Art Bell welcomes Sean Carroll, senior research associate in physics at the California Institute of Technology, for a wide-ranging discussion on the nature of time, space-time, and the possibility of time travel. Carroll explains that time has multiple definitions in physics, from the universal clock that labels events to the personal time measured by individual observers, a distinction Einstein revealed through relativity.

Carroll walks through the science of black holes, describing evidence for a supermassive black hole at the center of our galaxy with a mass millions of times that of the sun. He explains why real time travel, if possible, would resemble space travel rather than the Hollywood version of stepping into a machine and vanishing. The discussion covers upcoming experiments at CERN, why creating a small black hole there would pose no danger, and how gravity is really the curvature of space-time caused by mass and energy.

The conversation also addresses dark energy, the accelerating expansion of the universe, and the mystery of entropy and the arrow of time. Art and Sean explore why time appears to move in only one direction and what the low-entropy state of the early universe tells us about the origins of everything.