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.
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?
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.
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.
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.
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.
2006 Dirk Oden