The Physical Review begins publication at Cornell University’s Franklin Hall in Ithaca, New York
The American Physical Society is founded
Millikan determines the electron’s charge
Millikan’s oil-drop experiment proves that electric charge comes only in discrete, integer multiples of a fundamental constant, rather than in a continuum of values. Millikan determines this constant—the charge of the electron—to within half of a percent of today’s accepted value.
Compton shows light’s particle nature
Compton establishes the particle nature of light. He shows that when x rays and gamma rays scatter from electrons, they lose momentum as would be expected for corpuscular objects rather than for waves.
Matter is found to behave like waves
Davisson and Germer show experimentally that matter can be wavelike. The duo bombard nickel crystals with a beam of electrons, observing an interference pattern in the scattered electrons that could only be explained if the particles behave as waves.
Onsager presents theory for irreversible processes
Onsager’s two papers provide a general theory for irreversible processes such as heat transfer and current flow. Onsager derives a set of “reciprocal relations” that have, for example, been used to predict the behavior of thermoelectric and spintronic devices.
Discovery of deuterium is reported
Urey, Brickwedde, and Murphy report their discovery of deuterium, an isotope of hydrogen comprised of a proton and a neutron. Deuterium oxide, or “heavy water,” is later used during World War II in nuclear reactors. Today deuterium is widely used in nuclear magnetic resonance (NMR) experiments, as well as in many chemistry and particle physics experiments.
Anderson publishes observation of positron
Anderson discovers the positron, the electron’s antiparticle, by observing tracks left in a cloud chamber by a then-unknown particle in cosmic rays. Anderson’s experiments provide the first concrete evidence for Dirac’s prediction that every fermion has an antiparticle partner with the same mass but opposite charge.
Einstein-Podolsky-Rosen paradox challenges quantum theory
Einstein, Podolsky and Rosen (EPR) imagine a gedankenexperiment designed to point out a flaw in quantum mechanics: the theory conflicts with local realism. Experiments later vindicate quantum mechanics by confirming the violation of Bell inequalities. But the EPR paper also introduces the property now known as entanglement, which becomes the basis for the field of quantum information.
Nuclear magnetic resonance is discovered
Rabi and colleagues discover the phenomenon of nuclear magnetic resonance (NMR) and measure the magnetic moment of nuclei in molecular beams. Bloch, as well as Purcell and his co-workers, later extend Rabi’s techniques to the study of nuclei in liquids and solids, which eventually makes magnetic resonance imaging (MRI) possible.
Bohr and Wheeler describe fission with liquid-drop model
Less than a year after physicists reported their stunning discovery of nuclear fission, Bohr and Wheeler use the liquid-drop model of the nucleus to calculate fission parameters that agree well with experiments. The results become essential for the development of the atomic bomb and nuclear power.
Bethe predicts stellar nuclear reactions
Bethe shows that two types of helium-yielding nuclear reactions could power stars: the fusion of hydrogen and the so-called carbon-oxygen-nitrogen cycle. Nine years later, Bethe, Alpher, and Gamow propose an explanation for the abundance of the chemical elements using one of the first models of the post-big-bang Universe.
Lamb shift is discovered
Lamb and Retherford measure a small energy separation between two atomic orbitals of hydrogen that wasn’t predicted by Dirac’s theory. Bethe attributes this “Lamb shift” to interactions between the atom’s electrons and vacuum fluctuations. Within months, he describes the effect with a new “renormalization” approach that lays the foundation for quantum electrodynamics.
Schwinger and Feynman develop quantum electrodynamics
Working independently, Schwinger and Feynman each develops his own version of quantum electrodynamics (QED), with Feynman introducing his famous diagrams. Dyson shows that the two approaches are equivalent, and QED soon delivers unprecedentedly precise predictions—such as the electron’s anomalous magnetic moment—that are later confirmed in experiments.
Reines and Cowan announce first potential neutrino detection
Pauli had postulated the neutrino in the 1930s to explain missing energy in nuclear beta decay. In 1953 Reines and Cowan announce that they may have detected the ghostly particles using giant tanks of water placed near a nuclear reactor. They report the definitive detection of neutrinos in 1956 (in Science) and provide a full account of their experiments in 1960 (in the Physical Review).
Yang and Mills lay groundwork for field theory of strong and electroweak interactions
Yang and Mills formulate field theories in terms of photonlike particles. These Yang-Mills fields become central elements in the description of the electroweak interaction and in quantum chromodynamics, the theory that describes quarks
Parity violation is found in weak interaction
Mirror symmetry or, as physicists call it, parity symmetry, holds the status of a sacred principle until theorists Lee and Yang show that they can explain puzzling cosmic-ray data by assuming that the symmetry is violated in weak interactions. A year later, beta-decay experiments by Wu and her collaborators prove that parity is, in fact, violated.
Bardeen, Cooper, and Schrieffer develop theory of superconductivity
Nearly half a century after superconductivity was discovered, Bardeen, Cooper, and Schrieffer (BCS) provide a theory for the phenomenon in terms of electrons that pair up and condense into a single quantum state. BCS theory has been applied not only to problems in condensed matter, but also in particle and nuclear physics.
Spontaneous symmetry breaking emerges in theory for pions
Nambu relates the smallness of the pion mass to an approximate symmetry, which leads to an important insight: the symmetry of a physical system can be different from that of the interactions among the system’s components. Such “spontaneous symmetry breaking” is ubiquitous, occurring in, for example, magnets and solids. It is also related to the theory underlying the Higgs boson.
Gell-Mann classifies particles with “eightfold way”
Gell-Mann classifies light mesons and spin-1/2 baryons with a scheme known as the eightfold way. The scheme relies on an approximate symmetry that is ultimately explained in terms of the symmetries of the three lightest quarks: the up, down and strange quarks.
Giacconi discovers extrasolar x-ray source
To get past Earth’s x-ray absorbing atmosphere, Giacconi and his colleagues send up a rocket loaded with Geiger counters and make the surprising discovery of an x-ray source outside our Solar System. Giacconi is considered the father of x-ray astronomy, and his work led to the development of satellite telescopes for observing x-ray emissions from black holes and other cosmic sources.
Glauber formulates quantum theory for photons
Glauber develops the theory that describes the correlations among photons in a beam. His breakthrough is to realize that, because of quantum mechanics, the arrival of one photon in the beam affects the probability of detecting a subsequent photon. His approach proves instrumental for developing new methods in optical detection.
Cabibbo predicts particles mixing
Cabibbo’s theoretical ideas lead to the realization that quarks of a definite mass do not necessarily have definite flavor, that is, up, down, strange, etc. Rather, they can be mixtures of different flavors. Cabibbo’s insight explains why certain particle decays are suppressed, and it introduces the now-ubiquitous notion of mixing to particle physics.
Method of density functional theory is proposed
Hohenberger, Kohn, and Sham introduce density functional theory (DFT), a method that greatly simplifies calculations of the properties of molecules and solids, yet still provides high accuracy. The method uses an approximate solution to the quantum-mechanical equations of many-electron systems. Verlet soon develops the classical counterpart of DFT, a numerical method of solving Newton’s equations that is later used in computer simulations. Carr and Parrinello unify DFT and Verlet’s approach in 1985.
Higgs boson is predicted
Englert and Higgs independently derive a model that explains why fundamental particles have mass. Their theories require the existence of a new particle, now known as the Higgs boson, which becomes a critical building block of the standard model and is finally observed nearly 50 years later at CERN’s Large Hadron Collider.
Weinberg develops electroweak theory
Weinberg proposes a theory of electroweak interactions, which, when extended to include quarks and the strong interactions, becomes the standard model of particle physics. The central features detailed by Weinberg are all later confirmed by experiments, including, in 2012, the existence of the Higgs boson.
Experiments probe proton’s substructure
Electron-proton scattering experiments reported by Friedman, Kendall, Taylor and their colleagues give the first concrete evidence that protons are not fundamental particles. Instead, their data support the idea that protons are comprised of smaller particles, which we now know as quarks.
Wilson details renormalization group approach
Wilson’s two papers lay the foundations for the renormalization group approach, which is a mathematical framework for studying the properties of a system on various length scales. As a system makes a phase transition, the length scale over which correlations extend becomes infinite, and the renormalization group provides a powerful way to describe the correlations.
Superfluidity is observed in helium-3
Osheroff, Lee and Richardson observe that helium-3 cooled to near absolute zero becomes a superfluid, a liquid with zero viscosity. The finding demonstrates that fermions like helium-3 atoms can also form a superfluid phase, albeit one more complex than that of bosons like helium-4 atoms.
Theory of quark interaction is developed
Gross and Wilczek, and separately, Politzer, work out a theory for quarks that explains two seemingly contradictory observations: Quarks are always bound together to form other particles, such as protons or neutrons; and within one of these composite particles, they are only loosely bound.
Charm quark is discovered
Two groups—one led by Ting and the other by Richter—discover the charm quark by producing the charm-anticharm bound state, a particle that becomes known as the J/psi. The finding is hailed as the “November Revolution” because it promotes quarks from the status of a theoretical construct to an experimental reality.
Shell-model predictions are found to break down for light, unstable nuclei
The nuclear shell model had predicted that the most stable nuclei are those with certain “magic” numbers of neutrons and protons. Researchers studying unstable sodium nuclei at CERN find evidence that this picture breaks down for very neutron-rich nuclei.
Theorists describe topological phases
Thouless, Kosterlitz, and Haldane use the mathematics of topology to describe a number of exotic phases and phase transitions in solids and liquids. Their work provides insight into transport in thin films and the behavior of low-dimensional quantum magnets, superfluids, and superconductors.
Halperin and Nelson predict hexatic phase
Halperin and Nelson propose that the melting of a 2D solid involves an intermediate phase with both liquidlike and solidlike properties. The discovery of this so-called hexatic phase, which is later observed in experiments and simulations, demonstrates that melting in 2D systems (like thin films) is fundamentally different from that in 3D materials.
Quantum Hall effect is discovered
Von Klitzing, Dorda and Pepper find that the Hall conductance of a two-dimensional electron gas at low temperature jumps in integer multiples of e2/h as an external magnetic field is increased. The discovery of this quantum Hall effect (QHE) is closely followed by Tsui, Stormer and Gossard’s observation of the fractional QHE, in which excitations have fractional charge.
Guth proposes theory of inflation
Guth hypothesizes that, fractions of a second after the big bang, the Universe underwent a period of exponentially rapid expansion. This “inflation” model provides an explanation for two observed properties of the Universe that cosmologists had struggled to explain—its homogeneity and its flatness.
Bell tests confirm quantum predictions
Quantum theory had predicted that correlations between entangled particles exceed the value attainable by classical particles. Aspect and his colleagues confirm this prediction by performing a “Bell” test with photon pairs emitted from an atom, providing another win for the theory. Significantly, Aspect eliminates interactions among the measuring devices that had undermined the interpretation of earlier Bell tests.
Binnig and Rohrer report invention of scanning tunneling microscope
Binnig, Rohrer, and their colleagues demonstrate a scanning tunneling microscope (STM), which uses an atomically sharp tip to measure a miniscule tunneling current from a material’s surface. The probe can be scanned across a material’s surface to obtain an image with atomic resolution. Binnig later teams up with Quate and Gerber to develop the related technique of atomic force microscopy (AFM).
Quasicrystals are discovered
Schechtman and colleagues cause physicists to rethink the notion of a crystal with their discovery of an alloy whose atoms are arranged with fivefold symmetry (like a pentagon) but without any repeating units. In a follow-up paper, Levine and Steinhardt dub this arrangement of atoms a “quasicrystal” and explain how it is possible.
Chu advances laser-cooling techniques
Chu and colleagues use the radiation pressure of counterpropagating laser beams to confine atoms at a record low temperature (a few hundred microkelvin) and for up to almost a hundred microseconds. Their technique of cooling and trapping atoms leads to advances in precision atom spectroscopy and in studying quantum phases of matter such as the Bose-Einstein condensate.
Navier-Stokes equations are simulated on computer
Frisch, Hasslacher, and Pomeau devise a method of simulating the Navier-Stokes equations, which describe the behavior of fluids and are used in many areas of science and technology. Their approach consists of a virtual particle known as a “cellular automaton” whose movements on a hexagonal grid are related to the motions of fluid particles.
Superconductivity is achieved at record-high temperatures
Chu and colleagues synthesize a compound of yttrium-barium-copper-oxygen (YBCO) that superconducts at 93 kelvin—a record high. This transition temperature is balmy enough to be reached with an inexpensive cryogen (liquid nitrogen), suggesting that YBCO could be suitable for practical purposes such as power lines.
Giant magnetoresistance is discovered
Fert and Grünberg separately discover that they can significantly change the resistance between two magnetic layers by rotating one of the layers with respect to the other. This “giant” magnetoresistive effect is now used in hard drives and in spintronics devices, in which information is carried and stored via an electron’s spin, rather than its charge.
Photons are prepared in macroscopic quantum states
Haroche and his colleagues demonstrate the quantum superposition of macroscopic quantum states—also known as Schrödinger cat states—using photons that interact with highly excited atoms inside of a cavity. With a similar experimental setup, the team later observes the effect of quantum decoherence, a process that lies at the heart of quantum measurements.
Neutrino oscillations are observed
Researchers at Japan’s Super-Kamiokande experiment provide strong evidence that atmospheric muon neutrinos spontaneously transform into tau neutrinos (and vice versa)—that is, neutrinos “oscillate.” This finding and subsequent results from the Sudbury Neutrino Observatory imply that neutrinos have mass and explain the puzzling electron neutrino deficit from the Sun.
Pendry describes blueprint for perfect lens
Pendry imagines a “superlens” that would focus light beyond the limits set by classical wave optics. The proposed device would be made of materials that have a negative index of refraction. This structure amplifies evanescent waves (which normally die out), allowing for a theoretically perfect reconstruction of an imaged object.
Newman, Strogatz, and Watts implement random-graph model
Newman, Strogatz, and Watts develop a mathematical formalism for analyzing random graphs, which are good models for many real-world networks, such as epidemic spreading or social interactions. Their formalism applies to a more general class of graph compared with previous approaches and expands the types of problems that can be described with random-graph theory.
Synthesis of element 118 is reported
Scientists working at the Joint Institute for Nuclear Research in Dubna, Russia, had found hints in 2002 of a new superheavy chemical element with 118 protons. A 2006 paper from the team reports a series of experiments that confirm the new element, the heaviest ever produced in the lab. Element 118 is eventually named Oganesson in honor of one of its discoverers.
3D topological insulators are predicted
Fu and Kane predict a 3D version of a topological insulator, an exotic insulator with conducting surface states that are unusually robust even in the presence of impurities or defects. Until their prediction, a topological insulator had only been realized in 2D films. Fu and Kane show that the fascinating phase could exist in a much greater variety of materials.
Weyl fermions are observed in a solid
Hermann Weyl had predicted the existence of a massless fermion with a definite handedness, or “chirality,” in 1929. Physicists never found a fundamental particle with these characteristics, but a condensed-matter analog of the Weyl fermion is ultimately discovered in tantalum arsenide.
LIGO reports observation of gravitational waves
The collaborations behind the Laser Interferometer Gravitational-Wave Observatory (LIGO) and the Virgo experiment report that LIGO’s sensitive interferometers have picked up a gravitational-wave signal from the merger of two black holes—the first detection of the waves that Einstein had predicted in 1916. LIGO’s success sets the stage for a new era of gravitational-wave astronomy, and it is soon followed by a joint detection with Virgo of a binary neutron star merger.