The first and foremost issue that set Einstein's new view of relativity apart from the relativity of Galileo were the problems encountered with the new study of light.
In short, let us assume that the simple law of the constancy of the velocity of light “c” (in vacuum) is justifiably believed by the child at school. Who would imagine that this simple law has plunged the conscientiously thoughtful physicist into the greatest intellectual difficulties?
Albert Einstein - Relativity: The Special and General Theories
In order to understand how the speed of light led to Einstein’s theory of Special Relativity, we must first understand exactly what is meant by the assertion, as discovered in the 19th century, that the perception of the speed of light is constant.
Defining the Speed of Light as a Constant
It’s time for a thought experiment, just like the ones Einstein himself was famous for:
Imagine that you and two of your friends are traveling through space (in a complete vacuum – where all of the requirements of Special Relativity are duly in effect). You're probably wearing spacesuits, as otherwise you would surely die from either the cold or the lack of air; and equipped to your space suits are propulsion devices, which allow each of you to travel freely in any direction. Let's say further that you all decide to travel in the same direction, as if you were racing. You are all traveling at different speeds, one at 30 km/h, one at 300 km/h and one at 3000 km/h (clearly, one of you is going to win by a considerable margin).
Now, consider that as the three of you are racing at your various speeds, a ray of light should suddenly come zooming past you, in the same direction, at the speed of light (naturally) approx. 300,000 kilometers per second (in other words – much, much faster than any of you). If all three of you were somehow able to measure the speed of this light in relation to your own speeds, what do you suppose you would find? According to the standard law of the addition of velocities, you would surely expect that in order to find the speed of light relative to yourself, you would simply take the speed of light (c), and subtract from it your own speed, right? Not so, if the speed of light is a constant.
What was discovered in the 19th century which turned out to be so very peculiar was that in a situation such as this, all three of you would still measure the ray of light to be traveling at precisely the same speed – 300,000 km/h. In fact, no matter what speed one travels at, the law of the constancy of light always remains the same.
The Constant Speed of Light and Relativity
Now, what Einstein discovered was this: Because of the constant nature of the speed of light, Galilean relativity holds true. If this were not the case, then a fatal flaw would have been found with Galileo’s theory, as one could easily detect motion from within a given reference frame by measuring the relative speed of light. Surely, this would make life much simpler, most people probably thought, but it was simply not the way the universe worked.
It is from here that one finally finds themselves standing at the very gates of Einstein’s theory of Special Relativity. He built his theory off these two foundations:
1) Galilean relativity, which stated that there could be no “preferred” inertial reference frame – that within two reference frames moving at different speeds and different directions would nevertheless hold identical laws of physics.
2) The Constant speed of light, as determined by Maxwell, Michelson, Morley and the mathematics of H.A. Lorentz (who has not yet been discussed here), which showed that the theory of relativity appeared to be consistent after all, but which left some very important questions unanswered.
Perhaps the most important question soon to be asked by Albert Einstein is this: What effect does the constant speed of light have on us as we move along in our reference frames?
The answers are actually somewhat shocking.
References:
Einstein, A. (1961). Relativity: The Special and the General Theory - A clear Explanation that Anyone can Understand. New York, NY: Random House.
Gardner, M. (1962). Relativity Simply Explained. Mineola, NY: Dover Publications, Inc.
Davies, P. (1995). About Time - Einstein's Unfinished Revolution. New York: Simon & Schuster.
