Background and special relativity

by Wm. Robert Johnston
last updated 29 July 2001

In 1905 Albert Einstein published four scientific papers. One, which explained the photoelectric effect as a result of the quantum nature of light, brought him the Nobel Prize in physics in 1921. Two others set forth the special theory of relativity.

In the 1880s Michelson and Morley had attempted to measure hypothetical changes in the speed of light as the Earth moved around the Sun. At the time, light was believed to be a wave in a medium referred to as the ether. According to classical physics, the speed of light should measurably change as the Earth moved around the Sun. To their surprise, no variation in the speed of light could be detected.

Fitzgerald and Lorentz attempted to explain these results in the 1890s as the result of a change in the length of a moving object, measured along the direction of travel. The Lorentz transformations were one of several explanations of the results.

Einstein's special relativity invoked two postulates:

1. All inertial (i.e. non-accelerating) frames of reference are equally valid (i.e. any observation or experiment performed will produce equally valid results).
2. The speed of light is constant for all inertial frames of reference.

He showed that the Lorentz transformations followed mathematically from these assumptions, thus providing an explanation for the results of the Michelson-Morley experiment. Special relativity predicted that for an object traveling at speeds near the speed of light, time would slow down and length would contract. Additionally, as the speed approaches that of light, momentum will increase towards infinity. This last phenonema is sometimes interpreted as an increase in mass; it leads to the equivalence of mass and energy described by E=mc². A basic derivation of the Lorentz transformations requires no more math than second-year algebra, as you can see by clicking here for an introduction and more discussion of these aspects of relativity.

Special relativity also declared that space and time were not separate concepts, but intertwined or equivalent concepts:

If you are moving relative to me, then your time is a mixture of my space and time, and vice versa.
(This will be discussed more in the context of general relativity.)

These predictions are experimentally confirmed by many experiments and observations, including:

• Time dilation of high-speed subatomic particles (muons) created by cosmic rays striking the Earth's upper atmosphere.
• Time dilation of muons accelerated to near the speed of light in particle accelerators (1976).
• Time dilation measured by atomic clocks carried by aircraft in flight.