More Relativity: The Train and The Twins

Michael Fowler, UVa Physics.

Einstein’s Definition of Common Sense

As you can see from the lectures so far, although Einstein’s Theory of Special Relativity solves the problem posed by the Michelson-Morley experiment—the nonexistence of an ether—it is at a price.  The simple assertion that the speed of a flash of light is always c in any inertial frame leads to consequences that defy common sense.  When this was pointed out somewhat forcefully to Einstein, his response was that common sense is the layer of prejudices put down before the age of eighteen.  All our intuition about space, time and motion is based on childhood observation of a world in which no objects move at speeds comparable to that of light.  Perhaps if we had been raised in a civilization zipping around the universe in spaceships moving at relativistic speeds, Einstein’s assertions about space and time would just seem to be common sense.  The real question, from a scientific point of view, is not whether Special Relativity defies common sense, but whether it can be shown to lead to a contradiction.  If that is so, common sense wins.  Ever since the theory was published, people have been writing papers claiming it does lead to contradictions.  The previous lecture, the worked example on time dilation, shows how careful analysis of an apparent contradiction leads to the conclusion that in fact there was no contradiction after all.  In this lecture, we shall consider other apparent contradictions and think about how to resolve them.  This is the best way to build up an understanding of Relativity.