Written by | Lin Zhizhong (Institute of Physics and Department of Electronic Physics, Chiao Tung University, Taiwan)
Source | This article is from Physics, No. 2, 2021
01 Introduction
From the evening of February 7 to the early morning of February 8, 1922, when the temperature in the central German city was cold and snowy, the Stern-Gerlach experiment at the University of Frankfurt successfully measured for the first time the double splitting phenomenon of the electric neutral silver atomic beam in the non-uniform magnetic field, which clearly confirmed the nature of the quantization characteristics of the microscopic scale world, which is a major stroke in the history of modern physics!
The experiment was carried out by Walther Gerlach (1889-1979) alone, as Otto Stern (1888-1969) was briefly teaching at the University of Rostock in northern Germany (September 1921 to December 1922), returning to Frankfurt during the holidays to discuss data with Grach and further revise and improve the experimental design. According to Wilhelm Schütz, one of Grach's phD students at the time, Grach was a night owl who liked to go into the lab at 9 p.m. and work until the next morning. But part of the reason for this working time may also be due to the relatively rudimentary conditions of the experiment at the time and the limitations of the small laboratory space; and the experimental process had to run continuously for more than several hours to accumulate enough silver atoms on a condenser glass plate to be clearly developed. During the long nights, if the instruments were working smoothly, especially if the alignment of the vacuum system and the device remained normal and stable, Grach would read the literature, write papers, or prepare lecture notes for class, no different from the focused work of today's scientific researchers.
The enormous power of the 2-molecular-beam method
In 1911, french scientist Louis Dunoyer demonstrated for the first time that the motion of sodium atoms in a vacuum did indeed travel in a straight line like photons, confirming the basic hypothesis of Maxwell's gas dynamics theory. This sub-beam method/technique was subsequently refined, developed, and widely applied to the extreme by Stern, and many achievements were made in rewriting modern physics. The principle of the molecular beam method is very simple, in a high vacuum, a metal (such as sodium, silver, bismuth, etc.) is heated in a high temperature chamber above its boiling point, and then the flying metal vapor (atoms) fly out of a small hole or slender slit. Let the flying atoms pass through two collimated baffles with small holes or slits in succession to obtain a bundle of sparse (low density) atoms with a constant velocity, and these electrically neutral atoms are in a free state, far away from each other, without collision or collision with the vacuum glass tube wall. Thus, every atom has the same linear momentum1). Because the velocity of the atomic beam can be precisely regulated, Stern immediately came up with the idea of using this technique to verify the theoretical function of the Maxwell-Boltzmann distribution of molecular speeds in 1920. He then, in collaboration with Grach, tested theoretical predictions about the quantization of angular momentum space in 1922 — at a time when many prominent physicists believed that space quantization was merely a theoretical conjecture (meditation), or a mathematical notion, independent of physical reality. Thereafter, until 1933, during the period of about 10 and a half years after voluntarily resigning from the University of Hamburg and immigrating to the United States, Stern used the molecular beam method to conduct at least the following pioneering experiments, including: (1) using crystal surface scattering (diffraction), confirming the volatility of helium atoms and hydrogen molecules, and having the wavelengths of matter predicted by Louisde Broglie; (2) discovering and measuring the magnetic moment magnitude of protons, deuterium, and several other atoms. Since the mass of the proton is about 2,000 times larger than the mass of the electron, the magnetic moment signal is relatively weak by about 2,000 times, which is a difficult and ingenious experiment2). Each of these ingenious experiments and their immortal results deserves to be written into the history of quantum physics or in textbooks.
Fig. 1 Schematic diagram of the Stern-Grach experimental apparatus. O stands for high temperature chamber, SP1 and SP2 represent slits 1 and 2, M stands for non-uniform electromagnet, and P stands for glass deposition disc condensed by dry ice or liquid air. In later experiments, the slit is 800 μm long and 30 μm wide (image from https://www.mediatheque.lindau-nobel.org/research-profile/laureate-stern)
3 Fine instrumentation and experimental parameters of the Stern-Grach experiment
Today, when many teachers and students hear about an outstanding experiment that rewrote the history of science, they often unconsciously think of a spectacular and spacious laboratory and a huge and shiny expensive scientific instrument. However, the apparatus of the Stern-Grach experiment is not at all the case. Stern is acutely aware that the search for definitive answers to the most fundamental scientific questions of the moment must be carried out day in and day out in a craftsman/craft manner through rigorous technical details. Figure 1 is a schematic diagram of the molecular beam method experimental apparatus: the distance from the high temperature chamber (O) to the first slit (SP1) is about 3 cm, the distance between the first slit and the second slit (SP2) is about 3 cm, the length of the non-uniform electromagnet (M) is 3.5 cm, and the P is a condensed glass dish. That is, the entire key part of the instrument is only about 12 cm, which is the length of a ballpoint pen. Such a small instrument should be mainly limited by the vacuum technology at that time and the available space between the north and south poles of the electromagnet. Figure 2 is a historical photograph of Stern's figure, and the approximate size of the instrument can be compared to the approximate size of the instrument 3).
Figure 2 Steen experimented with the University of Hamburg (1923-1933). When Stern experimented, he often kept cigars in his mouth. There is a story that when the experiment was over and Grach took out a condensed glass dish, Stern stared at the plate in one hand and held a cigar in the other, and saw the deposition of silver atoms; it turned out that the (inferior) cigar contained a large amount of sulfur, and the silver was smoked and reacted into a black silver sulfide (AgS) (picture from the network)
In practice, the beam of silver atoms ejected from the high-temperature cavity bore must be collimated through Slit 1 and Slit 2 and parallel from the lower edge of the upper half (assuming the south pole) non-uniform electromagnet shown in Figure 1 before it can be deposited on the condensed glass dish. Due to the low beam density of silver atoms and the average free path of silver atoms greater than the length of the electromagnet, experimental measurements must be stable for more than a few hours, and the number of silver atoms at the deposit is sufficient to distinguish the image structure of the path from the optical microscope after development, whether it presents a continuous diffusion as predicted by classical theory, or double (multiple) splitting as predicted by quantum theory. Not only that, but the high-temperature chamber, slit 1, slit 2, and condensed glass discs (cooled by dry ice, acetone or liquid air) all have to be installed in a double-glazed vacuum chamber (10^(-5) torr) that requires good vacuum technology, and during experiments lasting more than several hours, the glass vacuum chamber, contacts, or seals cannot melt, crack, or leak. Another challenge was that the double-glazed vacuum walls had to be cooled with liquid air as the experiments were conducted. Fortunately, thanks to the bulb industry in the early 20th century, the vacuum pumping technology of the German industry at that time, if carefully adopted and applied, was enough to meet the requirements of these harsh experimental conditions.
All the above intricate details, as well as the meticulousness and hard work of the experimenters, are the practical parts of the Stern-Grach experiment, which is the most admirable and admirable place. The Stern-Grach experiment not only confirmed the quantization of angular momentum space, but also accurately measured the magnitude of the magnetic moment of the atom (Bohr magnetic moment). It is an epoch-making, epic experiment, designed and the result of which is a direct exploration of the mysteries of nature, leaving the essential qualities of the microscopic world invisible. Until the results of the experiment are released, no one knows exactly where the answer will point (continuous or split paths), so this is a "beautiful experiment" that is finalized! (For the statement that "this is a beautiful experiment", refer to Lin Zhizhong's Prism and Pendulum: Exploring the Beauty of Scientific Experiments.[ 5]
On the morning of February 8, 1922, after a night of smooth deposition, Grach carefully broke open the vacuum chamber and took out a condensed glass plate to rinse. He reacted a thin layer of invisible deposited silver atoms into a black silver sulfide (AgS), confirming for the first time the double splitting of electrically neutral metal atoms in the magnetic field — at a time when Stern was working at Rostock University. On the same day, Grach sent Bohr a postcard with a photograph of the experiment (Figure 3). The photograph clearly shows that, without magnetic field, silver atoms advance in a straight line and deposit directly in front of the slit, forming (after development) a slender dark mark (about 1.1 mm long and 60-100 μm wide, Figure 3 left). With the addition of a non-uniform magnetic field, the path of the silver atom splits into two lanes, and there is no deposition directly in front of the slit (in the middle blank) (Figure 3, right). In the image on the right, the dark marks coincide at the upper and lower ends because the path through the silver atoms at the upper and lower ends has fallen outside the non-uniform magnetic field and is therefore moving forward in a straight line without any magnetic force – in this measurement, Grach used a platinum slit 800 μm long and 30 μm wide. Conversely, the silver atom in the middle of the dark mark on the right is the strongest and the most deflected because it is close to the most non-uniform point of the magnetic field (the maximum Hz/z value). (There are different causes for the width of the two split marks, and part of it can be attributed to some microdistriction of the velocity values of the silver atoms emitted from the high temperature cavity.) )
Figure 3 A postcard sent by Grach to Bohr on February 8, 1922, with Grach's signature and date below the right. Notice the 1 mm width of the Grach mark in the image on the right (image from the network)
4Expensive experiments are supported by peers
Golden Triangle Combination: Stern's academic training background is that of a theoretical physical chemist, who excels at choosing to seek answers to major current scientific problems and "designing experiments and planning instruments with the brain" with the flavor of "thought experimenters" or "an experimenting theorist" in the experiment. Grach, on the other hand, was an able experimental physicist trained in orthodoxy, with delicate hands, delicate thoughts, and a willingness to devote himself wholeheartedly to pairing with Stern. Even after Stern left Frankfurt (September 1921), he undertook the experimental work alone and obtained epoch-making observations. In fact, after reading Dunoyer's paper, Grach became very interested in the molecular beam method in 1912 and began to have the idea of designing a large gradient non-uniform magnetic field. In addition, there is a young and skilled technician, Adolf Schmidt, who is willing to meet the endless and demanding needs of the experiment, constantly making and replacing instrument parts for the Schachers (and then assembling them by Grach). The highly specialized collimation techniques and techniques of the experimental apparatus (requiring misalignment errors of less than 10 mm) and the design of small, non-uniform electromagnets also received key advice, assistance and guidance from Max Born's successor, Professor Erwin Madelung (from 1921 to 1949, Professor Madelung was director of the Institute of Theoretical Physics at the University of Frankfurt).
Thus, the Stern-Grach experiment was able to be completed in Frankfurt in the early 1920s for a reason. Obviously, academic excellence is often the fruit of close cooperation between the brain and physical strength of the group, rather than the accidental personal show of a lone genius who relies on a sword and a long sky. Even for the gifted unborn genius Albert Einstein, if his academic life had not been grown up in Germany/Europe in the early 20th century, with the repeated promotion of Hendrik Lorentz, Max Planck, Walter Nernst, Emil Warburg and many others, and even the selfless help of excellent mathematician friends at several critical moments, he would not have been able to complete the photoelectric effect, radiation (light) Excellent theories of wave-particle duality and relativity.
Funding source: The Stern-Grach experiment was mainly carried out and completed between 1921 and 1922, shortly after the end of the First World War, when the German economy was depressed, inflation was serious, and university research funds were short. In order to support the experiment, which later became a landmark in modern physics, Born not only put all the laboratories, gold factories, and technicians of the (small) Institute of Theoretical Physics under his auspices, but he also gave a series of public lectures on relativity in the largest lecture hall of the University of Frankfurt, and charged an entrance fee—in those years, Stern was Born's assistant, and many people were obsessed with Einstein's fame and the mysteries of the theory of relativity. But after all, the income from the speech was limited, so Born later tried to find a way to unexpectedly and luckily raise a valuable amount of money from a German entrepreneur from Frankfurt who had moved to New York, USA, and successfully completed the experiment. Furthermore, the money for the production of non-uniform electromagnets was funded by Einstein using funds from the Kaiser Wilhelm Society for the Advancement of Science in Berlin, which he managed.5 In addition, the materials needed to make magnets and the constant consumption of liquid air have also been sponsored by local private companies in Frankfurt. Therefore, the success of the Stern-Grach experiment obviously did not come from a momentary coincidence, but from other lands with poor industrial strength and scientific knowledge!
5 personality traits
As the Nazi government came to power, Stern (of Jewish origin) voluntarily resigned from the University of Hamburg in August 1933 and moved to the United States, ending his golden years of scientific discovery of the molecular bundle method, which lasted for about 15 years. The year after the end of World War II, Stern retired from the Carnegie Institute of Technology in Pittsburgh, Pennsylvania, and since then he has taken boats almost every year6) to Visit Europe, attend conferences, and meet old friends such as Wolfgang Pauli (Wolfgang Pauli 7). He especially liked to stop in Zurich, Switzerland, but was reluctant to officially set foot on German soil for almost his life. A year before his death, he attended the Lindau Nobel Laureate Meeting in August 1968, which was probably his only trip back to public service. Every time he returns to Europe, Stern often invites friends, sometimes even pays for their travels, to reunite or vacation in Switzerland. Because the living conditions of many Germans after the war were extremely difficult, once the mail route was reopened, Stern often sent food and necessities from the United States to help old friends. He also sent clothes to Max von Laue because Laue's house had been blown up during the war. Due to his contempt for the evil deeds of the Nazi regime, Stern refused to receive an annuity owed to him by the German (Hamburg) government after his retirement. After Stern's death, his former senior peers, peers (including Grach), postdoctoral fellows, students, assistants, and technicians unanimously evaluated his personality traits as being open-minded and completely trustworthy. In the winter of 1933, before leaving Germany, Stern personally arranged for the completion of the chartered teaching qualification program for his assistant Friedrich Knauer, which was a small personal matter, but a major event that had followed him closely for many years.
6 Nobel Prizes and posthumous honors
In his lifetime, Stern published only more than 50 journal papers, but one after another was very loud and influential. In 1944, after being nominated as many as 82 times, Stern was finally awarded his contribution to the development of the molecular ray method and his discovery of the magnetic moment of the proton. One won the 1943 Nobel Prize in Physics. As for his close associates, Grach himself did not approve of the Fanatical Radicalism of the Nazis, was unwilling to criticize Einstein, and did not participate in the anti-Semitic (Jewish science) movement. But perhaps because From 1944 Grach was head of Germany's nuclear program, he was excluded from the Nobel Prize list. Moreover, the Nobel Prize Committee seemed to be prepared, and in the brief reasons for the award published, there was no mention of the Stern-Grach experiment at all. After the war, Grach contributed many ways to the reconstruction of German science and to the prohibition of the development of nuclear weapons. According to the Nobel Committee, There were two reasons why Stern was awarded the prize after many years of delay (the Nobel Prize from 1934 to 1940 was not awarded for war): (1) Arnold Sommerfeld had predicted the quantization of angular momentum space in 1916, so the measurements of the Stern-Grach experiment were not new; (2) the size of the proton magnetic moment measured by Stern in 1933 was similar to that of Paul Dirac. The theoretical predictions do not match those of The Rabbi in 1934. Paradoxically, Sommerfeld and Dirac's theories were wrong, while rabbi's experimental data were too in error, but Steadn's experimental values, under pressure from the Nazis to put a knife around their necks, had hurriedly concluded the experiment before leaving the University of Hamburg, which was closest to the correct values that are widely accepted today.
As the wheels of the times progressed, in order to commemorate the epic contribution of the Stern-Grach experiment to the development of modern physics, the German Physical Society established a Stern-Gerlach Medal in 1992 to recognize major achievements in experimental physics. Another medal of the society for major theoretical physical achievements is named the Max Planck Medal after Planck, the father of quantum physics.
7 Scientific concepts have gone down in history
During World War II, the Allies bombed Frankfurt for many years, and the instruments, laboratory notebooks, and raw data used in the Stern-Grach experiment were burned. Fortunately, natural science and art works have very different essences, paintings, calligraphy, sculpture and other artistic works, once destroyed, they are destroyed, ashes, irreplaceable. But the concepts and measurements of the natural sciences—the quantization of space and the magnetic moments of protons—once established, are integrated into the sea of scientific knowledge and the long river of scientific history, and will be learned, appreciated, respected, and further tested by generations of teachers and students.
8 Ends
In February 2002, the University of Frankfurt erected a commemorative plaque (picture 4) at the entrance to the building where the Schaegers were conducting their experiments and established a Stern-Gerlach Center for Experimental Physics. The commemorative plaque, with Stern's head on the left and Grach's on the right, is separated by a schematic diagram of the molecular beam method they developed, a metaphor for the physical authenticity of the quantization of space they have confirmed. In 2014, the European Physical Society designated the old physics museum where the Schaegers conducted their experiments as a site in the history of science, which is highly respected by them.
Fig. 4 In February 2002, 80 years after the completion of the experiment, a commemorative plaque was erected at the entrance of the old physics hall of the University of Frankfurt, where the Stern-Grach experiment was completed (image taken from https://physicstoday.scitation.org/doi/full/10.1063/1.1650229)
The epilogue is based on literature [1]-[4] and other sources. In the history of quantum theory/quantum mechanics in the early 20th century, heroes have emerged and many wonderful stories have been handed down, and science and engineering teachers and students and other industry people not only do not get tired of listening, but many people are familiar with the characters and plots. However, the narrative and interpretation of the history of the Stern-Grach experiment in textbooks and popular science articles is rare, and the names and deeds of the two Schengers are even unfamiliar to the scientific community, which is regrettable, not to mention that Stern's use of concise, intuitive, and clear molecular beam methods has repeatedly achieved many other masterpieces. In the past few years, when I teach the "Modern Physics" class, these forgotten and lamentable scientific processes have been haunting my mind, but now I have finally decided on a heart, hoping that through this article, readers can get a little understanding and inspiration for the nature of experimental physics and scientific discovery.
concentrate:
1) Another important tool for studying the intrinsic properties of protosons is spectroscopy, which measures photon absorption or emission during energy step transitions, and therefore involves excited states, which measure changes in physical quantities between two states. The electrically neutral proto-neutrino (fraction) advancing at medium speed in the molecular beam method is in its most natural state, the ground state, and therefore measures the absolute value of a physical quantity in a particular state.
2) Later in life, Stern recalled that when he and his assistant were preparing to measure the magnetic moment of protons, they were "strongly chided by the theoreticians" by many of their fellow theorists because they thought they already knew the answer.
3) Due to the continuous improvement and refinement of the experiment, the scale of each component and the experimental design and parameters of each time may have minor corrections or adjustments, which will not be repeated in this article. The next paragraph is only an order of magnitude estimate, intended to show the technical and technological characteristics of the Stern-Grach experiment, such as ingenuity, care, and patience.
4) In practice, how to accurately correct the magnetic field size and non-uniform magnetic field gradient is an important issue, which is omitted in this paper.
5) Einstein was the director at the time, and he and Stern were old acquaintances. After receiving his doctorate in 1912, Stern went to the University of Prague (Germany) and became Einstein's first postdoctoral researcher, and the following year Einstein took up a new position at the University of Zurich, with whom he invited Stern. It was his adventurous spirit, Stern said, that led him to decide to work with Einstein. He said that the first time he met Einstein in his office, Einstein was dressed like an Italian road builder.
6) During World War I, Stern volunteered for the army as a meteorological observation officer, and during a mission he was on was shot down by the Russians, and although he survived, he seemed to avoid flying thereafter.
7) It is said that Stern loves to watch movies and often goes with Pauli, but pauli needs to tell him if he has seen the movie before. Stern suffered a heart attack in a movie theater in Berkeley, California, in his later years and died a few days later.
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[2] Friedrich B，Herschbach D. Physics Today，2003，(12)：53
[3] Toennies J P,Schmidt-B cking H,Friedrich Bet al. Annals of Physics,2011,523(12):1045
[4] Serge E. Otto Stern：A Biographical Memoir. National Academy of Sciences (U.S.) ，1973
LIN Zhizhong. Physics,2008,37(10):749.]
This article is reprinted with permission from the WeChat public account "Chinese Physical Society Journal Network".