Hans Christian Oersted (1777-1851), Danish physicist and chemist, born in Rudköbing, and educated at the University of Copenhagen. He was appointed professor of physics at the University of Copenhagen in 1806. In 1819 he discovered that a magnetic needle is deflected at right angles to a wire carrying an electric current, thus initiating the study of electromagnetism. He was also the first to isolate (1825) aluminum. His Manual of Mechanical Physics appeared in 1844.
A Nobel Prize winner, Niels Bohr was known not only for his own theoretical work, but also as a mentor to younger physicists who themselves made important contributions to physical theory. As the director of the Institute for Theoretical Physics at the University of Copenhagen, Bohr gathered together some of the finest minds in the physics community, such as Werner Heisenberg and George Gawow. During the 1920’s, the Institute was the source of many important works in quantum mechanics, and theoretical physics in general.
Niels Bohr (1885-1962), Danish physicist and Nobel laureate, who made basic contributions to nuclear physics and the understanding of atomic structure.
Bohr was born in Copenhagen, the son of a physiology professor, and was educated at the University of Copenhagen, where he earned his doctorate in 1911. That same year he went to Cambridge, England, to study nuclear physics under British physicist Sir Joseph John Thomson, but he soon moved to Manchester to work with another British physicist, Ernest Rutherford.
Bohr’s theory of atomic structure (see Quantum Theory), for which he received the Nobel Prize in physics in 1922, was published in papers between 1913 and 1915. His work drew on Rutherford’s nuclear model of the atom, in which the atom is seen as a compact nucleus surrounded by a swarm of much lighter electrons (see Atom). Bohr’s atomic model made use of quantum theory and the Planck constant (the ratio between quantum size and radiation frequency). The model posits that an atom emits electromagnetic radiation only when an electron in the atom jumps from one quantum level to another. This model contributed enormously to future developments of theoretical atomic physics.
In 1916 Bohr returned to the University of Copenhagen as a professor of physics, and in 1920 he was made director of the university’s newly formed Institute for Theoretical Physics. There Bohr developed a theory relating quantum numbers to large systems that follow classical laws, and made other major contributions to theoretical physics. His work helped lead to the concept that electrons exist in shells and that the electrons in the outermost shell determine an atom’s chemical properties. He also served as a visiting professor at many universities.
In 1939, recognizing the significance of the fission experiments (see Nuclear Energy: Nuclear Energy from Fission) of German scientists Otto Hahn and Fritz Strassmann, Bohr convinced physicists at a scientific conference in the United States of the importance of those experiments. He later demonstrated that uranium-235 is the particular isotope of uranium that undergoes nuclear fission. Bohr then returned to Denmark, where he was forced to remain after the German occupation of the country in 1940. Eventually, however, he was persuaded to escape to Sweden, under peril of his life and that of his family. From Sweden the Bohrs traveled to England and eventually to the United States, where Bohr joined in the effort to develop the first atomic bomb, working at Los Alamos, New Mexico, until the first bomb’s detonation in 1945. He opposed complete secrecy of the project, however, and feared the consequences of this ominous new development. He desired international control.
In 1945 Bohr returned to the University of Copenhagen, where he immediately began working to develop peaceful uses for atomic energy. He organized the first Atoms for Peace Conference in Geneva, held in 1955, and two years later he received the first Atoms for Peace Award. In 1997 the International Union of Pure and Applied Chemistry announced that the chemical element with the atomic number 107 would be given the official name bohrium (Bh), in honor of Niels Bohr.
Canadian physicist Bertram Brockhouse won the Nobel Prize in physics in 1994. Brockhouse used neutron scattering to study atomic structure and movement.
Bertram N. Brockhouse (1918-2003), Canadian physicist and Nobel Prize winner. He helped to develop the technique of neutron scattering to study atomic structure and movement in various materials. He shared the 1994 Nobel Prize in physics with American physicist Clifford G. Shull for this work.
Brockhouse attended the University of British Columbia in Vancouver and received his Ph.D. degree from the University of Toronto in 1950. For more than a decade, he conducted nuclear-reactor research at the Chalk River Laboratories in Ontario. In 1962 he joined the faculty of McMaster University, also in Ontario, and remained there until retirement. He became professor emeritus in 1984.
Building on the work of Clifford Shull and other scientists who first developed neutron-scattering techniques, Brockhouse designed a new instrument for neutron-scattering research. Neutron scattering is based on the fact that beams of neutrons can pass readily into a substance, much as X rays can. The neutrons are scattered, or diffracted, by the atoms in the substance. By measuring the neutron change, scientists can gather information about a substance's atomic structure. Brockhouse designed an instrument called a triple-axis spectrometer, which can measure the energy and momentum of the neutrons as they enter and leave the sample substance. This allowed him to gather data about vibrations and other movements of the atoms within the substance, ultimately providing information about the substance's physical properties. His instrument is still widely used in neutron-scattering studies, which have applications in biology, chemistry, materials science, and engineering. Polymers, semiconductors, and superconductors are just three of the many materials analyzed and developed with the help of neutron-scattering techniques. Brockhouse's work is also credited with helping to form the basis of modern solid-state physics, also known as condensed-matter physics.
A pioneer in the area of quantum theory, Erwin Schrödinger is best known for his mathematical theory describing the wave mechanics of electrons. He and British physicist Paul Dirac shared the 1933 Nobel Prize in physics for their contributions to the understanding of quantum mechanics. Erwin Schrödinger (1887-1961), Austrian physicist and Nobel laureate. Schrödinger formulated the theory of wave mechanics, which describes the behavior of the tiny particles that make up matter in terms of waves. Schrödinger formulated the Schrödinger wave equation to describe the behavior of electrons (tiny, negatively charged particles) in atoms. For this achievement, he was awarded the 1933 Nobel Prize in physics with British physicist Paul Dirac. See also Quantum Theory; Atom. Schrödinger was born in Vienna, Austria. His father was an oilcloth manufacturer who had studied chemistry, and his mother was the daughter of a chemistry professor. He attended an elementary school in Innsbruck for a few weeks, but Schrödinger received most of his early education from a private tutor. In 1898 he entered the Gymnasium in Vienna, where he studied mathematics, physics, and ancient languages. He then attended the University of Vienna from 1906 to 1910, specializing in physics. Schrödinger obtained his doctoral degree in physics in 1910. After a year in military training, he returned to the university to teach a first-year physics laboratory class. His early research ranged over many topics in experimental and theoretical physics. During World War I (1914-1918) Schrödinger served as an artillery officer and then returned to his previous post at Vienna. Conditions were difficult in Austria after the war, and in 1920 Schrödinger decided to go to Germany. After a series of short-lived posts at the University of Jena, Stuttgart University, and the University of Breslau (now Wroclaw, Poland) in 1920 and 1921, he became a professor of physics at the University of Zürich in Switzerland in 1921. Schrödinger's most important work was done at Zürich, and his work received much attention. He succeeded German physicist Max Planck as professor of theoretical physics at the University of Berlin in 1927. Schrödinger remained there until the rise of the National Socialist (Nazi) movement in 1933, when he went to the University of Oxford in England. There he became a fellow of Magdalen College. Homesick, he returned to Austria in 1936 to take up a post at Graz University, but the Nazi takeover of Austria in 1938 placed Schrödinger in danger. Schrödinger was not Jewish, but his opposition to Nazi policies made him a potential target. The prime minister of Ireland, Eamon de Valera, helped Schrödinger get out of Austria. De Valera’s help also led to an appointment to a post at the Institute for Advanced Studies in Dublin in 1939. Schrödinger continued work in theoretical physics in Dublin until 1956, when he returned to Austria to a chair at the University of Vienna. He stayed at the University of Vienna until his death. Schrödinger's great discovery of wave mechanics originated with the work of French physicist Louis de Broglie. In 1923 de Broglie used ideas from German American physicist Albert Eintstein’s special theory of relativity to show that an electron, or any other particle, has a wave associated with it (see Albert Einstein). De Broglie’s work resulted in the equation ? = h/p, where ? is the wavelength of the associated wave, h is a number called Planck's constant, and p is the momentum of the particle. Physicists immediately deduced that if particles (particularly electrons) have waves, then a particular type of partial differential equation known as a wave equation should be able to describe their behavior. These ideas were taken up by both de Broglie and Schrödinger, and in 1926 each published the same wave equation. Unfortunately, while the equation is true, it was of very little help in explaining the behavior of particles. Later the same year Schrödinger used a new approach. He studied the mathematics of partial differential equations and the Hamiltonian function, a powerful idea in mechanics developed by British mathematician Sir William Rowan Hamilton in the mid-1800s. Schrödinger formulated an equation in terms of the energy of the electron and the energy of the electric field in which it was situated. Partial differential equations have many solutions, but solutions to Schrödinger’s equation had to meet strict conditions to be useful in describing the electron. Among other things, they had to be finite and possess only one value. These solutions were associated with special values of the electron’s energy level, known as proper values or eigenvalues. Schrödinger solved the equation for the hydrogen atom, V = –e2/r, in which V is the energy of the electric field surrounding the electron, e is the electron's charge, and r is its distance from the atom’s nucleus. He found that the eigenvalues of the electron’s energy corresponded with those of the energy levels given in the older theory of Danish physicist Niels Bohr. Bohr’s theory of the atom described electrons orbiting atoms in strict circular orbits at particular distances that corresponded to specific levels of energy. In the hydrogen atom (which consists of one electron and one proton), the wave function Schrödinger derived instead describes where physicists are most likely to find the electron. The electron is most likely to be where Bohr predicted it to be, but it does not follow a strictly circular orbit. The electron is described by the more complicated notion of an orbital—a region in space where the electron has varying degrees of probability of being found. Schrödinger's wave equation can describe atoms other than hydrogen as well as molecules and ions (atoms or molecules with electric charge), but such cases are very difficult to solve. In a few such cases physicists have found approximate solutions, usually with a computer carrying out the numerical work. Schrödinger's mathematical description of electron waves found immediate acceptance. The mathematical description matched what scientists had learned about electrons by observing them and their effects. In 1925, a year before Schrödinger published his results, German-British physicist Max Born and German physicist Werner Heisenberg developed a mathematical system called matrix mechanics. Matrix mechanics also succeeded in describing the structure of the atom, but it was totally theoretical. It gave no picture of the atom that physicists could verify observationally. Schrödinger's vindication of de Broglie's idea of electron waves immediately overturned matrix mechanics, though later physicists showed that wave mechanics is equivalent to matrix mechanics. During his later years Schrödinger became increasingly worried by the uncertain nature of quantum mechanics, of which wave mechanics is a part. Schrödinger believed he had produced a defining description of the atom in the same way that the three laws of English physicist Isaac Newton defined classical mechanics and the way that the equations of British physicist James Clerk Maxwell described electrodynamics. Instead, each new discovery about the structure of the atom only made atomic structure more complicated. Much of Schrödinger’s later work was concerned with philosophy, particularly as applied to physics and the atom.
Austrian-born Swedish physicist Lise Meitner is best known for her work in atomic physics. Meitner published the first paper on nuclear fission, in which atoms of an element split apart to produce energy and atoms of different elements. She also contributed important work to the study of radioactivity and atomic theory.
Lise Meitner (1878-1968), Austrian-Swedish physicist, who first identified nuclear fission. She was born in Vienna, Austria, and educated at the Universities of Vienna and Berlin. In 1918, in association with German physical chemist Otto Hahn, she helped discover the element protactinium. From 1926 to 1933 she was a professor of physics at the University of Berlin in Germany. In 1938 restrictions against Jews imposed by the Nazi regime led Meitner to leave Germany. She ended up in Sweden, joining the atomic research staff at the University of Stockholm. In 1939 Meitner and her nephew, the British physicist Otto Robert Frisch, published the first paper to provide a theoretical explanation for the splitting of the atom and named the process fission (see Nuclear Energy). In 1946 she was a visiting professor at Catholic University in Washington, D.C., and in 1959 she revisited the United States to lecture at Bryn Mawr College. In 1997 the International Union of Pure and Applied Chemistry announced that the chemical element with the atomic number 109 would be given the official name meitnerium (Mt) in her honor.
The writings of Austrian physicist and philosopher Ernst Mach did much to establish a scientific methodology that paved the way for the theory of relativity. His outstanding work in ballistics contributed greatly to the theory of flight.
Ernst Mach (1838-1916), Austrian physicist and philosopher, born in Turany (now in the Czech Republic), and educated at Vienna University. He served as professor successively at the universities of Graz, Prague, and Vienna from 1864 to 1901, when he retired from academic life. Mach felt that science should restrict itself to the description of phenomena that could be perceived by the senses. His writings did much to free science from metaphysical concepts and helped to establish a scientific methodology that paved the way for the theory of relativity. He studied the psychological phenomena of sensation and perceptions, and his outstanding work in ballistics contributed greatly to the theory of flight. The Mach number, which represents the ratio of the speed of an object to the speed of sound in the atmosphere, was named for him. See Aerodynamics; Fluid Mechanics.
Austrian-American physicist Victor Hess won the 1936 Nobel Prize in physics. He won the award for his research into cosmic rays.
Victor Franz Hess (1883-1964), Austrian-American physicist and Nobel laureate, one of the earliest workers in the field of cosmic rays. He taught in both Austria and the United States. As early as 1911 he measured cosmic-ray activity at altitudes as great as 9000 m (30,000 ft). He shared the 1936 Nobel Prize in physics with the American physicist Carl David Anderson. Hess wrote Conductivity of the Atmosphere (1928) and Cosmic Rays and Their Biological Effects (1949).
Christian Doppler (1803-53), Austrian physicist and mathematician. Christian Johann Doppler was born in Salzburg and educated there and in Vienna. He was a professor successively at the Technical Institute at Prague and at the Polytechnicum of Vienna, and he became director of the Physical Institute of Vienna University in 1850. He described the physical phenomenon now known as the Doppler effect in his monograph on the color effect of double stars, Über das farbige Licht der Doppelsterne (1842).
Austrian physicist Ludwig Boltzmann made advancements in statistical mechanics, a field of physics that uses average values and probabilities to describe systems made up of many bodies. Statistical mechanics is especially useful in the study of how temperature and pressure affect gases.
Ludwig Boltzmann (1844-1906), Austrian physicist, who helped lay the foundation for the field of physics known as statistical mechanics. Boltzmann was born in Vienna and educated at the universities of Vienna and Oxford. He was a professor of physics at various German and Austrian universities for more than 40 years. During the 1870s Boltzmann published a series of papers that showed that the second law of thermodynamics could be explained by statistically analyzing the motions of atoms. In these papers Boltzmann utilized the central principle of statistical mechanics: that large-scale, visible phenomena, such as the second law of thermodynamics, can be explained by statistically examining the microscopic properties of a system, such as the motions of atoms. Boltzmann also formulated the law of thermal radiation, named for him and the Austrian physicist Josef Stefan. The Stefan-Boltzmann law states that the total radiation from a blackbody, which is an ideal surface that absorbs all radiant energy that strikes it, is proportional to the fourth power of the absolute temperature of the blackbody. Boltzmann also made important contributions to the kinetic theory of gases. Boltzmann's work was strongly attacked by scientists of his time. However, much of Boltzmann's work was substantiated by experimental data soon after he committed suicide in 1906.
American theoretical physicist Val Fitch won the Nobel Prize in physics in 1980. Fitch showed that the K-mesons resulting from proton collisions did not obey the absolute principle of symmetry.
Val Logsdon Fitch, born in 1923, American experimental physicist and Nobel Prize winner. Fitch is noted for his studies of subatomic particles and for the discovery that one of the elementary particles, the neutral K-meson (now called the kaon), does not always follow the principles of symmetry, which were once thought to be universal traits. For their discovery of the violations of fundamental symmetry principles in the decay of neutral K-meson particles, Fitch and American physicist James Watson Cronin together were awarded the 1980 Nobel Prize in physics.
Fitch was born on a cattle ranch in Cherry County, Nebraska. His career as a physicist began during World War II (1939-1945) when, as a United States soldier, he was stationed in Los Alamos, New Mexico, to work on the Manhattan Project to design and build the atomic bomb. He developed excellent research skills and worked with future Nobel Prize winners, although he still had to complete his undergraduate degree, which he did in 1948 at McGill University in Montréal, Québec, Canada. Fitch earned his Ph.D. degree in 1954 at Columbia University in New York City. That same year, he joined the faculty at Princeton University in New Jersey, where he remained throughout his career.
At Princeton, Fitch teamed with Cronin to explore the characteristics and behavior of subatomic particles. Until the 1950s physicists believed that there was perfect balance, or symmetry, between matter and antimatter. This symmetry was described as CP, both a balance of positive and negative electrical charges (C) and parity, or equality, between left-handed and right-handed orientation (P). Fitch and Cronin found that this is not always true. In 1964 they observed that on rare occasions, the decay of neutral K-meson particles violates CP symmetry. The scientists modified the CP symmetry principle and confirmed CPT (charge, parity, time-reversal) symmetry, where there is a balance between matter and antimatter moving forward and backward in time, respectively.
Fitch and Cronin's research forced physicists to reexamine a number of theories. In particular, their discovery may explain the formation of sufficient matter to create our universe following the theoretical explosion known as the big bang. Without the violation of CP symmetry, matter and antimatter would have canceled each other out, and the big bang would have produced only gamma radiation, not our known universe.
American physicist Richard Feynman was well known for both his contributions to quantum electrodynamics and his enthusiastic teaching methods. Feynman reformulated quantum electrodynamic theory, which concerns the interactions between electromagnetic waves and matter. He is pictured here after winning the 1965 Nobel Prize in physics, which he shared with American physicist Julian S. Schwinger and Japanese physicist Tomonaga Shin’ichiro.
Richard Feynman (1918–1988), American physicist and Nobel laureate. Feynman shared the 1965 Nobel Prize in physics for his role in the development of the theory of quantum electrodynamics, the study of the interaction of light with atoms and their electrons. He also made important contributions to the theory of quarks (particles that make up elementary particles such as protons and electrons) and superfluidity (a state of matter in which a substance flows with no resistance). He created a method of mapping out interactions between elementary particles that became a standard way of representing particle interactions and is now known as Feynman diagrams. Feynman was a noted teacher, a notorious practical joker, and one of the most colorful characters in physics.
Richard Phillips Feynman was born in New York City. As a child he was fascinated by mathematics and electronics and became known in his neighborhood as “the boy who fixes radios by thinking.” He graduated with a bachelor’s degree in physics from the Massachusetts Institute of Technology (MIT) in 1939 and obtained a Ph.D. degree in physics from Princeton University in 1942. His advisor was John Wheeler, and his thesis, “A Principle of Least Action in Quantum Mechanics,” was typical of his use of basic principles to solve fundamental problems.
During World War II (1939-1945) Feynman worked at what would become Los Alamos National Laboratory in central New Mexico, where the first nuclear weapons were being designed and tested. Feynman was in charge of a group responsible for problems involving large-scale computations (carried out by hand or with rudimentary calculators) to predict the behavior of neutrons in atomic explosions.
After the war Feynman moved to Cornell University, where German-born American physicist Hans Bethe was building an impressive school of theoretical physicists. Feynman continued developing his own approach to quantum electrodynamics (QED) at Cornell and then at the California Institute of Technology (Caltech), where he moved in 1950.
Feynman shared the 1965 Nobel Prize in physics with American physicist Julian Schwinger and Japanese physicist Tomonaga Shin’ichiro for his work on QED. Each of the three had independently developed methods for calculating the interaction between electrons, positrons (particles with the same mass as electrons but opposite in charge) and photons (packets of light energy). The three approaches were fundamentally the same, and QED remains the most accurate physical theory known. In Feynman's space–time approach, he represented physical processes with collections of diagrams showing how particles moved from one point in space and time to another. Feynman had rules for calculating the probability associated with each diagram, and he added the probabilities of all the diagrams to give the probability of the physical process itself.
Feynman wrote only 37 research papers in his career (a remarkably small number for such a prolific researcher), but many consider the two discoveries he made at Caltech, superfluidity and the prediction of quarks, were also worthy of the Nobel Prize. Feynman developed the theory of superfluidity (the flow of a liquid without resistance) in liquid helium in the early 1950s. Feynman worked on the weak interaction, the strong force, and the composition of neutrons and protons later in the 1950s. The weak interaction is the force that causes slow nuclear reactions such as beta decay (the emission of electrons or positrons by radioactive substances). Feynman studied the weak interaction with American physicist Murray Gell-Mann. The strong force is the short-range force that holds the nucleus of an atom together. Feynman’s studies of the weak interaction and the strong force led him to believe that the proton and neutron were composed of even smaller particles. Both particles are now known to be composed of quarks.
The written version of a series of undergraduate lectures given by Feynman at Caltech, The Feynman Lectures on Physics (three volumes with Robert Leighton and Matthew Sands, 1963), quickly became a standard reference in physics. At the front of the lectures Feynman is shown indulging in one of his favorite pastimes, playing the bongo drum. Painting was another hobby. In 1986 Feynman was appointed to the Rogers Commission, which investigated the Challenger disaster—the explosion aboard the space shuttle Challenger that killed seven astronauts in 1986. In front of television cameras, he demonstrated how the failure of a rubber O-ring seal, caused by the cold, was responsible for the disaster. Feynman wrote several popular collections of anecdotes about his life, including “Surely You’re Joking Mr. Feynman” (with Ralph Leighton and Edward Hutchings, 1984) and What do YOU Care What Other People Think? (with Ralph Leighton, 1988).
Famous for producing the first controlled nuclear reaction in 1942, physicist Enrico Fermi worked as a consultant for the Manhattan Project during World War II, helping to design the atomic bomb. He won a Nobel Prize in 1938 for his work on artificial radioactivity. He inspired many students and continues to be honored through various awards and institutions that were established in his name, such as the Fermi National Accelerator Laboratory in Batavia, Illinois.
Enrico Fermi (1901-1954), Italian-born American physicist and Nobel Prize winner, who made important contributions to both theoretical and experimental physics. Fermi’s most well-known contribution was the demonstration of the first controlled atomic fission reaction. Atomic fission occurs when an atom splits apart (see Atom). Fermi was the first scientist to split an atom, although he misinterpreted his results for several years. He also had an important role in the development of fission for use as an energy source and as a weapon (see Nuclear Energy; see Atomic Bomb). He won the 1938 Nobel Prize in physics for his work in bombarding atoms with neutrons, subatomic particles with no electric charge. Initially, Fermi believed that this process created new chemical elements heavier than uranium (see Transuranium Elements), but other scientists showed that he actually split atoms to create fission reactions.
FERMI’S LIFE
Fermi was born in Rome, Italy. At age 17 he earned a scholarship to the prestigious Scuola Normale Superiore in Pisa by writing an essay on the characteristics of sound. He went on to the University of Pisa, where he earned his doctorate in 1922. Fermi studied with German physicist Max Born in Göttingen, Germany, from 1922 to 1924.
In 1924 Fermi returned to Italy to teach mathematics at the University of Florence. He became professor of theoretical physics at the University of Rome in 1927. He was 26 years old—the youngest professor in Italy since 16th-century Italian scientist Galileo. In the 1930s dictator Benito Mussolini introduced anti-Semitic laws to Italy and Fermi feared for the safety of his wife, who was Jewish. In 1938, after traveling to Sweden to accept the Nobel Prize, Fermi immigrated to the United States rather than return to Italy. Fermi became a professor at Columbia University in New York in 1939, and in 1941 moved to Chicago, Illinois, for a professorship at the University of Chicago. During World War II (1939-1945) he was involved in the Manhattan Project, the American effort to develop an atomic bomb. In 1945 Fermi became a U.S. citizen and returned to Chicago, where he remained until his death.
FERMI’S WORK
Fermi’s first important contributions to physics were theoretical. In 1926 he devised a method for calculating the behavior of a system composed of particles that obeyed the Pauli exclusion principle. The Pauli exclusion principle, developed by Austrian-born Swiss physicist Wolfgang Pauli, states that no two particles can have identical quantum numbers. Quantum numbers identify properties of a particle such as energy, angular momentum, magnetic properties, and spin, or direction of rotation. The method that Fermi developed became known as Fermi statistics, and the particles that obey the Pauli exclusion principle became known as fermions. Fermions include all three of the particles that make up atoms (electrons, protons, and neutrons) as well as many other particles. British physicist P. A. M. Dirac independently developed an equivalent theory with a different approach several months later.
In 1933 Fermi published a theory that explained beta decay, or the transformation of a neutron into a proton, an electron, and a neutrino. Neutrinos are neutral particles related to electrons. Beta decay is a form of radioactivity, a process in which particles in atoms release energy and other particles. Fermi’s explanation of beta decay introduced a fundamental force called the weak force, or weak nuclear interaction. Scientists recognized three fundamental forces of interactions at that time: The gravitational force controls interactions between masses, the electromagnetic force controls the interaction of electric charges, and the strong force controls the interaction of particles in the nucleus of an atom. The weak force is more obscure and removed from everyday experience than the other forces. It allows particles to change into other particles under certain circumstances.
Fermi then turned to experimental physics. In 1933 French physicists Irène Joliot-Curie and Frédéric Joliot-Curie had artificially produced radioactive elements by bombarding aluminum and boron with alpha particles. Radioactive elements are elements composed of atoms that decay, or easily release particles and energy. Alpha particles are the nuclei of helium atoms, which contain two protons and two neutrons. In 1934 Fermi showed that single neutrons were even more effective than alpha particles at creating radioactive elements and isotopes. Isotopes of an element are atoms that contain the same number of protons (the number of protons in an atom determines which element it is), but different numbers of neutrons. Fermi discovered that shooting neutrons through paraffin wax at a sample of atoms slowed the neutrons down and increased the intensity of the radioactivity. He bombarded uranium samples with these slow neutrons and interpreted the results as the creation of elements heavier than uranium, or transuranium elements. In 1938, however, Austrian-born Swedish physicist Lise Meitner and Austrian-born British physicist Otto Frisch proposed and confirmed a theory that the uranium atoms were actually splitting apart instead of forming heavier elements. Fermi won the 1938 Nobel Prize in physics for his work with neutrons and radioactivity.
Fermi and other scientists realized the potential power of fission, or the splitting of atoms. Atoms release energy in the form of heat and radiation when they split. Because fission is triggered by neutrons, and atoms release neutrons when they split, one fission reaction can start more reactions, creating a self-sustaining, or chain, reaction. The more fission reactions that occur, the more energy the system releases. In 1939 a group of physicists warned U.S. President Franklin D. Roosevelt that fission chain reactions could be used as weapons, and that Germany might be developing such a weapon—an atomic bomb. In 1942, the Manhattan Project, the American effort to develop an atomic bomb, officially began. By the end of the year Fermi had designed and presided over the first controlled fission reaction, which occurred in an unused squash court in the basement of Stagg Field at the University of Chicago. In July 1945 the United States tested the first atomic bomb, and in August of that year the United States dropped atomic bombs on two cities in Japan, Hiroshima and Nagasaki.
Fermi eventually returned to the University of Chicago and continued to research radioactivity and neutrons. He also consulted on the construction of the synchrocyclotron, a large particle accelerator at the University of Chicago, completed in 1951. Particle accelerators increase the speed of subatomic particles to allow scientists to study the particles at high energies. Fermi used the particle accelerator to study what happens to atoms when they break up under great force. In 1954 Fermi received the Atomic Energy Commission Award, which was later renamed the Fermi Award. In 1955, a year after his death, the element fermium was named in his honor.
Albert Einstein is considered one of the greatest and most popular scientists of all time. Three papers he published in 1905 were pivotal in the development of physics and, to a large degree, Western thought. These papers discussed the quantum nature of light, provided a description of molecular motion, and introduced the special theory of relativity. Einstein was famous for continually reexamining traditional scientific assumptions and coming to straightforward, elegant conclusions no one else had reached. He is less famous for his social involvement, although he was a staunch supporter of both pacifism and Zionism. Here, Einstein discusses Gandhi and commends nonviolence.
Albert Einstein (1879-1955), German-born American physicist and Nobel laureate, best known as the creator of the special and general theories of relativity and for his bold hypothesis concerning the particle nature of light. He is perhaps the most well-known scientist of the 20th century.
Einstein was born in Ulm on March 14, 1879, and spent his youth in Munich, where his family owned a small shop that manufactured electric machinery. He did not talk until the age of three, but even as a youth he showed a brilliant curiosity about nature and an ability to understand difficult mathematical concepts. At the age of 12 he taught himself Euclidean geometry.
Einstein hated the dull regimentation and unimaginative spirit of school in Munich. When repeated business failure led the family to leave Germany for Milan, Italy, Einstein, who was then 15 years old, used the opportunity to withdraw from the school. He spent a year with his parents in Milan, and when it became clear that he would have to make his own way in the world, he finished secondary school in Aarau, Switzerland, and entered the Swiss Federal Institute of Technology in Zürich. Einstein did not enjoy the methods of instruction there. He often cut classes and used the time to study physics on his own or to play his beloved violin. He passed his examinations and graduated in 1900 by studying the notes of a classmate. His professors did not think highly of him and would not recommend him for a university position.
For two years Einstein worked as a tutor and substitute teacher. In 1902 he secured a position as an examiner in the Swiss patent office in Bern. In 1903 he married Mileva Maric, who had been his classmate at the polytechnic. They had one daughter, who was born prior to their marriage and given up for adoption, and two sons. The couple eventually divorced, and Einstein later remarried.
Soviet physicist Igor Tamm won the 1958 Nobel Prize in physics. One of the great theoretical physicists, much of his ground-breaking research concerned the behavior of light, including his theoretical explanation of the Cherenkov effect. 
Igor Yevgenyevich Tamm (1895-1971), Soviet physicist and Nobel laureate, who based his work on the Einstein theory of relativity and on quantum mechanics. Tamm was born in Vladivostok and educated at Moscow University. Considered one of the outstanding theoretical physicists in the world, he developed (1924-30) the quantum theory of acoustical vibrations and the scattering of light in solid bodies, as well as the theory of interactions of light with electrons. In 1933 he theorized on the existence of surface states (Tamm's levels) of electrons in semiconductors. In 1937 he and Ilya Frank worked out a theoretical interpretation for the Cherenkov effect. Tamm suggested (1950) the use of electric charges in ionized gases as a means of obtaining controlled thermonuclear power. For their work on the Cherenkov effect, Tamm, Frank, and Pavel Cherenkov shared the 1958 Nobel Prize in physics.
Vitaly Ginzburg, born in 1916, Russian physicist and cowinner of the 2003 Nobel Prize in physics for his theories on the properties of superconductors (metals and other substances that conduct electricity without resistance). Although originally proposed in the 1950s, the theories of Ginzburg and his colleagues continue to anchor modern advances in superconducting technology. The magnetic resonance imaging (MRI) machines widely used in medical diagnosis and the high-energy accelerators used by physicists to hunt for subatomic particles are just two of the applications that owe at least a partial debt to Ginzburg’s theories.
Born in Moscow, Russia, Ginzburg received his doctoral degree in physics from Moscow State University in 1938. He began working at the P. N. Lebedev Physical Institute in Moscow in 1940 and became the head of its Theory Group. He remained at the institute for the duration of his career.
By the early 1950s, scientists were familiar with the phenomenon of superconductivity. This is the capacity of some compounds, when cooled to temperatures near absolute zero, -273.15°C (-459.67°F), to carry electricity with none of the resistance seen in conventional conductors. At the time, however, theoretical explanations for superconductivity were still lacking. Ginzburg, with colleague Lev Landau, who won the Nobel Prize in physics in 1962, proposed key theories to explain the relation between electrons and the magnetic field inside superconductors.
Ginzburg and Landau noted that some superconductors repel a magnetic field and they termed this class of superconductor Type I. Ginzburg also theorized, however, that other types of superconductors would be able to function in the presence of a magnetic field under certain conditions. Ultimately, Russian-born physicist Alexei A. Abrikosov, building on the Ginzburg-Landau theories, described more fully this second class of superconductor, now called Type II. Modern superconductors, including the ceramic varieties that function at higher temperatures, are Type II. Ongoing research in superconductivity is expected to find application in the design of generators and engines, in improved transmission of electrical power over long distances, and other uses.
Ginzburg shared his 2003 Nobel Prize with Abrikosov and with Anthony J. Leggett, a British-born American physicist who made essential theoretical breakthroughs in describing a related phenomenon known as superfluidity.
Swiss physicist Heinrich Rohrer won the 1986 Nobel Prize in physics. Rohrer shared credit for inventing the scanning tunneling microscope, a powerful microscope capable of producing three-dimensional images of materials at the atomic level.
Heinrich Rohrer, born in 1933, Swiss physicist and co-winner of the 1986 Nobel Prize for physics for his invention of the scanning tunneling microscope, a new type of powerful microscope capable of detecting images at the atomic level. Rohrer shared the prize with German physicists Gerd Karl Binnig and Ernst August Friedrich Ruska.
Rohrer was born in Buchs, Sankt Gallen, Switzerland, and studied at the Swiss Federal Institute of Technology, where he earned his Ph.D. degree in 1960. He joined the IBM Research Laboratory near Zürich, Switzerland, in 1963. There, he and Binnig turned their attention to an experiment that required the study of a microscopic surface. In so doing they created a new type of microscope.
The scanning tunneling microscope is based on the wavelike property of electrons. The microscope has a sharp probe that moves near the sample's surface in a vacuum, and emits electrons. The electrons tunnel, or flow through the vacuum, from the probe's tip to the sample's surface. The microscope records any change in distance between the probe and the sample—even as small as the diameter of a single atom. By moving the probe in a sweeping motion, a three-dimensional image of the sample can be produced. This image shows detail not possible with any other kind of microscope. It can reveal the surface of a material at the atomic level and also provide information about its atomic composition. The scanning tunneling microscope has been used to study biological samples, to analyze industrial materials (such as superconductors), and to test miniaturized electronic circuits.
Swiss oceanographic engineer Auguste Piccard was interested in deep-sea exploration and became known for his invention of an underwater vessel called the bathyscaphe. The bathyscaphe was designed to operate like a submarine and could reach great depths.
Auguste Piccard (1884-1962), Swiss physicist, known for his exploration of the stratosphere and the deep sea. He was born in Basel, Switzerland, and educated at the Federal Polytechnic School. He became professor of physics at the University of Brussels in 1922. In 1931 he attracted worldwide attention by making the first balloon ascension into the stratosphere, reaching an altitude of 15,787 m (51,793 ft), a new world record. During this flight Piccard acquired valuable information regarding the intensity of cosmic rays in the stratosphere; he also recorded stratospheric temperature ranging between -55° and -60° C (-67° and -76° F). In the following year he made another ascension, bettering his previous record by attaining an altitude of 16,940 m (55,577 ft). He later became interested in undersea exploration and in 1947 built his first bathyscaphe, with which he made a series of descents, including one in 1954 to a depth of 4000 m (13,125 ft). In 1953, he launched his second bathyscaphe, Trieste, with which he reached a depth of 3150 m (10,300 ft). In 1960 his son, Jacques Piccard in Trieste set the world record depth of 10,915 m (about 35,810 ft).
Swiss physicist K. Alex Müller won the 1987 Nobel Prize in physics. Müller discovered that certain materials can become superconductive (able to conduct an electrical current without resistance) at much higher temperatures than once thought possible.
Karl Alex Müller, born in 1927, Swiss physicist and cowinner of the 1987 Nobel Prize in physics for his discovery that copper oxide ceramic materials can achieve superconductivity (the ability of a material to carry an electrical current indefinitely without resistance) at temperatures well above the extremely low temperatures once associated with this remarkable property. Müller shared the prize with his colleague, German physicist Georg Johannes Bednorz.
Müller was born in Switzerland. He attended the Swiss Federal Institute of Technology, receiving his Ph.D. degree in physics there in 1958. He worked at the Battelle Institute in Geneva for several years and then in 1963 joined the International Business Machines (IBM) Research Laboratory near Zürich, Switzerland, where he spent most of his career. Müller was also on the faculty of the University of Zürich.
Superconductivity had been discovered in 1911. However, practical applications could not be developed because to achieve superconductivity materials had to be cooled to temperatures close to absolute zero (0 K, -273° C, -459° F). In the 1970s compounds containing the element niobium were found to be superconducting at 23 K (-250° C, -418° F), which theoretical physicists believed to be the upper temperature limit for superconductors. By systematically testing oxides containing nickel or copper, in 1986 Bednorz and Müller found a material, barium lanthanum copper oxide, that starts to become superconducting at temperatures higher than 100 K (-173° C, -279° F). This temperature is easier and cheaper to maintain, making it possible for researchers to design superconducting devices. Superconductors are now used in scientific and medical instruments and may find applications in the electronics industry and in electric power transmission and storage.
Johann Jakob Balmer (1825-1898), Swiss mathematician and physicist, born in Lausanne, Switzerland. In 1885 Balmer discovered a simple mathematical formula that generated the wavelength values for a certain series of spectral lines of the element hydrogen. This series of spectral lines is now called the Balmer series (see Spectroscopy). The reason that Balmer's formula generated the correct wavelength values was not understood until the development of quantum theory in the early 1900s.
