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Relativity, Special Theory of

The special theory of relativity (STR) is a fundamental theory of space and time that was formulated and published in 1905 by Albert Einstein (1879–1955). The theory gives up the notion of the absolute and independent character of space and time. Length and time measures change according to an observer's state of motion. Special relativity is based on the assumption that the laws of nature have the same form in any uniformly moving frame of reference (the principle of relativity). As a most popular result of the STR, Einstein derived the famous relation = mc2, which states the equivalence of matter and energy. Over the years the STR has been confirmed in many tests and experiments. Along with the quantum theory, it is considered a foundation pillar of modern physics.

The Principle of Relativity and Inertial Systems

Any quantitative law of mechanics, or rather physics, is expressed with respect to some fixed spatial and temporal frame of reference represented by three spatial coordinates plus a time coordinate. This can be realized in a terrestrial laboratory providing the frame for a measurement. An experiment may be performed equally well in a racing car, cutting a corner at high speed. Quite obviously these two systems are not equivalent: Due to the permanent change of direction (i.e., the velocity vector), pseudo forces appear within the racing car. These prevent, for example, a body, thrown straight upwards by the driver, from following a simple straight trajectory as it would in the stationary laboratory of an institute. The trajectory within the racing car reference does not follow the simple Newtonian law of motion. Among all possible frames of reference, physical laws take the simplest forms in so called inertial systems. These are systems in which bodies that are exposed to no pseudo forces whatsoever, like centrifugal or Coriolis forces, rest or move uniformly; that is, with constant speed and direction. To a good approximation, the earth's surface can be regarded as an inertial system. Although any point on it is moving in a complex way, the appearing pseudo forces are negligible.

Four centuries ago, the Italian naturalist Galileo Galilei (1564–1642) stated that uniform motion is not an inner property of a body but rather a quantity that is to be defined relative to a framework or an observer. Galileo illustrated this by the example of a body resting on a uniformly drifting boat on a silent lake. The question as to whether the concerning body is moving has to be answered in different ways, depending on whether an observer is sitting in the boat or resting on the bank. Both systems, boat and bank, can be considered as inertial systems in an idealized case. Isaac Newton (1642–1727), though sharing Galileo's view in considering relative motion as relevant exclusively, nevertheless assumed the existence of absolute space. He argued, however, on the basis of spiritual rather than physical criteria and admitted the impossibility of proving the existence of absolute space.

The Principle of Relativity beyond Mechanics

After centuries of empirical research, classical electrodynamicsthe theory of electromagnetic fields' and charged particles' dynamicswas brought to its early perfection in the 1860s by the Scottish physicist James Clerk Maxwell (1831–1879). In an accomplishment similar to Newton's reduction of classical mechanics to just three fundamental laws, Maxwell succeeded in expressing the nature and dynamics of electromagnetic fields with a system of just four equations. Following these Maxwell equations, the propagation of light should always occur with the same universally constant velocity. This was, however, in disagreement with Galileo's principle of relativity: The migration velocity of a light beam should certainly have different values depending on whether a resting or a moving observer is watching it. Furthermore, a constant universal speed of light should have the consequence of an absolute space, defined as the frame within which the light velocity adopts the well-known value of 300,000 kilometers per second.

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