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Since ancient times, people not only existed in time, but also tried to understand its essence. Heraclitus of Ephesus (VI and V centuries BC) believed that the world is full of contradictions and variability, but time flows invariably. His famous saying is, "You cannot step into the same river twice." Plato (427-347 BC) proclaimed the principle of its cyclicality. According to his teaching, everything in the world is repeated at certain intervals of time; that is, it goes in a circle.

However, Aristotle (384-322 BC) did not find a place for the concept of time in his system. The great philosopher was not interested in the dynamic processes of the Universe.

Galileo and Newton believed that the world is infinite and homogeneous. Time was established as one of the most important and special coordinates. Everything around was described as occurring in a continuous and endless space-time. Newton's mechanics required absolute and uniform time throughout the Universe, so its measurement accuracy became science's main technical task.

The theory of absolute space and time lasted only two centuries. At the turn of the 19th and 20th centuries, events occurred in physics that significantly changed man's understanding of the world around him and time in it. Schrödinger's quantum mechanics and Einstein's theory of relativity made it possible to realize that man no longer lives in a three-dimensional world. Still, a four-dimensional world, in which time, interconnected with space, plays a special role. Everything around became relative and probabilistic, many precise concepts began to dissolve, and time began to depend on the speed and degree of curvature of space.

In 1905, Albert Einstein proposed the special theory of relativity: time flows more slowly for a fast-moving object. That is, time became a relative quantity. Such concepts as "now", "today", and "tomorrow" have a simple, generally accepted meaning only for events that occur close to each other. And if, for example, you get into a spaceship and accelerate it to a speed close to the speed of light, then you can fly into the future to see what will happen on Earth in hundreds of years, but you will not be able to return to your present ... In 1908, the German mathematician G. Minkowski proved the inseparable unity of space and time and introduced a new concept of space-time. Thus, the world became four-dimensional. And in 1916, Einstein completed the creation of the general theory of relativity, according to which space-time can be curved under the action of gravitational forces (mathematically curved spaces were previously described by the Russian mathematician N. Lobachevsky). Since then, the geometry of curved spaces has been called non-Euclidean. However, the most interesting discovery in the general theory of relativity is that time flows more slowly in a strong gravitational field. This means that clocks near the surface of the Sun run more slowly than those near the surface of the Earth, while clocks in near-Earth orbit, on the contrary, run faster.

Today, scientists, astronomers, and engineers consider relativistic effects. Their influence on GPS (Global Positioning System) navigation satellites—the global coordinate determination system—affects orbit calculations, the distribution of navigation signals, and, of course, the rate of onboard atomic clocks. The last correction is the most significant and is expressed in the artificial "slowing down" of the atomic clocks of GPS satellites.

Many people note that time flows faster with age, but fairly young people also say the same thing.

Of course, this can be explained by the ever-accelerating pace of life. Trains and planes take us to other cities, countries, and continents faster and faster. The media bombards our poor heads with an ever-increasing stream of information. Thanks to radio, television, and the Internet, we learn about an event almost immediately after it has happened.

But Swiss scientists have found another explanation for the apparent acceleration of time. They believe that a person's biological clock begins to "lag" with age. And that is why it seems to him that the events around him are running faster. By analogy, try to walk along an underground passage in the metro slower than the pace of the crowd around you. And you will immediately notice that people are overtaking you from all sides. And so time overtakes us.

The arrow of time is the impossibility of moving from the future to the past. That is why we cannot turn an omelet into whole eggs, an ice cube cannot turn itself into a glass of water, and a broken cup or vase cannot be turned back into a whole one. Egyptian pharaohs built pyramids to preserve their memory for eternity. They have existed for about 5,000 years. But they are not eternal either. Gradually, the desert winds in which they stand will destroy them and turn them into sand. All this is a manifestation of entropy. Entropy, at the everyday level, is a measure of disorder or a measure of uncertainty. In physics, entropy is among such fundamental concepts as energy and temperature.

The existence of the arrow of time as a one-way movement from the past to the future was theoretically substantiated by I.R. Prigogine, a Belgian and American physicist and chemist of Russian origin, who was a Nobel Prize laureate in 1977. Prigogine Ilya Romanovich (1917-2003)

In the middle of the twentieth century, scientists - archaeologists, geologists, chemists, and physicists found ways to establish the lifespans of previously existing living organisms. The basis of the method is radiocarbon analysis, the founders of which are Pierre Curie and Ernest Rutherford.

Radiocarbon analysis is a physical method of dating biological remains, objects, and materials of biological origin by measuring the content of the radioactive isotope 14C in the material about stable isotopes of carbon.

In 1946, scientist Willard Libby presented a method based on radiocarbon analysis of objects of biological origin. Fifteen years later, the scientist received the Nobel Prize for this method, confirming radiocarbon analysis's usefulness. Radiocarbon analysis applies only to objects of biological origin and to the remains of people and animals. Archaeological objects of non-biological origin cannot be examined using this method.

Radiocarbon analysis is based on the fact that all living organisms are based on the chemical element carbon. Moreover, carbon has stable and radioactive isotopes as its components. Stable isotopes - 12C and 13C are present constantly, and the radioactive isotope 14C can accumulate under the influence of radiation, solar radiation, and space in the body, but gradually decompose after the death of a biological being.

During life, the balance of different carbon isotopes is the same, but this balance begins to be disturbed at death. Stable carbons remain unchanged in the body and do not decay, but the amount of radioactive carbon gradually decreases. Knowing the decay time of radioactive carbon, you can calculate the age of the remains with reliable accuracy. The maximum age that can be determined by this method is within sixty thousand years, while the error is from 70 to 300 years.

The radiocarbon method allows you to determine the age of Egyptian pharaohs from their mummies, fossil remains of people, animals, plants, and microorganisms.

Data on the sequence of the most important events in the history of planet Earth, on the change of geological eras, is summarized by historical geology. Nuclear geochronology is engaged in determining the absolute age of rocks. This is one of the youngest geological sciences, which studies the patterns of natural nuclear transformations in the Earth's matter and their manifestation in geological processes. The founder of this science is Academician V. I. Vernadsky (1935). It is based on the phenomenon of radioactive decay of uranium, thorium, potassium, strontium, etc. Geochronology establishes the absolute age of rocks. This age, expressed in time units, is usually millions of years.