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A century passed under the spell of the Special Theory of Relativity, but interested general public still does not have adequate knowledge about it. This five-minute presentation is for those who would like to understand what it is all about, but do not think that they would keep their minds on it for more than five minutes. If you become curious to learn more, you can find guidelines on how to continue at the end of the presentation.

The Special Theory of Relativity is connected with the properties of space and time. Any event is specified by a place and by a time of happening. In some class of frames the laws of physics take their simplest form. They are called inertial frames. If two inertial reference frames with observers move uniformly relative to each other, there is no way to tell, which of them is really at rest, because the laws of physics look the same for these observers.

The Classical Mechanics of Galileo and Newton is based on this statement, which is called the Principle of Relativity. Galileo was led to it by the observation that the laws of mechanics look the same in a ship moving uniformly and in a ship at rest. Galileo derived how coordinates of some event in one inertial frame should be connected with its coordinates in another frame so that the laws of mechanics look the same in both frames.

Electromagnetic and optical phenomena are described by Maxwell's equations based on experiment. Originally they were supposed to be written for a frame "at rest", and it was not clear whether the laws of electrodynamics look the same for observers moving uniformly relative to each other, like the laws of Classical Mechanics do.

According to the naive old understanding it was supposed that there exists a privileged frame at "absolute rest". First it was connected with the Earth, then with the Sun, then with the Galaxy. It was supposed that it is filled with ether - the medium, in which light is propagating. But experiments with electromagnetic and optical phenomena were not able to disclose the absolute motion of the Earth relative to the ether, and so the Principle of Relativity seemed to be a general law of Nature.

But Maxwell's equations were changing under the Galileo transformations. Trying to find transformations under which the laws of electrodynamics remain the same, Lorentz wrote a new set of transformations of coordinates and electromagnetic fields. Poincaré extended them by including also charge densities and currents emphasizing that they form a group.

In 1905 Einstein investigating electromagnetic phenomena suggested that they could be described without the notions "ether" and "absolute rest". He raised the Principle of Relativity to the status of a postulate.

The consequences based on the Maxwell's Theory of Electrodynamics appeared to be consistent and the developed theory later came to be called the Special Theory of Relativity in order to contrast it with the General Theory of Relativity that was extending the validity of the Principle of Relativity also on the gravity.

From Maxwell's equations follows the constancy of the speed of light for all observers, which means that if an observer with a source of light is moving with the velocity V relative to some other observer, the velocity c of light will be the same for both observers, and not c + V or c - V, as one could expect in accordance with the Classical Mechanics. The new formula of addition of velocities gives the correct result.

The validity of the Principle of Relativity for Electrodynamics led to a change of the concept of space and time. The concept of a universal, or absolute, time had to be abandoned, and the Classical Mechanics had to be adjusted to the requirement of the constancy of the speed of light.

Relativistic effects follow logically from the Theory and experiment verifies them. They are significant only at high speeds - mainly for the world of very large and for the world of very small and seem to be in conflict with our everyday experience. So called "paradoxes" of the Special Theory of Relativity are not paradoxical at all, when one considers them carefully. The key to that lies in correct determination of place and time of events in different frames.

Time and space can be measured in the same units. It should be familiar to you from everyday life. People often say: "My home is ten minutes by car from work." Such statement implies a speed of the car. Here we shall be implying the speed of light and shall measure both time and space in meters. For example, 10 meters of time will mean time in which the light covers the distance of 10 meters.

Imagine a latticework of sticks and clocks in some frame. We want them all to read the same time in this frame. In order to synchronize them, we pick one clock in the lattice. We set its pointer at zero time and at the same moment send a flash of light. When the flash of light gets to a clock 5 meters away, we want that clock to read 5 meters of time, and so on. The space location of an event will be the location of the clock nearest to the event, and the time of the event is taken to be the time recorded on the same clock.

Different observers may record different separations in space and time between two events, but some combination of them, called spacetime interval, will be the same.

It appears that events simultaneous for one observer might be not simultaneous for another observer, time is going slower in moving frames - this way you can, in principle, visit your descendants, and lengths of moving objects are shorter.

As one of the logical consequences of the Special Theory of Relativity Einstein found his famous formula

E = mc²,

expressing the equivalence of mass and energy. How to unlock the energy hidden inside mass became clear in 1938, when physicists managed to split an atomic nucleus.

The Principle of Relativity became a general criterion that all theories must satisfy. The Special Theory of Relativity initiated an appreciation of the role of symmetry criteria, which became a powerful guide in the search for the fundamental laws of Nature. Under the influence of the Special Theory of Relativity physicists are working towards having as few laws as possible. There exists even a hope to find a universal law, the complete theory of all forces - the Theory of Everything.

Nature is diverse and rich in phenomena, but the underlying laws appear to be beautiful and simple. So the ideas developed in 1905 are continuing to guide physicists today. The search for the fundamental laws of Nature continues…

If you are interested enough and would like to gain serious knowledge of the subject, you can use the famouse book of E. F. Taylor and J. A. Wheeler, Spacetime Physics, W.H Freeman and Company, 1992.