Synchronization of clocks and relativity of simultaneity...

Clock synchronization and simultaneity relativity

The concept of time is extremely important for physics. Therefore, it attracts the attention of all researchers who want to understand its conceptual arrangement. Among these researchers, A. Einstein acted as a genuine innovator. He drew particular attention to two concepts: clock synchronization and Simultaneousity & quot ;. If the length of processes is considered, and they are necessarily spatially extended, the time must be determined for different segments ( points) of the length. This requirement is quite natural in the light of the close interconnection of lengths and durations revealed in relativistic mechanics. This relationship must find its expression in some special concept. It just became for Einstein the concept of "clock synchronization". First, the clocks located at different points of a given frame of reference must be coordinated with each other. Secondly, this coordination should express the conceptual structure of the special theory of relativity.

Einstein proposed such a clock synchronization procedure. Assume that the clock at the point A, shows the time t 1. A ray of light is directed from the point A to the point B, where it is reflected from the mirror and returns to A at the time t 3. It is reasonable to assume that the points B light reached the time t 2:


Therefore, when the signal arrives on the clock at point B, the time t 2 should be set. Using the described synchronization procedure, it is possible to match all the clocks of a given frame of reference with each other. Of course, the requirement to place clocks on all points of the reference frame is unrealistic. But no one insists on it. It is sufficient to use the concept of clock synchronization whenever the duration of an object is determined. Einstein's clock synchronization procedure involves the use of some dynamic factors. Therefore, it is inherently dynamic in nature.

Historical excursion

The need to synchronize the clock seven years before Einstein pointed out in 1898, A. Poincaré. But, of course, at the same time he remained within his understanding of Maxwell's electrodynamics-Lorentz. Einstein also clearly followed in the wake of the concepts developed by him in the special theory of relativity. However, many attempts have been made, recognizing the special theory of relativity, nevertheless to correct his views.

In this connection, special attention was drawn to the arguments of G. Reichenbach (1928). Being an excellent logician, he drew attention to the following circumstance. It is logically unacceptable by means of light signals to first synchronize the clock, and then, based on their readings, determine the speed of the light itself in the forward and reverse directions (there is a logical circle). According to Reichenbach, the logical circle in discussions about the speed of light is overcome by the introduction of some agreements. The speed of light is postulated so that it travels all the way (from A to B and from B to A) for a time interval A t fixed by the clock A. In the formalogical key it is entirely permissible to assume that the speed of light in the forward and the opposite direction is not the same. In formula (3.7), the same speed of light in the forward and backward directions is fixed by the coefficient ε = 1/2. But, strictly speaking, its values ​​are located in the interval (0-1). The Reichenbach formula has the following form:


where ε is some arbitrary coefficient , t 3 - consecutive moments of time

The case ε = 1/3 corresponds to the condition that the light in the direction from A to B moved with a speed greater than the constant c, and in the direction from B to A - with a speed less than c.

Reichenbach's conclusion about the conditional value of the speed of light found support among many philosophers. But for the most part physicists rejected it. After all, according to the second postulate of the special theory of relativity, the speed of light is an invariant quantity. There are also many puzzling questions about the nature of light. Is it really such that, without any grounds, does the light move in the forward and reverse directions with different speeds that differ by several times? The reaction to this question, for example, of such outstanding scientists as A. Poincaré and A. Einstein, was fundamentally different.

Scientists argue

Poincare believed that the experimental data do not contradict the agreement on the invariance of the speed of light. Einstein also argued that the equality of the velocities of light for opposing directions "is not a prerequisite or hypothesis about the nature of light". Apparently, he still believed that the actual speed of light is quite certain. Reichenbach's reasoning refers, they say, to logic and have no physical meaning.

Consider another position on the issue of interest to us, which was especially clearly formulated by GB Malykin. It proceeds from the idea of ​​Bolotovskii and VL Ginzburg, who allowed the use of so-called light bunnies to synchronize clocks. Simple calculations show that if you direct a light beam to a plane that is a long distance from the signal source, then the speed of the bunny's movement along this plane can significantly exceed the speed of light in a vacuum. This fact does not contradict the provisions of the special theory of relativity, according to which interactions can not spread faster with, since it is obvious that the points illuminated by the rabbit are not related to each other by causal relations. Malykin's proposal is to use light bunnies to synchronize the clocks. In this case, it is possible to verify experimentally the equality of the velocities of light in opposite directions. The logical circle, about which Reichenbach wrote, seems to be breaking. Conventional opportunities that captivated so many philosophers are denied.

According to the author, the method of light bunnies does not reject the thesis about the conditional character of the speed of light. The difficulty in fact arose not because the speed of light c is low, but because it is necessary to determine the speed by reliance on the concept of time (speed is measured by time, and time by speed). Bunnies Do not deny this need. Their use does not lead beyond the special theory of relativity.

Thus, Einstein, not agreeing to accept the thesis of the convention of the speed of light in a vacuum, was right. In accordance with the dynamic principle, the physicist is obliged to explain the possibility of the propagation of light in a vacuum at various rates. Refusing this explanation, he, in fact, ignores the dynamic principle, which is unacceptable. It is wrong to recognize the special theory of relativity, together with its dynamic principle, to accompany the negation of this principle when describing the speed of light. The logical circle mentioned above occurs only as long as the content of the dynamic principle is ignored.

The concept of clock synchronization is organically connected with the concept of "simultaneity". Two events are considered simultaneous if they occurred at the same time as the synchronized clock. The thought experiment of Einstein convincingly testifies to the relativity of simultaneity. Suppose that inside the car moving uniformly along the rails of the platform, and exactly in its middle, there was a flash of light. The speed of light is the same as for the platform, and relative to the walls of the car. The observer, inside the car, fixes simultaneous achievement by the light beam of two opposite walls. A fundamentally different situation is observed by the person on the platform. Light first touches the wall, which does not move away from the observer, but approaches it.

Thus, simultaneity, like the speed of mechanical displacement, is a relation, and not an invariable property of time, which is supposedly independent of the frame of reference. Of course, there is no universal simultaneity, which many people, far from understanding the special theory of relativity, accept intuitively.


1. Simultaneity like speed is an attitude.

2. The meaning of time measurement is determined by the principle of invariance of the maximum speed of propagation of interactions.

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