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671 [1063] HAHN
HAHN [1063] could manufacture all such compounds from simple inorganic phosphates. McCollum also experimented on the im portance of fluorine, zinc, and man ganese to life. [1063] HAHN, Otto German physical chemist Born: Frankfurt-am-Main, March 8, 1879
Died: Gottingen, July 28, 1968 Hahn, the son of a glazier, was a me diocre student in college. Despite his fa ther’s wish that he become an architect, he grew interested in chemistry. He stud ied under Baeyer [718] at Munich, and it was not until he had entered graduate school that he found himself. He ob tained his Ph.D. in 1901 at the Univer sity of Marburg, and did so magna cum laude. He then continued his studies abroad.
He worked with Ramsay [832] in Lon don in 1904. Ramsay persuaded him to stay in research and in 1905 he went on to work with Ernest Rutherford [996] in Canada, succeeding Soddy [1052], who, in fair exchange, had returned to En gland to work with Ramsay. In 1906 Hahn returned to Germany, worked with Emil Fischer [833], achieved professorial rank in 1910, and served in World War I, working on poison gas under Haber [977], In 1928 he became director of the Kaiser Wilhelm Institute for Chemistry. During the early period of his research he helped work out some of the interme diate stages in the radioactive breakdown of thorium. Then, in 1917, with his long time associate Meitner [1060], he discov ered the new element protactinium. In 1921 the two also discovered nuclear isomers, atoms with nuclei that did not differ in content of subatomic particles but only in energy content and type of radioactive breakdown. However, Hahn’s real fame came fifteen years later in connection with the bombardment of uranium with neutrons, a project that had first been undertaken by Fermi [1243], in the mid 1930s. The results obtained by Fermi had been confusing, though it was suspected that artificial elements more complicated than uranium had been formed. Hahn and Meitner, among others, investigated the situation. They treated the bom barded uranium with barium, which car ried down a certain fraction of strongly radioactive material. This made them suspect that one of the products of neu tron-bombarded uranium was radium, which was chemically very similar to barium and would be expected to accom pany barium in any chemical manipu lations. However, no radium could be obtained in those barium-treated frac tions.
By 1938 Hahn, working with Fritz Strassman [1251], began to wonder if it was not barium itself—radioactive barium, of course, formed from the ura nium in the course of its bombardment by neutrons—that was being carried down by the barium he had added. How ever, the barium atom was much lighter than the uranium atom, so much lighter that it could have been formed only through the breaking in half of the ura nium atom. Such a breaking in half (uranium fission) was unheard of in nu clear work and Hahn hesitated to pub lish this suggestion. He published his findings in January 1939 but he carefully did not interpret them as representing fission. As a chemist he did not wish to fly in the face of physical dogma. Meitner, now in exile (and feeling per haps that she had little to lose), having received the news from Hahn himself, took the plunge and published the sug gestion of fission a month later. For his discovery of fission Hahn re ceived the 1944 Nobel Prize in chemistry. Very fortunately for the world, the Nazi government of Germany remained blind to the potentialities of fission and Hahn was left pretty much to himself and to minor experimentation during the course of World War II. In 1946, the war being over, Hahn became president of the West German Max Planck Society, hold ing this position till his retirement in 1960.
The news of fission had, however, been brought to the United States by 672 [1064] EINSTEIN
EINSTEIN [1064] Bohr [1101] and, thanks to Szilard [1208], American research got under way, culminating in the development of the atomic bomb. In 1945, after the end of the European phase of World War n, Hahn, who had taken no part in war research this time, was taken into custody by American forces, along with Laue [1068], Heisen berg [1245], and Weizsäcker [1376]. It was while he was in custody that Hahn received the news of the dropping of the atomic bomb on Hiroshima. Hahn conceived his personal respon sibility to be great and, for a while, even considered suicide. The fission bomb di minished, however, when compared with the fusion bomb whose theoretical basis had been forecast by Harkins [1022], In 1966 he was granted a share of the Fermi award issued by the Atomic En ergy Commission of the United States. He was the first foreigner to win this award. After his death (as the result of an ac cidental fall), element number 105 was named hahnium in his honor. [1064] EINSTEIN, Albert German-Swiss-American physicist Bom: Ulm, Germany, March 14, 1879
Died: Princeton, New Jersey, April 18, 1955 Einstein was the son of a chemical en gineer. Although Jewish, Einstein re ceived his earliest education in a Catho lic grammar school in Munich, Bavaria, to which city his family moved while he was still quite young. Like Newton [231], with whom he is often compared (and certainly he is the only scientist since Newton’s time who can bear the comparison), he showed no particular intellectual promise as a youngster. As a matter of fact, he was so slow in learn ing to speak that by the time he was three there was some feeling that he might prove retarded. In 1894 his father (who had failed in business) left for Milan, Italy, while Al bert stayed behind to finish his high school studies. However, he did very badly in Latin and Greek and was inter ested only in mathematics, so he left school by invitation of the teacher who said, “You will never amount to any thing, Einstein.” The young man thus became the most unusual dropout in the history of science. His uncle, Jakob, an other engineer, then began giving him mathematical puzzles that continued to feed his interest in this direction. After an Italian vacation (taken to avoid qualifying for military service in Germany—for he was a pacifist from the start) he began his college work in Swit zerland. This was not without difficulty, for only in mathematics was he really qualified for entrance. Nor did he enjoy the experience. He cut most of the lec tures, preferring to concentrate on inde pendent reading in theoretical physics. That he could pass his courses at all was due to the excellent lecture notes of a friend.
Once graduated, he tried to find a teaching post but that wasn’t easy, for he was not a Swiss citizen and he was Jew ish besides. In 1901, thanks to the influence of the father of the same friend whose lecture notes Einstein had used, Einstein accepted a position as a junior official at the patent office at Berne, Switzerland, and in that year became a Swiss citizen. Therefore, without any academic con nections, he began his work and for it, fortunately, he required no laboratory but only a pencil, some paper, and his mind. The year 1905 was his annus
five of his papers in the German Year book of Physics, involving three develop ments of major importance (and in that same year, he earned his Ph.D.). One paper dealt with the photoelectric effect, whereby light falling upon certain metals was found to stimulate the emis sion of electrons. Lenard [920] had in 1902 found that the energy of the emit ted electrons did not depend on the in tensity of the light. A bright light might bring about the emission of a greater number of electrons, but not of more en ergetic ones. There was no satisfactory
[1064] EINSTEIN
EINSTEIN [1064] explanation for this in terms of classical physics. Einstein, however, applied to the prob lem the quantum theory worked out five years earlier by Planck [887] and dis regarded since. Einstein maintained that a particular wavelength of light, being made up of quanta of fixed energy con tent, would be absorbed by a metallic atom and would force out an electron of fixed energy content and no other. Brighter light (more quanta) would then bring about the emission of more nu merous electrons, but all still of the same energy content. Light of shorter wave length, however, would have more ener getic quanta and would bring about the emission of more energetic electrons. Light that had wavelengths longer than a certain critical value would be made up of quanta so weak as to bring about no electron emission at all. The energy con tent of such long wavelength photons would be insufficient to break electrons away from the atoms of which they formed a part. This “threshold wave length” would be different for different metals, of course. Planck’s theory was thus, for the first time, applied to a physical phenomenon (other than the black-body problem that had occasioned its development in the first place) that it could explain and clas sical physics could not. This went a long way, perhaps even all the way, toward establishing the new quantum me chanics. For this feat Einstein was even tually awarded the 1921 Nobel Prize in physics, and yet it was not his greatest work of that year. In his second paper of 1905, published two months after the first, Einstein worked out a mathematical analysis of Brownian motion, first observed by Brown [403] three quarters of a century earlier. Einstein showed that if the water in which the particles were suspended was composed of molecules in random motion, according to the requirements of the kinetic theory of Maxwell [692] and Boltzmann [769], then the suspended particles would indeed jiggle as they were observed to do. Svedberg [1097] had suggested this molecular explanation of Brownian motion three years earlier, but it was Einstein who worked matters out in mathematical detail. All objects in water (or in any liquid or gas) are continually bombarded from all sides by molecules. Through the workings of chance, the number of mole cules striking any object of ordinary size from one angle is about the same as the number from another angle, the dif ferences in number that do exist being insignificant in comparison with the truly vast total numbers involved. For that reason there is no overall effect (or at least no detectable one) upon objects of ordinary size. As an object grows smaller, the total number of molecules bombarding it de creases and small differences in bom bardment from this direction or that grow appreciable. Grains of pollen or particles of dye are small enough to be pushed first this way by a slight excess of molecules striking in that direction, then in another, then in still another. The mo tion is quite random, attesting to the random motion of the molecules them selves. The larger the average size of the mol ecules, the larger the body for which this difference in bombardment can produce detectable effects. Therefore, the equa tion deduced by Einstein to describe Brownian motion could be used to work out the size of molecules and of the atoms that compose them. Three years later Perrin [990] conducted experiments on Brownian motion which confirmed Einstein’s theoretical work and which gave the first good values of atomic size. The atomic theory of Dalton [389] was a hundred years old by then and had been accepted by all but a few diehards such as Ostwald [840], and yet this was the first time the effect of individual mole cules could be directly observed. Even Ostwald gave in. Einstein’s greatest accomplishment of the year involved a new outlook on the universe, replacing the old Newtonian view, which had reigned supreme for two and a quarter centuries. Einstein’s work climaxed the famous experiment of Michelson [835] and Mor-
[1064] EINSTEIN
EINSTEIN [1064] ley [730], who had been unable to detect any difference in the velocity of light with changes in its direction through the ether. Einstein later claimed he had not yet heard of the experiment in 1905, but that he was troubled by a certain lack of symmetry in Maxwell’s equations con cerning electromagnetic effects. What ever the case, he began with the assump tion that the measured velocity of light in a vacuum is always constant despite any motion of its source or of the indi vidual measuring the light. Furthermore, he canceled out the ether as unnecessary by assuming that light traveled in quanta and therefore had particle-like properties and was not merely a wave that required some material to do the waving. This particle-like form of light was named a photon a decade later by Compton [1159], It represented a retreat from the extreme wave theory of light, moving back toward Newton’s old particle theory and taking up an intermediate po sition that was more sophisticated, and more useful, than either of the older theories. Einstein also pointed out that without the ether there was certainly nothing in the universe that could be viewed as at “absolute rest,” nor could any motion be considered an “absolute motion.” All motion was relative to some frame of reference chosen, usually, for its conve nience, and the laws of nature held un changed for all such frames of reference. His theory, because of the “all motion is relative” idea, is therefore called relativ ity. In this particular paper he dealt only with the special case of systems in uni form nonaccelerated motion, so it is called the special theory of relativity. He showed that from this simple as sumption of the constancy of the veloc ity of light and the relativity of motion, the Michelson-Morley experiment could be explained and Maxwell’s electromag netic equations could be kept. He showed also that the length-contraction effect of FitzGerald [821] and the mass enlargement effect of Lorentz [839] could be deduced, and that the velocity of light in a vacuum was therefore the maximum speed at which information could be transferred. All sorts of peculiar (in appearance) results followed. The rate at which time passed varied with velocity of motion; one had to give up notions of simul taneity, for one could no longer say, under certain conditions, whether A hap pened before B, after B, or simulta neously with B. Space and time vanished as single entities and were replaced by a fused “space-time.” All this was against “common sense” but common sense is based on a limited experience with ob jects of ordinary size moving at ordinary velocity. Under such conditions the difference between Einstein’s theory and the ordinary Newtonian view (which is “common sense”) becomes indetectably small. In the vast world of the universe as a whole and the tiny world within the atom, however, common sense is no guide; there is a detectable difference be tween the two views; and it is Einstein’s view and not Newton’s that is the more useful.
In the special theory of relativity, Ein stein worked out an interrelationship of mass and energy in a famous equation that goes: E=mc2, where E is energy, m mass, and c the velocity of light. Since the velocity of light is a huge quantity, a small amount of mass (multiplied by the square of the velocity) is equivalent to a large amount of energy. With mass and energy thus interpreted as different aspects of the same phenom enon, it was no longer sufficient to speak of Lavoisier’s [334] conservation of mass or of Helmholtz’s [631] conservation of energy. Instead there was the greater generalization of the conservation of mass-energy. Or, if one still speaks sim ply of the conservation of energy, it must be understood that mass is but one more aspect of energy. This new view at once explained the energies given off by radioactive ele ments as a consequence of the slight loss of mass involved, a loss so slight as to be indétectable by ordinary chemical proce dures. The interrelationship of mass and energy was quickly confirmed by a vari ety of nuclear measurements and has, 675 [1064] EINSTEIN
EINSTEIN [1064] ever since, proved fundamental in atomic studies. Once only did its usefulness seem to flag and then Pauli [1228] postulated the existence of the neutrino to save it. The value of the new generalization in everyday affairs, and not merely in the highly esoteric work of the atomic physi cists, was overwhelmingly shown when the conversion of mass to energy on a large scale made possible the devastation by atomic bombs a generation later, a denouement to which Einstein was to contribute directly, and which he was to find horrifying. Despite this triple thunderbolt of papers, it was four more years before Einstein could finally obtain a profes sorship (and a poorly paying one) at the University of Zürich. His reputation con tinued to grow, however, and in 1913 a position was created for him at the Kai ser Wilhelm Physical Institute in Berlin, thanks to Planck, who was greatly impressed by the young Einstein. For the first time Einstein was to be paid gen erously enough to make it possible for him to devote his life to science. World War I broke out but Einstein was little affected, since he was at the time a Swiss citizen. However, when many German scientists signed a nation alistic pro-war proclamation, Einstein was one of the few to sign a coun terproclamation calling for peace. Einstein was then working on the ap plication of his theory of relativity to the more general case of accelerated systems and in so doing worked out a new theory of gravitation of which Newton's classic theory was but a special case. He pub lished it in 1915 in another tremendous paper usually referred to as the “General Theory of Relativity.” The equations set up in this theory allowed grand conclu sions to be drawn about the universe as a whole and Sitter [1004] was to use those equations to better effect than Einstein himself. In the general theory, Einstein pointed out three places where his theory pre dicted effects that were not like those predicted by Newton’s theory. The phe nomena concerned could be measured and in that way a decision between the two theories could be reached. First, Einstein’s theory allowed for a shift of the position of the perihelion of a planet, a shift that Newton’s theory did not allow. Only in the case of Mercury (closest to the sun and its gravitational influence) was the difference large enough to be noticeable. And, as a mat ter of fact, the motion that Leverrier [564] had detected and tried to explain by supposing the existence of an infra Mercurian planet, was explained on the spot by Einstein’s theory. This, however, was not so impressive as it might be since Einstein knew about the discrep ancy of Mercury’s motion to begin with and could have “aimed” his theory at it. Secondly, however, Einstein pointed out that light in an intense gravitational field should show a red shift. This had never been looked for or observed so the coast was clear for a fair test. Only ex treme gravitational fields could show a shift large enough to measure at the time and, at Eddington’s [1085] suggestion, W. S. Adams [1045] demonstrated the existence of this Einstein shift in the case of the white-dwarf companion of Sirius, which had the intensest gravitational field then known. (In the 1960s, with improvement in measuring devices, the much smaller Einstein shift of the light of our own sun was measured and found to match Ein stein’s prediction. In addition, the shift in gamma-ray wavelength, worked out by Mossbauer [1483] in the late 1950s, was essentially an Einstein shift and it too has been measured and found to be in accord with the prediction.) Thirdly and most dramatically, Ein stein showed that light would be deflected by a gravitational field much more than Newton predicted. There was no way of testing this in the midst of World War I. However, with the war over (and Germany, but not Einstein, defeated) the opportunity arose on March 29, 1919, when a solar eclipse was scheduled to take place at just the time when more bright stars were in the vicinity of the eclipsed sun than would be there at any other time of year. The Royal Astronomical Society of London made ready for two expeditions, Download 17.33 Mb. Do'stlaringiz bilan baham: |
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