As a professor of natural philosophy at Glasgow University in Victorian era, Kelvin was the most influential theoretical physicist at that time. He calculated the age of the earth by using the thermodynamic principles known at that time. Kelvin knows that the interior of the earth is much hotter than the surface according to the observation of lava flow in the earth and the previous mining experience of deep deposits. Therefore, he expects to infer the age of the earth by observing the temperature difference between the surface and the ground (the so-called geothermal gradient). He assumed that the initial state of the earth was a mass of molten material with a temperature of about 3850 degrees Celsius. So his calculation concluded that it would take about 1 100 million years for the earth to reach the present geothermal gradient value. This time is regarded by him as the age of the earth.
Kelvin's above estimation results caused a big quarrel between his supporters-theoretical physicists and opponents-geomorphologists (geologists who study surface morphology). Geomorphologists have found that some features of the earth's surface reflect the age of the earth far beyond 1 100 million years when calculating the formation time of some geological features by using the variational principle. However, at that time, these geomorphologists could not convincingly prove that the geological phenomena they observed represented the older age of the earth, and their views were of course opposed by many physicists. On the issue of the age of the earth, the new understanding put forward by physicists at that time actually increased the uncertainty of scientists' understanding of the absolute age of the earth. But this situation only lasted for decades.
Sometimes, some new scientific insights or discoveries will lead to the revision of the paradigm and the reconciliation between some seemingly contradictory theories and observations. The debate about the age of the earth is like this: at the turn of the century from 65438 to 2009, the discovery of radioactivity made the age of the earth become a hot topic again. Lord Kelvin doesn't know that part of the heat generated by radioactivity in the earth's core. If this part of heat generated by radioactivity is taken into account in his calculation, the calculation result will be closer to the geologist's best estimate. The latter applies the principle of variational theory to the study of the evolution of surface morphology, and then infers the age of the earth. Radioactivity also provides an independent benchmark for geologists to determine the absolute age of the earth. Radioactive dating
Radioactive atoms will decay over time. The decay rate of radioactive atoms is considered to be a constant (unless the atom moves at a speed close to the speed of light), which is actually not affected by pressure, temperature, etc. , and will not be changed by the physical changes of compounds composed of radioactive atoms (such as rocks, water or air). The decay rate of radioactive elements is expressed by half-life, that is, the time required for half of the initial atomic number to decay into other elements and particles by naturally releasing mass and energy. If we know the constant rate of formation of sub-elements, then we can calculate the age of rocks by calculating the ratio of original elements to sub-elements. This knowledge enables scientists to calculate the age of formation of minerals containing primitive radioactive elements. Guided by radioactive decay in rocks, geologists and geochemists can determine the absolute ages of rocks, thus determining the absolute ages of various strata on the earth. With this radioactive dating technology, people have found a reliable clock to calculate the age of the earth.
Several elements are used for dating rocks, including uranium (U) isotope which decays into lead (Ph) isotope (half-life is between 700 million and 450 million years), rivet (Rb) isotope which decays into saw (Sr) (half-life is 50 billion years) and potassium (K) isotope which decays into ammonia (Ar) (half-life is 65.438+0.3 billion years).
In the early stage of isotope dating (1900 ~ 1939), the simple analysis method and the limitation of understanding the role of atomic nuclei hindered scientists' experimental work. Nevertheless, by measuring the U/pH ratio in uranium-bearing materials and the He-He/U ratio in various rocks and minerals, scientists can make a rough age estimate.
Because of the long half-life of Rb and K, Rb/Sr and K/AR dating techniques are one of the most reliable dating techniques, respectively, and their age range almost covers the history of the whole earth of about 4.5 billion years. However, if we want to determine the age of relatively new geological events (for example, the age of events thousands of years ago), we need to use some radioactive elements with much shorter half-lives.