices will enable them to hide planes, ships, spacecraft, tanks and other vehicles from radar. More advanced versions could ultimately be good enough to make objects or troops invisible to observers.

The Birmingham and Imperial team, led by theoretical physicist Sir John Pendry, has constructed a calcite cloak that can hide objects several centimetres in height. The cloak works only in the air.

Hopes of developing fully functional invisibility cloaks soon have been boosted by the fact that the Singapore and London devices turned out to be much cheaper to make than earlier attempts. Those first versions were built using intricately fabricated and highly expensive silicon microstructures.

But the materials needed to make the Smart cloak cost less than £1000 ($2060).

One of the concerns about cloaking used to be that it could be used to hide only microsize objects. But physicist Michal Lipson of Cornell University in New York says the breakthroughs point to an exciting future.

"We are close to cloaking objects that we are familiar with in everyday life."

- Observer