The Nobel Prize in Physics 1901-2000

The Nobel Prize in Physics 1901-2000

by Erik B. Karlsson*

What Is Physics?

Physics is considered to be the most basic of the natural sciences. It deals with the fundamental constituents of matter and their interactions as well as the nature of atoms and the build-up of molecules and condensed matter. It tries to give unified descriptions of the behavior of matter as well as of radiation, covering as many types of phenomena as possible. In some of its applications, it comes close to the classical areas of chemistry, and in others there is a clear connection to the phenomena traditionally studied by astronomers. Present trends are even pointing toward a closer approach of some areas of physics and microbiology.

Although chemistry and astronomy are clearly independent scientific disciplines, both use physics as a basis in the treatment of their respective problem areas, concepts and tools. To distinguish what is physics and chemistry in certain overlapping areas is often difficult. This has been illustrated several times in the history of the Nobel Prizes. Therefore, a few awards for chemistry will also be mentioned in the text that follows, particularly when they are closely connected to the works of the Physics Laureates themselves. As for astronomy, the situation is different since it has no Nobel Prizes of its own; it has therefore been natural from the start, to consider discoveries in astrophysics as possible candidates for Prizes in Physics.

From Classical to Quantum Physics

In 1901, when the first Nobel Prizes were awarded, the classical areas of physics seemed to rest on a firm basis built by great 19th century physicists and chemists. Hamilton had formulated a very general description of the dynamics of rigid bodies as early as the 1830s. Carnot, Joule, Kelvin and Gibbs had developed thermodynamics to a high degree of perfection during the second half of the century.

Maxwell's famous equations had been accepted as a general description of electromagnetic phenomena and had been found to be also applicable to optical radiation and the radio waves recently discovered by Hertz.

Everything, including the wave phenomena, seemed to fit quite well into a picture built on mechanical motion of the constituents of matter manifesting itself in various macroscopic phenomena. Some observers in the late 19th century actually expressed the view that, what remained for physicists to do was only to fill in minor gaps in this seemingly well-established body of knowledge.

However, it would very soon turn out that this satisfaction with the state of physics was built on false premises. The turn of the century became a period of observations of phenomena that were completely unknown up to then, and radically new ideas on the theoretical basis of physics were formulated. It must be regarded as a historical coincidence, probably never foreseen by Alfred Nobel himself, that the Nobel Prize institution happened to be created just in time to enable the prizes to cover many of the outstanding contributions that opened new areas of physics in this period.

One of the unexpected phenomena during the last few years of the 19th century, was the discovery of X-rays by Wilhelm Conrad Röntgen in 1895, which was awarded the first Nobel Prize in Physics (1901). Another was the discovery of radioactivity by Antoine Henri Becquerel in 1896, and the continued study of the nature of this radiation by Marie and Pierre Curie. The origin of the X-rays was not immediately understood at the time, but it was realized that they indicated the existence of a hitherto concealed world of phenomena (although their practical usefulness for medical diagnosis was evident enough from the beginning). The work on radioactivity by Becquerel and the Curies was rewarded in 1903 (with one half to Becquerel and the other half shared by the Curies), and in combination with the additional work by Ernest Rutherford (who got the Chemistry Prize in 1908) it was understood that atoms, previously considered as more or less structureless objects, actually contained a very small but compact nucleus. Some atomic nuclei were found to be unstable and could emit the  or  radiation observed. This was a revolutionary insight at the time, and it led in the end, through parallel work in other areas of physics, to the creation of the first useful picture of the structure of atoms.

In 1897, Joseph J. Thomson, who worked with rays emanating from the cathode in partly evacuated discharge tubes, identified the carriers of electric charge. He showed that these rays consisted of discrete particles, later called "electrons". He measured a value for the ratio between their mass and (negative) charge, and found that it was only a very small fraction of that expected for singly charged atoms. It was soon realized that these lightweight particles must be the building blocks that, together with the positively charged nuclei, make up all different kinds of atoms. Thomson received his Prize in 1906. By then, Philipp E.A. von Lenard had already been acknowledged the year before (1905) for elucidating other interesting properties of the cathodic rays, such as their ability to penetrate thin metal foils and produce fluorescence. Soon thereafter (in 1912) Robert A. Millikanmade the first precision measurement of the electron charge with the oil-drop method, which led to a Physics Prize for him in 1923. Millikan was also rewarded for his works on the photoelectric effect.

In the beginning of the century, Maxwell's equations had already existed for several decades, but many questions remained unanswered: what kind of medium propagated electromagnetic radiation (including light) and what carriers of electric charges were responsible for light emission? Albert A. Michelson had developed an interferometric method, by which distances between objects could be measured as a number of wavelengths of light (or fractions thereof). This made comparison of lengths much more exact than what had been possible before. Many years later, the Bureau International de Poids et Mesures, Paris (BINP) defined the meter unit in terms of the number of wavelengths of a particular radiation instead of the meter prototype. Using such an interferometer, Michelson had also performed a famous experiment, together with E. W. Morley, from which it could be concluded that the velocity of light is independent of the relative motion of the light source and the observer. This fact refuted the earlier assumption of an ether as a medium for light propagation. Michelson received the Physics Prize in 1907.

The mechanisms for emission of light by carriers of electric charge was studied by Hendrik A. Lorentz, who was one of the first to apply Maxwell's equations to electric charges in matter. His theory could also be applied to the radiation caused by vibrations in atoms and it was in this context that it could be put to its first crucial test. As early as 1896 Pieter Zeeman, who was looking for possible effects of electric and magnetic fields on light, made an important discovery namely, that spectral lines from sodium in a flame were split up into several components when a strong magnetic field was applied. This phenomenon could be given a quite detailed interpretation by Lorentz's theory, as applied to vibrations of the recently identified electrons, and Lorentz and Zeeman shared the Physics Prize in 1902, i.e. even before Thomson's discovery was rewarded. Later, Johannes Stark demonstrated the direct effect of electric fields on the emission of light, by exposing beams of atoms ("anodic rays", consisting of atoms or molecules) to strong electric fields. He observed a complicated splitting of spectral lines as well as a Doppler shift depending on the velocities of the emitters. Stark received the 1919 Physics Prize.

With this background, it became possible to build detailed models for the atoms, objects that had existed as concepts ever since antiquity but were considered more or less structureless in classical physics. There existed already, since the middle of the previous century, a rich empirical material in the form of characteristic spectral lines emitted in the visible domain by different kinds of atoms, and to this was added the characteristic X-ray radiation discovered by Charles G. Barkla (Physics Prize in 1917, awarded in 1918), which after the clarification of the wave nature of this radiation and its diffraction by Max von Laue (Physics Prize in 1914), also became an important source of information on the internal structure of atoms.

Barkla's characteristic X-rays were secondary rays, specific for each element exposed to radiation from X-ray tubes (but independent of the chemical form of the samples). Karl Manne G. Siegbahn realized that measuring characteristic X-ray spectra of all the elements would show systematically how successive electron shells are added when going from the light elements to the heavier ones. He designed highly accurate spectrometers for this purpose by which energy differences between different shells, as well as rules for radiative transitions between them, could be established. He received the Physics Prize in 1924 (awarded in 1925). However, it would turn out that a deeper understanding of the atomic structure required a much further departure from the habitual concepts of classical physics than anyone could have imagined.

Classical physics assumes continuity in motion as well as in the gain or loss of energy. Why then, do atoms send out radiations with sharp wavelengths? Here, a parallel line of development, also with its roots in late 19th century physics, had given important clues for interpretation. Wilhelm Wien studied the "black-body" radiation from hot solid bodies (which in contrast to radiation from atoms in gases, has a continuous distribution of frequencies). Using classical electrodynamics, he derived an expression for the frequency distribution of this radiation and the shift of the maximum intensity wavelength, when the temperature of a black body is changed (the Wien displacement law, useful for instance in determining the temperature of the sun). He was awarded the Physics Prize in 1911.

However, Wien could not derive a distribution formula that agreed with experiments for both short and long wavelengths. The problem remained unexplained until Max K.E.L. Planck put forward his radically new idea that the radiated energy could only be emitted in quanta, i.e. portions that had a certain definite value, larger for the short wavelengths than for the long ones (equal to a constant  times the frequency for each quantum). This is considered to be the birth of quantum physics. Wien received the Physics Prize in 1911 and Planck some years later, in 1918 (awarded in 1919). Important verifications that light comes in the form of energy quanta came also through Albert Einstein's interpretation of the photoelectric effect (first observed in 1887 by Hertz) which also involved extensions of Planck's theories. Einstein received the Physics Prize for 1921 (awarded in 1922). The prize motivation cited also his other "services to theoretical physics," which will be referred to in another context.

Later experiments by James Franck and Gustav L. Hertz demonstrated the inverse of the photoelectric effect (i.e. that an electron that strikes an atom, must have a specific minimum energy to produce light quanta of a particular energy from it) and showed the general validity of Planck's expressions involving the constant . Franck and Hertz shared the 1925 prize, awarded in 1926. At about the same time, Arthur H. Compton (who received one-half of the Physics Prize for 1927) studied the energy loss in X-ray photon scattering on material particles, and showed that X-ray quanta, whose energies are more than 10,000 times larger than those of light, also obey the same quantum rules. The other half was given to Charles T.R. Wilson (see later), whose device for observing high energy scattering events could be used for verification of Compton's predictions.

With the concept of energy quantization as a background, the stage was set for further ventures into the unknown world of microphysics. Like some other well-known physicists before him, Niels H. D. Bohr worked with a planetary picture of electrons circulating around the nucleus of an atom. He found that the sharp spectral lines emitted by the atoms could only be explained if the electrons were circulating in stationary orbits characterized by a quantized angular momentum (integer units of Planck's constant divided by ) and that the emitted frequencies corresponded to emission of radiation with energy equal to the difference between quantized energy states of the electrons. His suggestion indicated a still more radical departure from classical physics than Planck's hypothesis. Although it could only explain some of the simplest features of optical spectra in its original form, it was soon accepted that Bohr's approach must be a correct starting point, and he received the Physics Prize in 1922.

It turned out that a deeper discussion of the properties of radiation and matter (until then considered as forming two completely different categories), was necessary for further progress in the theoretical description of the microworld. In 1923 Prince Louis-Victor P. R. de Broglie proposed that material particles may also show wave properties, now that electromagnetic radiation had been shown to display particle aspects in the form of photons. He developed mathematical expressions for this dualistic behavior, including what has later been called the "de Broglie wavelength" of a moving particle. Early experiments by Clinton J. Davisson had indicated that electrons could actually show reflection effects similar to that of waves hitting a crystal and these experiments were now repeated, verifying the associated wavelength predicted by de Broglie. Somewhat later, George P. Thomson (son of J. J. Thomson) made much improved experiments on higher energy electrons penetrating thin metal foils which showed very clear diffraction effects. de Broglie was rewarded for his theories in 1929 and Davisson and Thomson later shared the 1937 Physics Prize.

What remained was the formulation of a new, consistent theory that would replace classical mechanics, valid for atomic phenomena and their associated radiations. The years 1924-1926 was a period of intense development in this area. Erwin Schrödinger built further on the ideas of de Broglie and wrote a fundamental paper on "Quantization as an eigenvalue problem" early in 1926. He created what has been called "wave mechanics". But the year before that, Werner K. Heisenberg had already started on a mathematically different approach, called "matrix mechanics", by which he arrived at equivalent results (as was later shown by Schrödinger). Schrödinger's and Heisenberg's new quantum mechanics meant a fundamental departure from the intuitive picture of classical orbits for atomic objects, and implied also that there are natural limitations on the accuracy by which certain quantities can be measured simultaneously (Heisenberg's uncertainty relations).

Heisenberg was rewarded by the Physics Prize for 1932 (awarded 1933) for the development of quantum mechanics, while Schrödinger shared the Prize one year later (1933) with Paul A.M. Dirac. Schrödinger's and Heisenberg's quantum mechanics was valid for the relatively low velocities and energies associated with the "orbital" motion of valence electrons in atoms, but their equations did not satisfy the requirements set by Einstein's rules for fast moving particles (to be mentioned later). Dirac constructed a modified formalism which took into account effects of Einstein's special relativity, and showed that such a theory not only contained terms corresponding to the intrinsic spinning of electrons (and therefore explaining their own intrinsic magnetic moment and the fine structure observed in atomic spectra), but also predicted the existence of a completely new kind of particles, the so-called antiparticles with identical masses but opposite charge. The first antiparticle to be discovered, that of the electron, was observed in 1932 by Carl D. Anderson and was given the name "positron" (one-half of the Physics Prize for 1936).

Other important contributions to the development of quantum theory have been distinguished by Nobel Prizes in later years. Max Born, Heisenberg's supervisor in the early twenties, made important contributions to its mathematical formulation and physical interpretation. He received one-half of the Physics Prize for 1954 for his work on the statistical interpretation of the wave function. Wolfgang Pauli formulated his exclusion principle (which states that there can be only one electron in each quantum state) already on the basis of Bohr's old quantum theory. This principle was later found to be associated with the symmetry of wave functions for particles of half-integer spins in general, distinguishing what is now called fermions from the bosonic particles whose spins are integer multiples of . The exclusion principle has deep consequences in many areas of physics and Pauli received the Nobel Prize in Physics in 1945.

The study of electron spins would continue to open up new horizons in physics. Precision methods for determining the magnetic moments of spinning particles were developed during the thirties and forties for atoms as well as nuclei (by Stern, Rabi, Bloch and Purcell, see later sections) and in 1947 they had reached such a precision, that Polykarp Kusch could state that the magnetic moment of an electron did not have exactly the value predicted by Dirac, but differed from it by a small amount. At about the same time, Willis E. Lamb worked on a similar problem of electron spins interacting with electromagnetic fields, by studying the fine structure of optical radiation from hydrogen with very high resolution radio frequency resonance methods. He found that the fine structure splitting also did not have exactly the Dirac value, but differed from it by a significant amount. These results stimulated a reconsideration of the basic concepts behind the application of quantum theory to electromagnetism, a field that had been started by Dirac, Heisenberg and Pauli but still suffered from several insufficiencies. Kusch and Lamb were each awarded half the the Physics Prize in 1955.

In quantum electrodynamics (QED for short), charged particles interact through the interchange of virtual photons, as described by quantum perturbation theory. The older versions involved only single photon exchange, but Sin-Itiro Tomonaga, Julian Schwinger and Richard P. Feynman realized that the situation is actually much more complicated, since electron-electron scattering may involve several photon exchanges. A "naked" point charge does not exist in their picture; it always produces a cloud of virtual particle-antiparticle pairs around itself, such that its effective magnetic moment is changed and the Coulomb potential is modified at short distances. Calculations starting from this picture have reproduced the experimental data by Kusch and Lamb to an astonishing degree of accuracy and modern QED is now considered to be the most exact theory in existence. Tomonaga, Schwinger and Feynman shared the Physics Prize in 1965.

This progress in QED turned out to be of the greatest importance also for the description of phenomena at higher energies. The notion of pair production from a "vacuum" state of a quantized field (both as a virtual process and as a real materialization of particles), is also a central building block in the modern field theory of strong interactions, quantum chromodynamics (QCD).

Another basic aspect of quantum mechanics and quantum field theory is the symmetries of wave functions and fields. The symmetry properties under exchange of identical particles lie behind Pauli's exclusion principle mentioned above, but symmetries with respect to spatial transformations have turned out to play an equally important role. In 1956, Tsung-Dao Leeand Chen Ning Yang pointed out, that physical interactions may not always be symmetric with respect to reflection in a mirror (that is, they may be different as seen in a left-handed and a right-handed coordinate system). This means that the wave function property called "parity", denoted by the symbol "P", is not conserved when the system is exposed to such an interaction and the mirror reflection property may be changed. Lee's and Yang's work was the starting point for an intense search for such effects and it was shown soon afterwards that the  decay and the  decay, which are both caused by the so-called "weak interaction" are not parity-conserving (see more below). Lee and Yang were jointly awarded the Physics Prize in 1957.

Other symmetries in quantum mechanics are connected with the replacement of a particle with its antiparticle, called charge conjugation (symbolized by "C"). In the situations discussed by Lee and Yang it was found that although parity was not conserved in the radioactive transformations there was still a symmetry in the sense that particles and antiparticles broke parity in exactly opposite ways and that therefore the combined operation "C"x"P" still gave results which preserved symmetry. But it did not last long before James W. Cronin and Val L. Fitch found a decay mode among the "K mesons" that violated even this principle, although only to a small extent. Cronin and Fitch made their discovery in 1964 and were jointly awarded the Physics Prize in 1980. The consequences of their result (which include questions about the symmetry of natural processes under reversal of time, called "T") are still discussed today and touch some of the deepest foundations of theoretical physics, because the "P"x"C"x"T" symmetry is expected always to hold.

The electromagnetic field is known to have another property, called "gauge symmetry", which means that the field equations keep their form even if the electromagnetic potentials are multiplied with certain quantum mechanical phase factors, or "gauges". It was not self-evident that the "weak" interaction should have this property, but it was a guiding principle in the work by Sheldon L. Glashow, Abdus Salam, and Steven Weinberg in the late 1960s, when they formulated a theory that described the weak and the electromagnetic interaction on the same basis. They were jointly awarded the Physics Prize in 1979 for this unified description and, in particular, for their prediction of a particular kind of weak interaction mediated by "neutral currents", which had been found recently in experiments.

The last Physics Prize (1999) in the 20th century was jointly awarded to Gerhardus 't Hooft and Martinus J. G. Veltman. They showed the way to renormalize the "electro-weak" theory, which was necessary to remove terms that tended to infinity in quantum mechanical calculations (just as QED had earlier solved a similar problem for the Coulomb interaction). Their work allowed detailed calculations of weak interaction contributions to particle interactions in general, proving the utility of theories based on gauge invariance for all kinds of basic physical interactions.

Quantum mechanics and its extensions to quantum field theories is one of the great achievements of the 20th century. This sketch of the route from classical physics to modern quantum physics, has led us a long way toward a fundamental and unified description of the different particles and forces in nature, but much remains to be done and the goal is still far ahead. It still remains, for instance, to "unify" the electro-weak force with the "strong" nuclear force and with gravity. But here, it should also be pointed out that the quantum description of the microworld has another main application: the calculation of chemical properties of molecular systems (sometimes extended to biomolecules) and of the structure of condensed matter, branches that have been distinguished with several prizes, in physics as well as in chemistry.

Microcosmos and Macrocosmos

"From Classical to Quantum Physics", took us on a trip from the phenomena of the macroscopic world as we meet it in our daily experience, to the quantum world of atoms, electrons and nuclei. With the atoms as starting point, the further penetration into the subatomic microworld and its smallest known constituents will now be illustrated by the works of other Nobel Laureates.

It was realized, already in the first half of the 20th century, that such a further journey into the microcosmos of new particles and interactions would also be necessary for understanding the composition and evolution histories of the very large structures of our universe, the "macrocosmos". At the present stage elementary particle physics, astrophysics, and cosmology are strongly tied together, as several examples presented here will show.

Another link connecting the smallest and the largest objects in our universe is Albert Einstein's theories of relativity. Einstein first developed his special theory of relativity in 1905, which expresses the mass-energy relationship . Then, in the next decade, he continued with his theory of general relativity, which connects gravitational forces to the structure of space and time. Calculations of effective masses for high energy particles, energy transformations in radioactive decay as well as Dirac's predictions that antiparticles may exist, are all based on his special theory of relativity. The general theory is the basis for calculations of large scale motions in the universe, including discussions of the properties of black holes. Einstein received the Nobel Prize in Physics in 1921 (awarded in 1922), motivated by work on the photo-electric effect which demonstrated the particle aspects of light.

The works by Becquerel, the Curies, and Rutherford gave rise to new questions: What was the source of energy in the radioactive nuclei that could sustain the emission of  and  radiation over very long time intervals, as observed for some of them, and what were the heavy particles and the nuclei themselves actually composed of? The first of these problems (which seemed to violate the law of conservation of energy, one of the most important principles of physics) found its solution in the transmutation theory, formulated by Rutherford and Frederick Soddy (Chemistry Prize for 1921, awarded in 1922). They followed in detail several different series of radioactive decay and compared the energy emitted with the mass differences between "parent" and "daughter" nuclei. It was also found that nuclei belonging to the same chemical element could have different masses; such different species were called "isotopes". A Chemistry Prize was given in 1922 to Francis W. Aston for his mass-spectroscopic separation of a large number of isotopes of non-radioactive elements. Marie Curie had by then already received a second Nobel Prize (this time in Chemistry in 1911), for her discoveries of the chemical elements radium and polonium.

All isotopic masses were found to be nearly equal to multiples of the mass of the proton, a particle also first seen by Rutherford when he irradiated nitrogen nuclei with  particles. But the different isotopes could not possibly be made up entirely of protons since each particular chemical element must have one single value for the total nuclear charge. Protons were actually found to make up less than half of the nuclear mass, which meant that some neutral constituents had to be present in the nuclei. James Chadwick first found conclusive evidence for such particles, the neutrons, when he studied nuclear reactions in 1932. He received the Physics Prize in 1935.

Soon after Chadwick's discovery, neutrons were put to work by Enrico Fermiand others as a means to induce nuclear reactions that could produce new "artificial" radioactivity. Fermi found that the probability of neutron-induced reactions (which do not involve element transformations), increased when the neutrons were slowed down and that this worked equally well for heavy elements as for light ones, in contrast to charge-particle induced reactions. He received the Physics Prize in 1938.

With neutrons and protons as the basic building blocks of atomic nuclei, the branch of "nuclear physics" could be established and several of its major achievements were distinguished by Nobel prizes. Ernest O. Lawrence, who received the Physics Prize in 1939, built the first cyclotron in which acceleration took place by successively adding small amounts of energy to particles circulating in a magnetic field. With these machines, he was able to accelerate charged nuclear particles to such high energies that they could induce nuclear reactions and he obtained important new results. Sir John D. Cockcroft and Ernest T.S. Walton instead, accelerated particles by direct application of very high electrostatic voltages and were rewarded for their studies of transmutation of elements in 1951.

Otto Stern received the Physics Prize in 1943 (awarded in 1944), for his experimental methods of studying magnetic properties of nuclei, in particular for measuring the magnetic moment of the proton itself. Isidor I. Rabiincreased the accuracy of magnetic moment determinations for nuclei by more than two orders of magnitude, with his radio frequency resonance technique, for which he was awarded the Physics Prize for 1944. Magnetic properties of nuclei provide important information for understanding details in the build-up of the nuclei from protons and neutrons. Later, in the second half of the century, several theoreticians were rewarded for their work on the theoretical modelling of this complex many-body system: Eugene P. Wigner(one-half of the prize), Maria Goeppert-Mayer (one-fourth) and J. Hans D. Jensen (one-fourth) in 1963 and Aage N. Bohr, Ben R. Mottelson and L. James Rainwater in 1975. We will come back to these works under the heading "From Simple to Complex Systems".

As early as 1912, it was found by Victor F. Hess (awarded half the Prize in 1936 and the other half to Carl D. Anderson) that highly penetrating radiation is also reaching us continuously from outer space. This "cosmic radiation" was first detected by ionization chambers and later by Wilson's cloud chamber referred to earlier. Properties of particles in the cosmic radiation could be inferred from the curved particle tracks produced when a strong magnetic field was applied. It was in this way that C. D. Anderson discovered the positron. Anderson and Patrick M.S. Blackett showed that electron positron pairs could be produced by  rays (which needed a photon energy equal to at least ) and that electrons and positrons could annihilate, producing  rays as they disappeared. Blackett received the Physics Prize in 1948 for his further development of the cloud chamber and the discoveries made with it.

Although accelerators were further developed, cosmic radiation continued for a couple of decades to be the main source of very energetic particles (and still surpasses the most powerful accelerators on earth in this aspect, although with extremely low intensities), and it provided the first glimpses of a completely unknown subnuclear world. A new kind of particles, called mesons, was spotted in 1937, having masses approximately 200 times that of electrons (but 10 times lighter than protons). In 1946, Cecil F. Powell clarified the situation by showing that there were actually more than one kind of such particles present. One of them, the "meson", decays into the other one, the "µ meson". Powell was awarded the Physics Prize in 1950.

By that time, theoreticians had already been speculating about the forces that keep protons and neutrons together in nuclei. Hideki Yukawasuggested in 1935, that this "strong" force should be carried by an exchange particle, just as the electromagnetic force was assumed to be carried by an exchange of virtual photons in the new quantum field theory. Yukawa maintained that such a particle must have a mass of about 200 electron masses in order to explain the short range of the strong forces found in experiments. Powell's meson was found to have the right properties to act as a "Yukawa particle". The µ particle, on the other hand, turned out to have a completely different character (and its name was later changed from "µmeson" to "muon"). Yukawa received the Physics Prize in 1949. Although later progress has shown that the strong force mechanism is more complex than what Yukawa pictured it to be, he must still be considered as the first one who led the thoughts on force carriers in this fruitful direction.

More new particles were discovered in the 1950s, in cosmic radiation as well as in collisions with accelerated particles. By the end of the 50s, accelerators could reach energies of several GeV (109 electron volts) which meant that pairs of particles, with masses equal to the proton mass, could be created by energy-to-mass conversion. This was the method used by the team of Owen Chamberlain and Emilio Segrè when they first identified and studied the antiproton in 1955 (they shared the Physics Prize for 1959). High energy accelerators also allowed more detailed studies of the structures of protons and neutrons than before, and Robert Hofstadter was able to distinguish details of the electromagnetic structure of the nucleons by observing how they scattered electrons of very high energy. He was rewarded with half the Physics Prize for 1961.

One after another, new mesons with their respective antiparticles appeared, as tracks on photographic plates or in electronic particle detectors. The existence of the "neutrino" predicted on theoretical grounds by Pauli already as early as the 1930s, was established. The first direct experimental evidence for the neutrino was provided by C. L. Cowan and Frederick Reines in 1957, but it was not until 1995 that this discovery was awarded with one-half the Nobel Prize (Cowan had died in 1984). The neutrino is a participant in processes involving the "weak" interaction (such as  decay and meson decay to muons) and, as the intensity of particle beams increased, it became possible to produce secondary beams of neutrinos from accelerators. Leon M. Lederman, Melvin Schwartz and Jack Steinberger developed this method in the 1960s and demonstrated that the neutrinos accompanying µ emission in  decay were not identical to those associated with electrons in decay; they were two different particles,  and .

Physicists could now start to distinguish some order among the particles: the electron (e), the muon (µ), the electron neutrino (), the muon neutrino () and their antiparticles were found to belong to one class, called "leptons". They did not interact by the "strong" nuclear force, which on the other hand, characterized the protons, neutrons, mesons and hyperons (a set of particles heavier than the protons). The lepton class was extended later in the 1970s when Martin L. Perl and his team discovered the lepton, a heavier relative to the electron and the muon. Perl shared the Physics Prize in 1995 with Reines.

All the leptons are still considered to be truly fundamental, i.e. point-like and without internal structure, but for the protons, etc, this is no longer true.Murray Gell-Mann and others managed to classify the strongly interacting particles (called "hadrons") into groups with common relationships and ways of interaction. Gell-Mann received the Physics Prize in 1969. His systematics was based on the assumption that they were all built up from more elementary constituents, called "quarks". The real proof that nucleons were built up from quark-like objects came through the works of Jerome I. Friedman, Henry W. Kendall and Richard E. Taylor. They "saw" hard grains inside these objects when they studied how electrons (of still higher energy than Hofstadter could use earlier) scattered inelastically on them. They shared the Physics Prize in 1990.

It was understood that all strongly interacting particles are built up by quarks. In the middle of the 1970s a very short-lived particle, discovered independently by the teams of Burton Richter and Samuel C.C. Ting, was found to contain a so far, unknown type of quark which was given the name "charm". This quark was a missing link in the systematics of the elementary particles and Burton and Ting shared the Physics Prize in 1976. The present standard model of particle physics sorts the particles into three families, with two quarks (and their antiparticles) and two leptons in each: the "up" and "down" quarks, the electron and the electron-neutrino in the first; the "strange" and the "charm" quark, the muon and the muon neutrino in the second; the "top" and the "bottom" quark, the tauon and the tau neutrino in the third. The force carriers for the combined electro-weak interaction are the photon, the Z-particle and the W-bosons, and for the strong interaction between quarks the so-called gluons.

In 1983, the existence of the W- and Z-particles was proven by Carlo Rubbia's team which used a new proton-antiproton collider with sufficient energy for production of these very heavy particles. Rubbia shared the 1984 Physics Prize with Simon van der Meer, who had made decisive contributions to the construction of this collider by his invention of "stochastic cooling" of particles. There are speculations that additional particles may be produced at energies higher than those attainable with the present accelerators, but no experimental evidence has been produced so far.

Cosmology is the science that deals with the structure and evolution of our universe and the large-scale objects in it. Its models are based on the properties of the known fundamental particles and their interactions as well as the properties of space-time and gravitation. The "big-bang" model describes a possible scenario for the early evolution of the universe. One of its predictions was experimentally verified when Arno A. Penzias and Robert W. Wilson discovered the cosmic microwave radiation background in 1960. They shared one-half of the Physics Prize for 1978. This radiation is an afterglow of the violent processes assumed to have occurred in the early stages of the big bang. Its equilibrium temperature is 3 kelvin at the present age of the universe. It is almost uniform when observed in different directions; the small deviations from isotropy are now being investigated and will tell us more about the earliest history of our universe.

Outer space has been likened to a large arena for particle interactions where extreme conditions, not attainable in a laboratory, are spontaneously created. Particles may be accelerated to higher energies than in any accelerator on earth, nuclear fusion reactions proliferate in the interior of stars, and gravitation can compress particle systems to extremely high densities. Hans A. Bethe first described the hydrogen and carbon cycles, in which energy is liberated in stars by the fusion of protons into helium nuclei. For this achievement he received the Physics Prize in 1967.

Subramanyan Chandrasekhar described theoretically the evolution of stars, in particular those ending up as "white dwarfs". Under certain conditions the end product may also be a "neutron star", an extremely compact object, where all protons have been converted into neutrons. In supernova explosions, the heavy elements created during stellar evolution are spread out into space. The details of some of the most important nuclear reactions in stars and heavy element formation were elucidated by William A. Fowlerboth in theory and in experiments using accelerators. Fowler and Chandrasekhar received one-half each of the 1983 Physics Prize.

Visible light and cosmic background radiation are not the only forms of electromagnetic waves that reach us from outer space. At longer wavelengths, radio astronomy provides information on astronomical objects not obtainable by optical spectroscopy. Sir Martin Ryle developed the method where signals from several separated telescopes are combined in order to increase the resolution in the radio source maps of the sky. Antony Hewish and his group made an unexpected discovery in 1964 using Ryle's telescopes: radio frequency pulses were emitted with very well-defined repetition rates by some unknown objects called pulsars. These were soon identified as neutron stars, acting like fast rotating lighthouses emitting radiowaves because they are also strong magnets. Ryle and Hewish shared the Physics Prize in 1974.

By 1974, pulsar search was already routine among radio astronomers, but a new surprise came in the summer of the same year when Russell A. Hulseand Joseph H. Taylor, Jr. noticed periodic modulations in the pulse frequencies of a newly discovered pulsar, called PSR 1913+16. It was the first double pulsar detected, so named because the emitting neutron star happened to be one of the components of a close double star system, with the other component of about equal size. This system has provided, by observation over more than 20 years, the first concrete evidence for gravitational radiation. The decrease of its rotational frequency is in close agreement with the predictions based on Einstein's theory, for losses caused by this kind of radiation. Hulse and Taylor shared the Physics Prize in 1993. However, the direct detection of gravitational radiation on earth still has to be made.

From Simple to Complex Systems

If all the properties of the elementary particles as well as the forces that may act between them were known in every detail, would it then be possible to predict the behavior of all systems composed of such particles? The search for the ultimate building blocks of nature and of the proper theoretical description of their interactions (on the macro as well as the micro scale), has partly been motivated by such a reductionistic program. All scientists would not agree that such a synthesis is possible even in principle. But even if it were true, the calculations of complex system behavior would very soon be impossible to handle when the number of particles and interactions in the system is increased. Complex multi-particle systems are therefore described in terms of simplified models, where only the most essential features of their particle compositions and interactions are used as starting points. Quite often, it is observed that complex systems develop features called "emergent properties", not straightforwardly predictable from the basic interactions between their constituents.

Atomic Nuclei

The first complex systems from the reductionist's point of view are the nucleons, i.e. neutrons and protons composed of quarks and gluons. The second is the atomic nuclei, which to a first approximation are composed of separate nucleons. The first advanced model of nuclear structure was the nuclear shell model, put up by the end of the 1940s by Maria Goeppert-Mayer and Johannes D. Jensen who realized that at least for nuclei with nearly spherical shape, the outer nucleons fill up energy levels like electrons in atoms. However, the order is different; it is determined by another common potential and by the specific strong spin-orbit coupling of the nuclear forces. Their model explains why nuclei with so-called "magic numbers" of protons or neutrons are particularly stable. They shared the Physics Prize in 1963 together with Eugene Wigner, who had formulated fundamental symmetry principles important in both nuclear and particle physics.

Nuclei with nucleon numbers far from the magic ones are not spherical. Niels Bohr had already worked with a liquid drop model for such deformed nuclei which may take ellipsoidal shapes, and in 1939 it was found that excitation of certain strongly deformed nuclei could lead to nuclear fission, i.e. the breakup of such nuclei into two heavy fragments. Otto Hahn received the Chemistry Prize in 1944 (awarded in 1945) for the discovery of this new process. The non-spherical shape of deformed nuclei allows new collective, rotational degrees of freedom, as do also the collective vibrations of nucleons. Models describing such excitations of the nuclei were developed by James Rainwater, Aage Bohr (son of Niels Bohr) and Ben Mottelson, who jointly received the Physics Prize in 1975.

The nuclear models mentioned above, were based not only on general, guiding principles, but also on the steadily increasing information from nuclear spectroscopy. Harold C. Urey discovered deuterium, a heavy isotope of hydrogen, for which he was awarded the Chemistry Prize in 1934. Fermi, Lawrence, Cockcroft, and Walton mentioned in the previous section developed methods for the production of unstable nuclear isotopes. For their extension of the nuclear isotope chart to the heaviest elements, Edwin M. McMillan and Glenn T. Seaborg were awarded, again with a Chemistry Prize (in 1951). In 1954, Walther Bothe received one-half of the Physics Prize and the other half was awarded to Max Born, mentioned earlier. Bothe developed the coincidence method, which allowed spectroscopists to select generically related sequences of nuclear radiation from the decay of nuclei. This turned out to be important, particularly for the study of excited states of nuclei and their electromagnetic properties.

Atoms

The electronic shells of the atoms, when considered as many-body systems, are easier to handle than the nuclei (which actually contain not only protons and neutrons but also more of other, short-lived "virtual" particles than the atoms). This is due to the weakness and simplicity of the electromagnetic forces as compared to the "strong" forces holding the nuclei together. With the quantum mechanics developed by Schrödinger, Heisenberg, and Pauli, and the relativistic extensions by Dirac, the main properties of the atomic electrons could be reasonably well described. However, a long standing problem has remained, namely to solve the mathematical problems connected with the mutual interactions between the electrons after the dominating attraction by the positive nuclei has been taken into account. One aspect of this was addressed in the work by one of the most recent Chemistry Laureates (1998), Walter Kohn. He developed the "density functional" method which is applicable to free atoms as well as to electrons in molecules and solids.

At the beginning of the 20th century, the periodic table of elements was not yet complete. The early history of the Nobel Prizes includes the discoveries of some of the then missing elements. Lord Rayleigh (John William Strutt)noticed anomalies in the relative atomic masses when oxygen and nitrogen samples were taken directly from the air that surrounds us, instead of separating them from chemical compounds. He concluded that the atmosphere must contain a so far unknown constituent, which was the element argon with atomic mass 20. He was awarded the Physics Prize in 1904, the same year that Sir William Ramsay obtained the Chemistry Prize for isolating the element helium.

In the second half of the 20th century, there has been a spectacular development of atomic spectroscopy and the precision by which one can measure the transitions between atomic or molecular states that fall in the microwave and optical range. Alfred Kastler (who received the Physics Prize in 1966) and his co-workers showed in the 1950s that electrons in atoms can be put into selected excited substates by the use of polarized light. After radiative decay, this can also lead to an orientation of the spins of ground-state atoms. The subsequent induction of radio frequency transitions opened possibilities to measure properties of the quantized states of electrons in atoms in much greater detail than before. A parallel line of development led to the invention of masers and lasers, which are based on the "amplification of stimulated emission of radiation" in strong microwave and optical (light) fields, respectively (effects which in principle would have been predictable from Einstein's equations formulated in 1917 but were not discussed in practical terms until early in the 1950s).

Charles H. Townes developed the first maser in 1958. Theoretical work on the maser principle was made by Nikolay G. Basov and Aleksandr M. Prokhorov. The first maser used a stimulated transition in the ammonia molecule. It emitted an intense microwave radiation, which unlike that of natural emitters, was coherent (i.e. with all photons in phase). Its frequency sharpness soon made it an important tool in technology, for time-keeping and other purposes. Townes received half the Physics Prize for 1964 and Basov and Prokhorov shared the other half.

For radiation in the optical range, lasers were later developed in several laboratories. Nicolaas Bloembergen and Arthur L. Schawlow were distinguished in 1981 for their work on precision laser spectroscopies of atoms and molecules. The other half of that year's prize was awarded to Kai M. Siegbahn (son of Manne Siegbahn), who developed another high-precision method for atomic and molecular spectroscopy based on electrons emitted from inner electron shells when hit by X-rays with very well-defined energy. His photo- and Auger-electron spectroscopy is used as an analytical tool in several other areas of physics and chemistry.

The controlled interplay between atomic electrons and electromagnetic fields has continued to provide ever more detailed information about the structure of electronic states in atoms. Norman F. Ramsey developed precision methods based on the response to external radio frequency signals by free atoms in atomic beams and Wolfgang Paul invented atomic "traps", built by combinations of electric and magnetic fields acting over the sample volumes. Hans G. Dehmelt's group was the first to isolate single particles (positrons) as well as single atoms in such traps. For the first time, experimenters could "communicate" with individual atoms by microwave and laser signals. This enabled the study of new aspects of quantum mechanical behavior as well as further increased precision in atomic properties and the setting of time standards. Paul and Dehmelt received the 1989 Physics Prize and the other half was awarded to Ramsey.

The latest step in this development has involved the slowing down of the motion of atoms in traps to such an extent that it would correspond to micro-kelvin temperatures, had they been in thermal equilibrium in a gas. This is done by exposing them to "laser cooling" through a set of ingenious schemes designed and carried out in practice by Steven Chu, Claude Cohen-Tannoudji and William D. Phillips, whose research groups manipulated atoms by collisions with laser photons. Their work, which was recognized by the 1997 Physics Prize, promises important applications in general measurement technology in addition to a still more increased precision in the determination of atomic quantities.

Molecules and Plasmas

Molecules are composed of atoms. They form the next level of complexity when considered as many-body systems. But molecular phenomena have traditionally been viewed as a branch of chemistry (as exemplified by the Chemistry Prize in 1936 to Petrus J.W. Debye), and have only rarely been in the focus for Nobel Prizes in Physics. One exception is the recognition of the work by Johannes Diderik van der Waals, who formulated an equation of state for molecules in a gas taking into account the mutual interaction between the molecules as well as the reduction of the free volume due to their finite size. van der Waals' equation has been an important starting point for the description of the condensation of gases into liquids. He received the 1910 Physics Prize. Jean B. Perrin studied the motion of small particles suspended in water and received the 1926 Physics Prize. His studies allowed a confirmation of Einstein's statistical theory of Brownian motion as well as of the laws governing the equilibrium of suspended particles under the influence of gravity.

In 1930, Sir C. Venkata Raman received the Physics Prize for his observations that light scattered from molecules contained components which were shifted in frequency with respect to the infalling monochromatic light. These shifts are caused by the molecules' gain or loss of characteristic amounts of energy when they change their rotational or vibrational motion. Raman spectroscopy soon became an important source of information on molecular structure and dynamics.

A plasma is a gaseous state of matter in which the atoms or molecules are strongly ionized. Mutual electromagnetic forces, both between the positive ions themselves and between the ions and the free electrons, are then playing dominant roles, which adds to the complexity as compared to the situation in neutral atomic or molecular gases. Hannes Alfvén demonstrated in the 1940s that a new type of collective motion, called "magneto-hydrodynamical waves" can arise in such systems. These waves play a crucial role for the behavior of plasmas, in the laboratory as well as in the earth's atmosphere and in cosmos. Alfvén received half of the 1970 Physics Prize.

Condensed Matter

Crystals are characterized by a regular arrangement of atoms. Relatively soon after the discovery of the X-rays, it was realized by Max von Laue that such rays were diffracted when passing through crystalline solids, like light passing an optical grating. This effect is related to the fact that the wavelength of common X-ray sources happens to coincide with typical distances between atoms in these materials. It was first used systematically by Sir William Henry Bragg and William Lawrence Bragg (father and son) to measure interatomic distances and to analyse the geometrical arrangement of atoms in simple crystals. For their pioneering work on X-ray crystallography (which has later been developed to a high degree of sophistication), they received the Nobel Prize in Physics; Laue in 1914 and the Braggs in 1915.

The crystalline structure is the most stable of the different ways in which atoms can be organized to form a certain solid at the prevalent temperature and pressure conditions. In the 1930s Percy W. Bridgman invented devices by which very high pressures could be applied to different solid materials and studied changes in their crystalline, electric, magnetic and thermal properties. Many crystals undergo phase transitions under such extreme circumstances, with abrupt changes in the geometrical arrangements of their atoms at certain well-defined pressures. Bridgman received the Physics Prize in 1946 for his discoveries in the field of high pressure physics.

Low-energy neutrons became available in large numbers to the experimenters through the development of fission reactors in the 1940s. It was found that these neutrons, like X-rays, were useful for crystal structure determinations because their associated de Broglie wavelengths also fall in the range of typical interatomic distances in solids. Clifford G. Shullcontributed strongly to the development of the neutron diffraction technique for crystal structure determination, and showed also that the regular arrangement of magnetic moments on atoms in ordered magnetic materials can give rise to neutron diffraction patterns, providing a new powerful tool for magnetic structure determination.

Shull was rewarded with the Physics Prize in 1994, together with Bertram N. Brockhouse, who specialized in another aspect of neutron scattering on condensed material: the small energy losses resulting when neutrons excite vibrational modes (phonons) in a crystalline lattice. For this purpose, Brockhouse developed the 3-axis neutron spectrometer, by which complete dispersion curves (phonon energies as function of wave vectors) could be obtained. Similar curves could be recorded for vibrations in magnetic lattices (the magnon modes).

John H. Van Vleck made significant contributions to the theory of magnetism in condensed matter in the years following the creation of quantum mechanics. He calculated the effects of chemical binding on the paramagnetic atoms and explained the effects of temperature and applied magnetic fields on their magnetism. In particular, he developed the theory of crystal field effects on the magnetism of transition metal compounds, which has been of great importance for understanding the function of active centers in compounds for laser physics as well as in biomolecules. He shared the Physics Prize in 1977 with Philip W. Anderson and Sir Nevill F. Mott (see below).

Magnetic atoms can have their moments all ordered in the same direction in each domain (ferromagnetism), with alternating "up" and "down" moments of the same size (simple antiferromagnets) or with more complicated patterns including different magnetic sublattices (ferrimagnets, etc). Louis E.F. Néel introduced basic models to describe antiferromagnetic and ferrimagnetic materials, which are important components in many solid state devices. They have been extensively studied by the aforementioned neutron diffraction techniques. Néel obtained one-half of the Physics Prize in 1970.

The geometric ordering of atoms in crystalline solids as well as the different kinds of magnetic order, are examples of general ordering phenomena in nature when systems find an energetically favorable arrangement by choosing a certain state of symmetry. The critical phenomena, which occur when transitions between states of different symmetry are approached (for instance when temperature is changed), have a high degree of universality for different types of transitions, including the magnetic ones. Kenneth G. Wilson, who received the Physics Prize in 1982, developed the so-called renormalization theory for critical phenomena in connection with phase transitions, a theory which has also found application in certain field theories of particle physics.

Liquid crystals form a specific class of materials that show many interesting features, from the point of view of fundamental interactions in condensed matter as well as for technical applications. Pierre-Gilles de Gennesdeveloped the theory for the behavior of liquid crystals and their transitions between different ordered phases (nematic, smectic, etc). He used also statistical mechanics to describe the arrangements and dynamics of polymer chains, thereby showing that methods developed for ordering phenomena in simple systems can be generalized to the complex ones occurring in "soft condensed matter". For this, he received the Physics Prize in 1991.

Another specific form of liquid that has received attention is liquid helium. At normal pressures, this substance remains liquid down to the lowest temperatures attainable. It also shows large isotope effects, since condenses to liquid at 4.2 K, while the more rare isotope  remains in gaseous form down to 3.2 K. Helium was first liquefied by Heike Kamerlingh-Onnes in 1909. He received the Physics Prize in 1913 for the production of liquid helium and for his investigations of properties of matter at low temperatures. Lev D. Landau formulated fundamental concepts (e.g. the "Landau liquid") concerning many-body effects in condensed matter and applied them to the theory of liquid helium, explaining specific phenomena occurring in such as the superfluidity (see below), the "roton" excitations, and certain acoustic phenomena. He was awarded the Physics Prize in 1962.

Several of the experimental techniques used for the production and study of low temperature phenomena were developed by Pyotr L. Kapitsa in the 1920s and 30s. He studied many aspects of liquid and showed that it was superfluid (i.e. flowing without friction) below 2.2 K. The superfluid state was later understood to be a manifestation of macroscopic quantum coherence in a Bose-Einstein type of condensate (theoretically predicted in 1920) with many features in common with the superconducting state for electrons in certain conductors. Kapitsa received one-half of the Physics Prize for 1978.

In liquid , additional, unique phenomena show up because each nucleus has a non-zero spin in contrast to those of . Thus, it is a fermion type of particle, and should not be able to participate in Bose-Einstein condensation, which works only for bosons. However, like in superconductivity (see below) pairs of spin-half particles can form "quasi-bosons" that can condense into a superfluid phase. Superfluidity in , whose transition temperature is reduced by a factor of a thousand compared to that of liquid , was discovered by David M. Lee, Douglas D. Osheroff and Robert C. Richardson, who received the Physics Prize in 1996. They observed three different superfluid phases, showing complex vortex structures and interesting quantum behavior.

Electrons in condensed matter can be localized to their respective atoms as in insulators, or they can be free to move between atomic sites, as in conductors and semiconductors. In the beginning of the 20th century, it was known that metals emitted electrons when heated to high temperatures, but it was not clear whether this was due only to thermal excitation of the electrons or if chemical interactions with the surrounding gas were also involved. Through experiments carried out in high vacuum, Owen W. Richardson could finally establish that electron emission is a purely thermionic effect and a law based on the velocity distribution of electrons in the metal could be formulated. For this, Richardson received the Physics Prize in 1928 (awarded in 1929.)

The electronic structure determines the electric, magnetic, and optical properties of solids and is also of major importance for their mechanical and thermal behavior. It has been one of the major tasks of physicists in the 20th century to measure the states and dynamics of electrons and model their behavior so as to understand how they organize themselves in various types of solids. It is natural that the most unexpected and extreme manifestations of electron behavior have attracted the strongest interest in the community of solid state physicists. This is also reflected in the Nobel Prize in Physics: several prizes have been awarded for discoveries connected with superconductivity and for some of the very specific effects displayed in certain semiconducting materials.

Superconductivity was discovered as early as 1911 by Kamerlingh-Onnes, who noticed that the electrical resistivity of mercury dropped to less than one billionth of its ordinary value when it was cooled well below a transition temperature of , which is about 4 K. As mentioned earlier, he received the Physics Prize in 1913. However, it would take a very long period of time before it was understood why electrons could flow without resistance in certain conductors at low temperature. But in the beginning of the 1960s Leon N. Cooper, John Bardeen and J. Robert Schrieffer formulated a theory based on the idea that pairs of electrons (with opposite spins and directions of motion) can lower their energy by an amount  by sharing exactly the same deformation of the crystalline lattice as they move. Such "Cooper pairs" act as bosonic particles. This allows them to move as a coherent macroscopic fluid, undisturbed as long as the thermal excitations (of energy ) are lower in energy than the energy  gained by the pair formation. The so-called BCS-theory was rewarded with the Physics Prize in 1972.

This breakthrough in the understanding of the quantum mechanical basis led to further progress in superconducting circuits and components: Brian D. Josephson analysed the transfer of superconducting carriers between two superconducting metals, separated by a very thin layer of normal-conducting material. He found that the quantum phase, which determines the transport properties, is an oscillating function of the voltage applied over this kind of junction. The Josephson effect has important applications in precision measurements, since it establishes a relation between voltage and frequency scales. Josephson received one-half of the Physics Prize for 1973. Ivar Giaever, who invented and studied the detailed properties of the "tunnel junction", an electronic component based on superconductivity, shared the second half with Leo Esaki for work on tunneling phenomena in semiconductors (see below).

Although a considerable number of new superconducting alloys and compounds were discovered over the first 75 years that followed Kamerlingh-Onnes' discovery, it seemed as if superconductivity would forever remain a typical low temperature phenomenon, with the limit for transition temperatures slightly above 20 K. It therefore came as a total surprise when J. Georg Bednorz and K. Alexander Müller showed that a lanthanum-copper oxide could be made superconducting up to 35 K by doping it with small amounts of barium. Soon thereafter, other laboratories reported that cuprates of similar structure were superconducting up to about 100 K. This discovery of "high temperature superconductors" triggered one of the greatest efforts in modern physics: to understand the basic mechanism for superconductivity in these extraordinary materials. Bednorz and Müller shared the Physics Prize in 1987.

Electron motion in the normal conducting state of metals has been modeled theoretically with increasing degree of sophistication ever since the advent of quantum mechanics. One of the early major steps was the introduction of the Bloch wave concept, named after Felix Bloch (half of the Physics Prize for magnetic resonance in 1952). Another important concept, "the electron fluid" in conductors, was introduced by Lev Landau (see liquid He). Philip W. Anderson made several important contributions to the theory of electronic structures in metallic systems, in particular concerning the effects of inhomogeneities in alloys and magnetic impurity atoms in metals. Nevill F. Mott worked on the general conditions for electron conductivity in solids and formulated rules for the point at which an insulator becomes a conductor (the Mott transition) when composition or external parameters are changed. Anderson and Mott shared the 1977 Physics Prize with John H. Van Vleck for their theoretical investigations of the electronic structure of magnetic and disordered systems.

An early Physics Prize (1920) was given to Charles E. Guillaume for his discovery that the thermal expansion of certain nickel steels, so-called "invar" alloys, was practically zero. This prize was mainly motivated by the importance of these alloys for precision measurements in physics and geodesy, in particular when referring to the standard meter in Paris. The invar alloys have been extensively used in all kinds of high-precision mechanical devices, watches, etc. The theoretical background for this temperature independence has been explained only recently. Also very recently (1998), Walter Kohn was recognized by a Nobel Prize in Chemistry for his methods of treating quantum exchange correlations, by which important limitations for the predictive power of electronic structure calculations, in solids as well as molecules, have been overcome.

In semiconductors, electron mobility is strongly reduced because there are forbidden regions for the energy of the electrons that take part in conduction, the "energy gaps". It was only after the basic roles of doping of ultra-pure silicon (and later other semiconducting materials) with chosen electron-donating or electron-accepting agents were understood, that semiconductors could be used as components in electronic engineering. William B. Shockley, John Bardeen (see also BCS-theory) and Walter H. Brattain carried out fundamental investigations of semiconductors and developed the first transistor. This was the beginning of the era of "solid state electronics". They shared the Physics Prize in 1956.

Later, Leo Esaki developed the tunnel diode, an electronic component that has a negative differential resistance, a technically interesting property. It is composed of two heavily and  doped semiconductors, that have an excess of electrons on one side of the junction and a deficit on the other. The tunneling effect occurs at bias voltages larger than the gap in the semi-conductors. He shared the Physics Prize for 1973 with Brian D. Josephson.

With modern techniques it is possible to build up well-defined, thin-layered structures of different semiconducting materials, in direct contact with each other. With such "heterostructures" one is not limited to the band-gaps provided by semi-conducting materials like silicon and germanium. Herbert Kroemer analysed theoretically the mobility of electrons and holes in heterostructure junctions. His propositions led to the build up of transistors with much improved characteristics, later called HEMTs (high electron mobility transistors), which are very important in today's high-speed electronics. Kroemer suggested also, at about the same time as Zhores I. Alferov, the use of double heterostructures to provide conditions for laser action. Alferov later built the first working pulsed semiconductor laser in 1970. This marked the beginning of the era of modern optoelectronic devices now used in laser diodes, CD-players, bar code readers and fiber optics communication. Alferov and Kroemer recently shared one-half of the Physics Prize for the year 2000. The other half went to Jack S. Kilby, co-inventor of the integrated circuit (see the next section on Physics and Technology).

By applying proper electrode voltages to such systems one can form "inversion layers", where charge carriers move essentially only in two dimensions. Such layers have turned out to have some quite unexpected and interesting properties. In 1982, Klaus von Klitzing discovered the quantized Hall effect. When a strong magnetic field is applied perpendicularly to the plane of a quasi two-dimensional layer, the quantum conditions are such that an increase of magnetic field does not give rise to a linear increase of voltage on the edges of the sample, but a step-wise one. Between these steps, the Hall resistance is , where i's are integers corresponding to the quantized electron orbits in the plane. Since this provides a possibility to measure the ratio between two fundamental constants very exactly, it has important consequences for measurement technology. von Klitzing received the Physics Prize in 1985.

A further surprise came shortly afterwards when Daniel C. Tsui and Horst L. Störmer made refined studies of the quantum Hall effect using inversion layers in materials of ultra-high purity. Plateaus appeared in the Hall effect not only for magnetic fields corresponding to the filling of orbits with one, two, three, etc, electron charges, but also for fields corresponding to fractional charges! This could be understood only in terms of a new kind of quantum fluid, where the motion of independent electrons of charge e is replaced by excitations in a multi-particle system which behave (in a strong magnetic field) as if charges of , , etc were involved. Robert B. Laughlin developed the theory that describes this new state of matter and shared the 1998 Physics Prize with Tsui and Störmer.

Sometimes, discoveries made in one field of physics turn out to have important applications in quite different areas. One example, of relevance for solid state physics, is the observation by Rudolf L. Mössbauer in the late 50s, that nuclei in "absorber" atoms can be resonantly excited by  rays from suitably chosen "emitter" atoms, if the atoms in both cases are bound in such a way that recoils are eliminated. The quantized energies of the nuclei in the internal electric and magnetic fields of the solid can be measured since they correspond to different positions of the resonances, which are extremely sharp. This turned out to be important for the determination of electronic and magnetic structure of many substances and Mössbauer received half the Physics Prize in 1961 and R. Hofstadter the other half.

Physics and Technology

Many of the discoveries and theories mentioned so far in this survey have had an impact on the development of technical devices; by opening completely new fields of physics or by providing ideas upon which such devices can be built. Conspicuous examples are the works of Shockley, Bardeen, and Brattain which led to the transistors and started a revolution in electronics, and the basic research by Townes, Basov, and Prokhorov which led to the development of masers and lasers. It could also be mentioned that particle accelerators are now important tools in several areas of materials science and in medicine. Other works honored by Nobel Prizes have had a more direct technical motivation, or have turned out to be of particular importance for the construction of devices for the development of communication and information.

An early Physics Prize (1912) was given to Nils Gustaf Dalén for his invention of an automatic "sun-valve", extensively used for lighting beacons and light buoys. It was based on the difference in radiation of heat from reflecting and black bodies: one out of three parallel bars in his device was blackened, which gave rise to a difference in heat absorption and length expansion of the bars during sunshine hours. This effect was used to automatically switch off the gas supply in daytime, eliminating much of the need for maintenance at sea.

Optical instrumentation and techniques have been the topics for prizes at several occasions. Around the turn of the century, Gabriel Lippmanndeveloped a method for colour photography using interference of light. A mirror was placed in contact with the emulsion of a photographic plate in such a way that when it was illuminated, reflection in the mirror gave rise to standing waves in the emulsion. Developing resulted in a stratification of the grains of silver and when such a plate was looked at in a mirror, the picture was reproduced in its natural colours. The Physics Prize in 1908 was awarded to Lippmann. Unfortunately, Lippmann's method requires very long exposure times. It has later been superseded by other techniques for photography but has found new applications in high-quality holograms.

In optical microscopy it was shown by Frits Zernike that even very weakly absorbing (virtually transparent) objects can be made visible if they consist of regions with different refractive indices. In Zernike's "phase-contrast microscope" it is possible to distinguish patches of light that have undergone different phase changes caused by this kind of inhomogeneity. This microscope has been of particular importance for observing details in biological samples. Zernike received the Physics Prize in 1953. In the 1940s, Dennis Gabor laid down the principles of holography. He predicted that if an incident beam of light is allowed to interfere with radiation reflected from a two-dimensional array of points in space, it would be possible to reproduce a three-dimensional picture of an object. However, the realization of this idea had to await the invention of lasers, which could provide the coherent light necessary for such interference phenomena to be observed. Gabor was awarded the Physics Prize in 1971.

Electron microscopy has had an enormous impact on many fields of natural sciences. Soon after the wave nature of electrons was clarified by C. J. Davisson and G. P. Thomson, it was realized that the short wavelengths of high energy electrons would make possible a much increased magnification and resolution as compared to optical microscopes. Ernst Ruska made fundamental studies in electron optics and designed the first working electron microscope early in the 1930s. However, it would take more than 50 years before this was recognized by a Nobel Prize.

Ruska obtained half of the Physics Prize for 1986, while the other half was shared between Gerd Binnig and Heinrich Rohrer, who had developed a completely different way to obtain pictures with extremely high resolution. Their method is applicable to surfaces of solids and is based on the tunneling of electrons from very thin metallic tips to atoms on the surface when the tip is moved at very close distance to it (about 1 nm). By keeping the tunneling current constant a moving tip can be made to follow the topography of the surface, and pictures are obtained by scanning over the area of interest. By this method, single atoms on surfaces can be visualized.

Radio communication is one of the great technical achievements of the 20th century. Guglielmo Marconi experimented in the 1890s with the newly discovered Hertzian waves. He was the first one to connect one of the terminals of the oscillator to the ground and the other one to a high vertical wire, the "antenna", with a similar arrangement at the receiving station. While Hertz' original experiments were made within a laboratory, Marconi could extend signal transmission to distances of several kilometers. Further improvement was made by Carl Ferdinand Braun (also father of the "Braunian tube", an early cathode ray oscilloscope), who introduced resonant circuits in the Hertzian oscillators. The tunability and the possibility to produce relatively undamped outgoing oscillations greatly increased the transmission range, and in 1901 Marconi succeeded in establishing radio connection across the Atlantic. Marconi and Braun shared the 1909 Nobel Prize in Physics.

At this stage, it was not understood how radio waves could reach distant places (practically "on the other side of the earth"), keeping in mind that they were known to be of the same nature as light, which propagates in straight lines in free space. Sir Edward V. Appleton finally proved experimentally that an earlier suggestion by Heaviside and Kennelly, that radio waves were reflected between different layers with different conductance in the atmosphere, was the correct explanation. Appleton measured the interference of the direct and reflected waves at various wavelengths and could determine the height of Heaviside's layer; in addition he found another one at a higher level which still bears his name. Appleton received the Physics Prize in 1947.

Progress in nuclear and particle physics has always been strongly dependent on advanced technology (and sometimes a driving force behind it). This was already illustrated in connection with the works of Cockcroft and Walton and of Lawrence, who developed linear electrostatic accelerators and cyclotrons, respectively. Detection of high energy particles is also a technological challenge, the success of which has been recognized by several Nobel Prizes.

The Physics Prize in 1958 was jointly awarded to Pavel A. Cherenkov, Il'ja M. Frank and Igor Y. Tamm for their discovery and interpretation of the Cherenkov effect. This is the emission of light, within a cone of specific opening angle around the path of a charged particle, when its velocity exceeds the velocity of light in the medium in which it moves. Since this cone angle can be used to determine the velocity of the particle, the work by these three physicists soon became the basis for fruitful detector developments.

The visualization of the paths of particles taking part in reactions is necessary for the correct interpretation of events occurring at high energies. Early experiments at relatively low energies used the tracks left in photographic emulsions. Charles T.R. Wilson developed a chamber in which particles were made visible by the fact that they leave tracks of ionized gas behind them. In the Wilson chamber the gas is made to expand suddenly, which lowers the temperature and leads to condensation of vapour around the ionized spots; these drops are then photographed in strong light. Wilson received half of the Physics Prize in 1927, the other half was awarded to Arthur H. Compton.

A further step in the same direction came much later when Donald A. Glaserinvented the "bubble chamber". In the 1950s accelerators had reached energies of 20-30 GeV and earlier methods were inadequate; for the Wilson chamber the path lengths in the gas would have been excessive. The atomic nuclei in a bubble chamber (usually containing liquid hydrogen) are used as targets, and the tracks of produced particles can be followed. At the temperature of operation the liquid is superheated and any discontinuity, like an ionized region, immediately leads to the formation of small bubbles. Essential improvements were made by Luis W. Alvarez, in particular concerning recording techniques and data analysis. His work contributed to a fast extension of the number of known elementary particles then known, in particular the so-called "resonances" (which were later understood as excited states of systems composed of quarks and gluons). Glaser received the Physics Prize in 1960 and Alvarez in 1968.

Bubble chambers were, up to the end of the 80s, the work horses of all high energy physics laboratories but have later been superseded by electronic detection systems. The latest step in detector development recognized by a Nobel Prize (in 1992) is the work of Georges Charpak. He studied in detail the ionization processes in gases and invented the "wire chamber", a gas-filled detector where densely spaced wires pick up electric signals near the points of ionization, by which the paths of particles can be followed. The wire chamber and its followers, the time projection chamber and several large wire chamber/scintillator/Cherenkov detector arrangements, combined into complex systems, has made possible the selective search for extremely rare events (like heavy quark production), which are hidden in strong backgrounds of other signals.

The first Nobel Prize (year 2000) in the new millennium was awarded in half to Jack S. Kilby for achievements that laid the foundations for the present information technology. In 1958, he fabricated the first integrated circuit where all electronic components are built on one single block of semiconducting material, later called "chip". This opened the way for miniaturization and mass production of electronic circuits. In combination with the development of components based on heterostructures described in an earlier section (for which Alferov and Kroemer shared the other half of the Prize), this has led to the "IT-revolution" that has reshaped so much our present society.

Further Remarks

In reading the present survey, it should be kept in mind that the number of Nobel awards is limited (according to the present rules, at most 3 persons can share a Nobel Prize each year). So far, 163 laureates have received Nobel Prizes for achievements in physics. Often, during the selection process, committees have had to leave out several other important, "near Nobel-worthy" contributions. For obvious reasons, it has not been possible to mention any of these other names and contributions in this survey. Still, the very fact that a relatively coherent account of the development of physics can be formulated, hinging as here on the ideas and experiments made by Nobel Laureates, can be taken as a testimony that most of the essential features in this fascinating journey towards an understanding of the world we inhabit have been covered by the Nobel Prizes in Physics.

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* Erik B. Karlsson was born in 1931. He was professor of physics at Uppsala University in 1975-1996 (now retired). He spent the years 1978-1980 as scientific associate at CERN, Geneva and in 1989 as Professeur invité at Université Joseph Fourier, Grenoble. He was elected as member of the Swedish Royal Academy of Sciences in 1982 and as member of its Nobel Committee for Physics 1987-1998 (chairman in 1998). Publications include: The Use of Positive Muons in Metal Physics (1981); Solid State Phenomena, as seen by Muons, Protons and Excited Nuclei (Oxford University Press, 1995); Modern Studies of Basic Quantum Concepts and Phenomena (organizer and editor, World Scientific, 1998) as well as numerous articles on nuclear and solid state magnetism, metal-hydrogen systems, tunneling phenomena, etc.

This article is published as a chapter of the book "The Nobel Prize: The First 100 Years", Agneta Wallin Levinovitz and Nils Ringertz, eds., Imperial College Press and World Scientific Publishing Co. Pte. Ltd., 2001.

First published 9 February 2000

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