The
Speed of 
Light travels at a speed of 186,000 miles a second or 700 million
miles an hour. For scale, the distance from the Earth to the
Moon is about 239,000 miles. This seems pretty fast and indeed
theory says that nothing can travel faster than the speed of
light.
In our every day lives light seems to travel
from one place to another instantaneously. When we flip on the
light in a room there is no delay between when we first see
the bulb start glowing and when light illuminates the far corners
of the chamber. Our nervous systems are much too slow to notice
the rays of light that appear from the bulb and move like a
wave washing over the room.
When we deal with the immense distances of space,
though, even light seems slow. When astronauts were on the Moon
it took over a second for the radio waves (which travel at the
speed of light) carrying their voices to reach us. Light coming
from the sun takes eight and one half minutes to hit Earth.
(This means that if the sun were suddenly to go dark, it would
take over eight minute for us to notice) Light from the nearest
stars, other than the sun, takes four and a half years to get
here. From the farthest stars in distant galaxies it can take
billions of years for the light to arrive..
The distance light can travel in a year is called
a "light year." The light year is one of the basic measures
of distance for astronomy.
When designing probes for trips to other planets
in our solar system it is important for the planners to keep
the communications time lag, caused by the speed of light, in
mind. For example, a probe designed to land on Mars must be
smart enough to handle problems in the flight on it's own without
instructions from Earth. If a course change is needed during
landing the probe would have to do it automatically. The delay
caused by the probe requesting instructions from Earth and getting
commands back might be nearly an hour, plenty of time for the
probe to crash.
The delay caused by the speed of light can sometimes
be noticed here on Earth during telephone calls. Long distance
calls that have been routed over one or more space satellites
may cause a half second or so delay between the speaker and
the listener.
The speed of light has several properties which
may seem counter-intuitive to us, but are true:
-Nothing travels faster than the speed of light.
-No matter how fast you are moving the speed of
light seems to be the same speed as if you were not moving at
all.
-As an object or person is accelerated toward
the speed of light time slows down for it/him.
This last property leads to the "twins" effect:
Twin brothers live on Earth. One brother takes a trip to a distant
star traveling at a high percentage of the speed of light. When
the twin returns he will be younger than his brother because
for him time slowed down during the trip.
This effect, called "time dilation," helps explain
why the speed of light is the same no matter how fast you are
going. As a traveler accelerates time slows down for him. This,
in turn, affects his measurements.
Copyright Lee
Krystek 1996. All Rights Reserved.
