"To travel in time, we'll have to go more than 2,000 times faster [than the Apollo 10 spacecraft, which reached 25,000 mph]. And to do that, we'd need a much bigger ship, a truly enormous machine ... that would have to be big enough ... to accelerate it to nearly the speed of light. Getting to just beneath the cosmic speed limit would require six whole years at full power."

Kaku, on the other hand, offers two alternative scenarios in which you or I might enter a time machine.

"One is if aliens from outer space have already done it and come to Earth; and the second is maybe our descendants, who are very advanced, come backward in time.

"So, I say that, one day, if somebody knocks on your door and claims to be your great-great-great-great-great-granddaughter, don't slam the door."

And if Kaku, one of the most respected scientists in the world, could do it, which direction in time would he prefer to travel?

"I'd actually prefer to go to the future. You know, nature gives you a finite life span, but all the good stuff -- antimatter drives and parallel universes -- takes place beyond your lifetime. So, I want to see beyond my years."

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