William Henry Bragg (1862— 1942), a British physicist, is one of the founders of modern solid state physics. He studied mathematics at Trinity College in Cambridge in his early years. He was a professor at the University of Adelaide in Australia, the University of Leeds in England and the University of London, and became the president of the Royal Society from 65438 to 0940. He shared the 19 15 Nobel Prize in Physics with his son, W.L. Prague, in recognition of his pioneering contribution to the study of crystal atomic and molecular structures by X-ray diffraction. It is unique in history that father and son won the Nobel Prize together. At the same time, as an outstanding social activist, he was a man of the hour in British public affairs in the 1920s and 1930s.

1895 After the discovery of X-rays, many physicists thought that this is a special kind of light-you can shoot nails in wood or bones in the palm of your hand with X-rays-and its properties should be consistent with waves. But no one can be sure, because no one can clearly prove that X-rays have diffraction and other properties. The key problem is that the grating gap should be equal to the wavelength of the test object when diffraction test is carried out. A grating with 20,000 lines per inch is suitable for visible light. But the energy of X-ray is much greater than that of visible light, which means that according to the interpretation of classical physics, its wavelength is much shorter-maybe only one thousandth of the wavelength of visible light. It is absolutely impossible to make such a fine grating.

German physicist von Laue believes that if such gratings cannot be made by hand, natural creation may work. Crystals in nature are thought to be made up of atoms arranged according to certain rules, and each layer is only a few atoms thick. Laue thinks that the gap between these atomic layers may be appropriate and can be used as an X-ray diffraction grating. However, because the atom is a solid composed of atomic layers, the pattern formed at the other end will be very complicated, just like stacking several gratings together. Arnold sommerfeld, the boss of Laue and a professor at Munich University, thought the idea was ridiculous and advised him not to waste time on it. But in 19 12, two students confirmed Laue's prediction. They shot a beam of X-rays at the zinc sulfide crystal and captured the scattering phenomenon on the photosensitive plate, which was later called Laue's photo. After the photosensitive plate was developed, they found the circular arrangement of bright spots and dark spots-diffraction pattern. Laue proved that X-rays have the property of waves. Nature magazine called this discovery "the greatest and most profound discovery of our time". Two years later, this discovery won Laue the Nobel Prize.

This discovery has two major meanings. First, it shows that X-rays are waves, so that scientists can determine their wavelengths and make instruments to distinguish different wavelengths. Like visible light, X-rays have different wavelengths. But the second field advocated by Laue has produced more fruitful results. Once a certain wavelength beam is obtained, researchers can use X-rays to study the spatial arrangement of crystal gratings: X-ray crystallography has become the first detector to study the three-dimensional material structure at the atomic level.

Humphry davy, one of the founders of modern chemistry, said a century before Pauling entered California Institute of Technology: "In the process of acquiring knowledge, the use of new tools is very important. The key factor for people of different times to achieve different achievements is not their natural intelligence level, but the various means and artificial resources they have mastered. " X-ray crystallography will become a powerful artificial resource.

The theory behind it is simple. Researchers are faced with three factors: X-ray with a certain wavelength, crystal grating with a certain structure and diffraction pattern-there is a simple mathematical relationship between them. Knowing the map and another factor, we can deduce the third factor. Many of the original mathematical and practical skills were developed by Prague and his son. Their laboratories in Cambridge and Manchester have become the most famous X-ray crystallography research centers in the world. 19 12, Laue's paper on X-rays was published, which attracted the attention of the Prague father and son. At that time, henry bragg was a professor of physics at Leeds University, and lawrence bragg had just graduated from Cavendish Laboratory of Cambridge University, stayed in the laboratory and started to engage in scientific research.

It is not complicated in theory, but in practice, because the diffraction pattern is quite complicated, the process of piecing together the crystal structure needs a lot of time and energy. The early instruments were all made by ourselves, and the quality was very unstable. Crystals are usually very large and need to be carefully refined, cut at a certain angle and placed accurately to obtain satisfactory diffraction patterns. If Laue photos are successfully obtained, the position and distribution of each point should be carefully measured. Then there is the mathematical calculation. Even simple crystals, in the era without computers, need several months to calculate the structure of each crystal. If the crystal is too complex and the number of atoms in the unit cell of the basic crystal structure exceeds ten, then the diffraction pattern of X-ray will be extremely complex and difficult to crack. The whole process is a bit like shooting wrought iron for decoration with a homemade shotgun, and then inferring the shape of wrought iron by analyzing the trajectory of ricochet.

For these reasons, the research object can only be limited to very simple crystals. However, the study of these simple crystals produced surprising results. For the first time, researchers can use tools to understand the arrangement of individual atoms in crystals and accurately measure the distance and angle between atoms. The first crystal structure solved by Bragg and his son was rock salt, and the result was unexpected. The whole crystal forms a huge grid, each sodium ion is surrounded by six equidistant chloride ions, and each chloride ion is surrounded by six equidistant sodium ions. There is no single sodium chloride "molecule". This discovery shocked the field of theoretical chemistry and immediately triggered new thinking on the behavior of salt in solution. Another early success of the Prague laboratory was the discovery of the structure of diamonds, which confirmed the theory of early chemists. It is purely a tetrahedron composed of carbon atoms. Prague and his son went on to solve the structure of several other crystals, and they shared the Nobel Prize a year after Laue.

When it comes to the contribution of Bragg and his son to science, we can't help but mention the key role of X-ray diffraction technology in the development of modern molecular biology. The so-called "X-ray diffraction technology" is to accurately determine the spatial position of atoms in the crystal through the mutual transformation relationship (mutual Fourier transform) between the X-ray diffraction pattern of the crystal and the arrangement of crystal atoms. In the early 1950s, Watson and Crick of Cavendish Laboratory of Cambridge University proposed the double helix model of DNA with the help of this technology. So far, this technology is still the main means to study the structure of biological macromolecules.

Old Prague is a scientist. On the one hand, he insists on the "value neutrality" of science, on the other hand, he firmly believes that science will benefit mankind. Not only that, as a social activist, "how science benefits society" is the theme of his life. Due to the negative effects of science and technology, his belief may be doubted by some people, but this humanistic tradition has its eternal value, especially the science-technology-business alliance will still dominate human life, at least in the foreseeable future.

In life, he treats the world well, agrees to get along, and then goes his own way independently. Perhaps out of shyness, he doesn't seem to pursue close friendship. From 1904 to 1907, in his close correspondence with Rutherford (some of them were as long as 34 pages), we only saw discussions about scientific research. He often reads the diary of his predecessor Faraday, just like reading a letter from a friend, and he has great respect and admiration for it. This is a kind of "spiritual intimacy".

His humility and fraternity are especially manifested in his attitude towards children. His basic view is: "Children must be free, absolutely free!" Whenever the children ask him for advice on important issues, he will appear very uneasy. "Walk back and forth in the chair, murmur sympathetically, then get up from the chair and try to change the conversation until he finally feels exhausted." He will say "let me think about it", and then in a day or two, he will send a detailed proposal letter, in which "all the objections have been carefully considered"; Sometimes even in order to show his neutrality, he puts forward some bizarre suggestions, trying to make children "judge for themselves" and "not be bound by his views". Perhaps the most legendary thing is that old Prague is a scientist who started his research activities in middle age. In his early years, he worked conscientiously as a teacher in an unknown university in Australia until he was 42 years old. After returning to Britain, he became a scientific spokesman in just a few years. What the hell is going on here? The answer is memorable: "The answer may lie in the long and happy wandering life." "Perhaps the busy and happy 20 years spent in Australia are as precious to a prophet as the years in the desert, giving him time to prepare calmly." "He has time to discover the principles guiding his life and organize his thoughts", and once "there are clear principles, his life is as thoughtful as his manuscript, with almost no deletion"!

His "practical religious view" is very interesting: "You have a good idea, and you work hard to realize it; If the result confirms your idea, then you can take this conclusion as further basis. In the laboratory, in any training in education, literature and cooking, and in religion. " For him, religious belief makes him willing to risk his life to assume that Christ is right and test it through lifelong fraternity experiments. "