The Special Theory of Relativity
The Principle of Relativity
When Einstein formulated his special theory of relativity—it is "special" in the sense that it deals with Euclidean, or flat-space geometry—he developed two simple, yet profound principles: one dealt with the nature of light, and will be described a bit later, while the second dealt with the nature of physical laws in a more general aspect.
The principle of relativity is not concerned with a specific physical law, but rather with all physical laws and is based on one very simple premise: When describing an object's speed or velocity (the object's speed and direction of motion), it is essential to specify who or what is doing the measuring. In other words, we can speak about the motion of an object, but only relative to or by comparison to another object. Saying "Marianne is traveling at 65 miles per hour" is meaningless, but saying "Marianne is traveling at 65 miles per hour past Ginger" does have meaning. In other words, there is no absolute notion of motion. All motion is relative to some external reference.
Imagine Marianne has boarded an airplane and settled comfortably into her seat. After picking up a magazine she begins reading while waiting for the airplane to depart for its destination. Momentarily she looks out the window and notices the aircraft next to her plane moving forward slowly. Unsure of what has happened, she cannot be sure if the airplane seen out the window is slowly taxiing forward or if her own airplane has been pushed back from the gate and is slowly backing up. Likewise, as Ginger looks out the window of the other plane, she may experience the same momentary confusion. Is her plane moving forward or is Marianne's plane pushing back from the gate?
This example describes one and the same situation but from two different and equally valid points of view. If the push-back from the gate is very gentle, both passengers will still feel she is stationary and perceives that the other is moving. As stated, there is no one view point that is correct and the other incorrect.
Although the example described above is a bit contrived, it does serve to highlight the essential aspects of the principle of relativity because airplanes taxi slowly and it is quite possible that passengers may not feel the initial "push back" from the boarding gate. The example can easily be reshaped to describe two astronauts floating freely in the vacuum of space. Marianne would have no way of knowing whether she is motionless in space or if she is moving. Even if she suddenly notices Ginger in the distance and sees that the distance between the two of them is decreasing, Marianne would have no way of knowing if she is stationary and Ginger is approaching her, if Ginger is stationary and Marianne is moving towards her, or if both are moving.
You may have figured out the essential element in the scenarios described so far and that key element is that neither Marianne or Ginger is being acted upon by an external force. Both exist in their own inertial frame of reference in which they are either stationary or moving at a constant velocity. In this respect, inertial reference frames are force-free frames of reference. This is a crucial consideration because if either Marianne or Ginger had a rocket pack, she could fire the rocket and feel the force that would give away the fact that she is moving. Constant velocity motion is relative, but accelerated motion is not.
Einstein realized that there was no experiment that a person in an inertial reference frame could perform that would give away whether the experimenter was stationary or moving at a constant velocity. He then went on to make an even grander claim about nature: the laws of physics must be absolutely identical for all observers undergoing constant velocity motion.
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