Understanding Time

Astronomer Carl Sagan had it right when he said that time is "resistant to simple definition." Lots of us think we know what time is, but it is hard to define. You can not literally see or touch time, but you can see its effects. The evidence that we are moving through time is found in everything -- our bodies age, buildings weather and crumble, trees grow. Most of us feel the pressure of time as we are pushed to meet deadlines and make appointments. Our lives are often dictated by what time we need to be somewhere.

­Ask most people to define time and they are likely to look at their watch or a clock. We see time as the ticking of the hands on these devices. We know that there are 60 seconds in a minute, 60 minutes in an hour, 24 hours in a day and 365 days in a year. These are the basic numbers of time that we all learned in grade school.

Time is also defined as being the fourth dimension of our universe. The other three dimensions are of space, including up-down, left-right and backward-forward. Time cannot exist without space, and likewise, space cannot exist without time. This interconnected relationship of time and space is called the spacetime continuum, which means that any event that occurs in the universe has to involve both space and time.

According to Einstein's theory of special relativity, time slows as an object approaches the speed of light. This leads many scientists to believe that traveling faster than the speed of light could open up the possibility of time travel to the past as well as to the future. The problem is that the speed of light is believed to be the highest speed at which something can travel, so it is unlikely that we will be able to travel into the past. As an object nears the speed of light, its relativistic mass increases until, at the speed of light, it becomes infinite. Accelerating an infinite mass any faster than that is impossible, or at least it seems to be right now.

But time travel in the other direction is not as difficult, and the future may one day be a possible destination...

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.

There may be no other concept that captures the imagination more than the idea of time travel -- the ability to travel to any point in the past or future. What could be cooler? You could jump into your time machine to go back and see major events in history and talk to the people who were there! Who would you travel back to see? Julius Caesar? Leonardo da Vinci? Elvis? You could go back and meet yourself at an earlier age, go forward and see how you look in the future... It's these possibilities that have made time travel the subject of so many science fiction books and movies.

­It turns out that, in some sense, we are all time travelers. As you sit at your desk, doing nothing more than clicking your mouse, time is traveling around you. The future is constantly being transformed into the past with the present only lasting for a fleeting moment. Everything that you are doing right now is quickly moving into the past, which means we continue to move through time.

Ideas of time travel have existed for centuries, but when Albert Einstein released his theory of special relativity, he laid the foundation for the theoretical possibility of time travel. As we all know, no one has successfully demonstrated time travel, but no one has been able to rule it out either.

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.