There's a small explanation here of what he meant: http://www.robertgraboyes.com/writings_files/Bank%20at%20the%20End.htm http://www.robertgraboyes.com/writings

There's a small explanation here of what he meant: http://www.robertgraboyes.com/writings_files/Bank%20at%20the%20End.htm http://www.robertgraboyes.com/writings_files/Bank%20at%20the.... His point definitely seems to be more intelligible after reading that article, but he seems to have a very limited definition of time travel. First, it must allow travel both ways. Second, one must be able to change the past. Third, the changed past must be the same as our past (Think of each new time travel instance as a new stack frame, with the stack variables being history. If one were to go back in time and change the past, one could create a new "stack frame" that leaves the history intact in its own timeline.) However, one doesn't need a rather weak economic argument for the impossibility of time travel -- analyzing a light cone shows that causality will break down at speeds greater than that of light. Thus, traveling greater than the speed of light is equivalent to time travel. Using the Lorentz factor, we can trivially derive the fact that achieving a speed greater than that of light for a particle with finite mass requires an infinite amount of energy -- thus, time travel is impossible (assuming Einstein's Theory of Relativity is correct, which we are pretty damn certain about).