What did Einstein claim in the special theory of relativity

Amit Hagar

From this it follows that the structure of the world is not the one we are used to

The history of science includes very few cases where a single article managed to change a world view. Albert Einstein's article on the theory of special relativity, sent to the monthly "Annalen der Physik" in June 1905 under the name "On the Electrodynamics of Moving Bodies", is such a case.

The article on special relativity was the product of a change in Einstein's thinking pattern. If in the previous articles that he published in "The Year of the Wonder" he tried to explain physical phenomena with the help of assembling models that relied on assumptions and hypotheses regarding the "building blocks" of the world of unobservable matter, then in the article on the special theory of relativity he turned to the "principle" way, in which the physicist does not make assumptions anything about the structure of matter, but proceeds from a limited number of indisputable observational principles and asks himself what the nature of the world is if these principles are muscles in it.

The special theory of relativity begins with two indisputable phenomena, which Einstein raised to the level of a principle. According to the first phenomenon, which was named the "Principle of Relativity", it is not possible to distinguish experimentally between a physical system moving in steady motion (for example, a train moving at a constant speed) and a system that is at rest (for example, the same train, which is motionless at the station). Another way to describe the principle of relativity is to say that it is not possible to point to a point of view that is "truly" at rest: every physical event can be equally described both from a point of view where the observer is in some steady motion and from a point of view where the observer is not in motion at all. In the case of the train, the description of the event in which the train is approaching the station (described as such by an observer standing motionless in the station and seeing the train approaching him) is physically equivalent to a corresponding description from the point of view of a passenger in the train, from which it appears as if the station is the one approaching the train (since For the ticket holder, the train is the one that is at rest).

The principle of relativity was already known from the days of Galileo Galilei and adopted by Newton, but combining it with a second principle, the "principle of light", according to which the speed of light in a vacuum is constant (a little less than 300,000 km per second) and does not depend on the speed of the light source, led to a contradiction - Presumably: imagine a night train that passes at a speed of 120 km/h past a shovel that is at rest relative to it on some platform. Inside the train, a bored conductor walks at a speed of 2 km/h and shines his flashlight for pleasure. According to the principle of relativity and Galileo and Newton's picture of the world, in order to calculate the speed of the light beam passing by in the dead of night, the shovel has to connect it with the speed of the ticket holder's walk and with the speed of the train. But the principle of light (which according to the principle of relativity should be acceptable in any situation, both at rest and in steady motion) states that the speed of light should always be constant.

Galileo's and Newton's picture of the world (and it must be admitted, also that of common sense) carries with it "inflation" in the speeds of light that different observers moving at different speeds will think. Therefore, so that the principle of relativity does not violate the principle of light, the shovel must measure the distance traveled by the ticket in a different way than the ticket itself. From the shovel's point of view, this distance will be shorter and the amount of time it will take for the ticket to cross it will be longer.

From similar considerations, Einstein came to another conclusion, according to which the measurement of periods of time, and therefore also the answer to the question of whether two events occur "simultaneously", must change when moving from the point of view of the shovel to the point of view of the ticket holder. Einstein claimed, therefore, that the conclusion from the combination of the principle of relativity and the principle of light is that the structure of space and time is not the good old structure of Galileo and Newton, since distances and time periods are not absolute but change according to the observer's point of view.

According to the special theory of relativity, the changes in the measurement of distances and time periods depend solely on the speed of the measurer (or more precisely, on the relative speed of the measurer and the system it measures). In cases where the relative speed approaches the speed of light, the relativistic effects (the shortening of distance and the slowing down of time) become significant. If, for example, the train travels at a speed that is half the speed of light, the walking distance of the ticket holder that will be measured by the shovel will be shortened by about 13%. Conversely, if the train travels at one-tenth of the speed of light, the ticket holder's walking distance will be shortened by 0.5%. In cases where the relative speed is much smaller than the speed of light (for example speeds familiar to us from everyday life), the relative effects are negligible, but we must not forget that they always exist and are even taken into account when designing systems we are familiar with. It is hard to imagine, for example, the synchronization of a GPS satellite system without the effects predicted by the theory of relativity.

The seeming contradiction arising from the combination of the principle of relativity and the principle of light preoccupied scientists even before Einstein. Einstein's contemporaries - the Dutch physicist Hendrik Lorentz and the Irish physicist George Francis Fitzgerald - claimed more than ten years before the publication of the article on the theory of special relativity, that the source of the contradiction between the two principles is in actual deformations that bodies go through while they are moving at a certain speed - deformations that come Expressed in the shortening of rulers or the slowing down of clocks. That is, according to Lorenz and Fitzgerald, two observers moving at different speeds will measure identical distances and periods of time - so to speak - in a different way, but the difference between the two measurements does not indicate that the Galilean-Newtonian picture of the world is incorrect, but rather it arises from the fact that different speeds distort the rulers and clocks The measurement is different. The two observers, Lorentz and Fitzgerald continued, will always agree with each other about the physical events they are measuring, if only they factor the various distortions into their measurements. The two physicists formulated the laws according to which this kind of weighting must be carried out, and these are called "Lorentz transformations".

Einstein was the first to try to reconcile the principle of relativity with the principle of light using the "principle" approach - he claimed that the fact that two observers moving at different speeds would describe the world differently and yet agree between them about the physical events taking place in it, indicates that the structure of the world is not the one we are used to.

The structure of the world according to Einstein is such that the concept of time and the concept of space coalesce into one unit. From a geometric point of view, the world "as it really is" according to the theory of relativity consists of a four-dimensional canvas of spatio-temporal events; Each event, which occurs in a certain place and at a certain time, is a point on the canvas, and the four-dimensional "distance" between any two events (for example, the four-dimensional "distance" between the event when this article is read and the event when the newspaper is thrown into the trash can) is not Only distance in the spatial sense of the word but also includes the time dimension within it. As observers of the four-dimensional world, we have become accustomed to separately measuring the same four-dimensional "distance" between the two events - a three-dimensional spatial distance to each other (with the help of a ruler) and a temporal one-dimensional distance to each other (with the help of a clock). According to Einstein, even though two observers moving at different speeds will measure with the help of rulers and clocks three-dimensional distances and one-dimensional periods of time that are different from each other, they will agree among themselves on the four-dimensional "distance" between the physical events. This four-dimensional "distance" between the events will remain constant regardless of the separate and different descriptions of the viewers.

Apart from the conceptual revolution inherent in the theory of relativity, Einstein's greatness was that he was able to derive from the principle of relativity and the principle of light the Lorentz transformations, the "laws of translation" that allow a transition between different measurements made from different points of view and preserve the four-dimensional "distance" between physical events in space-time. However, unlike Lorentz and Fitzgerald, Einstein saw the shortening of the rulers and the slowing down of the clocks as a product of the four-dimensional space-time structure and not of material physical changes that bodies moving at constant speed in separate and absolute Newtonian space and time undergo.

These two approaches - Lorentz's assembly approach and Einstein's principle approach - imply completely different conceptions of the world. According to Einstein, not only is it not possible to point to a point of view that has priority over other points of view - that is, a stationary point of view from which the physical events are seen "as they really are" and relative to which the other points of view are wrong - but such a point of view does not exist at all. Any description from a certain point of view is just as "correct" as a description from another point of view, provided that we move from one point to another with the help of Lorentz transformations and remember that this is a three-dimensional spatial description and a separate one-dimensional temporal description, and not a description of the four-dimensional world "as it really is".

Lorenz and Fitzgerald, on the other hand, argued that a preferred point of view exists, but that the laws of physics themselves (ie the shortening of rulers and the slowing of clocks) prevent us from noticing it. With Einstein's publication, the crucial contribution of the two (and of other scientists such as the French mathematician Henri Poincaré) to the creation of the special theory of relativity was forgotten.

The structure of the world derived from the theory of special relativity, according to which periods of time and distances are not absolute sizes but dependent on the point of view and the concept of simultaneity is relative, is today called "Minkowski space-time", after the mathematician Herman Minkowski, Einstein's teacher, and it became, like the photons , to the iron sheep property of modern physics, so much so that physicists dismiss any theory that there is a fear that it violates the special theory of relativity. The path to this was not a bed of roses: the committee that decided to award Einstein the Nobel Prize in Physics in 1921, for example, conditioned the acceptance of the prize on the fact that Einstein would not say anything about the special theory of relativity (as well as the general theory) during the ceremony itself. The members of the committee did not expect that a day would come when special relativity, despite the dramatic change it imposes on common sense, would be revived in any particle accelerator.

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