Time is familiar to everyone, yet it's hard to define and understand.
Science, philosophy, religion, and
the arts have different definitions of time, but the system of measuring it is relatively consistent.
Clocks are based on seconds, minutes, and hours. While the basis for these units has changed
throughout history, they trace their roots back to ancient Sumeria. The modern international unit of
time, the second, is defined by the electronic transition of the cesium atom. But what, exactly, is time?
Scientific Definition
Long exposure of a colorful rainbow of light
Artur Debat / Getty Images
Physicists define time as the progression of events from the past to the present into the future.
Basically, if a system is unchanging, it is timeless. Time can be considered to be the fourth dimension of
reality, used to describe events in three-dimensional space. It is not something we can see, touch, or
taste, but we can measure its passage.
The Arrow of Time
Post-it notes reading past, now, and future
Bogdan Vija / EyeEm / Getty Images
Physics equations work equally well whether time is moving forward into the future (positive time) or
backward into the past (negative time.) However, time in the natural world has one direction, called the
arrow of time. The question of why time is irreversible is one of the biggest unresolved questions in
science.
One explanation is that the natural world follows the laws of thermodynamics. The second law of
thermodynamics states that within an isolated system, the entropy of the system remains constant or
increases. If the universe is considered to be an isolated system, its entropy (degree of disorder) can
never decrease. In other words, the universe cannot return to exactly the same state in which it was at
an earlier point. Time cannot move backward.
Time Dilation
Light trails on modern building background in Shanghai
zhuyufang / Getty Images
In classical mechanics, time is the same everywhere. Synchronized clocks remain in agreement. Yet we
know from Einstein's special and general relativity that time is relative. It depends on the frame of
reference of an observer. This can result in time dilation, where the time between events becomes
longer (dilated) the closer one travels to the speed of light. Moving clocks run more slowly than
stationary clocks, with the effect becoming more pronounced as the moving clock approaches light
speed. Clocks in jets or in orbit record time more slowly than those on Earth, muon particles decay more
slowly when falling, and the Michelson-Morley experiment confirmed length contraction and time
dilation.
Time Travel
Globes stretching in space
MARK GARLICK / SCIENCE PHOTO LIBRARY / Getty Images
Time travel means moving forward or backward to different points in time, much like you might move
between different points in space. Jumping forward in time occurs in nature. Astronauts on the
International Space Station jump forward in time when they return to Earth because of its slower
movement relative to the station.
The idea of traveling back in time, however, poses problems. One issue is causality or cause and effect.
Moving back in time could cause a temporal paradox. The "grandfather paradox" is a classic example.
According to the paradox, if you travel back in time and kill your grandfather before your mother or
father was born, you could prevent your own birth. Many physicists believe time travel to the past is
impossible, but there are solutions to a temporal paradox, such as traveling between parallel universes
or branch points.
Time Perception
Young and old hands
Catherine Falls Commercial / Getty Images
The human brain is equipped to track time. The suprachiasmatic nuclei of the brain is the region
responsible for daily or circadian rhythms. But neurotransmitters and drugs affect time perceptions.
Chemicals that excite neurons so they fire more quickly than normal speed up time, while decreased
neuron firing slows down time perception. Basically, when time seems to speed up, the brain
distinguishes more events within an interval. In this respect, time truly does seem to fly when one is
having fun.
Time seems to slow down during emergencies or danger. Scientists at Baylor College of Medicine in
Houston say the brain doesn't actually speed up, but the amygdala becomes more active. The amygdala
is the region of the brain that makes memories. As more memories form, time seems drawn out.
The same phenomenon explains why older people seem to perceive time as moving faster than when
they were younger. Psychologists believe the brain forms more memories of new experiences than that
of familiar ones. Since fewer new memories are built later in life, time seems to pass more quickly.
The Beginning and End of Time
Time in a never-ending spiral
Billy Currie Photography / Getty Images
As far as the universe is concerned, time had a beginning. The starting point was 13.799 billion years ago
when the Big Bang occurred. We can measure cosmic background radiation as microwaves from the Big
Bang, but there isn't any radiation with earlier origins. One argument for the origin of time is that if it
extended backward infinitely, the night sky would be filled with light from older stars.
Will time end? The answer to this question is unknown. If the universe expands forever, time would
continue. If a new Big Bang occurs, our time line would end and a new one would begin. In particle
physics experiments, random particles arise from a vacuum, so it doesn't seem likely the universe would
become static or timeless. Only time will tell.
Key Points
Time is the progression of events from the past into the future.
Time moves only in one direction. It's possible to move forward in time, but not backward.
Scientists believe memory formation is the basis for human perception of time.
Sources
Carter, Rita. The Human Brain Book. Dorling Kindersley Publishing, 2009, London.
Richards, E. G. Mapping Time: The Calendar and its History. Oxford University Press, 1998, Oxford.
Schwartz, Herman M. Introduction to Special Relativity, McGraw-Hill Book Company, 1968, New York.
Cite this Article
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