Time is not completely separate from and
independent of space as you would ordinarily assume. In his **Special
Relativity** theory, Einstein assumed
that the fundamental laws of
physics do not depend on your location or motion. Two
people, one in a
stationary laboratory and another in a laboratory aboard a train or rocket
moving in a straight line at uniform speed, should get the same results in any
experiment they conduct. In fact, if the laboratory in the train or rocket is
soundproof and has no windows, there is no experiment a person could conduct
that would show he/she is moving.

The laws of physics include the
laws of
electromagnetism developed by James
Maxwell and Maxwell found that electromagnetic
waves should travel at a speed given by the combination of two universal
constants of nature. Since the laws of physics do not depend on your location
or motion, Einstein reasoned that the *speed* of
light will be measured to be the *same* by any two observers
regardless of their velocity relative to each other.
For example, if one observer is in a rocket moving toward another person at
half the speed of light and both observers measure the speed of a beam of light
emitted by the rocket, the person at rest will get the *same* value the
person in the rocket ship measures (about 300,000 kilometers/second) instead of
1.5 times the speed of light (=rocket speed + speed of beam of light).
This assumption
has now been shown to be correct in many experiments. To get the same
value of the *speed* (= distance/time) of light, the two
observers moving with respect to each other would not only disagree
on the *distance* the light travelled as Newton said, they would
also disagree on the *time* it took.

Einstein found that what you
measure for length, time, and mass depends on your motion relative to
a chosen frame of reference. Everything is in motion. As you sit in
your seat, you are actually in motion around the center of the Earth
because of the rapid rotation of the Earth on its axis. The Earth is
in motion around the Sun, the Sun is in orbit around the center of
our Galaxy, the Galaxy is moving toward a large group of galaxies,
etc. When you say something has a *velocity,* you are measuring
its change of position relative to some reference point which may
itself be in motion. All motion is *relative* to a chosen frame of
reference. That is what the word ``relativity'' means in Einstein's
Relativity theories. The only way observers in motion relative to
each other can *measure* a single light ray to travel the same
distance in the same amount of time *relative to their own
reference frames* is if their ``meters'' are different and their
``seconds'' are different! Seconds and meters are *relative*
quantities.

Two consequences of Special Relativity are a stationary observer will find (1)
the length of a fast-moving object is *less* than if the object was
at rest, and (2) the passage of time on the fast-moving object is slower than
if the object was at rest. However, an observer *inside* the
fast-moving object sees everything inside as their normal length and
time passes normally, but all of the lengths in the world outside are
shrunk and the outside world's clocks are running slow.

One example of the slowing of time at high speeds that is observed
all of the time is what happens when *cosmic rays* (extremely
high-energy particles, mostly protons) strike the Earth's atmosphere.
A shower of very fast-moving muon particles are created very high up
in the atmosphere. Muons have very short lifetimes---only a couple of
*millionths* of a second. Their short lifetime should allow them
to travel at most 600 meters. However they reach the surface after
travelling more than 100 kilometers! Because
they are moving close to the speed of light, the muons' internal
clocks are running much slower than stationary muons. But in their
own reference frame, the fast-moving muons's clocks run forward
``normally'' and the muons live only a couple of millionths of a second.

Time
and space are relative to the motion of an observer and they are not
independent of each other.
Time and space are connected to make four-dimensional **spacetime**
(three dimensions for space and one dimension for time).
This is not that strange---we often define *distances* by the
*time* it takes light to travel between two points. For example, one
light year is the *distance* light will travel in a *year.*
To talk about an *event,* you will usually tell where (in space)
and when (in time) it happened. The event happened in spacetime.

Another consequence of Special Relativity is that *nothing* can
travel faster than the speed of light. Any object with mass moving
near the speed of light would experience an increase in its mass. That
mass would approach infinity as it reached light speed and would,
therefore, require an infinite amount of energy to accelerate it to
light speed. The fastest possible speed any form of information or
force (including gravity) can operate is at the speed of light.
Newton's law of gravity seemed
to imply that the force of gravity would *instantly* change
between two objects if one was moved---Newton's gravity had infinite
speed (a violation of Special Relativity). The three strange effects of
Special Relativity (shrinking lengths, slowing time, increasing mass)
are only noticeable at speeds that are greater than about ten percent of the
speed of light. Numerous experiments using very high-speed objects have shown
that Special Relativity is correct.

Special Relativity also predicts that matter can be converted into
energy and energy in to matter. By applying Newton's second law of
motion to the energy of motion for something moving at high speed
(its ``kinetic energy''), you will find that energy = mass ×
(speed of light)^{2}. More concisely, this is Einstein's famous
equation, *E = mc ^{2}.* This result also applies
to an object at rest
in which case, you will refer to its ``rest mass'' and its ``rest
energy'', the energy equivalent of mass. The amount of rest energy in
something as small as your astronomy textbook, for example, is
tremendous. If all of the matter in your textbook was converted to
energy, it would be enough energy to send a million

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last updated: 17 May 2001

Author of original content: Nick Strobel