You know, if you asked Christiaan Huygens in 01671 about the possibility of detecting a 4-km-long tunnel getting shorter by a factor of one in a sextillion for a hundredth of a second, he surely would have doubted that it was possible. But it's only been 350 years, and when LIGO detected its first gravitational waves five years ago, that's precisely how it did it.

By contrast, there's ample reason to believe that black-hole power plants are possible. In fact, the spatial precision required is actually the same order of magnitude as that of LIGO, about an attometer. So we might not even have to wait 350 years for it.