Byline: PJ Slinger

Could Harry Potter's invisibility cloak actually work? Would the "Star Wars" Death Star really be capable of blowing up an entire planet? Is it possible for Scotty from "Star Trek" to beam people up?

Of course, in fictional worlds, anything is possible. But in the real world, where is the line between possible and impossible? Dr. Michio Kaku, a theoretical physicist, attempts to answer this question in his latest book, "Physics of the Impossible" (Anchor Books, $15.95 paperback).

"When reading or watching science fiction (as a child), I realized most of the technologies are impossible," Kaku said recently in a phone interview. "But impossibility is sometimes relative. It's relative to what era you live in. About one hundred years ago, people thought that airplanes were impossible. But that's because we didn't understand a lot of the physics, and if we understand the laws of physics, we realize that most of the technologies of science fiction are actually possible."

In his book, Kaku places "the impossible" into three categories.

Class I...