Kaku divides his book into three sections, based on the level of difficulty presented by the technologies in question:
- Class I impossibilities, which don't violate the laws of physics and aren't quite achievable yet. These are mostly technologies we're halfway there with already (robots), or for which we've got all the components, just no (affordable) means of combining them (starships, forcefields).
- Class II impossibilities, which are possible only with vastly improved technology and scientific understanding, some of which we can't begin to predict or imagine (time machines, faster-than-light travel, parallel universes).
- Class III impossibilities, which violate the laws of physics (perpetual motion machines, precognition).
Kaku goes into the history and development of each technology, explains where we are right now, and discusses ways we could make (or approximate) the technology. Usually the tech that's actually possible doesn't quite line up with what we get on TV, but I'm taking that as a good thing. Might as well write what's accurate, right?
For example, his chapter of phasers is mostly about current and near future developments in powerful lasers, how to make said lasers portable, and the cool things you can do with nuclear fusion (including at the star level). Yes, they're all ray guns. No, they're not exactly phasers à la Star Trek. I'm pretty sure you can't set a laser to stun.
Something else I liked about this book is that Kaku hesitates to call anything impossible. The two Class III's I mentioned above? They're his only completely impossible tropes. The rest he says is possible, either within the next couple hundred years or after a couple thousand, because we'll need that much time to further our understanding or organize the building efforts.* There's a nice sense of hope to the book, which is a pleasant contrast to other predictive books I've read, which don't account for sudden jumps in technology or the various ways an invention can be taken.
Physics of the Impossible isn't an in-depth study of the science needed to come up with the Science Fiction Universe, and shouldn't be taken as one. It's a starting point, a book to be read for ideas and basic understanding. It's also a good book to read if you want to get an overview of where we are now**, or if you want to see how someone can take scientific fact and work forward to really cool stuff like invisibility shielding for spaceships.
Final verdict (and this'll probably go for all the books I discuss here): check it out!
*For instance, one way to travel by wormhole is to link a black hole to a white hole, but we'd have to create the white hole and then find a way to stick it to the black hole indefinitely without making everything blow up. We're nowhere near that ability yet, but we could be in, say, the year 4000.
** More or less. It was published in 2008.