Physics of the Impossible 5

Class II impossibilities
Class II Impossibilities are “technologies that sit at the very edge of our understanding of the physical world," possibly taking thousands or millions of years to become available.[8]
Such a technology is time travel. Einstein’s equations do show that time travel is possible. However, this would not be developed for a time scale of centuries or even millennia from now. This would make it a Class II impossibility. The two major physical hurdles are energy and stability. Traveling through time would require the entire energy of a star or black hole. Questions of stability are: will the radiation from such a journey kill you and will the “aperture” remain open so you can get back?[7] According to Dr. Kaku in an interview, “the serious study of the impossible has frequently opened up rich and unexpected domains of science”.[9]
[edit]Class III impossibilities
Class III Impossibilities are “technologies that violate the known laws of physics." Kaku writes about only two of these, perpetual motion machines and precognition. Development of these technologies would represent a fundamental shift in human understanding of physics.[3][8]