Published online 19 September 2008 | Nature | doi:10.1038/news.2008.1113


Opening the door to Hogwarts

Scientists demonstrate how to make a hidden portal.

Phil BallDr Ball ponders the relevant technological nicheSarah Loriot/Nature

For readers bored with stories about how physicists are developing invisibility cloaks1 just like the one that teenage wizard Harry Potter uses to sneak around Hogwarts School of Witchcraft and Wizardry, here's a welcome respite: physicists developing a way of hiding entrances to things — just like the entrance to the railway platform that Harry Potter uses to get to Hogwarts in the first place.

As our illustration shows, signs for Platform 93/4 already exist at King's Cross station in London — a stone's throw from the Nature offices — but visitors attempting to push a trolley through to the mystical platform itself will be in for a rude shock. Xudong Luo and his colleagues at Shanghai Jiao Tong University have now figured out how the whole thing could be made real. In two preprints, they describe a method for concealing an entrance so that what looks like a blank wall actually contains invisible openings2,3.

Physicist John Pendry of Imperial College in London, whose theoretical work on metamaterials laid the foundations for the trick, says he feels that the effect is even more remarkable than the invisibility cloak, because it seems so counter-intuitive that an object can project itself into empty space. "It is a bit like magic — terrific fun," he says.

Speak, friend, and enter

Metamaterials are structures pieced together from tiny electrical devices that allow them to interact with light in ways that are impossible for ordinary substances. Some metamaterials have a negative refractive index, meaning that they bend light the 'wrong' way. This means that an object within the metamaterial can appear to float above it. A metamaterial invisibility shield, meanwhile, bends light smoothly around an object at its centre, like water flowing around a rock in a river. The Shanghai group recently showed how the object can be revealed again with an anti-invisibility cloak4.

Now they have worked out in theory how to hide a doorway. The trick is to create an object that, because of its unusual interactions with light, looks bigger than it really is. A pillar made of such stuff, placed in the middle of an opening in a wall, could appear to fill the gap completely, whereas in fact there are open spaces to each side.

Pendry and S. Anantha Ramakrishna demonstrated the basic principle in 2003, when they showed that a cylinder of metamaterial could act as a magnifying lens for an object inside it5.

Milk bigger than its bottle

"When you look at a milk bottle, you don't see the glass", Pendry explains. Because of the way in which the milk scatters light, "the milk seems to go right to the edge of the bottle." Now Luo and colleagues have shown that an even more remarkable effect is possible: the milk can appear to be outside the bottle. "It's like a three-dimensional projector", says Pendry. "I call it a super-milk bottle."

The Chinese team opts for the rather more prosaic 'superscatterer'. They show that such an object could be made from a metal core surrounded by a metamaterial with a negative refractive index1.


The researchers have calculated how light interacts with a rectangular superscatterer placed in the middle of a wide opening in a wall, and find that, for the right choice of sizes and metamaterial properties, the light bounces back just as it does if there was no opening2.

If someone passes through the concealed opening, they find, it becomes momentarily visible before disappearing again once they are on the other side.

But the calculations so far only show concealment for microwave radiation, not visible light. Pendry says that the problem in using visible-light metamaterials — which were reported last month6,7 — is that they tend to absorb some light rather than scattering it all into the magnified image, making it hard to project the image a significant distance beyond the object's surface. Openings hidden from the naked eye aren't likely "until we get on top of these materials", Pendry says. 

  • References

    1. Schurig, D. et al. Science 314, 977–980 (2006).
    2. Yang, T., Chen, H., Luo, X. & Ma, H. Preprint at (2008).
    3. Luo, X., Yang, T., Gu, Y. & Ma, H. Preprint at (2008).
    4. Chen, H., Luo, X., Ma, H. & Chan, C. T. Preprint at (2008).
    5. Pendry, J. B. & Ramakrishna, S. A. J. Phys. Cond. Matter 15, 6345–6364 (2003).
    6. Valentine, J. et al. Nature 455, 376–379 (2008).
    7. Yao, J. et al. Science 321, 930 (2008). | Article | PubMed | ChemPort |
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