American Scientist

To the Editors:

Regarding the September–October article on “cloaking” ("A Protective Cloak Against Earthquakes and Storms"), gravitational lensing and quantum tunneling are forms of cloaking, albeit on massively large and infinitesimally small scales. They may have much less practical relevance than the ones mentioned in the essay.

Alan Hull
Conyers, GA

Dr. Gbur responds:

Actually, gravitation works as a sort of “anticloak,” pulling light inwards instead of deflecting it outwards! Gravitational lensing results from light that would otherwise miss the stellar object being deflected toward it, because of the gravitational pull created by its large mass. However, because the mathematics of transformation optics (which can redirect light along a specific path) used to design cloaks is based on the same mathematical warping of space as general relativity, the phenomena are closely related. Researchers have used transformation optics to make, in essence, “optical black holes”: objects that pull in and trap light much like the gravity of a black hole does. The analogy is not perfect, however, as gravity warps space and time, whereas transformation optics devices usually only involve warpings of space.

Quantum tunneling, where particles can pass through materials on a quantum scale that they could not normally surpass, has proven very important to electron microscopy and computer chips. The idea of a tunneling and cloaking relationship is interesting—I’ll have to think more about that!