The idea that excites me the most concerns the two greatest puzzles in science: the origin of the universe, and the origin of consciousness. The origin of the universe is what I do for a living, working on string theory. But I am also fascinated by consciousness.

No one knows when a robot will approach human intelligence, but I suspect it will be late in the 21st century. Will they be dangerous? Possibly. So I suggest we put a chip in their brain to shut them off if they have murderous thoughts.

In the 1950s, we had all these B-grade science-fiction movies. The point was to scare the public and get them to buy popcorn. No attempt was made to create movies that were somewhat inherent to the truth.

Think of all the nonsense you had to learn in psychology courses. None of which was testable. None of which was measurable. We had behaviorism, Freudian psychology, all of these theories that you learn in psychology. Totally untestable. Now, we can test it, because physics allows us to calculate energy flows in the brain.

Chances are, the aliens will not want to land on our backyard, or even the White House lawn, with their flying saucers. They may have tiny, robotic self-replicating probes which can reach near light speed and can proliferate around the galaxy. So instead of the Enterprise and huge star ships, the aliens might actually send tiny probes to explore the universe. One might land on our lawn and we won't even know.

What we usually consider as impossible are simply engineering problems... there's no law of physics preventing them.

Time travel and teleportation will have to wait. It may take centuries to master these technology.