The ancient Romans may have built structures that acted as seismic protection screens long before physicists were aware of metamaterials or transform optics. This is what French researchers suggest in their study Role of nanophotonics in the birth of seismic megastructures .
In it they state that the pattern of the foundations of some Roman theaters and amphitheaters closely resembles the characteristics of modern electromagnetic cloaking devices. They say that these archaeological metamaterials they may have indirectly provided earthquake protection by deflecting seismic waves around the sand.
Metamaterials are artificial structures that manipulate electromagnetic waves or sound in ways not normally found in nature. A mathematical framework called transformation optics has been developed to design new devices made of metamaterials, including invisibility cloaks that deflect microwaves around objects.
One possible application of metamaterials is the creation of structures that deflect seismic waves around buildings to protect them from earthquakes. The idea is to surround a building with a network of holes or solid objects buried in the ground. When seismic waves within a certain range of wavelengths pass through that network, multiple reflections in the network destructively interfere with each other to create a band gap resulting in a significant reduction in building shaking.
Stéphane Brûlé and civil engineers from the Ménard company in Lyon, together with researchers from the Fresnel Institute in Marseille, demonstrated this idea in 2012 when they drilled a two-dimensional network of wells into the ground, each 5 meters deep. By generating acoustic waves using a nearby source, they discovered that much of the wave energy was reflected back to the source by the first two rows of holes.
However, Brûlé considers that the ancient Romans may have discovered this first, albeit without knowing it. He was on vacation looking at the archaeological remains of the city of Autun, in central France, when he saw an aerial photograph showing the foundations of a Gallo-Roman theater buried under a field just off the road. Although barely noticeable, the markings on the field showed the outline of the 1st century AD building. and he considered the semicircular structure to bear an uncanny resemblance to half of an invisibility cloak .
This was confirmed by a photo of an archaeological study carried out a few years earlier that mapped the foundations of the theater much more clearly. Superimposing that photo and one of a 20-centimetre-diameter invisibility cloak constructed by Brûlé and his colleagues at the Fresnel Institute, he found that the theater's pillars and cloak elements were almost exactly aligned, each arranged in a series. of concentric (semicircular) circles that approach each other at smaller radii.
Brûlé found that the same thing happened with the foundations of the Colosseum in Rome and other amphitheatres, which, unlike the semicircular theaters, are completely closed. In particular, he found that the ratio of the radii of neighboring concentric circles (or ellipses, in the case of most amphitheatres) was nearly identical.
As for how the Romans arrived at this design, Brûlé suggests that they may have gradually modified their amphitheaters thanks to experience gained over several centuries in highly seismic areas. But it could also just have been a matter of luck.
Sébastien Guenneau of the Fresnel Institute has run computer simulations to test the idea, but says the complexity of the real ground and its influence on the passage of seismic waves makes ground experiments essential. He hopes to build a model consisting of buried concrete piles arranged in characteristic concentric ellipses with an outer diameter of at least 20 meters. This scale, he explains, would be suitable for seismic waves (of short wavelength) propagating in soft soils.