'A Brief History of Time' is more than a lengthy commentary on the complex concepts and theories described in the legendary 'Brief History.' Seventeen years later, the authors, Stephen Hawking and Leonard Mlodinow, take a fresh look at age-old questions: the nature of space and time, God's role in creation, and the past and future of the universe. Data obtained by astronomical observatories, space telescopes, and physics laboratories have become a solid foundation for theoretical physics, which has been struggling for decades to develop a coherent, unified field theory that would unite all known forms of interaction. Many believe this is string theory, the propositions and consequences of which Professors Hawking and Mlodinow expound at length. Dark matter and dark energy, known to us only through gravitational effects, and time travel, which the authors believe 'is not worth betting on yet'—all of this is at the center of their attention.