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The Postulates of Special Relativity

Gennady Sokolov, Vitali Sokolov

Creating his special theory of relativity, Einstein expanded Galileoís principle of relativity to electromagnetic and optical phenomena. Einsteinís postulate of relativity asserts that not only mechanical but optical observations and experiments do not allow determination of the direction and speed of movement in an inertial frame of reference. In order to adapt his postulate of relativity to known experiments of that time, Einstein proposed his second postulate Ė the postulate of invariability of the light speed. Einsteinís postulate of relativity and the postulate of invariability of the light speed are the basis of special relativity.

Einsteinís postulate of relativity.

Galileoís principle of relativity states that the laws of the mechanics are identical in all inertial frames and therefore the observations and mechanical experiments carried out in the inertial frame do not allow detection of the movement of the frame. In his well-known example with the ship moving with constant speed in a calm lake, Galileo shows that the observer cannot determine the direction and the speed of the ship if he carries out experiments inside the ship.

Einsteinís postulate of relativity states that not only mechanical but and optical experiments and observations do not allow distinguishing the movement of oneís own frame from the movement of another inertial frame. In accordance with this postulate, Einstein drew the wrong conclusion; that the movement of the observer was equivalent to the movement of the source of light. Einstein assumed this because he did not understand the difference between observations mad inside the inertial frame and observations of external light signals coming from other frames. In the special theory of relativity, all main conclusions are drawn on the basis of the analysis of the situations with movement of the light source or the observer. And in all situations with the movement of the observer or the light source, the observer receives the light signals coming to his inertial frame from another frame connected with the light source. That is, the observer receives the signals which are external to his own inertial frame.

However, as Galileo warned, the principle of relativity means that only the observations and the experiments carried out inside the inertial frame do not allow detection of the movement of the frame. If you go on deck of the ship, said Galileo, you will see that your ship moves relative to shore, relative to the air, relative to the water, and so on. The observation of the external light signals coming from other frames allow detection the movement of own inertial frame relative to those frames. There is no equivalence of the movements of the light source and the observer because the observer receives external signals. Receiving the light signals from the different inertial frames, the observer can detect his own movement relative to those frames and some observations already prove that. The aberration of light coming from the stars, allows determining the speed of orbital motion of the Earth. This fact contradicts Einsteinís postulate of relativity. The observation of so-called relict radiation allows determining the direction and the speed of the movement of the Earth relative to the stars. Stellar aberration takes place when the observer moves (this has been known about 300 years), but does not take place when the light source moves (this is proven by the observations of double star systems which has been known about 70 years). This fact convincingly proves that the movement of the observer is not equivalent to the movement of the source of light.

All optical and electromagnetic experiments, carried out on the Earth, that is, inside the inertial frame, prove the principle of relativity. The best example is Michelsonís interference experiment which, in accordance with Galileoís principle of relativity, does not allow detecting the movement of the Earth. It proves with great accuracy only that the light propagates in the earthís atmosphere in all directions with identical speed and that this speed does not depend on the speed and the direction of the movement of the Earth.

The postulate of the Invariability of Light Speed

The equivalence of the movements of the light source and the observer means that the observer measuring the speed of the light will receive the same result both in the case when he is immovable but the light source moves at speed V and in the case when the light source is immovable but the observer moves at the same speed V in the opposite direction.

In order to adapt his postulate of relativity and the equivalence of the movements of the light source and the observer to the known experiments, Einstein established his second postulate Ė the postulate of the constancy or the invariability of the speed of the light. This states that the speed of the light depends neither on the movement of the light source nor the movement of the observer measuring this speed. The speed is always equal to C=299 792 458 mps in vacuo.

The only experiment proving the influence of the movement of the observer was carried out in 1810-1813 by D.Arago. Observing the light of the star toward which at that time the Earth moved on the circumsolar orbit, D.Arago wanted to detect the change of the focal distance in his telescope or the change of the angle of refraction in a prism. The experiment showed that the speed with which the light enters the object lens or the prism does not depend on the speed of the Earthís movement. Now Aragoís experiment has a simple explanation and cannot be considered as the proof of the independence of the light speed on the movement of the observer.

When the special theory of relativity was already created, De Sitter convincingly proved that the speed with which light came to Earth from binary stars did not depend on the movement of the stars. The independence of light speed on the movement of the source is proven by numerous laboratory experiments. Because in special relativity the movement of the observer is equivalent to the movement of the light source, it is generally assumed that the speed of light does not depend, as the postulate of invariability states, on the movement of the observer. After Aragoís experiment there was no experiment proving the independence of the light speed on the movement of the observer.

What does the independence of light speed on the movement of the observer mean? It means, for example, that the observer moving in space at a speed of 8000 meters per second toward the star and measuring the speed of light coming from that star, will determine the value at 299 792 458 mps but not 299 800 458 mps.

The postulate of the invariability of light speed and, first of all, the independence of light speed on the movement of the observer led to a revolution in our ideas of time and space. In special relativity, with the help of very simple examples involving the movements of the light source and two observers, it is proven that the events simultaneous in one inertial frame are not simultaneous in the other frame; that time dilation and length contraction take place in the moving frames and so on. It is proven simply and logically. It is logical if the postulate of invariability is right and if the speed of light does not depend on the movement of the observer.

In order to prove the falsity of the special theory of relativity it is necessary to carry out the experiments proving that the speed of the light depends on the movement of the observer and that in the case when the observer moves toward the light beam he meets the photons with more speed than in the case when he moves in the opposite direction. At first it is necessary to carry out a simple orbital experiment in which the observer will measure the speed of the light coming to him from some bright star or from a laser located on another spaceship. This experiment will exert more influence on developments in science than Michelsonís experiment did in its time.