Albert Einstein – the name conjures-up a multitude of images. Along with the familiar and numerous likenesses born of camera and pen, are the mental images of genius, perhaps best exemplified in the public mind by his famous theories of relativity. Everyone knows of these, yet relatively few know much about them. It is my goal in this post to “explain” in two brief and simple statements, the essence of his famous 1905 theory of special relativity. Discussion of his 1916 general theory is deferred for perhaps another post. Yes, special relativity can be very effectively summarized in the following two premises: Premise #1: There is no reference point in space itself which allows for the measurement of absolute motion through space. Accordingly, measurements of motion and velocity made from one “frame of reference” (such as earth or a spacecraft) are meaningful only relative to other similarly “non-privileged” distinct frames of reference (such as the sun, another planet, a distant star, an orbiting spacecraft, etc.). Premise #2: The speed of light is constant for all observers no matter what the motion/velocity between light source and observer. Congratulations, you now understand the essence of special relativity. That was easy! Premise #1 states that there are no markers in empty space, no coordinate grid-lines which allow for determining position “in space” or velocity “through space.” How fast is the earth traveling “through space?” This, we cannot determine; motion “through space” has no validity. Motion relative to another distinct body (another frame of reference) is a valid concept, and subject to measurement. Premise #2 of special relativity relates Einstein’s pronouncement of a new and completely counter-intuitive universal law of nature. Here is the simple but amazing ramification: Imagine a sender of light and a receiver of that light, and both are stationary with respect to the earth. The sender measures the departing speed of his flash of light at 186,000 miles per second as it departs in a straight line toward the receiver who measures its arrival at 186,000 miles per second – as expected. Now the receiver accelerates until he is traveling toward the still-stationary sender at 93,000 miles per second – half the speed of light. Both the sender and the receiver continue to measure the light beam at exactly 186,000 miles per second with respect to themselves! Amazing, but true, and it required no less than the drastic re-structuring of time and space in the special theory to accommodate the constant speed of light as just exemplified in the above example. The two simple assertions by Einstein in premise #1 and premise #2 fomented an upheaval in physics which is virtually unprecedented. Not since Isaac Newton published his monumental book, The Principia, in 1687, had physics seen anything like the revolution Einstein created with his mundane-appearing, but bombshell paper, On the Electrodynamics of Moving Bodies which appeared in the 1905 German scientific journal, Annalen Der Physik. Recall from my earlier post on Einstein (Feb.21, 2013) that he was, at the time, an unknown, part-time, kitchen-table physicist doing physics on weekends and during evenings! With the easy task of stating the essence of special relativity behind us, the difficulty begins for those who wish to probe more deeply into relativity. That difficulty is not inherent in deciphering complex mathematics as might be pre-supposed. Rather, the difficulty is rooted in the task of accepting the necessary but extremely counter-intuitive implications that result from the twin premises of special relativity. These implications, that Einstein insisted must be true and real, include slowing clocks and shrinking rulers resident in one frame of reference (on earth, for example) when viewed from another frame of reference (an uber-speed rocket, for example) passing by at a significant percentage of the constant speed of light, 186,000 miles per second! Coming to terms with such counter-intuitive ideas of non-absolute space and non-absolute time requires considerable imagination and logic. We won’t go there in this post – see my book, The Elusive Notion of Motion, for the details. It took considerable courage on Einstein’s part to insist that the abandonment of Newton’s absolute space and absolute time be considered not merely hazy implications of the twin premises, but, rather, dictates of physical reality. It took well over ten years for even the learned physics establishment to grudgingly fall in line with Einstein’s radical suggestions. In the many years since 1905, not one of the numerous high-level experiments conducted to prove (or disprove) special relativity has countered Einstein’s assertions!
The Big Question and the Key Clue
Now that you “know” the special theory, you might be interested in the simple central question that led Einstein to his dramatic proposals in 1905: Is there a “medium” in space which is required to support the propagation of light (as electromagnetic waves) as postulated by James Clerk Maxwell in 1865? When we reflect upon the fact that air is required to propagate sound waves and water is necessary to produce the spreading wave-ripples in a pond when we drop a stone, we can see the logic of the question. Which answer did physics wish to see? If yes, then the seemingly common-sense need for a supporting medium for any wave propagation could be extended to light-waves. That would be the “good” news of a “yes” answer. Now the “bad” news: Such a proposed medium extending throughout space, tentatively called an “ether sea” at the time, would dictate the speed of light through it and make the observed speed of light dependent on the observer’s motion/speed relative to the ether sea – contrary to important contemporary astronomical observations and laboratory experiments indicating that the speed of light seemed constant to all observers in all states of motion! The famous Michelson-Morley physics experiments in 1881 and 1887 disproved the conceptually difficult concept of an “ether sea” and supported the prevailing belief in the constant speed of light. Today, physics accepts the fact that “empty” space can nevertheless support the transverse electromagnetic waves which comprise light without having the kind of physical nature exemplified by air or water. I hope you will take-away the following from this post: That Einstein revolutionized physics through special relativity (and in other instances) by asking simple, real-world questions and doggedly pursuing the answers to them. By his own admission, he asked the kind of curiosity-based questions that children would ask and adults might be afraid to – a marvelous aspect of Einstein, the man and the scientist! His focus on the important question of a propagation medium for light (and, ultimately, its non-existence) led him to his sublime and beautiful theory of relativity. The question was simple, and the twin premises of the outcome, special relativity, are easily stated. The devil is in the details, and there the difficulty resides – the difficulty of questioning, of thinking outside the box, of overturning the familiar, the comfortable, the accepted. Therein, in the daring application of simple but precise logic while doggedly pursuing nature’s “truths,” lies the true genius of an Albert Einstein.