It is said that the epiphany came to Albert Einstein in an instant. The scientist allegedly was riding a tram in Bern (Switzerland), looked at the street clock and suddenly realized that if the tram now accelerated to the speed of light, then in his perception, this clock would stop - and there would be no time around. This led him to formulate one of the central postulates of relativity - that different observers perceive reality differently, including such fundamental quantities as distance and time.
Scientifically speaking, that day, Einstein realized that the description of any physical event or phenomenon depends on the frame of reference in which the observer is located (see Coriolis effect). If a passenger on a tram, for example, drops glasses, then for her they will fall vertically downward, and for a pedestrian standing on the street, glasses will fall in a parabola, since the tram is moving while the glasses are falling. Each has its own frame of reference.
But although the descriptions of events change during the transition from one frame of reference to another, there are also universal things that remain unchanged. If, instead of describing the fall of glasses, we ask a question about the law of nature that causes them to fall, then the answer to it will be the same for an observer in a fixed coordinate system, and for an observer in a moving coordinate system. The law of distributed traffic is equally valid on the street and in the tram. In other words, while the description of events depends on the observer, the laws of nature do not depend on him, that is, as they say in scientific language, they are invariant. This is the principle of relativity.
Like any hypothesis, the principle of relativity had to be tested by correlating it with real natural phenomena. From the principle of relativity, Einstein derived two separate (albeit related) theories. The special, or particular, theory of relativity proceeds from the assumption that the laws of nature are the same for all frames of reference moving at a constant speed. General relativity extends this principle to any frame of reference, including those that move with acceleration. The special theory of relativity was published in 1905, and the more complex from the point of view of the mathematical apparatus, the general theory of relativity was completed by Einstein by 1916.
Special theory of relativity
Most of the paradoxical and contradicting intuitive ideas about the world of effects that arise when moving with a speed close to the speed of light are predicted by the special theory of relativity. The most famous of them is the effect of slowing down the clock, or the effect of slowing down time. A clock moving relative to the observer runs slower for him than exactly the same clock in his hands.
Time in a coordinate system moving with speeds close to the speed of light is stretched relative to the observer, while the spatial extent (length) of objects along the axis of the direction of motion, on the contrary, is compressed. This effect, known as the Lorentz-Fitzgerald contraction, was described in 1889 by the Irish physicist George Fitzgerald (1851-1901) and completed in 1892 by the Dutchman Hendrick Lorentz (1853-1928). The Lorentz-Fitzgerald abbreviation explains why the Michelson-Morley experiment to determine the speed of the Earth's motion in outer space by measuring the "ether wind" gave a negative result. Later, Einstein included these equations in special relativity and supplemented them with a similar transformation formula for mass,according to which the mass of the body also increases as the speed of the body approaches the speed of light. So, at a speed of 260,000 km / s (87% of the speed of light), the mass of an object from the point of view of an observer in the resting frame of reference will double.
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Since the time of Einstein, all these predictions, no matter how contrary to common sense they may seem, find complete and direct experimental confirmation. In one of the most revealing experiments, scientists at the University of Michigan put an ultra-precise atomic clock on board an airliner that made regular transatlantic flights, and after each flight back to the home airport, they checked their readings against the control clock. It turned out that the clock on the plane gradually lagged behind the control ones more and more (so to speak, when it comes to fractions of a second). For the past half century, scientists have been researching elementary particles in huge hardware complexes called accelerators. In them, beams of charged subatomic particles (such as protons and electrons) are accelerated to speeds close to the speed of light,then they are fired at various nuclear targets. In such experiments on accelerators, it is necessary to take into account the increase in the mass of the accelerated particles - otherwise the results of the experiment will simply not lend themselves to a reasonable interpretation. And in this sense, the special theory of relativity has long passed from the category of hypothetical theories to the field of tools of applied engineering, where it is used on a par with Newton's laws of mechanics.
Returning to Newton's laws, I would like to emphasize that the special theory of relativity, although it outwardly contradicts the laws of classical Newtonian mechanics, in fact, practically exactly reproduces all the usual equations of Newton's laws, if applied to describe bodies moving with a speed significantly less than the speed of light. That is, the special theory of relativity does not cancel Newtonian physics, but expands and complements it (this idea is discussed in more detail in the Introduction).
The principle of relativity also helps to understand why the speed of light, and not any other, plays such an important role in this model of the structure of the world - this question is asked by many of those who first encountered the theory of relativity. The speed of light stands out and plays a special role as a universal constant, because it is determined by a natural science law (see Maxwell's equations). By virtue of the principle of relativity, the speed of light in a vacuum, c, is the same in any frame of reference. This seemingly contradicts common sense, since it turns out that light from a moving source (no matter how fast it moves) and from a stationary source reaches the observer simultaneously. However, this is so.
Due to its special role in the laws of nature, the speed of light is central to general relativity.
General theory of relativity
The general theory of relativity is already applied to all frames of reference (and not only to those moving at a constant speed relative to each other) and looks mathematically much more complicated than the special one (which explains the eleven-year gap between their publication). It includes, as a special case, the special theory of relativity (and, therefore, Newton's laws). Moreover, the general theory of relativity goes much further than all its predecessors. In particular, it provides a new interpretation of gravity.
General relativity makes the world four-dimensional: time is added to the three spatial dimensions. All four dimensions are inseparable, so we are no longer talking about the spatial distance between two objects, as is the case in the three-dimensional world, but about the space-time intervals between events that unite their distance from each other - both in time and in space … That is, space and time are considered as a four-dimensional space-time continuum or, simply, space-time. In this continuum, observers moving relative to each other may even disagree about whether two events happened simultaneously - or one preceded the other. Fortunately for our poor minds, the matter does not come to a violation of cause-and-effect relationships - that is, the existence of coordinate systems,in which two events do not occur simultaneously and in a different sequence, even the general theory of relativity does not allow.
Newton's law of gravity tells us that there is a force of mutual attraction between any two bodies in the universe. From this point of view, the Earth revolves around the Sun, since the forces of mutual attraction act between them. General relativity, however, forces us to look at this phenomenon differently. According to this theory, gravity is a consequence of the deformation ("curvature") of the elastic tissue of space-time under the influence of mass (in this case, the heavier a body, for example, the Sun, the more the space-time "bends" under it and, accordingly, the stronger its gravitational field). Imagine a canvas stretched tightly (a kind of trampoline) with a massive ball on it. The web deforms under the weight of the ball, and a funnel-shaped depression forms around it. According to general relativity,The Earth revolves around the Sun like a small ball set to roll around the cone of a funnel formed as a result of "forcing" space-time by a heavy ball - the Sun. And what seems to us to be the force of gravity, in fact, is, in fact, a purely external manifestation of the curvature of space-time, and not at all a force in Newtonian understanding. To date, no better explanation of the nature of gravity than the general theory of relativity gives us has been found. To date, no better explanation of the nature of gravity than the general theory of relativity gives us has been found. To date, no better explanation of the nature of gravity than the general theory of relativity gives us has been found.
It is difficult to test the general theory of relativity, since in ordinary laboratory conditions its results almost completely coincide with what Newton's law of universal gravitation predicts. Nevertheless, several important experiments have been carried out, and their results allow the theory to be considered confirmed. In addition, general relativity helps explain the phenomena that we observe in space - for example, minor deviations of Mercury from a stationary orbit, which are inexplicable from the point of view of classical Newtonian mechanics, or the curvature of electromagnetic radiation from distant stars when it passes in close proximity to the Sun.
In fact, the results predicted by general relativity differ markedly from the results predicted by Newton's laws only in the presence of super-strong gravitational fields. This means that for a full-fledged test of the general theory of relativity, either ultra-precise measurements of very massive objects are needed, or black holes, to which none of our usual intuitive ideas are applicable. So the development of new experimental methods for testing the theory of relativity remains one of the most important tasks of experimental physics.