The Principle of Relativity

The principle of relativity introduced by Galileo first of all extended to mechanical systems. He said that no mechanical experiments can determine whether the system is at rest or straight and evenly moving. In other words, when the same mechanical experiments are performed in different inertial coordinate systems (with acting inertia forces) , the results will be similar.

Galileo noticed that the mechanics of movements, or rather collisions, impacts, flight of projectiles and other phenomena, gives the same results: both in uniformly and rectilinearly moving laboratories, and in those at rest.

This mechanical principle of relativity can be explained in the following example. Let's say that one car passes near another without any jolts, that is, at a constant speed, evenly. And all around is shrouded in such thick dense fog that near absolutely nothing is visible. The question is: can the passengers in the car determine which of them is moving? Can they be helped by making experiments on mechanics?

It turns out that in this case passengers can only observe relative movement. Despite the fact that all the laws of motion and vector addition rules are developed with the help of moving laboratories, they do not detect, "do not feel" any influence of this movement on themselves. The principle of relativity also indicates that no mechanical experiments will allow us to detect a rectilinear uniform motion of the reference frame relative to the stars and the sun. However, when the reference frame is accelerated relative to the stars and the sun, the results of the experiments are affected.

The Galilean principle of relativity in mechanics deserves special attention. None of the Galilean systems can be given preference in principle, in spite of the fact that from a practical point of view it is advisable to consider this or that reference system as the preferred one depending on the situation.

So, for the passenger in the car of the passenger the coordinate system that is connected with the machine, will be a reference system more natural than that associated with the road. And the latter system, in turn, will become more convenient for a person watching the movement of the car, standing near the road. Different Galilean systems have a fundamental equivalence, which is expressed in the fact that for the transition between systems there are the same formulas, and the variable value is only the value of the relative velocity.

This principle of relativity is considered from the point of view of kinematics, but such equivalence of different systems is characteristic for dynamics. This is the classical principle of relativity.

There is also a special principle that applies to any physical phenomena, not just mechanical movements. Its essence lies in the fact that for any coordinate system that moves uniformly and rectilinearly relative to each other, any physical phenomena proceed in the same way, and any physical experiments give a similar result.

This provision is defined as a special relativity principle, since it refers to special cases of rectilinear uniform motion. In such a case, all laws look the same both for the coordinate systems relating to the stars, and for any other systems that move uniformly and rectilinearly relative to the stars.

There is also a more general principle that covers cases of coordinate systems with accelerated motion. It is called the general principle of relativity.