The special theory of relativity, published in 1905 by Einstein and 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 for it in a completely unusual way: its length decreases, its mass increases, and time slows down. Immediately after the publication, attempts began to discredit the theory, which continue today, although more than a hundred years have passed. This is not surprising, because the question of what time is has long worried mankind and attracted everyone's attention.
What is relativism
The essence of relativistic mechanics (it is also the special theory of relativity, hereinafter referred to as SRT) and its difference from classical mechanics is vividly expressed by the direct translation of its name: Latin relativus means "relative". SRT postulates the inevitability of time dilation for an object as it moves relative to the observer.
The differenceof this theory, proposed by Albert Einstein, from Newtonian mechanics and lies in the fact that all ongoing processes can only be considered relative to each other or to some outside observer. Before describing what relativistic time dilation is, it is necessary to delve a little into the question of the formation of the theory and determine why its formulation became possible and even mandatory at all.
The Origins of Relativity
By the end of the 19th century, scientists came to understand that some experimental data did not fit into the picture of the world based on classical mechanics.
Fundamental contradictions resulted in attempts to combine Newton's 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 within the framework of electrodynamics, but it was extremely problematic to argue with visual and, most importantly, time-tested mechanics.
The contradiction, however, was evident. Suppose that a lantern is fixed in front of a moving train, which shines forward. According to Newton, the speeds of the train and the light coming from the lantern must add up. Maxwell's equations in this hypothetical situation simply "broke". An entirely new approach was needed.
Special Relativity
It would be incorrect to believe that Einstein invented the theory of relativity. In fact, he turned to the works and hypotheses of scientists who worked before him. However, the author approachedquestion on the other hand and instead of Newton's mechanics recognized Maxwell's equations as "a priori correct".
In addition to the famous principle of relativity (in fact, formulated by Galileo, however, within the framework of classical mechanics), this approach led Einstein to an interesting statement: the speed of light is constant in all frames of reference. And it is this conclusion that allows us to talk about the possibility of changing the time standards when the object moves.
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 that you are standing still and watching the light move away from you at a fixed speed. You follow the beam, but it continues to move away from you at exactly the same speed. Moreover, turning around and flying in the opposite direction from the beam, you will not change the speed of your distance from each other in any way!
How is this possible? Here begins the conversation about the relativistic effect of time dilation. Interesting? Then read on!
Relativistic time dilation according to Einstein
When an object's speed approaches the speed of light, the object's internal time is calculated to slow down. If we assume that a person moves parallel to the sunbeam with a similar speed, time for him will stop running at all. There is a formula for relativistic time dilation, reflecting its relationship with the speed of an object.
However, when studying this issue, it should be remembered that no body with mass can even theoretically reach the speed of light.
Paradoxes related to theory
Special relativity is a scientific work and not easy to understand. However, public interest in the question of what time is regularly gives rise to ideas that at the everyday level seem to be insoluble paradoxes. For example, the following example baffles most people who are introduced to SRT without any knowledge of physics.
There are two planes, one of which flies straight, and the second takes off and, having described an arc at a speed close to the speed of light, catches up with the first. Predictably, it turns out that time for the second apparatus (which flew at near-light speed) passed more slowly than for the first. However, in accordance with the SRT postulate, the frames of reference for both aircraft are equal. This means that time can pass more slowly for both one and the other device. It would seem that this is a dead end. But…
Resolving paradoxes
In fact, the source of this kind of paradoxes is a misunderstanding of the mechanism of the theory. This contradiction can be resolved using a well-known speculative experiment.
We have a shed with two doors forming a through passage and a pole slightly longer than the length of the shed. If we stretch the pole from door to door, they will not be able to close or they will simply break our pole. If the pole, flying into the barn,will have a speed close to the speed of light, its length will decrease (remember: an object moving at the speed of light will have a length of zero), and at the moment 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 the plane, it is the barn that should decrease relative to the pole. The paradox is repeated, and, it would seem, there is no way out - both objects are synchronously reduced in length. However, remember that everything is relative, and solve the problem by changing the time.
Relativity of Simultaneity
When the front edge of the pole is 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 it at the same time, and the experiment failed, but here the main thing turns out: in accordance with the special theory of relativity, the closing moments of both doors are located at the same point on the time axis.
This happens because 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 the absolutely everyday generalization of Einstein's theory: everything is relative.
There is one more detail: the equality of reference systems is relevant in SRT, when both objects move uniformly and rectilinearly. As soon as one of the bodies starts accelerating or decelerating, its frame of reference becomes uniquepossible.
Twin Paradox
The most famous paradox that explains 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 the speed of light, while the other remains on the ground. Upon returning, the astronaut brother discovers that he himself has aged 10 years, and his brother, who remained at home, has aged as much as 20.
The overall picture should already be clear to the reader from the previous explanations: for the brother on the spaceship, time slows down because his speed is close to the speed of light; we cannot accept the frame of reference relative to the brother-on-ground, since it will turn out to be non-inertial (only one brother experiences overloads).
I would like to note something else: no matter what degree the opponents reach in 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 age exactly at the same rate as time passes in his frame of reference, and the second brother will age at exactly the same rate - the difference will be revealed only when they meet, and not in any other case.
Gravity time dilation
In conclusion, it should be noted that there is a second type of time dilation, already associated with the general theory of relativity.
Even in the 18th century, Mitchell predicted the existence of the reddisplacement, which means that when an 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 that, as they say, is a completely different story.
