The impossible physics of Harry Potter

Physicist Michio Kaku explores the possibilities of invisibility cloaks, time travel, and other sci-fi wonders.

Michio Kaku is a physicist specializing in string theory at the City College of New York; it would seem he's also a major Star Trek fan. The show, along with other science fiction classics, is a constant touchstone in "Physics of the Impossible," his latest book, which invites readers to take a romp through the barely possible. Kaku even uses Star Trek's chief engineer, Scotty, to state the book's starting point: "I canna' change the laws of physics, Captain!"

Physics of the Impossible: A Scientific Exploration into the World of Phasers, Force Fields, Teleportation, and Time Travel is a popularization of physics and it reads like nothing so much as a thought-provoking manual for science fiction writers who want to get it right. Kaku takes 15 sci-fi staples, from invisibility cloaks to robots to parallel universes, and divides them up into Class I, II, and III impossibilities. Class I impossibilities are impossible at present, but could be just around the corner. No laws of physics forbid them. Class II impossibilities might be realized if humanity lives long enough and becomes advanced enough to take on such minor public works projects as particle accelerators 10 light-years long or "laser beams as large as a solar system or star cluster."

Class III impossibilities are just plain impossible. Sorry.

But there are fewer truly impossible things than one might expect. Time travel is a mere Class II impossibility, for example. Kaku walks us through the history of and current thinking on each problem, making clear the technical hurdles and sketching out the possible solutions – all with a minimum of jargon and confusion. As long as one is somewhat familiar with, for example, what an electron is, the waters of incomprehensibility will only occasionally close overhead, and never for long. The latter chapters, dealing as they do with things at the very edge or on the other side of impossibility, are a bit heavier going.

Science fiction writers, even science-minded daydreamers, will find that instead of becoming disappointed with what is not possible they will more likely be inspired to new plot twists by the limitations of nature. Laser guns are perfectly possible ... if they are plugged in. (Until a portable power pack capable of delivering the oomph of a commercial power station is invented, space duels won't be able to stray far from the surge protector. )

Similarly, an invisibility shield is potentially possible, and Kaku explains how, but since it can't be seen through from the inside, eyeholes would be needed. Perhaps disembodied floating eyes would be less inconspicuous than a whole visible person. But then again, one could listen, or even mind-read. While we might someday be able to read the vague tenor of another's thoughts, Kaku says, the details may be forever inaccessible.

And though it may not spark any story ideas, there is a certain grim satisfaction to finding out that to make Harry Potter invisible without a special shield "one would have to liquefy him, boil him to create steam, crystallize him, heat him again, and then cool him." Take that, boy wizard.

Kaku covers a lot of ground, including a fair bit of biology for sections on telepathy and a subchapter on suspended animation, and he moves at a decent clip. This book is not for those seeking to deeply understand string theory or quantum mechanics. It delivers just enough science to back up and make sense of the upshot: Can we or can't we? If we can't, why not? And if we can, how? Because Kaku doesn't get weighed down by details, this is a good choice for someone interested in physics but timid about the nuts and bolts. It's like those cupcake tops they sell in bakeries – the yummy bits without the foundation.

"Physics of the Impossible" is an invigorating experience. Much that seems impossible, isn't. The book's reach in time and space is so immense, that there's even a slight danger of light-headedness – especially when Kaku writes of nano-spacecraft that would replicate themselves on distant planets and send more nano-spacecraft further out, creating "a sphere of trillions of these robots, multiplying exponentially as it grows in size, expanding at nearly the speed of light." Not for the faint of heart, images like that.

Kaku does sail from topic to topic a bit abruptly, and his prose has a few irritating quirks – all his sentences are roughly the same length and he's absolutely addicted to parentheses – but these are minor quibbles. It's a small price to pay for a real sense of the vastness of space, the infinite length of time, and the admirable cleverness of humanity.

• Emma Marris is a freelance writer in Columbia, Mo.