"I think we have the best chance of any species and actually the best chance that humanity has ever had," he says. "We now have something in our hands that may destroy the Earth and possibly a chain reaction that destroys the universe." He adds: "If you keep thinking, well, we survive, we go to the next sun ... Eventually the question will be, if you just keep doing that forever at some point, the universe either continues to expand or could collapse back in itself"