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It's About Time

      It dominates our lives. If we don't know what to do with it we just kill it. If we can't find it, we still use it. We can spend it trying to save it. Whether we take it, keep it, or give it, we still run out of it. We often lose track of it when we are getting the most out of it. Some seem to have plenty while for others there is never enough. On any given day, though, we all have the same amount. You have probably guessed what it is, and it's about time.

 What is Time?

      Time is one of those seemingly familiar concepts that is difficult to describe. Most dictionaries define time as "the space between two events" or a "measurable period during which an action, process, or condition exists." My Webster's Collegiate Dictionary gives a total of twenty-one definitions for time followed by fifty-three word entries related to time ranging from "time and a half" to "time zone." Time is certainly an important part of our lives. But when we measure time, what are we really measuring?

 The Arrow of Time

      While both distance and time are measured in lengths, time is unique in that its apparent movement in only one direction affects our ability to measure it. Think about a world-class runner setting a record for the hundred meter dash. We could measure the distance ran over and over again and get the same results. We could measure either from  the start to the finish or from the finish to the start, and we would always get a hundred meters. But we only get one chance to measure the time it took for the runner to complete the sprint. We cannot have the runner repeat the race and expect to get identical results; nor can we start timing when the runner crosses the line and time backwards until reaching the start of the race. This directional quality is known simply as the arrow of time.

 How Do We Measure Time?

      From calendar sticks to sundials to the first mechanical clocks, people have developed increasingly more accurate ways of measuring time. Today's atomic clocks are accurate to within one second over a 200,000 year period! As our clocks have become more accurate, though, we have discovered the difficulty in defining a specific unit of time.

      Solar time, used until the seventeenth century, was based on the idea that the sun reaches its highest point in the sky at noon and that the period between one noon to the next could be divided into twenty four equal parts (hours--which could be divided into minutes and seconds) . The development of more accurate clocks (for that time), though, led to the discovery that the total length of individual days could vary by as much as sixteen minutes.

      An improved version of measuring time, mean solar time, was then introduced which led to the development of standard time in the late nineteenth century. Although precise enough for everyday use, calibrating clocks with standard time is not accurate enough for some scientific measurements. Mean solar time does not take into account some minor fluctuations in the earth's rotation or the fact that the earth's rotation is slowing down (measurably, but minutely).

      In 1940, scientists introduced ephemeris time, which was based not on the earth's rotation on its axis but rather its revolution around the sun. The "second" was then officially (and more accurately) redefined not as a fraction of a mean solar day but instead as a fraction of a specific ephemeral year.

 Einstein and Relativity

      With the improvements in the accuracy of measuring time, we can now be sure that a second (minute, hour, etc.) measured in one place is exactly the same as a second (minute, hour, etc.) measured in a different place, right? Well, not exactly. This assumption of absolute time (a minute in Monte Vista is the same as a minute on the moon) was, for a long time, believed to be a fundamental concept of time. In the early 1900's, though, Albert Einstein shattered that idea with his theory of relativity. One implication of his theory was that time could not be absolute; time was affected by both gravity and motion.

      While Einstein's theory was based on complex mathematical computations, testable predictions based on relativity have supported his theory. Several interesting experiments have been conducted involving time. According to relativity, time passes more slowly the closer an observer is to the surface of the earth. The closer an observer is to the surface of the earth, the stronger the force of gravity. The greater the force of gravity, according to Einstein, the slower time passes.

      Two very accurate clocks were used to test this idea. One was mounted at the base of a tall tower; the other at the top of the tower. Sure enough, the clock at the top ran faster. While measurable, though, the difference is generally not significant (the extra time I would appear to gain by writing this on my rooftop--when an hour had passed on the ground, slightly more than an hour would have passed on the roof--would not be worth the climb). Still, the effect does have important implications. Satellites used for precision land-based navigation systems are high enough above the Earth that the corrections for differences in the passage of time must be made or calculated results can be off by several miles!

      Relativity also predicts that the greater the difference in motion between two objects, the greater the differences in the passage of time for those objects. Another experiment involving three very accurate clocks showed this to be true. One clock was placed in an airplane traveling in the same direction as the earth's rotation, and one in an airplane traveling opposite the earth's rotation. After a period of time, these clocks were compared to a third clock on earth. The moving clocks either gained or lost time (depending on the direction traveled) when compared to the third clock.

 A Trip of (More Than) A Lifetime?

      Perhaps one of the most interesting implications of Einstein's theory relates to space travel. Suppose future technologies allow travel at speeds near that of light. At such speeds, time passes much more slowly. If travelers left the Earth and zipped through space for, by their measure, several years, they would return to find that much more time, perhaps hundreds of years, had passed on Earth! Even on relatively short trips at such high speeds, astronauts could conceivably return to earth to find their children older than themselves.

       We live in an interesting time. We are surrounded by time saving devices and yet are forced to manage our time more carefully than at any time in the past. Balancing family, career, and personal interests is no easy task. In the end, though, our lives will be measured not by how we saved time, but by how we spent it. The next time you glance at your watch to see how much time you have, why not spend just a little of it pondering the wondrously mysterious nature of our world.

1997, 2006  Dirk Oden

   
   
 

This site was last updated 03/08/07