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Unwinding the Clockwork Universe

Between the 18th and 19th centuries, several problems arose in astronomy and physics that would later challenge Newton's universe.

Mercury's Time-keeping

The first problem concerned the planet Mercury. In the late 1800's, astronomers noted that there were slight differences between Mercury's observed orbit and that predicted by Newton's theory of gravitation. Mercury's elliptical path around the Sun shifts slightly forward with each orbit such that its closest point ("perihelion") advances by 5 arc minutes per century. Newton's theory could explain all but 42 arc seconds. No one could account for this tiny but significant discrepancy. Until Einstein.

All's Not Well with the Aether

The second area of theoretical disquiet concerned the brilliant new unified theory of electromagnetism, developed in the 1870's by the Scottish physicist James Clerk Maxwell. His theory, which predicted the existence of electromagnetic waves moving at the speed of light, was highly successful in all experimental tests. But it seemed to require that such waves move through a fixed reference frame which, in contrast to Newton's empty space, was believed at that time to consist of an invisible substance known as the aether.

When describing systems at rest in absolute space, Maxwell's laws were simple and elegant, but when they were applied to moving systems, the mathematics grew much more complex and difficult to interpret. Scientists, particularly Einstein, began to explore Maxwell's laws further. Its equations turned out to be essentially correct, but Einstein attempted to explain why in his Special Theory of Relativity.

Bending the Rules

A third problem in the Newtonian universe concerned the speed of light. To an observer at rest in Newton's absolute space, light travels (through the aether) at a constant speed. But logic dictates that when the observer is moving through space, the observed speed of light should vary, depending on whether the observer is moving toward the source of light or away from it.

By the end of the 19th century, more sensitive instrumentation allowed this notion to be put to the test. In 1881, using a very sensitive apparatus, Albert Michelson found no difference at all between the speeds of light measured for opposite directions of travel. Working with chemist Edward Morley in 1887, he repeated the experiment using yet more accurate instruments; they obtained the same results. The truth of observations were undeniable. Scientists began to explain the results by theorizing that distance through which light travelled might appear to stretch or shrink, depending on the direction of travel.

The notion of the aether as a fixed reference frame through which light must propagate was beginning to unravel. No experiments had demonstrated its existence. Einstein's Special Theory of Relativity was to dispense with the aether altogether.

Opportunity Knocks

As a young man in college, Einstein seized upon these theoretical difficulties as interesting puzzles to probe. His interest in them did not wane when he failed to get a teaching post and eventually secured a position with the Swiss patent office. Einstein continued to study the new physics on his own time and in relative isolation, though he enthusiastically discussed philosophical and scientific topics with a circle of college friends. He was working on several ideas at once, including his Ph.D. thesis research and a separate paper on special relativity.

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Copyright 1995, The Board of Trustees of the University of Illinois

NCSA. Last modified 11/14/95.