Traveling Through Time
What is time? The question is
as hard to answer as whether or not time travel will ever be possible.
by Clifford Pickover
What is time? Is time travel possible? For centuries, these questions have
intrigued mystics, philosophers, and scientists. Much of ancient Greek
philosophy was concerned with understanding the concept of eternity, and the
subject of time is central to all the world's religions and cultures. Can the
flow of time be stopped? Certainly some mystics thought so. Angelus Silesius, a
sixth-century philosopher and poet, thought the flow of time could be suspended
by mental powers:
Time is of your own making;
The line between science and mysticism sometimes grows thin. Today physicists
would agree that time is one of the strangest properties of our universe. In
fact, there is a story circulating among scientists of an immigrant to America
who has lost his watch. He walks up to a man on a New York street and asks,
"Please, Sir, what is time?" The scientist replies, "I'm sorry, you'll have
to ask a philosopher. I'm just a physicist."
its clock ticks in your head.
The moment you stop thought
time too stops dead.
Most cultures have a grammar with past and future tenses, and also
demarcations like seconds and minutes, and yesterday and tomorrow. Yet we
cannot say exactly what time is. Although the study of time became scientific
during the time of Galileo and Newton, a comprehensive explanation was given
only in this century by Einstein, who declared, in effect, time is simply what
a clock reads. The clock can be the rotation of a planet, sand falling in an
hourglass, a heartbeat, or vibrations of a cesium atom. A typical grandfather
clock follows the simple Newtonian law that states that the velocity of a body
not subject to external forces remains constant. This means that clock hands
travel equal distances in equal times. While this kind of clock is useful for
everyday life, modern science finds that time can be warped in various ways,
like clay in the hands of a cosmic sculptor.
Science-fiction authors have had various uses for time machines, including
dinosaur hunting, tourism, visits to one's ancestors, and animal collecting.
Ever since the time of H.G. Wells' famous novel The Time Machine (1895),
people have grown increasingly intrigued by the idea of traveling through time.
(I was lucky enough to have chats with H.G. Wells' grandson, who told me that
his grandfather's book has never been out of print, which is rare for a book a
century old.) In the book, the protagonist uses a "black and polished brass"
time machine to gain mechanical control over time as well as return to the
present to bring back his story and assess the consequences of the present on
the future. Wells was a graduate of the Imperial College of Science and
Technology, and scientific language permeates his discussions. Many believe
Wells' book to be the first story about a time machine, but seven years before
22-year-old Wells wrote the first version of The Time Machine, Edward
Page Mitchell, an editor of the New York Sun, published "The Clock That
The first science-fiction story
about time travel appeared in the 1880s.
One of the earliest methods for fictional time travel didn't involve a
machine; the main character in Washington Irving's "Rip van Winkle" (1819)
simply fell asleep for decades. King Arthur's daughter Gweneth slept for 500
years under Merlin's spell. Ancient legends of time distortion are, in fact,
quite common. One of the most poetic descriptions of time travel occurs in a
popular medieval legend describing a monk entranced for a minute by the song of
a magical bird. When the bird stops singing, the monk discovers that several
hundred years have passed. Another example is the Moslem legend of Muhammad
carried by a mare into heaven. After a long visit, the prophet returns to Earth
just in time to catch a jar of water the horse had kicked over before starting
Time travel is possible
Today, we know that time travel need not be confined to myths, science fiction,
Hollywood movies, or even speculation by theoretical physicists. Time travel is
possible. For example, an object traveling at high speeds ages more slowly than
a stationary object. This means that if you were to travel into outer space and
return, moving close to light speed, you could travel thousands of years into
the Earth's future.
Newton's most important contribution to science was his mathematical
definition of how motion changes with time. He showed that the force causing
apples to fall is the same force that drives planetary motions and produces
tides. However, Newton was puzzled by the fact that gravity seemed to operate
instantaneously at a distance. He admitted he could only describe it without
understanding how it worked. Not until Einstein's general theory of relativity
was gravity changed from a "force" to the movement of matter along the shortest
space in a curved spacetime. The Sun bends spacetime, and spacetime tells
planets how to move. For Newton, both space and time were absolute. Space was a
fixed, infinite, unmoving metric against which absolute motions could be
measured. Newton also believed the universe was pervaded by a single absolute
time that could be symbolized by an imaginary clock off somewhere in space.
Einstein changed all this with his relativity theories, and once wrote,
"Newton, forgive me."
Einstein's first major contribution to the study of time occurred when he
revolutionized physics with his "special theory of relativity" by showing how
time changes with motion. Today, scientists do not see problems of time or
motion as "absolute" with a single correct answer. Because time is relative to
the speed one is traveling at, there can never be a clock at the center of the
universe to which everyone can set their watches. Your entire life is the blink
of an eye to an alien traveling close to the speed of light. Today, Newtonian
mechanics have become a special case within Einstein's theory of relativity.
Einstein's relativity will eventually become a subset of a new science more
comprehensive in its description of the fabric of our universe. (The word
"relativity" derives from the fact that the appearance of the world depends on
our state of motion; it is "relative.")
Albert Einstein, whose
theories of relativity changed our understanding of time and space, once wrote
"Newton, forgive me."
We are a moment in astronomic time, a transient guest of the Earth. Our wet,
wrinkled brains do not allow us to comprehend many mysteries of time and space.
Our brains evolved to make us run from saber-toothed cats on the American
savanna, to hunt deer, and to efficiently scavenge from the kills of large
carnivores. Despite our mental limitations, we have come remarkably far. We
have managed to pull back the cosmic curtains a crack to let in the light.
Questions raised by physicists, from Newton to Kurt Gödel to Einstein to
Stephen Hawking, are among the most profound we can ask.
Is time real? Does it flow in one direction only? Does it have a beginning or
an end? What is eternity? None of these questions can be answered to
scientists' satisfaction. Yet the mere asking of these questions stretches our
minds, and the continual search for answers provides useful insights along the
Continue: The future of time travel
Sagan on Time Travel |
Traveling Through Time
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© | Updated November 2000