Physics of the Impossible

Kaku, a theoretical physicist and professor, builds the book around a simple question: is what science fiction imagines pure fantasy, or technology humanity just hasn't mastered yet? To answer it, he sorts each idea into three classes, according to the distance between the dream and what current physics allows.

Class I covers what is impossible today but doesn't violate any known law: force fields, invisibility, ray guns, particle teleportation, telepathy through brain reading, psychokinesis, robots, extraterrestrials and UFOs, starships, and antimatter. According to the author, some of this could become everyday technology in this century or the next.

Class II is tougher: faster-than-light travel, time travel, and parallel universes sit at the edge of what physics allows, and would require engineering thousands or millions of years ahead of ours. Class III, meanwhile, groups perpetual motion and precognition, ideas that run straight into well-confirmed laws, such as the second law of thermodynamics.

Movies love an invisible shield that stops bullets midair. The closest physics has come to that is plasma, a gas so hot that electrons break loose from atoms, held in place by a magnetic field, as already happens in experimental fusion reactors.

Today that field only holds plasma; it doesn't block bullets or rocks. Kaku compares the situation to having the car's engine ready but still lacking the body and the brakes: the central piece exists in theory, but the whole assembly is yet to be built, which could take decades of materials research.

Invisibility in novels is magic; in the lab, it's materials engineering. So-called metamaterials already exist, structures smaller than the wavelength of light, able to steer light around an object instead of letting it hit and bounce off, a bit like a river flowing around a rock without changing its course as seen from far away.

The difficulty, according to the book, is making this work for the entire range of visible light and for an object the size of a person, without costing a fortune or requiring materials beyond industrial scale. It's Class I because physics allows it; engineering just has to catch up to real size.