As we discussed earlier, the theory of relativity states that as the velocity of an object nears the speed of light, time slows down. Scientists have discovered that even at the speeds of the space shuttle, astronauts can travel a few nanoseconds into the future. To understand this, picture two people, person A and person B. Person A stays on Earth, while person B takes off in a spacecraft. At takeoff, their watches are in perfect sync. The closer person B's spacecraft travels to the speed of light, the slower time will pass for person B (relative to person A). If person B travels for just a few hours at 50 percent the speed of light and returns to Earth, it will be obvious to both people that person A has aged much faster than person B. This difference in aging is because time passed much faster for person A than person B, who was traveling closer to the speed of light. Many years might have passed for person A, while person B experienced a time lapse of just a few hours. Find out more about this twin paradox in How Special Relativity Works.

Wormholes could allow you to travel into the past and the future.

If wormholes could be discovered, it might allow us to travel to the past as well as the future. Here's how it would work: Let's say the mouth of the wormhole is portable. Then person B in the example above, who traveled at 50 percent of light speed into space for a few hours, could carry one wormhole mouth into space, while the mouth at the opposite end of the wormhole would stay with person A on Earth. The two people would continue to see one another as person B traveled into space. When person B returned to Earth a few hours later, a few years may have passed for person A. Now, when person A looks through the wormhole that traveled into space, that person will see him or herself at a younger age, the age he or she was when person B launched into space. The cool thing about it is that the older person A would be able to step into the past by entering the wormhole, while the younger person B could step into the future.

If we are ever able to develop a workable theory for time travel, we would open up the ability to create very complicated problems called paradoxes. A paradox is defined as something that contradicts itself. Here are two common examples:

• Let's say, for the sake of argument, that you could travel back to a time before you were born. The mere fact that you could exist in a time before you were born creates a paradox. If you were born in 1960, how could you exist in 1955?

• Possibly the most famous paradox is the grandfather paradox. What would happen if a time traveler went back and killed one of his or her ancestors before the traveler was born? If the person killed his or her grandfather, then how could that person be alive to go back and kill his or her grandfather? If we could change the past, it would create an infinite number of paradoxes.

Another theory regarding time travel brings up the idea of parallel universes, or alternative histories. Let's say that you do travel back to meet your grandfather when he was a boy. In the theory of parallel universes, you may have traveled to another universe, one that is similar to ours, but has a different succession of events. For instance, if you were to travel back in time and kill one of your ancestors, you've only killed that person in one universe, which is no longer the universe that you exist in. And if you then try to travel back to your own time, you may end up in another parallel universe and never be able to get back to the universe you started in.

The idea here is that every action causes the creation of a new universe, and that there are an infinite number of universes that exist. When you killed your ancestor, you created a new universe, a universe that was identical to your own up until the time you changed the original succession of events.

Confused yet? Welcome to the world of time travel. Just imagine how complicated the ticket prices will be.

Mullu Merkabah