The 100 Most Influential Scientists of All Time (33 page)

During the academic year 1926–27, Werner Heisenberg served as Bohr's assistant in Copenhagen, where he formulated the fundamental uncertainty principle as a consequence of quantum mechanics. Bohr, Heisenberg, and a few others then went on to develop what came to be known as the Copenhagen interpretation of quantum mechanics, which still provides a conceptual basis for the theory. A central element of the Copenhagen interpretation is Bohr's complementarity principle, presented for the first time in 1927 at a conference in Como, Italy.

According to complementarity, on the atomic level a physical phenomenon expresses itself differently depending on the experimental setup used to observe it. Thus, light appears sometimes as waves and sometimes as particles. For a complete explanation, both aspects, which according to classical physics are contradictory, need to be taken into account. The other towering figure of physics in the 20th century, Albert Einstein, never accepted the Copenhagen interpretation, famously declaring against its probabilistic implications that “God does not play dice.” The discussions between Bohr and Einstein, especially at two of the renowned series of Solvay Conferences in physics, in 1927 and 1930, constitute one of the most fundamental and inspired discussions between physicists in the 20th century. For the rest of his life, Bohr worked to generalize complementarity as a guiding idea applying far beyond physics.

In the early 1930s Bohr, together with Hevesy and the Danish physiologist August Krogh, applied for support from the Rockefeller Foundation to build a cyclotron—a kind of particle accelerator recently invented by Ernest O. Lawrence in the United States—as a means to pursue biological studies. Although Bohr intended to use the cyclotron primarily for investigations in nuclear physics, it could also produce isotopes of elements involved in organic processes, making it possible in particular to extend the radioactive indicator method, invented and promoted by Hevesy, to biological purposes.

After the German physicists Otto Hahn and Fritz Strassmann in late 1938 had made the unexpected and
unexplained experimental discovery that a uranium atom can be split in two approximately equal halves when bombarded with neutrons, a theoretical explanation based on Bohr's recently proposed theory of the compound nucleus was suggested by two Austrian physicists close to Bohr—Lise Meitner and her nephew Otto Robert Frisch; the explanation was soon confirmed in experiments by Meitner and Frisch at the institute. By this time, at the beginning of 1939, Bohr was in the United States, where a fierce race to confirm experimentally the so-called fission of the nucleus began after the news of the German experiments and their explanation had become known. In the United States, Bohr did pathbreaking work with his younger American colleague John Archibald Wheeler at Princeton University to explain fission theoretically.

After the discovery of fission, Bohr was acutely aware of the theoretical possibility of making an atomic bomb. In early 1943 Bohr received a secret message from his British colleague James Chadwick, inviting Bohr to join him in England to do important scientific work. Although Chadwick's letter was vaguely formulated, Bohr understood immediately that the work had to do with developing an atomic bomb. Convinced of the infeasibility of such a project, Bohr answered that there was greater need for him in occupied Denmark.

In the fall of 1943, the political situation in Denmark changed dramatically after the Danish government's collaboration with the German occupiers broke down. After being warned about his imminent arrest, Bohr escaped by boat with his family across the narrow sound to Sweden. In Stockholm the invitation to England was repeated, and
Bohr was brought by a military airplane to Scotland and then on to London.

Upon being briefed about the state of the Allied atomic bomb project on his arrival in London, Bohr changed his mind immediately about its feasibility. Concerned about a corresponding project being pursued in Germany, Bohr willingly joined the Allied project. Taking part for several weeks at a time in the work in Los Alamos, N.M., to develop the atomic bomb, he made significant technical contributions, notably to the design of the so-called initiator for the plutonium bomb. His most important role, however, was to serve, in J. Robert Oppenheimer's words, “as a scientific father confessor to the younger men.”

Bohr was allowed to return home only after the atomic bomb had been dropped on Japan in August 1945. He later took part in the establishment of CERN, the European experimental particle physics facility near Geneva, Switz., as well as of the Nordic Institute for Atomic Physics (Nordita) adjacent to his institute.

(b. Aug. 12, 1887, Vienna, Austria—d. Jan. 4, 1961, Vienna)

A
ustrian theoretical physicist Erwin Schrödinger contributed to the wave theory of matter and to other fundamentals of quantum mechanics. He shared the 1933 Nobel Prize for Physics with the British physicist P.A.M. Dirac.

Schrödinger entered the University of Vienna in 1906 and obtained his doctorate in 1910, upon which he accepted a research post at the university's Second Physics Institute. He saw military service in World War I and then went to the University of Zürich in 1921, where he remained for the next six years. There, in a six-month period in 1926, at
the age of 39, a remarkably late age for original work by theoretical physicists, he produced the papers that gave the foundations of quantum wave mechanics. In those papers he described his partial differential equation that is the basic equation of quantum mechanics and bears the same relation to the mechanics of the atom as Newton's equations of motion bear to planetary astronomy.

Adopting a proposal made by Louis de Broglie in 1924 that particles of matter have a dual nature and in some situations act like waves, Schrödinger introduced a theory describing the behaviour of such a system by a wave equation that is now known as the Schrödinger equation. The solutions to Schrödinger's equation, unlike the solutions to Newton's equations, are wave functions that can only be related to the probable occurrence of physical events. The definite and readily visualized sequence of events of the planetary orbits of Newton is, in quantum mechanics, replaced by the more abstract notion of probability. (This aspect of the quantum theory made Schrödinger and several other physicists profoundly unhappy, and he devoted much of his later life to formulating philosophical objections to the generally accepted interpretation of the theory that he had done so much to create.)

In 1927 Schrödinger accepted an invitation to succeed Max Planck, the inventor of the quantum hypothesis, at the University of Berlin, and he joined an extremely distinguished faculty that included Albert Einstein. He remained at the university until 1933, at which time he reached the decision that he could no longer live in a country in which the persecution of Jews had become a national policy. He then began a seven-year odyssey that took him to Austria, Great Britain, Belgium, the Pontifical Academy of Science in Rome, and—finally in 1940—the Dublin Institute for Advanced Studies, founded under the
influence of Premier Eamon de Valera, who had been a mathematician before turning to politics. Schrödinger remained in Ireland for the next 15 years, doing research both in physics and in the philosophy and history of science. During this period he wrote
What Is Life?
(1944), an attempt to show how quantum physics can be used to explain the stability of genetic structure. Although much of what Schrödinger had to say in this book has been modified and amplified by later developments in molecular biology, his book remains one of the most useful and profound introductions to the subject. In 1956 Schrödinger retired and returned to Vienna as professor emeritus at the university.

Of all of the physicists of his generation, Schrödinger stands out because of his extraordinary intellectual versatility. He was at home in the philosophy and literature of all of the Western languages, and his popular scientific writing in English, which he had learned as a child, is among the best of its kind. His study of ancient Greek science and philosophy, summarized in his
Nature and the Greeks
(1954), gave him both an admiration for the Greek invention of the scientific view of the world and a skepticism toward the relevance of science as a unique tool with which to unravel the ultimate mysteries of human existence. Schrödinger's own metaphysical outlook, as expressed in his last book,
Meine Weltansicht
(1961;
My View of the World
), closely paralleled the mysticism of the Vedānta.

Because of his exceptional gifts, Schrödinger was able in the course of his life to make significant contributions to nearly all branches of science and philosophy, an almost unique accomplishment at a time when the trend was toward increasing technical specialization in these disciplines.

(b. July 22, 1888, Priluka, Ukraine, Russian Empire [now Pryluky, Ukraine]—d. Aug. 16, 1973, Hyannis, Mass., U.S.)

U
krainian-born American biochemist Selman Abraham Waksman was one of the world's foremost authorities on soil microbiology. After the discovery of penicillin, he played a major role in initiating a calculated, systematic search for antibiotics among microbes. His consequent codiscovery of the antibiotic streptomycin, the first specific agent effective in the treatment of tuberculosis, brought him the 1952 Nobel Prize for Physiology or Medicine.

A naturalized U.S. citizen (1916), Waksman spent most of his career at Rutgers University, New Brunswick, New Jersey, where he served as professor of soil microbiology (1930–40), professor of microbiology and chairman of the department (1940–58), and director of the Rutgers Institute of Microbiology (1949–58). During his extensive study of the actinomycetes (filamentous, bacteria-like microorganisms found in the soil), he extracted from them antibiotics (a term he coined in 1941) valuable for their killing effect not only on gram-positive bacteria, against which penicillin is effective, but also on gram-negative bacteria, of which the tubercle bacillus (
Mycobacterium tuberculosis
) is one.

In 1940 Waksman, along with Albert Schatz and Elizabeth Bugie, isolated actinomycin from soil bacteria but found it to be extremely toxic when given to test animals. Three years later they extracted the relatively nontoxic streptomycin from the actinomycete
Streptomyces griseus
and found that it exercised repressive influence on tuberculosis. In combination with other chemotherapeutic agents, streptomycin has become a major factor in
controlling the disease. Waksman also isolated and developed several other antibiotics, including neomycin, that have been used in treating many infectious diseases of humans, domestic animals, and plants.

Among Waksman's books are
Principles of Soil Microbiology
(1927), regarded as one of the most exhaustive works on the subject, and
My Life with the Microbes
(1954), an autobiography.

(b. Nov. 20, 1889, Marshfield, Mo., U.S.—d. Sept. 28, 1953, San Marino, Calif.)

A
merican astronomer Edwin Powell Hubble is considered the founder of extragalactic astronomy. He provided the first evidence of the expansion of the universe.

Hubble's interest in astronomy flowered at the University of Chicago, where he was inspired by the astronomer George E. Hale. At Chicago, Hubble earned both an undergraduate degree in mathematics and astronomy (1910) and a reputation as a fine boxer. Upon graduation, however, Hubble turned away from both astronomy and athletics, preferring to study law as a Rhodes Scholar at the University of Oxford (B.A., 1912). He joined the Kentucky bar in 1913 but dissolved his practice soon after, finding himself bored with law. A man of many talents, he finally chose to focus them on astronomy, returning to the University of Chicago and its Yerkes Observatory in Wisconsin. After earning a Ph.D. in astronomy (1917) and serving in World War I, Hubble settled down to work at the Mount Wilson Observatory near Pasadena, Calif., and began to make discoveries concerning extragalactic phenomena.

While at Mount Wilson, Hubble discovered (1922–24) that not all nebulae in the sky are part of the Milky Way Galaxy, the vast star system to which the Sun belongs. He found that certain nebulae contain stars called Cepheid variables, for which a correlation was already known to exist between periodicity and absolute magnitude. Using the further relationship among distance, apparent magnitude, and absolute magnitude, Hubble determined that these Cepheids are several hundred thousand light-years away and thus outside the Milky Way system and that the nebulae in which they are located are actually galaxies distinct from the Milky Way. This discovery, announced in 1924, forced astronomers to revise their ideas about the cosmos.