The special theory of relativity, published in 1905 by Einstein and became an important generalization of a number of earlier hypotheses, is one of the most resonant and discussed in physics.
Indeed, it is difficult to imagine that when an object moves at near-light speed, physical processes begin to proceed in a completely unusual way for it: its length is reduced, its mass is increased, and time slows down. Immediately after publication, attempts began to discredit the theory, which continues today, although more than a hundred years have passed. This is not surprising, because the question of what time is, has long worried humanity and attracted universal attention.
What is relativism?
The essence of relativistic mechanics (it is also a special theory of relativity, hereinafter referred to as SRT) and its difference from classical is clearly expressed by a direct translation of its name: Latin relativus means "relative". In the framework of SRT, the inevitability of time dilation for an object when it moves relative to the observer is postulated.
The difference between this theory, proposed by Albert Einstein, from Newtonian mechanics lies in the fact that all the processes that occur can be considered only relative to each other or to some outside observer. Before describing what the relativistic time dilation consists of, it is necessary to delve somewhat into the question of the formation of the theory and determine why its formulation in general has become possible and even mandatory.
The origins of the theory of relativity
By the end of the XIX century, scientists came to the understanding that some experimental data do not fit into the picture of the world, based on the basis of classical mechanics.
Fundamental contradictions ended in attempts to combine Newtonian mechanics with Maxwell's equations describing the motion of electromagnetic waves in vacuum and continuous media. It was already known that light is just such a wave, and it should be considered in the framework of electrodynamics, but to argue with visual and, most importantly, time-tested mechanics was extremely problematic.
The controversy, however, was obvious. Suppose that a lantern is fixed in front of a moving train, which shines forward. According to Newton, the speed of the train and the light coming from the lantern should add up. Maxwell's equations in this hypothetical situation simply “broke”. The need was ripened for a completely new approach.
Special theory of relativity
To believe that Einstein came up with the theory of relativity would be incorrect. In fact, he turned to the works and hypotheses of scientists who worked before him. However, the author approached the issue from a different perspective and, instead of Newtonian mechanics, recognized Maxwell's equations as “a priori true”.
In addition to the famous principle of relativity (in fact, formulated by Galileo, however, in the framework of classical mechanics), this approach led Einstein to the most interesting statement: the speed of light is constant in all reference frames. And it is precisely this conclusion that allows us to talk about the possibility of changing the standards of time when moving an object.
The constancy of the speed of light
It would seem that the statement “the speed of light is constant” is not surprising. But try to imagine: you are standing still and watch the light moving away from you at a fixed speed. You fly after the beam, but it continues to move away from you at exactly the same speed. Moreover, turning and flying in the opposite direction from the beam, you will not change the speed of your distance from each other!
How is this possible? This is where the conversation begins about the relativistic effect of time dilation. Interesting? Then read on!
Einstein's relativistic time dilation
When the speed of an object approaches the speed of light, the internal time of the object, according to calculations, slows down. If we assume that a person moves at the same speed in parallel with the sun's ray, time for him will cease to go at all. There is a formula for relativistic time dilation, reflecting its relationship with the speed of the object.
However, when studying this issue, it should be remembered that not a single body with mass can even theoretically reach the speed of light.
Theory-related paradoxes
The special theory of relativity is a scientific work and is not easy to understand. However, public interest in the question of what time is, regularly gives rise to ideas that seem insoluble paradoxes at the everyday level. For example, the following example baffles most people who become familiar with SRT without any knowledge of physics.
There are two aircraft, one of which flies straight, and the second takes off and, having described the arc at a speed close to the speed of light, catches up with the first. Predictably, it turns out that the time for the second vehicle (which flew at near-light speed) was slower than for the first. However, in accordance with the STO postulate, the reference systems for both aircraft are equal. This means that time can flow more slowly for both one and the other device. It would seem to be a dead end. But...
Paradox Resolution
In fact, the source of this kind of paradox is a misunderstanding of the mechanism of the theory. The mentioned contradiction can be resolved using a well-known speculative experiment.
We have a barn with two doors forming a through passage, and a pole, the length of which is slightly greater than the length of the barn. If we extend the pole from door to door, they will not be able to close or just break our pole. If the pole, flying into the barn, has a speed close to the speed of light, its length will decrease (recall: for an object moving at the speed of light, the length will be zero), and when it is inside the barn, we can close and open the doors, without breaking our props.
On the other hand, as in the example with an airplane, the shed should be reduced relative to the pole. The paradox is repeated, and, it would seem, there is no way out - both objects are simultaneously reduced in length. However, we recall that everything is relative, and we will solve the problem by changing the time.
Simultaneity relativity
When the front edge of the pole will be inside, in front of the front door, we can close and open it, and at the moment when the pole flies into the shed completely, we will do the same with the back door. It would seem that we are not doing this at the same time, and the experiment failed, but the main thing is found out here: in accordance with the special theory of relativity, the closing times of both doors are located at one point on the time axis.
This is because the events occurring simultaneously in one frame of reference will not be simultaneous in another. Relativistic time dilation is manifested in the relationship of objects, and we return to an absolutely general generalization of Einstein's theory: everything is relative.
There is one more detail: the equality of reference systems is relevant in service stations, when both objects move uniformly and rectilinearly. As soon as one of the bodies proceeds to accelerate or decelerate, its reference frame becomes the only possible one.
Twins paradox
The most famous paradox explaining the relativistic time dilation “in a simple way” is a thought experiment with two twin brothers. One of them flies away on a spaceship at a speed close to light, and the other remains on the ground. Upon returning, the cosmonaut brother discovers that he himself aged 10 years, and his brother, who remained at home - by as much as 20.
The general picture should already be clear to the reader from previous explanations: for a brother on a spaceship, time slows down, since its speed is close to the speed of light; we cannot accept the reference frame relative to the brother-on-earth, since it will turn out to be non-inertial (only one brother experiences overload).
I would like to point out something else: no matter how much the opponents reach the dispute, the fact remains: time in its absolute value remains constant. No matter how many years a brother flies on a spaceship, he will continue to grow old at exactly the speed with which his frame of reference goes, and the second brother will age at exactly the same speed - the difference will be revealed only when they meet, and in no other case.
Gravitational time dilation
In conclusion, it should be noted that there is a second type of time dilation associated with the general theory of relativity.
As far back as the 18th century, Mitchell predicted the existence of the redshift effect, which consists in the fact that when the object moves between areas with strong and weak gravity, the time for it will change. Despite attempts to study the issue by Laplace and Zoldner, only Einstein presented a full-fledged work on this topic in 1911.
This effect is no less interesting than the relativistic time dilation, but it requires a separate study. And this, as they say, is a completely different story.