Collection of Personal 'REAL' life as well as my VIRTUAL Second Life. How do we know one or the other is real life? The short answer is: we don't. We can never prove that we're not all hallucinating, or simply living in a computer simulation. But that doesn't mean that we believe that we are. (Sean Carroll Cosmologist and Physics professor specializing in dark energy and general relativity, research professor in the Department of Physics at the California Institute of Technology)