In an article on "Brownian motion" Einstein accurately predicted the movement pattern of atoms and finally convinced the doubters of their existence
Amit Hagar
Illustration depicting the random movement of powder grains in a liquid ("Brownian motion"). Incessant "bombardment" of invisible atoms
In May 1905, ten days after he submitted the article on the diameter of the sugar molecules, thanks to which he would be awarded a doctorate in physics in the summer of that year, Albert Einstein completed another article, in which he sought to show how "according to the molecular-kinetic theory of heat, microscopic bodies floating in a liquid and which can be distinguished We must, due to thermal molecular movement, move in a way that can be measured". Thus, in one sentence, Einstein staged the last act in the great scientific drama at the end of which the hypothesis that the world of matter consists of atoms was accepted without question.
To understand how fateful this seemingly innocent sentence was, one must go back to the middle of the 19th century. At that time, three scientists - Rudolf Clausius, Ludwig Boltzmann and James Clerk Maxwell - worked on the development of two new theories: thermodynamics, which dealt with the conversion of energy into heat and mechanical work (for example, moving a weight from one place to another) and the kinetic theory of gases, which tried to explain thermodynamics on The basis of Newtonian mechanics and the basis of the atomic hypothesis.
Clausius is responsible, among other things, for the formulation of the first and second laws of thermodynamics.
The first is the law of conservation of energy, according to which it can simply and take shape and produce work, radiation, movement or heat, but its quantity will forever remain constant. Although the analogy is a little misleading, the first law can be understood without difficulty if you think of energy in terms of matter trapped inside a container. As long as the container remains closed, the amount of material in it remains constant. It does not get lost, no matter what form it takes.
The first law states that energy can roll from one form to another, but it does not force the rolls of energy in any direction. This task is assigned to the second law. Anyone who has ever put their hand on a hot pot and felt pain knows that an immutable rule is that heat "flows" spontaneously from a hot body to a cold body and not the other way around, and that this process takes place until the temperature of the two bodies equalizes.
The second law of thermodynamics states that spontaneous thermodynamic processes are irreversible and occur only in one direction: at room temperature ice melts and turns into water spontaneously, but to turn water into ice you have to put it in the refrigerator; Milk and coffee mix easily spontaneously, but they cannot be returned to the state of separate milk and separate coffee without investment of work. The laughter that accompanies the projection of a video film backwards shows how accustomed we are to such irreversible spontaneous phenomena in everyday life.
At the same time as Clausius' work, Boltzmann and Maxwell tried to develop mechanical models for the laws of thermodynamics, the basis of which was the atomic hypothesis. According to these models, heat is nothing less than atoms in motion. But the movement of these atoms should be dictated by the equations of motion of Newtonian mechanics, and these do not distinguish between a body moving forward in time and the same body moving "backward in time". In other words, a video of atoms colliding won't make anyone laugh if played backwards, since we won't be able to tell any difference between the two directions.
Maxwell and Boltzmann's solution to the apparent contradiction between the laws of thermodynamics and the atomic models was a probabilistic solution: "reverse" macroscopic phenomena such as the spontaneous separation of coffee into milk and coffee or the spontaneous freezing of water into ice are not impossible but simply improbable, and the probability of their occurrence exists but She is very small. This means that the second law of thermodynamics is not a law but a statistical rule, which has exceptions.
Many of Boltzmann's and Maxwell's contemporaries did not accept the "package deal" that the two proposed, a deal that included the belief in the existence of atoms, the claim that at the microscopic level the movement of atoms can be treated as random and the laws of probability imposed on it, and the conclusion that thermodynamics is not an all-embracing theory - since it is not Applies to the microscopic world where phenomena occur that do not conform to its predictions. Boltzmann, for example, was often attacked by his contemporaries for his ideas and about a decade before the publication of Einstein's article he was even invited to Britain to defend his views in a debate that quickly turned personal.
The greatness of Einstein's article was that it made it possible, for the first time, to decide the debate empirically. In the paper, Einstein offered a brilliant mathematical analysis of random molecular motion that has consequences observed under the microscope. In other words, Einstein proposed a "prescription" for an experiment that could support Maxwell and Boltzmann's theory with all its implications. According to Einstein, if Maxwell and Boltzmann's theory is correct, powder particles floating in a liquid should be constantly "bombarded" by fast-moving molecules, and these collisions cause them to move from place to place in a "zigzag" movement that can be measured with a microscope; This movement depends on the movement of the molecules, which in turn depends on their size. This meant that if an experiment was indeed carried out that would confirm Einstein's predictions, it would be more difficult than ever to reject the Boltzmann and Maxwell package deal.
It is important to note that the phenomenon of which Einstein proposed a mathematical analysis was already known decades before and was nicknamed "Brownian motion", after a British botanist of that name. Brown, who in 1828 observed under a microscope the "zigzag" and random movement of pollen grains floating in a liquid, was the first to show that the explanation for the phenomenon must be physical and not biological (until then the random movement was considered the product of "animal" force of the grains). But such a physical model did not exist. Einstein was the first to provide it, and he did it in such a precise way that to confirm it all that was left was to look through the microscope and measure with a stopwatch and a ruler the speed of movement of those grains of powder floating in the liquid, a movement caused by their collision with the invisible molecules.
In 1909, the French physicist Jean Perrin managed to perform a series of experiments according to Einstein's "prescription". Einstein's predictions matched him with incredible accuracy, and the impact on the scientific world was enormous. With the exception of Ernst Mach, the well-known skeptic of the atomic hypothesis, who died in 1916 while still doubting its truth, physicists were convinced that atoms, more than a useful fiction, did exist. Einstein had no doubt about it long before.
Next week: special relativity; 
General Relativity
