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all rights reserved 2002
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Einstein’s Postulates Einstein's
theory of special relativity results from two statements -- the two basic
postulates of special relativity: 1.
The laws of physics are the same in any inertial (that is,
non-accelerated) frame of reference. This means that the laws of physics
observed by a hypothetical observer traveling with a relativistic particle must
be the same as those observed by an observer who is stationary in the
laboratory. 2.
The speed of light is the same for all observers, no matter what their
relative speeds. Given
these two statements, Einstein showed how definitions of momentum and energy
must be refined and how quantities such as length and time must change from one
observer to another in order to get consistent results for physical quantities
such as particle half-life. To decide whether his postulates are a correct
theory of nature, physicists test whether the predictions of Einstein's theory
match observations. Indeed many such tests have been made -- and the answers
Einstein gave are right every time! The
First Postulate of the Special Theory of Relativity: The
first postulate of the theory of special relativity is not too hard to swallow: The
laws of physics hold true for all frames of reference. This is the simplest
of all relativistic concepts to grasp. The physical laws help us understand how
and why our environment reacts the way it does. They also allow us to predict
events and their outcomes. Consider a yardstick and a cement block. If you
measure the length on the block, you will get the same result regardless of
whether you are standing on the ground or riding a bus. Next, measure the time
it takes a pendulum to make 10 full swings from a starting height of 12 inches
above its resting point. Again, you will get the same results whether you are
standing on the ground or riding a bus. Note that we are assuming that the bus
is not accelerating, but traveling along at a constant velocity on a smooth
road. Now if we take the same examples as above, but this time measure the block
and time the pendulum swings as they ride past us on the bus, we will get
different results than our previous results. The difference in the results of
our experiments occurs because the laws of physics remain the same for all
frames of reference. The discussion of the Second Postulate will explain this in
more detail. It is important to note that just because the laws of physics are
constant, it does not mean that we will get the same experimental results in
differing frames. That depends on the nature of the experiment. For example, if
we crash two cars into each other, we will find that the energy was conserved
for the collision regardless of whether we were in one of the cars or standing
on the sidewalk. Conservation of energy is a physical law and therefore, must be
the same in all reference frames. The
Second Postulate of the Special Theory of Relativity: The
second postulate of the special theory of relativity is quite interesting and
unexpected because of what it says about frames of reference. The postulate is: The
speed of light is measured as constant in all frames of reference. This can
really be described as the first postulate in different clothes. If the laws of
physics apply equally to all frames of reference, then light (electromagnetic
radiation) must travel at the same speed regardless of the frame. This is
required for the laws of electrodynamics to apply equally for all frames.
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