### 5.6 Romulan cloaking devices

A wonderful application of analogue gravity techniques is the design of cloaking devices [385, 498, 387].
How to achieve invisibility, or more properly, low observability has been a matter of extensive study for
decades. With the appearance of a technology capable of producing and controlling meta-materials and
plasmonic structures [3], cloaking is becoming a real possibility.
To achieve cloaking, one needs to ensure that light rays (beyond geometric optics it is impossible to
produce perfect invisibility [449, 690]) effectively behave as if they were propagating in Minkowski
spacetime, although in reality they are bending around the invisible compact region. One way of producing
this is, precisely, to make rays propagate in Minkowski spacetime but using non-Cartesian coordinates. Take
the Minkowski metric in some Cartesian coordinates , , and apply a diffeomorphism, which is
different from the identity only inside the compact region. One then obtains a different representation
of the flat geometry. Now take the to be the Cartesian coordinates of the real
laboratory spacetime, and build this metric with the meta-material. By construction, the scattering
process with the compact region will not change the directions of the rays, making everything
within the compact region invisible. Recent implementations of these ideas have investigated the
concept of a “spacetime cloak” or “history editor” that cloaks a particular event, not a particular
region [439].

To end this brief account we would like to highlight the broad scope of application of these ideas:
Essentially the cloaking techniques can be applied to any sort of wave, from acoustic cloaking [133] to
earthquake damage prevention [192]. More radically, and with enough civil engineering, one might adapt
the ideas of Berry [67, 68] to anti-tsunami cloaking.