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."
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."
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.
"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."