1. The age of the earth calculated by the Archbishop
Before the Enlightenment, medieval Europe was controlled by religion. There is a concept of "creation" in Christianity, so based on biblical chronology, the congregation believes that God created the world around 3000-4000 B.C. However, for various reasons, it was not possible to agree on a common view and no consensus was reached on the specific year. Until the appearance of an Irishman in the seventeenth century.
James Uzzie
The Irishman James Ukher was not a scientist, but an Archbishop of the Catholic Church of Ireland, but James Ukher was very interested in science. James Usher used the biblical approach of the Bible to link the timeline of the Bible with the timeline of the canonical history, and he also went through many non-Christian ancient historical documents. If you can find a clear record of events in both the Bible and the canonical history, then you can map the timeline in the Bible to the timeline in the canonical history. Soon he found an event with historical references, namely that the third monarch of the Neo-Babylonian state thought that The ascension of Miróda to the throne occurred in 563 BC. Through this event, after a simple calendar calculation, Archbishop James Uschet finally came to an incredibly accurate conclusion that the world was created at around 6 p.m. on Saturday, October 22, 4004 BC. Archbishop James Usher was so pleased with the result that he wanted the whole world to know about his work that he recorded in the Bible. Due to the dominance of Christianity over people's society at that time, this "age of the earth" quickly became the mainstream of the age of the earth in Western society.
2. Physicists calculate the age of the Earth
After the Enlightenment, theocratic rule had begun to falter, physics and geology began to develop gradually, and these scholars used more scientific methods to calculate the age of the earth. Supported by many strong evidence, the British geologist Hutton proposed the "uniformitarian theory", he believes that the evolution of the earth is an extremely long process, and we can only explain the specific changes of each geological period, but can not do the beginning and end, this concept of geological gradient theory was gradually carried forward by the Duke of Lyle and became the mainstream view at that time. Influenced by the Duke of Lyle, Darwin thus began his circumnavigation of the globe, preparing the premise for the birth of The Origin of Species. Darwin believed that some geological changes were more than 300 million years old or more, so the age of the earth was far more than 300 million years, but this view was not recognized by the scientific community at the time. At a time when geologists and biologists were helpless, physicists stepped forward.
Baron Kelvin
Baron Kelvin, a famous physicist, proposed the famous first law of thermodynamics and the second law of thermodynamics. Through the second law of thermodynamics, we can know that the Earth was born as a high-heat magma ball, and its temperature gradually decreased with time until the heat was completely dissipated. Then through three coefficients: the earth's initial temperature, the rock thermal conductivity and the earth temperature gradient, the initial temperature of the earth is Kelvin's own estimate of 3870 degrees Celsius, the rock thermal conductivity and the ground temperature gradient use a rough average, and finally the age of the earth can be derived with the help of the formula. Kelvin calculations by this method show that the age of the Earth is between about 20 million and 400 million years. Over the next 35 years, Kelvin answered this question again and again, and the range of the Earth's age became smaller and smaller. Later, by calculating the rate of sedimentary rock formation, geologists concluded that the age of the Earth was about 100 million years in Kelvin, making Kelvin's statement that the Earth was 98 million years old was widely accepted. But then Kelvin's calculations were proved wrong by an accident.
Kelvin's method of calculating the age of the Earth is based on two basic assumptions: first, that there is no other source of heat in the Earth's interior; and second, that the Earth's interior is a homogeneous solid. With the subsequent discovery of radioactive phenomena by french scientists and French physicist Becquerel, and later through seismic waves, it was discovered that the Earth was not a homogeneous sphere, but was divided into core, mantle and crust. Thus confirming that both of Kelvin's basic assumptions are incorrect. Even more interesting is that Rutherford, a student at Kelvin, successfully overturned Kelvin's calculation of the age of the Earth by discovering that the heat generated by radioactive elements affects the cooling of the Earth itself. Although the radioactivity dealt a heavy blow to Kelvin's calculations, it also provided us with the tools to solve the age of the earth once and for all.
Rutherford
3. Isotope method to calculate the age of the earth
In 1946, Patterson's doctoral supervisor, Harrison Brown, and his colleagues formed the first team in the United States to study geochemistry. Brown focused his team's research efforts on three main areas: estimating the relative abundance of elements in the solar system by determining the composition of meteorites, determining crustal magma by measuring the age of uranium-lead in ordinary igneous rocks, the time course of evolution, and determining the age of The Earth by measuring the isotopic composition of lead in iron meteorites. So Brown was looking for a student who could use a mass spectrometer to help him analyze the isotopes of lead in ancient meteorites and modern rocks today, and Patterson was selected by Brown to use the mass spectrometer to measure the age of the Earth.
Patterson's mentor Brown decided to use the decay of uranium to measure the age of the Earth. Both uranium-235 and uranium-238 undergo a series of decays with different probabilities, decaying into lead 207 and lead 206, respectively. So theoretically, you only need to know how much lead and uranium are in a sample, and how much lead is in formation, to calculate the age of the earth. However, how to determine the initial amount of lead is a difficult problem, a good way is to measure the amount of uranium in zircon, because when the magma cools and crystallizes to form zircon, the vast majority of lead atoms are squeezed out, so it can be basically treated as the initial lead content of zero. Patterson and his colleagues then began a lengthy experiment measuring uranium and lead levels in zircon. Patterson's data will always have too much lead bias. Following standard chemical practices, he conducted blank experiments and found that not only was there lead in the blank experiments, but sometimes 200 times more than they expected. Patterson began to think of ways to eliminate the source of extra lead during sample processing, eventually achieving only 0.1 μg of lead in a blank experiment. In June 1951, although Patterson did not yet know the age of the Earth, he had developed a technique for accurately measuring microgram-scale samples, determined the lead isotopic composition of zircon, and obtained the age of the zircon used in the experiment to be 1.05 billion years.
Patterson in the lab
In 1951, Patterson followed his mentor to Caltech, where he continued experiments measuring the age of the Earth, which was in line with Patterson's intentions, and because the lead was inaccurate and the age of the Earth could not be measured, Patterson established the first clean laboratory in the field of lead isotope chemistry. Due to Patterson's attitude toward scientific rigor, the pure lead was eventually separated. But since it is impossible to actually find zircon that formed at the same time as the birth of the earth, it is not feasible to use zircon to determine the age of the earth. To measure the age of the Earth, it is necessary to find material that formed at the same time as the Earth. There is one kind of rock on the earth's surface that meets this condition, and that is meteorites.
The vast majority of meteorites formed simultaneously in the solar system and on Earth, and one type of iron meteorite in meteorites has a particularly excellent property: they have a high lead content and a low uranium content, which means that the lead produced by uranium decay is insignificant here. Therefore, the isotopic relative content of lead that is now measured, close to the amount of lead at the time of the birth of the Earth, is the initial content of lead. In 1953, Patterson isolated the iron meteorites of Diablo Canyon, Arizona, and finally got enough ultra-clean raw lead samples that he used for mass spectrometry analysis, and when he finished the experiment, he knew that the meteorite, the Earth, and the solar system were all 4.5 billion years old. In 1956, Patterson published a paper in the Journal of Geochemistry and Cosmic Chemistry under the title Meteorites and the Age of the Earth, which accurately defined the age of the Earth to 4.55 ± 0.7 billion years. It has been more than 60 years since the results of this study were published, and the age of the Earth has undergone only a slight adjustment, and the age of the Earth is currently considered to be 45.4 ± 0.5 billion years. This benchmark work is one of the greatest achievements in the history of geochemistry in the 20th century.