Radiation is a potential problem to any time travelers using wormholes to try to go into the past, says Hawking. "As soon as the wormhole expands, natural radiation will enter it and end up in a loop. The feedback will become so strong, it destroys the wormhole. So, although tiny wormholes exist ... it won't last long enough to be of use as a time machine."

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And what about this whole time paradox thing portrayed in movies or TV shows -- in which, for example, I go back in time, meet my own father before I was born and accidentally kill him. Would that mean I'd suddenly cease to exist?

According to Kaku, if that old river of time has forks in it, there's no problem: "You've just killed somebody else's father. In that timeline, you don't exist, but you exist because you jumped the stream."

A man who would've been happy to jump that stream was Gordon Cooper, one of America's original "Right Stuff" astronauts.

Thirty years ago, I interviewed him several times on a variety of subjects, including time travel. Cooper said he knew scientists who, back then, tried to figure out how to build a time-traveling vehicle. He told me, "It's theoretically possible, although there are a lot of questions that can't be answered without trying it.

"It would take some exceptional engineering and physics talent -- people who are progressive in their thinking and far out in their technology, but I think it could be done."

Cooper added that his science buddies thought it would only be a one-way ticket, that the potential time traveler wouldn't be able to return to home base. When asked who would be stupid enough to take that kind of no-way-back trip, he calmly replied, "You're looking at him."

"Sure, I'd volunteer," said Cooper, who along with other astronauts was among America's most fearless test pilots. "But we haven't even started counting hardware yet." Cooper, who died in 2004, never had the chance to take that historic trip.

Why would scientists even want to unlock the key to time travel?

"We want to push the laws of physics until they break," explained Kaku. "Because that's where you make new discoveries, by looking at where the old theory breaks down."

Another challenge with this entire time travel issue is, even if we had the technology to utilize a black hole, we still don't have the means to actually get us there to try these theories out.

"Some people think, 'Well, maybe a backyard inventor may invent a time machine,' " said Kaku. "I don't think so -- I don't think a backyard inventor's going to find a time machine anytime soon."

Hawking imagines that time travel into the future is absolutely possible. "If we want to travel into the future, we just need to go fast. Really fast. And I think the only way we're ever likely to do that is by going into space.

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