Happy Birthday, Relativity!

On June 30, 1905, Albert Einstein published a paper in the scientific journal Annalen der Physik, "On the Electrodynamics of Moving Bodies." This paper is the foundation of Einstein's theory of relativity, though the theory would not be "proven" for nearly 15 years.

Before this time, it was commonly believed that light was a wave (see Young's double slit experiment and wave particle duality), which meant that it had to travel through some sort of medium, a theoretical substance which came to be called the aether. The famed Michelson-Morley experiments of the 1880s, however, had failed to detect motion within this aether. (Recent experiments, with greater precision, have confirmed these results.)

Hendrik Lorentz had analyzed this and come up with translational models (known as Lorentz transformations) which predicted contraction of physical matter moving through the ether, in an effort to reconcile the inconsistency between the aether model and the Michelson-Morley results.

Albert Einsten's 1905 paper, however, took an entirely different approach. He assumed two postulates:

• First Postulate: The laws of physics are the same for all inertial reference frames.
• Second Postulate: Light always propagates through a vacuum (i.e. empty space or "free space") at a definite velocity, c, which is independent of the state of motion of the emitting body.

By assuming that the speed of light is a constant, he therefore eliminated any need for the aether description. His previous work (also in 1905) in explaining the photoelectric effect had already proposed the photon theory of light, which allowed light to move as a particle instead of a wave.

The consequences of these assumptions and the conclusions drawn from them are astounding. In relativity, gravity is seen as the curvature of spacetime and is not a force, but is rather the motion along this curvature (which can be represented fairly faithfully, in most common circumstances, using Newton's law of gravity).

Furthermore, matter can be seen as a form of energy, resulting in the most famous equation in the history of science - E = mc² - and the creation of the atomic bomb, as well as more benign uses of nuclear energy.

The first experiment to confirm Einstein's theory of general relativity was performed by Arthur Eddington and a British expedition to Africa in 1919, during a solar eclipse. There they observed that stars near the sun appeared to shift, which fit with the theoretical prediction of relativity that the light would be bent by the gravitational influence of the sun itself. Since then, many of its predictions have been confirmed.

Needless to say, such a revolutionary theory has always had its critics. Even today, there are some who oppose relativity (or Einstein himself), on grounds that may be either scientific or philosophical or both. Do you have questions about relativity? Do you understand it, but disagree with its interpretations? Tell us what you think of relativity, either in the blog comments or the About Physics Forums.

Learn more about relativity:

• Einstein's Theory of Relativity
• Special Relativity and Lorentz Transformations
• General Relativity
• Fundamental Concepts & Controversies of Relativity
• Books about Albert Einstein & Relativity
• Albert Einstein - Biographical Profile
• Relativistic Doppler Effect - Red Shift & Blue Shift

Image: Albert Einstein in 1905 (public domain due to age)