To fully understand the mysteries of Albert Einstein's Special Theory of Relativity, one must first understand the theory on which it was based: Galileo's theory.
When Albert Einstein attempted to explain to laymen the Special Theory of Relativity he came up with in 1905, he often used analogies involving trains. In fact, Einstein was not alone in this. Pick up almost any book on the subject and you’re more than likely to find some train analogy popping up in one form or another (actually, these days you are just as likely to find analogies involving trains or space ships, but the basic principle remains the same).
The reason for this is rather simple: Special Relativity has everything to do with the idea of motion.
The Train Analogy of Relativity
Trains, to Einstein, were a good example of the kind of motion necessary to think about Special Relativity for two reasons. 1) They moved ahead (generally) in a perfectly straight path, stuck to rails, and 2) Once they were moving, they generally maintained a consistent speed. Both of these requirements are why this theory is referred to as “special” – it required certain very specific conditions, or else it would not work.
When Galileo Galilei proposed the first theory of relativity (upon which Einstein would later base his own work) in his 1632 work, “Dialogue Concerning Two Chief World Systems,” he actually used a sailboat to get his point across, as this was the best form of constant motion available at the time (presumably he assumed that the wind speed was fairly constant and that the water was calm during any experiments). Had trains been invented in 1632, it can be assumed that Galileo probably would have used them in expressing his ideas instead.
The Relativity of Galileo
In short, Galilean Relativity (or, as it is often referred, Galilean Invariance) is simply this: If one is standing within an object moving at a perfectly consistent speed and in a straight line, any experiment performed will have the exact same results as if one were standing perfectly still on solid ground.
In other words, if one was to stand on a train (using Einstein’s analogy) and drop a ball to the ground, to any observer on board the train, the ball would be seen to travel straight downward, just as it would as if he/she were standing on solid ground. To an observer outside the train, however (providing they could see through the train car’s walls), the ball would appear to be falling with a curve in the direction of the train’s motion.
The question thus asked via Galilean Relativity (and later in Special Relativity) is which of these two viewpoints (the observer on the train or the observer on land) is preferred? Is the ball actually moving in a straight line, as anyone on the train would observe, or is it moving in a curve, as it would be relative to the Earth? In the parlance of relativity, these two different perspectives are actually equal and are referred to as “Reference Frames” or “Inertial Frames.” In the theory of relativity, all reference frames are equal.
Put still another way (anyone attempting to explain physics is by nature prone to repetition. It’s just the nature of the work), according to Galileo’s theory of relativity, there is no experiment a person can perform in a given reference frame (as long as it’s speed and direction remain perfectly constant) that will prove the existence of motion. If I am inside an airplane moving at 600 mph with all the windows closed, there is no experiment I can perform on the plane (short of opening a window) that will prove that the plane is, indeed moving. And besides, if I did open a window, who’s to say that it is not the Earth itself which is moving while I am standing still?
This idea of the equality of Inertial Frames is the key to unlocking the secrets and mysteries inherent in Einstein’s own theory of Special Relativity which would come almost 300 years later.
For more information on relativity, see Also:
Relativity and Electromagnetism
References:
Einstein, A. (1961). Relativity: The Special and the General Theory - A clear Explanation that Anyone can Understand. New York, NY: Random House.
Galilei, G. (1632). Dialogue Concerning the Two Chief World Systems. Pisa.
