Simultaneity and Relativity
Proof of The Flexibility of Time and Space
In striving to develop his theory of Special Relativity, Einstein was forced to prove that truly simultaneous events could never possibly be measured.
In order for Albert Einstein to develop his theory of Special Relativity, he first had to try and connect the tenants of Galilean Relativity with the peculiar findings regarding the speed of light developed throughout the 19th century. To do this, he explored a concept known as “simultaneity.” How, Einstein asks, can we ever truly tell if two events occur at exactly the same instant? How can we ever be sure that two things are simultaneous? In the physicist’s own words from his book, “Relativity: The Special and the General Theory”:
As long as this requirement is not satisfied, I allow myself to be deceived as a physicist (and of course the same applies if I am not a physicist), when I imagine that I am able to attach a meaning to the statement of simultaneity.
Einstein uses the following thought experiment to explain his assertion that if relativity is true, then true simultaneity is, by definition, non-existent.
Imagine that there are two observers, one on board a moving train and one on stationary ground beside the train tracks. The observer on the ground is standing at the precise point in between two distant poles beside the railroad tracks in either direction. Now, imagine that at the very instant that the observer on the train is also equally in between both poles, the two poles were struck by lightning at precisely the same instance. If these two observers were both attempting to determine if the bolts had indeed been simultaneous, what do you suppose they would find?
Surely, the observer on the ground would indeed view the events as having occurred simultaneously, as light from both events would have reached his position at exactly the same moment. The observer on the moving train, however, was at that moment moving toward one of the poles and away from the other, meaning that the light from one of the events reached him first, while the other took a little bit longer, so from his perspective the bolts were not simultaneous but occurred one after the other. You might say to yourself that the first person is obviously correct, as he was not moving so could make more accurate observations. While this is true, it is important to remember that according to the theory of relativity, neither of these “reference frames,” whether moving or stationary, can be viewed as “preferred.” They are both to be treated equal in value and weight. Thus, in this instance, which can then be extrapolated into any number of other situations, true simultaneity appears rather impossible to measure.
And what is the consequence of this little thought experiment by Einstein? It is actually rather profound. Using just this rather simple thought regarding the theory of relativity, and assuming the speed of light to be a constant, Einstein had begun to realize that neither time nor distance could be given any absolute meaning. Both of these elements were perhaps not as “fixed” as science had always assumed.
And, when a physicist of Einstein's calibre took a step back to view the universe in light of this, he finally began to understand how the speed of light could be a constant after all.
Einstein’s answer? Two of the most interesting elements in all of relativity: Time and space dilation.
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.
