The laughter of the ancient alchemists
Since ancient times, researchers have dreamed of finding a way to transform some metals into others, especially tin and lead, into gold. Around this dream arose the parascience of alchemy, in which practical knowledge of the reactions between substances was mixed with mystical teaching. For example, alchemists tried to create a philosopher’s stone that not only glorified the metal, but also cured all kinds of diseases and restored youth when taken orally.
Alchemists were not successful in transmuting metals; At best, they created a golden alloy with the help of sulfur. But from their experiments emerged scientific chemistry, which derived an unshakable axiom about the conservation of matter. INSIDE expressions According to the 18th century chemist Lavoisier, the situation is as follows:
“Nothing is created (created from nothing) neither by artificial processes nor by natural processes, and it can be said that in every process (chemical reaction), the same amount of matter is present before and after, the quality and quantity of the principles are the same. (the elements) remain the same. At the same time, there were only displacements and regroupings. The whole art of experimentation in chemistry is based on this premise.”
In a simpler formulation, this means that at the end of the reaction the same atoms remain in the same amount as at the beginning. If anything other than water emerges inside the container during the combustion of hydrogen in oxygen, this is pollution coming from outside. This is still taught in the first courses of school chemistry.
Lavoisier would have been surprised to hear Nobel laureate Niels Bohr speak at the opening of the Fifth Washington Conference on Theoretical Physics on January 26, 1939. When bombarded with neutrons he stated: (an atom consists of a nucleus and a shell of negatively charged electrons; the nucleus consists of positively charged protons, the number of which determines the type of matter, and neutrons, which are necessary to give stability to the nucleus) Uranium nuclei can fuse into two barium nuclei with about half the mass. As physicist Edward Teller said, the day before the conference, he received a phone call from his colleague Georgiy Gamow, who knew the content of the talk and aforementioned to him: “Bohr has gone mad. “It says uranium is fissile.”
But during the speech, Bohr outlined a simple method by which anyone could obtain experimental proof of his thesis. While he was talking, someone in the audience whispered to the other: “I need to put a new sample in the accelerator urgently.” When Bohr finished his work, physicists rushed to their phones to instruct their colleagues in the laboratory. Some scientists decided to leave the conference immediately to check on their own whether uranium was really suitable for fission.
Within weeks, many scientific groups independently repeated what Bohr said. It is often said that scientists later discovered the transformation of some metals into others; This is something they have been trying to achieve for thousands of years. True, ancient alchemists would have laughed at such a transformation, because it transformed rare and expensive uranium into cheaper and more abundant barium.
Was this the first conversion?
In fact, physicists began to detect violations of Lavoisier’s postulate long before the discovery of uranium nuclear fission. At the end of the 19th century, scientists discovered that some chemical elements (including uranium and thorium) inherently emit radiation, a property called radioactivity. By the 1900s, it was revealed that radioactive elements actually emit three types of rays: alpha, beta and gamma. As Ernest Rutherford proved, beta rays are electrons and alpha rays are the nuclei of helium atoms.
Experiments have shown that radioactive elements, for some reason, break down over time, as if they were about to decay. Rutherford and his student Frederick Soddy noticed that during decay, some chemical elements transform into others, always changing according to the same law: with alpha decay, matter moves back two positions in the periodic table and the atomic mass decreases by 4; In beta decay, matter moves forward one position, but the atomic mass remains unchanged.
That is, by “firing” an alpha particle, uranium turns into thorium, thorium turns into radium, radium turns into radon, radon turns into polonium, polonium turns into lead. By emitting a beta particle, thorium transformed into proactinium, actinium into thorium, and bismuth into polonium. It also turns out that chemically identical atoms of radioactive substances can decay at different rates and have different nuclear masses; Such modifications of chemical elements were called isotopes.
With this data at hand, it was not difficult to understand that all chemical substances are actually of the same nature and that atomic nuclei consist of the same components. In the 1930s, physicists came to the conclusion that the nucleus of any atom resembles a liquid drop consisting of a certain number of protons and neutrons. Like a liquid, this drop can split and combine, so chemical elements can pass into each other. So if you separate two protons from radium, you get radon, and the two protons are the nucleus of the helium atom. The chemical properties of an atom depend on the number of protons in the nucleus, and the existence of isotopes is explained by different numbers of neutrons.
In the 1920s and 1930s, physicists discovered many transformations, and not just in metals. For example, during the experiment, nitrogen was converted to oxygen. So if the nucleus is like a drop of liquid and can split and combine, then what was the shock of uranium fission?
new energy source
All experiments pointed to the same truth: The nucleus of an atom is extremely strong, and the forces that hold its components together are incredibly strong (these were called strong interactions). It was believed that it was impossible to separate anything larger than the alpha particle from the nucleus, and therefore chemical elements could only be transformed into those adjacent to them in the periodic table.
Therefore, when German scientists Otto Hahn, Fritz Strassmann, Lise Meitner and Otto Frisch irradiated uranium with a stream of neutrons in 1938, they were sure that they would obtain radium as a result. It is shifted four positions relative to uranium in the periodic table and can be produced by two alpha decays. But scientists actually faced the same challenge as the Curies who discovered radium. Radium and barium are very similar chemically and differ only in the rate at which they accumulate from solution. Using this method, Hahn and Strassmann tested “radium” produced by repeatedly irradiating uranium, regularly behaving like barium. They eventually even tested the method on real radium from a store, and it worked well.
Later, physicists realized that an “explosion” occurred in the atomic nucleus, but they did not believe it.
“As chemists, we should replace the name Ba with Ra. As “nuclear chemists” who are very close to physics, we cannot yet accept this step, which contradicts all previous experiences in physics.” Wrote They appear in a scientific paper published in the journal Naturwissenschaften before Christmas 1938.
There was also another problem. “After separation, the two drops (two nuclei) will separate due to mutual electrical repulsion and achieve high speed and therefore very high energy, only about 200 MeV. So where can this energy come from? – According to Otto Frisch, physicists reasoned during a ski trip. (Frisch, Otto. “The few things I remember”)
But they suddenly realized that the two barium atoms that appeared would be 1/5 the mass of the proton lighter than the original mass of the uranium. According to Einstein’s mass-energy balance formula, 1/5 of a proton equals exactly 200 MeV energy. In other words, the fission of the uranium nucleus “suddenly” released a huge amount of energy.
This is precisely why physicists were shocked by Bohr’s report, submitted with the permission of Hahn and Strassmann in January 1939. It turns out that with certain manipulations, thousands of times more energy can be extracted from a piece of metal than from the same amount of oil or gas. And on the one hand, this energy can be gradually extracted and used, for example, to generate electricity. If you suddenly force it to release, an explosion will occur, the power of which should be measured in thousands of tons of TNT. All that remained was to figure out how to release this energy, and fortunately for the first time humanity managed to do this outside of Nazi Germany.
As for gold and medieval alchemy, it can be obtained from lighter and simpler elements through nuclear fusion reactions. This was confirmed during experiments: for example, in 1947, American physicists obtained 35 micrograms of gold from 100 mg of mercury; now stored in the Chicago Museum of Science and Industry. However, the price of obtaining this gold is much higher than buying it on the market: so real physical transformation would give nothing to medieval alchemists and kings dreaming of getting rich.