Cognition of the surrounding world is impossible without understanding the age of historical antiquities and how long the world itself - our Universe - has existed. Scientists have created many methods for determining the age of archaeological finds and establishing the dates of historical events. Today, the chronological timeline marks both the dates of the eruptions of ancient volcanoes and the time of birth of the stars that we see in the night sky. Today we will tell you about the main dating methods.
Archaeological finds
When it comes to the age of archaeological finds, then, of course, everyone remembers the radiocarbon method. This is perhaps the most famous, though not the only, method of dating antiquities. Known also for the constant criticism he is subjected to. So what is this method, what and how is it used?
To begin with, it must be said that this method is used, with very rare exceptions, only for dating objects and materials of biological origin. That is, the age of everything that was once alive. Moreover, we are talking about dating exactly the moment of death of a biological object. For example, a person found under the rubble of a house destroyed by an earthquake, or a tree felled to build a ship. In the first case, this allows you to determine the approximate time of the earthquake (if it was not known from other sources), in the second - the approximate date of the ship's construction. So, for example, they dated a volcanic eruption on Santorini Island, one of the key events in ancient history, a possible cause of the Bronze Age apocalypse. For the analysis, the scientists took an olive tree branch found during excavations of volcanic soil.
Why does the moment of death of an organism matter? Carbon compounds are known to form the basis of life on our planet. Living organisms get it primarily from the atmosphere. With death, carbon exchange with the atmosphere stops. But carbon on our planet, although it occupies one cell of the periodic table, is different. There are three carbon isotopes on Earth, two stable ones - 12C and 13C and one radioactive, decaying - 14C. As long as an organism is alive, the ratio of stable and radioactive isotopes in it is the same as in the atmosphere. As soon as carbon exchange stops, the amount of the unstable isotope 14C (radiocarbon) begins to decrease due to decay and the ratio changes. After about 5700 years, the amount of radiocarbon is halved, a process called half-life.
Radiocarbon is born in the upper atmosphere from nitrogen, and then it turns into nitrogen in the process of radioactive decay
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The radiocarbon dating method was developed by Willard Libby. Initially, he assumed that the ratio of carbon isotopes in the atmosphere in time and space does not change, and the ratio of isotopes in living organisms corresponds to the ratio in the atmosphere. If so, then by measuring this ratio in the available archaeological sample, we can determine when it corresponded to atmospheric. Or get the so-called "infinite age" if there is no radiocarbon in the sample.
The method does not allow looking far into the past. Its theoretical depth is 70,000 years (13 half-lives). In about this time, the unstable carbon will completely decay. But the practical limit is 50,000-60,000 years. It is no longer possible, the accuracy of the equipment does not allow. They can measure the age of the "Ice Man", but it is no longer possible to look into the history of the planet before the appearance of man and determine, for example, the age of the remains of dinosaurs. In addition, the radiocarbon method is one of the most criticized. The controversy surrounding the Shroud of Turin and the analysis of the method for determining the age of the relic is just one of the illustrations of the imperfection of this method. What is the argument about the contamination of samples with a carbon isotope after the termination of carbon exchange with the atmosphere. It is not always certain that the object taken for analysis is completely free of carbon,introduced after, for example, bacteria and microorganisms that have settled on the subject.
It is worth noting that after the start of the application of the method, it turned out that the ratio of isotopes in the atmosphere changed over time. Therefore, scientists needed to create a so-called calibration scale, on which changes in the content of radiocarbon in the atmosphere are noted over the years. For this, objects were taken, the dating of which is known. Dendrochronology, a science based on the study of tree rings of wood, came to the aid of scientists.
At the beginning, we mentioned that there are rare cases when this method applies to objects of non-biological origin. A typical example is ancient buildings, in the mortar of which quicklime CaO was used. When combined with water and carbon dioxide in the atmosphere, lime was converted to calcium carbonate CaCO3. In this case, carbon exchange with the atmosphere stopped from the moment the mortar hardened. In this way, you can determine the age of many ancient buildings.
Remains of dinosaurs and ancient plants
Now let's talk about dinosaurs. As you know, the era of the dinosaurs was a relatively small (of course, by the standards of the geological history of the Earth) period of time, which lasted 186 million years. The Mesozoic era, as it is designated on the geochronological scale of our planet, began about 252 million years ago and ended 66 million years ago. At the same time, scientists confidently divided it into three periods: Triassic, Jurassic and Cretaceous. And for each they have identified their own dinosaurs. But how? After all, the radiocarbon method is not applicable for such periods. In most cases, the age of the remains of dinosaurs, other ancient creatures, as well as ancient plants is determined by the time in which rocks were found. If the remains of a dinosaur were found in the rocks of the Upper Triassic, and this is 237-201 million years ago, then the dinosaur lived at that time. Now the question is,how to determine the age of these rocks?
Dinosaur remains in ancient rock
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We have already said that the radiocarbon method can be used not only to determine the age of objects of biological origin. But the carbon isotope has too short a half-life, and in determining the age of the same geological rocks, it is not applicable. This method, although it is the most famous, is just one of the methods of radioisotope dating. There are other isotopes in nature whose half-lives are longer and known. And minerals that can be used to age, such as zircon.
It is a very useful mineral for age determination using uranium-lead dating. The starting point for determining the age will be the moment of crystallization of zircon, similar to the moment of death of a biological object with the radiocarbon method. Zircon crystals are usually radioactive, as they contain impurities of radioactive elements and, above all, uranium isotopes. By the way, the radiocarbon method could also be called the carbon-nitrogen method, since the decay product of the carbon isotope is nitrogen. But which of the nitrogen atoms in the sample were formed as a result of decay, and which were there initially, scientists cannot determine. Therefore, unlike other radioisotope methods, it is so important to know the change in the concentration of radiocarbon in the planet's atmosphere.
Zircon crystal
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In the case of the uranium-lead method, the decay product is an isotope, which is interesting because it could not have been in the sample earlier or its initial concentration was initially known. Scientists estimate the decay time of two isotopes of uranium, the decay of which ends with the formation of two different isotopes of lead. That is, the ratio of the concentration of the initial isotopes and daughter products is determined. Radioisotope methods are applied by scientists to igneous rocks and show the time that has passed since solidification.
Earth and other celestial bodies
Other methods are used to determine the age of geological rocks: potassium-argon, argon-argon, lead-lead. Thanks to the latter, it was possible to determine the time of formation of the planets of the solar system and, accordingly, the age of our planet, since it is believed that all the planets in the system were formed almost simultaneously. In 1953, the American geochemist Claire Patterson measured the ratio of lead isotopes in samples of a meteorite that fell about 20-40 thousand years in the territory now occupied by the state of Arizona. The result was a refinement of the estimate of the age of the Earth to 4.550 billion years. Analysis of terrestrial rocks also gives figures of a similar order. So, stones discovered on the shores of Hudson Bay in Canada are 4.28 billion years old. And located also in Canada gray gneisses (rocks,chemically similar to granites and clay shales), which for a long time held the lead in age, had an estimate of 3.92 to 4.03 billion years. This method is applicable to everything that we can "reach" in the solar system. Analysis of the samples of lunar rocks brought to Earth showed that their age is 4.47 billion years.
But with the stars, everything is completely different. They are far from us. Getting a piece of a star to measure its age is unrealistic. But, nevertheless, scientists know (or are sure) that, for example, the closest star to us, Proxima Centauri, is only slightly older than our Sun: it is 4.85 billion years old, the Sun is 4.57 billion years old. But the diamond of the night sky, Sirius, is a teenager: he is about 230 million years old. The North Star is even less: 70-80 million years old. Relatively speaking, Sirius lit up in the sky at the beginning of the era of the dinosaurs, and the North Star already at the end. So how do scientists know the age of the stars?
We cannot receive anything from distant stars except their light. But this is already a lot. In fact, this is the piece of the star that allows you to determine its chemical composition. Knowing what a star is made of is necessary to determine its age. During their lifetime, stars evolve, going through all stages from protostars to white dwarfs. As a result of thermonuclear reactions occurring in the star, the composition of the elements in it is constantly changing.
Immediately after birth, the star falls into the so-called main sequence. Main sequence stars (including our Sun) are composed primarily of hydrogen and helium. In the course of thermonuclear reactions of hydrogen burnout in the core of a star, the content of helium increases. The hydrogen burning stage is the longest period in the life of a star. At this stage, the star is about 90% of the time allotted to it. The speed of passing through the stages depends on the mass of the star: the larger it is, the faster the star contracts and the faster it "burns out". The star stays on the main sequence as long as hydrogen burns out in its core. The duration of the remaining stages, at which the heavier elements burn out, is less than 10%. Thus, the older a star on the main sequence, the more helium and less hydrogen it contains.
A couple of hundred years ago, it seemed that we would never be able to find out the composition of the stars. But the discovery of spectral analysis in the middle of the 19th century gave scientists a powerful tool for studying distant objects. But first, Isaac Newton at the beginning of the 18th century with the help of a prism decomposed white light into separate components of different colors - the solar spectrum. 100 years later, in 1802, the English scientist William Wollaston looked closely at the solar spectrum and discovered narrow dark lines in it. He did not attach much importance to them. But soon the German physicist and optician Josef Fraunhofer investigates them and describes them in detail. In addition, he explains them by the absorption of rays by the gases of the Sun's atmosphere. In addition to the solar spectrum, he studies the spectrum of Venus and Sirius and finds similar lines there. They are also found near artificial light sources. And only in 1859, German chemists Gustav Kirchhoff and Robert Bunsen conducted a series of experiments, which resulted in the conclusion that each chemical element has its own line in the spectrum. And, therefore, according to the spectrum of celestial bodies, conclusions can be drawn about their composition.
Solar photosphere spectrum and Fraunhofer absorption lines
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The method was immediately adopted by scientists. And soon an unknown element was discovered in the composition of the Sun, which was not found on Earth. It was helium (from "helios" - the sun). Only a little later it was discovered on Earth.
Our Sun is 73.46% hydrogen and 24.85% helium, the proportion of other elements is insignificant. By the way, there are also metals among them, which speaks not so much about the age, but rather about the "heredity" of our star. The Sun is a young third generation star, which means that it was formed from what remains of the stars of the first and second generations. That is, those stars in the cores of which these metals were synthesized. In the Sun, for obvious reasons, this has not happened yet. The composition of the Sun allows us to say that it is 4.57 billion years old. By the age of 12.2 billion years, the Sun will leave the main sequence and become a red giant, but long before this moment, life on Earth will be impossible.
The main population of our Galaxy is stars. The age of the Galaxy is determined by the oldest objects that have been discovered. Today the oldest stars in the Galaxy are the red giant HE 1523-0901 and the Methuselah Star, or HD 140283. Both stars are in the direction of the constellation Libra, and their age is estimated at about 13.2 billion years.
By the way, HE 1523-0901 and HD 140283 are not just very old stars, they are stars of the second generation, which have an insignificant metal content in their composition. That is, the stars belonging to the generation that preceded our Sun and its "peers".
Another oldest object, according to some estimates, is the globular star cluster NGC6397, whose stars are 13.4 billion years old. In this case, the interval between the formation of the first generation of stars and the birth of the second is estimated by researchers at 200-300 million years. These studies allow scientists to argue that our Galaxy is 13.2-13.6 billion years old.
Universe
As with the Galaxy, the age of the Universe can be assumed by determining how old its oldest objects are. To date, the galaxy GN-z11, located in the direction of the constellation Ursa Major, is considered the oldest among the objects known to us. The light from the galaxy took 13.4 billion years, meaning it was emitted 400 million years after the Big Bang. And if light has come such a long way, then the Universe cannot have a smaller age. But how was this date determined?
For 2016, the galaxy GN-z11 is the most distant known object in the Universe.
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The number 11 in the designation of the galaxy indicates that it has a redshift of z = 11.1. The higher this indicator, the further the object is from us, the longer the light went from it and the older the object. The previous age champion, the Egsy8p7 galaxy, has a redshift of z = 8.68 (13.1 billion light years distant from us). The contender for the seniority is the galaxy UDFj-39546284, probably has z = 11.9, but this has not yet been fully confirmed. The universe cannot have an age less than these objects.
Earlier we talked about the spectra of stars, which determine the composition of their chemical elements. In the spectrum of a star or galaxy that is moving away from us, there is a shift in the spectral lines of chemical elements to the red (long-wave) side. The further an object is from us, the greater its redshift. The shift of lines to the violet (shortwave) side, due to the approach of an object, is called blue or violet shift. One explanation for this phenomenon is the ubiquitous Doppler effect. They, for example, explain the lowering of the tone of the siren of a passing car or the sound of the engine of a flying plane. The work of most cameras for fixing violations is based on the Doppler effect.
Spectral lines have shifted to the red side
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So, it is known that the universe is expanding. And knowing the rate of its expansion, you can determine the age of the universe. The constant that shows the speed at which two galaxies, separated by a distance of 1 Mpc (megaparsec), fly in different directions, is called the Hubble constant. But in order to determine the age of the universe, scientists needed to know its density and composition. For this purpose, the space observatories WMAP (NASA) and Planck (European Space Agency) were sent into space. The WMAP data made it possible to determine the age of the universe at 13.75 billion years. Data from a European satellite launched eight years later made it possible to refine the necessary parameters, and the age of the universe was determined at 13.81 billion years.
Space Observatory Planck
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Sergey Sobol
