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Principle of Relativity |
The Principle of Relativity asserts that the laws of physics are the same in
every inertial frame of reference.
In other words, nature shows no favoritism. Everybody gets a front seat when it
comes to viewing reality. The most fundamental properties of the universe are
independent of whoever observes them.
Because the laws of nature are the same in all inertial reference frames, it is impossible to use natural laws to identify an absolute rest frame. Therefore, motion is meaningless in the absolute sense. Motion is a relative term. This allows any observer to declare himeself to be at rest without any danger of being proven otherwise.
Although the principle of relativity is most commonly used in reference to motion, it does not pertain exclusively to motion. It is a very powerful principle that applies to the whole universe. It allows us to predict with confidence what has happened, what is happening, and what will happen in other parts of the universe - places and environments that we will never be able to encounter personally. It allows us to make astronomical observations billions of light-years away of events billions of years in the past, and to gain a deep understanding of what is happening because the laws of physics are the same there and then as they are here and now.
However, one should remember that the principle of relativity pertains particularly to inertial frames of reference. It is not valid for accelerating frames or rotating frames. Therefore, even though it is impossible to determine whether or not a frame is moving, it is possible to determine whether or not it is accelerating or rotating and to what extent. Nature does distinguish between accelerating and non-accelerating frames, between rotating and non-rotaing frames. So it is not true that all motion is relative. Some aspects of motion are absolute in character and one should never forget that fact. All reference frames are not equivalent, only inertial frames.
Q1. It is impossible to use the
laws of physics to determine whether or not a particular reference frame is
really (A) moving, (B) accelerating, (C) rotating. (D) Two of these (E) Three
of these.
Q2. Which are true? (A)
Perpendicular lines in spacetime will have slopes on normalized spacetime
diagrams that are positive reciprocals of one another. (B) Perpendicular lines
in spacetime always have one timelike line and one spacelike line. (C) Lines in
spacetime perpendicular to an observer’s worldline always connect events
that occur simultaneously according to that observer. (D) Two of these. (E)
Three of these.
Q3. Which of the following are
valid expressions of the principle of relativity? (A) Nature shows no favoritism.
(B) All reference frames are equivalent. (C) Natural laws are universal and
unchanging. (D) Two of these. (E) Three of these.
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