Mystery of Time

(THEME)

ANNCR:

This is SCIENCE IN THE NEWS, in VOA Special English. I'm Steve
Ember. This week our program is about a mystery as old as time. Bob
Doughty and Sarah Long tell about the mystery of time.

(THEME)

VOICE ONE:

If you can read a clock, you can know the time of day. But no one
knows what time itself is. We cannot see it. We cannot touch it. We
cannot hear it. We know it only by the way we mark its passing.

For all our success in measuring the smallest parts of time, time
remains one of the great mysteries of the universe.

VOICE TWO:

One way to think about time is to imagine a world without time.
There could be no movement, because time and movement cannot be
separated.

A world without time could exist only as long as there were no
changes. For time and change are linked. We know that time has
passed when something changes.

VOICE ONE:

In the real world -- the world with time -- changes never stop.
Some changes happen only once in a while, like an eclipse of the
moon. Others happen repeatedly, like the rising and setting of the
sun. Humans always have noted natural events that repeat themselves.
When people began to count such events, they began to measure time.

In early human history, the only changes that seemed to repeat
themselves evenly were the movements of objects in the sky. The most
easily seen result of these movements was the difference between
light and darkness.

The sun rises in the eastern sky, producing light. It moves
across the sky and sinks in the west, causing darkness. The
appearance and disappearance of the sun was even and unfailing. The
periods of light and darkness it created were the first accepted
periods of time. We have named each period of light and darkness --
one day.

VOICE TWO:

People saw the sun rise higher in the sky during the summer than
in winter. They counted the days that passed from the sun's highest
position until it returned to that position. They counted
three-hundred sixty-five days. We now know that is the time Earth
takes to move once around the sun. We call this period of time a
year.

VOICE ONE:

Early humans also noted changes in the moon. As it moved across
the night sky, they must have wondered. Why did it look different
every night? Why did it disappear? Where did it go?

Even before they learned the answers to these questions, they
developed a way to use the changing faces of the moon to tell time.

The moon was "full" when its face was bright and round. The early
humans counted the number of times the sun appeared between full
moons. They learned that this number always remained the same --
about twenty-nine suns. Twenty-nine suns equaled one moon. We now
know this period of time as one month.

(MUSIC)

VOICE TWO:

Early humans hunted animals and gathered wild plants. They moved
in groups or tribes from place to place in search of food. Then,
people learned to plant seeds and grow crops. They learned to use
animals to help them work, and for food.

They found they no longer needed to move from one place to
another to survive.

As hunters, people did not need a way to measure time. As
farmers, however, they had to plant crops in time to harvest them
before winter. They had to know when the seasons would change. So,
they developed calendars.

No one knows when the first calendar was developed. But it seems
possible that it was based on moons, or lunar months.

When people started farming, the wise men of the tribes became
very important. They studied the sky. They gathered enough
information so they could know when the seasons would change. They
announced when it was time to plant crops.

(MUSIC)

VOICE ONE:

The divisions of time we use today were developed in ancient
Babylonia four-thousand years ago. Babylonian astronomers believed
the sun moved around the Earth every three-hundred-sixty-five days.
They divided the trip into twelve equal parts, or months. Each month
was thirty days. Then, they divided each day into twenty-four equal
parts, or hours. They divided each hour into sixty minutes, and each
minute into sixty seconds.

VOICE TWO:

Humans have used many devices to measure time. The sundial was
one of the earliest and simplest.

A sundial measures the movement of the sun across the sky each
day. It has a stick or other object that rises above a flat surface.
The stick, blocking sunlight, creates a shadow. As the sun moves, so
does the shadow of the stick across the flat surface. Marks on the
surface show the passing of hours, and perhaps, minutes.

The sundial works well only when the sun is shining. So, other
ways were invented to measure the passing of time.

VOICE ONE:

One device is the hourglass. It uses a thin stream of falling
sand to measure time. The hourglass is shaped like the number eight
--- wide at the top and bottom, but very thin in the middle. In a
true "hour" glass, it takes exactly one hour for all the sand to
drop from the top to the bottom through a very small opening in the
middle. When the hourglass is turned with the upside down, it begins
to mark the passing of another hour.

By the eighteenth century, people had developed mechanical clocks
and watches. And today, many of our clocks and watches are
electronic.

VOICE TWO:

So, we have devices to mark the
passing of time. But what time is it now. Clocks in different parts
of the world do not show the same time at the same time. This is
because time on Earth is set by the sun's position in the sky above.

We all have a twelve o'clock noon each day. Noon is the time the
sun is highest in the sky. But when it is twelve o'clock noon where
I am, it may be ten o'clock at night where you are.

VOICE ONE:

As international communications and travel increased, it became
clear that it would be necessary to establish a common time for all
parts of the world.

In eighteen-eighty-four, an international conference divided the
world into twenty-four time areas, or zones. Each zone represents
one hour. The astronomical observatory in Greenwich, England, was
chosen as the starting point for the time zones. Twelve zones are
west of Greenwich. Twelve are east.

The time at Greenwich -- as measured by the sun -- is called
Universal Time. For many years it was called Greenwich Mean Time.

VOICE TWO:

Some scientists say time is governed by the movement of matter in
our universe. They say time flows forward because the universe is
expanding. Some say it will stop expanding some day and will begin
to move in the opposite direction, to grow smaller. Some believe
time will also begin to flow in the opposite direction -- from the
future to the past. Can time move backward?

Most people have no trouble agreeing that time moves forward. We
see people born and then grow old. We remember the past, but we do
not know the future. We know a film is moving forward if it shows a
glass falling off a table and breaking into many pieces. If the film
were moving backward, the pieces would re-join to form a glass and
jump back up onto the table. No one has ever seen this happen.
Except in a film.

VOICE ONE:

Some scientists believe there is one reason why time only moves
forward. It is a well-known scientific law -- the second law of
thermodynamics. That law says disorder increases with time. In fact,
there are more conditions of disorder than of order.

For example, there are many ways a glass can break into pieces.
That is disorder. But there is only one way the broken pieces can be
organized to make a glass. That is order. If time moved backward,
the broken pieces could come together in a great many ways. Only one
of these many ways, however, would re-form the glass. It is almost
impossible to believe this would happen.

VOICE TWO:

Not all scientists believe time is governed by the second law of
thermodynamics. They do not agree that time must always move
forward. The debate will continue about the nature of time. And time
will remain a mystery.

(THEME)

ANNCR:

Our program was written by Marilyn Christiano and read by Sarah
Long and Bob Doughty. I'm Steve Ember. You can download all of our
program script text and audio files from WWW.testbig.com.
Listen again next week for Science in the News, in VOA Special English.