He has many good articles. http://www.mkaku.org/articles/physics_of_space_travel.shtml
Excerpt: "The first realistic attempt to analyze extra-terrestrial civilizations from the point of view of the laws of physics and the laws of thermodynamics was by Russian astrophysicist Nicolai Kardashev. He based his ranking of possible civilizations on the basis of total energy output which could be quantified and used as a guide to explore the dynamics of advanced civilizations: Type I: this civilization harnesses the energy output of an entire planet. Type II: this civilization harnesses the energy output of a star, and generates about 10 billion times the energy output of a Type I civilization. Type III: this civilization harnesses the energy output of a galaxy, or about 10 billion time the energy output of a Type II civilization." <more>
