Scientist and TV personality Michio Kaku attends the premiere of the Discovery Channel's "Life" at Alice Tully Hall in New York's Lincoln Center on March 4.

If you could travel backward or forward in time, which direction would you go, and what would you do when you got there? It's the stuff of so many books, television shows, movies and daydreams.

From H.G. Wells' "The Time Machine" to the "Back to the Future" trilogy -- not to mention "Bill & Ted's Excellent Adventure," "Star Trek," "Lost" and "Hot Tub Time Machine" -- there's no doubt that the concept of time travel has become a well-established theme in science fiction.

But two of the world's most respected scientific minds now say that time travel could become a part of science reality.

American theoretical physicist Michio Kaku, who is featured on the Science Channel's "SciQ Sundays," told AOL News that in physicist Albert Einstein's theory of relativity, the concept of time is like a river.

"Einstein taught us that there's a fourth dimension, known as time, and that we freely move back and forth in space, but doing that in time is a little more complicated."

Kaku says that as early as 1935, Einstein considered the possibility of using wormholes -- theoretical holes in time -- as potential roadways for time travel. He also suggests a black hole -- a region of space that transforms space and time -- as another method for traveling through time.

"Black holes distort this fabric of space-time maximally, and you can show that black holes can be time machines," Kaku said. "If you go through a black hole, in principle, you can go to another point in space-time, just like [the fictional] Alice's looking-glass."

Across the pond, England's leading physicist, Stephen Hawking, agrees that there's a fourth dimension, or "a length in time as well as space."

Hawking suggests in his Discovery Channel series, "Into the Universe," that wormholes and black holes could be useful in achieving time travel.

Writing in the U.K's Daily Mail, Hawking, who refers to himself as a "physicist, cosmologist and something of a dreamer," wonders if it's possible to open "a portal to the past or find a shortcut to the future ... and can we ultimately use the laws of nature to become masters of time itself?

"Time travel was once considered a scientific heresy. I used to avoid talking about it for fear of being labeled a crank," Hawking wrote. "But these days I'm not so cautious."

Hawking says he's obsessed with time and suggests that, to understand time the way most physicists do, it's important to consider the fourth dimension.

Putting aside for the moment all of the scientific rules, equations and technology that would be required to make time travel a practical reality, what about the dangers of moving into a black hole or wormhole to try and get to the other side of the "looking-glass"?

"The question is, if you go into this wormhole, does the wormhole stay open, is it stable as you go through it?" Kaku said. "And Hawking goes back and forth on this question."

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