- American Physical Society Sites
- Meetings & Events
- Policy & Advocacy
- Careers In Physics
- About APS
- Become a Member
Ever since the days of Galileo and Isaac Newton, physicists had observed that laboratory measurements of mechanical processes could never show any difference between an apparatus at rest and one moving at constant speed in a straight line. Objects behaved the same way on a uniformly moving ship as on a ship docked in the harbor. They called this the principle of relativity. But they didn't believe this principle should apply to light.
At the end of the 19th century, light was assumed to be a wave. To scientists, this meant it had to travel in some sort of medium, just like sound or water waves. They decided there must a special substance called the ether permeating the world, and this would explain why light could travel through seemingly empty space. But if this were true, then the speed of light should not be constant, as predicted in Maxwell's equations; it should show the effects of motion. Experiment after experiment failed to turn up any supporting evidence for the assumption.
The most well-known of these experiments was done in Cleveland, Ohio, by Albert Michelson and Edward Morley in 1887. Their experimental apparatus was a massive stone block with mirrors and crisscrossing light beams, giving an accurate measurement of any change in the velocity of light. Michelson and Morley expected to see their light beams shifted by the swift motion of the earth in space. To their surprise, they could not detect any change. It is debatable whether Einstein paid heed to this particular experiment, but his work provided an explanation of the unexpected result through a new analysis of space and time.
In the wake of Michelson and Morley's results, (or lack thereof), Einstein decided to dispense with the notion of the ether altogether for his theory of special relativity. He began with two fundamental assumptions.
First, any observer moving at a constant speed would have the same laws of physics. Second, the speed of light c is always constant, no matter how fast or in what direction the light source was moving. If both of those assumptions held, then our notion of time must be incorrect: specifically, two events that are simultaneous in one frame of reference would not be simultaneous in another. Time was not absolute, but relative.
Since length measurement involves determining the front and back positions of an object at the same time, this same relative principle must apply to length as well. It also applies to the quantities of matter and energy. So time beats at different rates depending on how fast an object (or person) is moving; the faster you move, the slower time progresses. And the faster an object moves, the more distances contract, and the heavier an object becomes. In fact, in the limit that the speed of a massive object approaches c, time slows down to a stop, distances contract to nothing, and the object's energy becomes infinite.
Einstein wasn't the only scientist or philosopher to question the absoluteness of time. He developed his ideas in an era that was obsessed with the issue of synchronizing time frames through space. For one thing, it was critical to coordinating the schedules of railway companies. By the time Einstein was employed in the Swiss patent office in Bern, developing networks of clocks running in sync was a major precision industry, according to Peter Galison, author of Einstein's Clocks, Poincare's Maps. There were a large number of patents submitted dealing with clocks linked by signals.
And by the 1890s it was routine for astronomers and engineers to figure in the time an electrical signal took to travel from one place to another in their calculations. Some engineers even sent their time signals on round trips to compensate for the inevitable errors.At the Bureau of Longitude in Paris, Henri Poincare was among those worried about this "time of transmission." In January 1898, he wrote a famous philosophical article, "The Measure of Time," in which he discussed the possibility that simultaneity is little more than the exchange of signals between two clocks, taking into account the time of transfer between them at the speed of an electrical signal, or of light. Poincare didn't apply his ideas to physics until 1900, when he was invited to speak at a gathering to honor H.A. Lorentz, who was a major figure in the electrodynamics of moving bodies. Poincare realized he could reinterpret Lorentz's purely mathematical ideal of time as a physical coordination procedure.
Yet Poincare couldn't bring himself to discard the fundamental distinction between true time (in the frame of the ether) and "apparent time," as measured in any other frame of reference, nor could he discard the notion of the ether. Einstein did away with both, and the result was truly revolutionary.
Next Month: Einstein's Most Famous Formula
See the special exhibit on Albert Einstein's life and work by the American Institute of Physics: http://www.aip.org/history/einstein/
©1995 - 2021, AMERICAN PHYSICAL SOCIETY
APS encourages the redistribution of the materials included in this newspaper provided that attribution to the source is noted and the materials are not truncated or changed.