Friedel Weinert
Technological Applications of the Theory of Relativity
- Introduction.
The 20th century saw the birth of two famous theories: relativity and quantum theory. This article deals with the theory of relativity, which was developed by Albert Einstein (1879-1955) in two stages. In 1905 Einstein proposed his Special Theory of relativity. It is called ‘special’ because it is restricted to uniform motion. That is, either rest or constant motion. Out of this Special Theory grew the most famous equation of all of physics: E=mc2. In 1916 Einstein went a step further and proposed his General Theory of Relativity. This theory was no longer restricted to uniform or inertial motion. It applies to both uniform and non-uniform motion. Non-uniform motion occurs when a system undergoes acceleration or rotation. Gravity, for instance, accelerates objects toward the Earth. The General Theory is truly revolutionary because it replaces Newton’s theory of gravitation by the notion of space-time curvature. The mathematics of the General Theory is also vastly more complicated than that of the Special Theory. This article is not be concerned with the mathematical or philosophical aspects of the Theory of Relativity, but with its technological applications. It may not be obvious to the reader but both theories have led to many technological innovations, which are encountered in daily life, from cell phones to GPS, from speed guns to nuclear power. As Atkins put it Einstein “moved civilization forward in two steps.”[1] It is convenient to start with the applications of the Special Theory of Relativity (STR) before turning to the General Theory of Relativity (GTR).
- Technological Applications of the Special Theory.
From a historical perspective, the Special Theory is basically the crowing of classical physics. But it has vast implications for our understanding of space and time. That is, it affects the measurement of spatial and temporal intervals. In particular, the running of clocks lies at the root of the technological application of the Special Theory.
2. 1. One of the great pleasures of modern life is the freedom of movement afforded by the use of motor vehicles. But as so often there is a sting in the tail. Modern cars can move very fast, and often people break speed limits, resulting in fines. But how is the speed of objects, such as a car or a tennis ball, measured? Speed guns use the so-called Doppler Effect to perform measurements. The effect is named after the Austrian physicist Christian Doppler (fig. 1).
Doppler discovered how the observed frequency of sound waves is affected by the relative motion of the source (for instance, a car) and the detector (ears, speed guns). “Legend has it that he grasped the phenomenon thanks to a type of tuba.”[2] Hence, the effect was known well before Einstein developed his Theory of Relativity. The siren of an ambulance, for instance, has a higher pitch (or frequency) when it is approaching than when it is recedin