## Special Relativity

**Einstein lecturing in Vienna in 1921**

The 1890’s were a troubling time for physicists. Even though this time period followed the great success of Maxwell’s unification of electricity and magnetism, and Newton’s laws of motion had been a hallmark mathematical description of physical behavior for centuries, a big problem could not be ignored: the experimentally verified constancy of the speed of light in a vacuum. How could the speed of light be the same, regardless of the speed of the light source to an observer without physics breaking down?

Albert Einstein (1879 – 1955), then a patent clerk in Switzerland, became the first to have a sound answer. It came with his groundbreaking **Theory of Special Relativity** (published in 1905). It was based on two simple postulates: (1) the speed of light is a constant and (2) the principle of relativity, as demonstrated below. It was an update to Newton’s laws, which could now only be treated as an approximation for velocities far below the speed of light. However, Einstein was left unsatisfied as special relativity only worked for inertial frames—that is, frames of reference traveling at constant speed. This was eventually resolved 11 years later by his publication of the Theory of General Relativity, which included accelerated reference frames.

**In this diagram, we have one scientist dropping a ball in an closed box accelerating at 1g (left) and another scientist performing the same experiment, but stationary on Earth with a gravitational force of 1g (right)—the laws of physics will appear the same to both observers, a demonstration of the principle of relativity**

The effects of special relativity begin to make a difference when speeds exceed one-tenth of the speed of light (0.1 c). It involves strange concepts, such as time dilation and length contraction. Special relativity also ensures that the speed of light is the ultimate speed limit for which matter and information can travel in the Universe.

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