Scientists are hard at work developing real-world "invisibility cloaks" thanks to a special class of exotic manmade "metamaterials." Now a team of French scientists has suggested in a recent preprint on the physics arXiv that certain ancient Roman structures, like the famous Roman Colosseum, have very similar structural patterns, which may have protected them from damage from earthquakes over the millennia.
Falling within the broader class of photonic band gap materials, a "metamaterial" is technically defined as any material whose microscopic structure can bend light in ways it doesn't normally bend. That property is called an index of refraction, i.e., the ratio between the speed of light in a vacuum and how fast the top of the light wave travels. Natural materials have a positive index of refraction; certain manmade metamaterials—first synthesized in the lab in 2000—have a negative index of refraction, meaning they interact with light in such a way as to bend light around even very sharp angles.
That's what makes metamaterials so ideal for cloaking applications—any "invisibility cloak" must be able to bend electromagnetic waves around whatever it's supposed to be cloaking. (They are also ideal for making so-called "super lenses" capable of seeing objects at much smaller scales than is possible with natural materials, because they have significantly lower diffraction limits.) Most metamaterials consist of a highly conductive metal like gold or copper, organized in specific shapes and arranged in carefully layered periodic lattice structures. When light passes through the material, it bends around the cloaked object, rendering it "invisible." You can see anything directly behind it but never perceive the object itself.
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