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...

Back to the Future Noel Vasquez/Getty Images The DeLorean Time Machine The spacetime continuum was frequently mentioned in the movie "Back to the Future." Doc Brown and Marty travel from 1985 to 1955 in a DeLorean that has been converted into a time machine. What Marty and the Doc soon realize is that interactions with Marty's parents in 1955 threaten to unravel the fabric of the spacetime continuum. Check out more information about DeLorean DMC-12.

­ ­