The field of *special relativity* is founded with one of four papers that Einstein published during his annus mirabilis, 1905. The theory of special relativity is based on the idea that two systems that are travelling at a given, constant speed should not be distinguishable. The laws of physics should be the same in both of them. Such a system is called an **inertial frame of reference**. Together with one other fundamental assumption, we obtain the theory of special relativity: 1. All inertial frames of reference are equivalent. 2. The speed of light in free space $c$ is constant in all inertial frames of reference. At low speeds like the ones that we experience in everyday life, effects of special relativity do not really matter. As soon as velocities are high (as in half of the speed of light), effects like **time dilation** (moving clocks tick slower), **synchronization issues** and **length contraction** (a moving ruler is shorter). >[!read]- Further Reading >- [[Speed of Light]] >[!ref]- References >- A. Einstein, Zur Elektrodynamik bewegter Körper, Annalen Der Physik **322**, 891 (1905).