- Nucleon-nucleon correlations, short-lived excitations, and the quarks within

Author(s): Or Hen, Gerald A. Miller, Eli Piasetzky, and Lawrence B. WeinsteinWhat happens to nucleon-nucleon interactions at very short distances, below the radius of the proton? Does the structure of … - Coupling functions: Universal insights into dynamical interaction mechanisms

Author(s): Tomislav Stankovski, Tiago Pereira, Peter V. E. McClintock, and Aneta StefanovskaDynamical systems in science with examples ranging from physics, chemistry, and biology up to population dyn … - Colloquium: Quantum coherence as a resource

Author(s): Alexander Streltsov, Gerardo Adesso, and Martin B. PlenioThe dictum that “information is physical” indicates that we should understand how features of quantum physics, in particular, the ph … - Colloquium: Astromaterial science and nuclear pasta

Author(s): M. E. Caplan and C. J. HorowitzWhat is the nature of matter in the interior of cold white dwarfs and in the crust of neutron stars? To shed light on this question, theoretical simulations h … - Colloquium: Strong optical forces on atoms in multifrequency light

Author(s): Harold MetcalfOne of the fundamental processes in physics is the interaction of matter with light. One learns early in quantum mechanics how electrons in a hydrogen atom absorb light with w … - Nobel Lecture: Topological defects and phase transitions

Author(s): John Michael KosterlitzThe 2016 Nobel Prize for Physics was shared by David J. Thouless, F. Duncan M. Haldane, and John Michael Kosterlitz. These papers are the text of the address given in … - Nobel Lecture: Topological quantum matter

Author(s): F. Duncan M. HaldaneThe 2016 Nobel Prize for Physics was shared by David J. Thouless, F. Duncan M. Haldane, and John Michael Kosterlitz. These papers are the text of the address given in co … - Multiboson interactions at the LHC

Author(s): D. R. Green, P. Meade, and M.-A. PleierAt the LHC the production of two or more gauge bosons is completely predicted by the standard model to high precision. Departures from those predictio … - Colloquium: Toward living matter with colloidal particles

Author(s): Zorana Zeravcic, Vinothan N. Manoharan, and Michael P. BrennerLiving systems are undoubtedly complicated. The complicated behaviors result from interactions between many different component … - The $^{12}\mathbf{C}(α,γ)^{16}\mathbf{O}$ reaction and its implications for stellar helium burning

Author(s): R. J. deBoer, J. Görres, M. Wiescher, R. E. Azuma, A. Best, C. R. Brune, C. E. Fields, S. Jones, M. Pignatari, D. Sayre, K. Smith, F. X. Timmes, and E. UbersederThe 12C(α,γ)16O reaction is … - Universal few-body physics and cluster formation

Author(s): Chris H. Greene, P. Giannakeas, and J. Pérez-RíosA comprehensive account of the theoretical analyses of the Efimov effect and the universal properties of three-body bound states is provided … - Yield stress materials in soft condensed matter

Author(s): Daniel Bonn, Morton M. Denn, Ludovic Berthier, Thibaut Divoux, and Sébastien MannevilleThis review discusses disordered materials that only flow if the imposed stress is beyond a certain th … - Electron vortices: Beams with orbital angular momentum

Author(s): S. M. Lloyd, M. Babiker, G. Thirunavukkarasu, and J. YuanThis article presents a review on electron vortex beams highlighting both its experimental and its theoretical aspects. The unique c … - Simulation and understanding of atomic and molecular quantum crystals

Author(s): Claudio Cazorla and Jordi BoronatSolids formed by light atoms or molecules have, at experimentally realizable low temperatures, a kinetic energy per particle that is large compared to the t … - Quantum sensing

Author(s): C. L. Degen, F. Reinhard, and P. CappellaroQuantum technologies are increasingly driving the field of precision metrology. While current techniques for sensing and recording time rely on cl … - Strongly coupled quark-gluon plasma in heavy ion collisions

Author(s): Edward ShuryakThe field of relativistic heavy ion collisions spans over five decades ranging from the formulation of scientific goals and early experiments at Berkeley in the 1960s with mos … - Perpendicular magnetic anisotropy at transition metal/oxide interfaces and applications

Author(s): B. Dieny and M. ChshievIn spintronics devices, magnetic materials are used as polarizers or analyzers for electron spin. Magnetic anisotropy defines the orientation for magnetization and po … - Frontiers of chaotic advection

Author(s): Hassan Aref, John R. Blake, Marko Budišić, Silvana S. S. Cardoso, Julyan H. E. Cartwright, Herman J. H. Clercx, Kamal El Omari, Ulrike Feudel, Ramin Golestanian, Emmanuelle Gouillart, GertJ … - Interface-induced phenomena in magnetism

Author(s): Frances Hellman et al.Magnetism at interfaces often takes on a fundamentally completely different character when compared to magnetism in bulk. This review focuses on these differences and … - Quantum Hall physics: Hierarchies and conformal field theory techniques

Author(s): T. H. Hansson, M. Hermanns, S. H. Simon, and S. F. ViefersThe quantum Hall effects by now are recognized as prime examples of the importance of topological considerations in condensed-matte … - Colloquium: Strongly interacting photons in one-dimensional continuum

Author(s): Dibyendu Roy, C. M. Wilson, and Ofer FirstenbergPhotons, the particles of light, are in most conditions very weakly interacting. Nevertheless, it is possible to make them interact by alteri … - Graviton mass bounds

Author(s): Claudia de Rham, J. Tate Deskins, Andrew J. Tolley, and Shuang-Yong ZhouIf gravitation propagates via a massive field, the velocity of gravitational waves (gravitons) depends on their frequ … - Quantum spin liquid states

Author(s): Yi Zhou, Kazushi Kanoda, and Tai-Kai NgThe concept of a quantum spin liquid is important for problems ranging from quantum spin chains to high-temperature superconductivity. This review giv … - Experimental soft-matter science

Author(s): Sidney R. NagelSoft condensed matter refers to materials where the constituent building blocks are larger than atoms but smaller than the system itself. The large size of the constituent pa … - Testing black hole candidates with electromagnetic radiation

Author(s): Cosimo BambiCan one determine the black hole nature of an observed object by electromagnetic observations? As astrophysical black holes are expected to result from collapse with nonzero ang … - Erratum: Carrier dynamics in semiconductors studied with time-resolved terahertz spectroscopy [Rev. Mod. Phys. 83, 543 (2011)]

Author(s): Ronald Ulbricht, Euan Hendry, Jie Shan, Tony F. Heinz, and Mischa Bonn[Rev. Mod. Phys. 89, 029901] Published Thu Apr 06, 2017 - Colloquium: Atomic quantum gases in periodically driven optical lattices

Author(s): André EckardtDynamics of quantum many-body systems is one of the most complex problems in physics since it involves the time evolution of a large number of particles that interact with each … - Equations of state for supernovae and compact stars

Author(s): M. Oertel, M. Hempel, T. Klähn, and S. TypelWhat are the thermodynamic properties of matter at extreme densities, even exceeding nuclear matter density severely? How can we describe the com … - Stimulated Raman adiabatic passage in physics, chemistry, and beyond

Author(s): Nikolay V. Vitanov, Andon A. Rangelov, Bruce W. Shore, and Klaas BergmannBy use of a pulse sequence, the technique of stimulated Raman adiabatic passage transfers population without loss be … - Light fields in complex media: Mesoscopic scattering meets wave control

Author(s): Stefan Rotter and Sylvain GiganWave front shaping, the ability to manipulate light fields both spatially and temporally, in complex media is an emerging field with many applications. This r … - Quantum random number generators

Author(s): Miguel Herrero-Collantes and Juan Carlos Garcia-EscartinIn mathematics and computer science, random numbers have the role of a resource for assisting proofs, making cryptography secure, and … - Electron-phonon interactions from first principles

Author(s): Feliciano GiustinoThe electron-phonon interaction in solids is important for many interesting properties of solids, among them the critical temperature of phonon-mediated superconductors, t … - Entropic uncertainty relations and their applications

Author(s): Patrick J. Coles, Mario Berta, Marco Tomamichel, and Stephanie WehnerThe Heisenberg uncertainty principle has a more precise formulation in terms of inequalities involving quantum entropies … - Editorial: From the APS Editor in Chief

Author(s): Pierre Meystre[Rev. Mod. Phys. 89, 010001] Published Fri Feb 03, 2017 - Colloquium: An algebraic model of localized surface plasmons and their interactions

Author(s): T. J. Davis and D. E. GómezLocalized surface plasmons are collective modes of the conduction electrons excited by light in metal nanoparticles. Although plasmons have been known for some ti … - Colloquium: Status of $α$-particle condensate structure of the Hoyle state

Author(s): Akihiro Tohsaki, Hisashi Horiuchi, Peter Schuck, and Gerd RöpkeIn 1954, Fred Hoyle made a prediction regarding the existence of a resonant state of carbon-12 that would be associated with a … - Dynamics of non-Markovian open quantum systems

Author(s): Inés de Vega and Daniel AlonsoThis review gives a summary of the many techniques that are used in the analysis of open quantum systems. Emphasis is on those cases where it is unsuitable to … - Colloquium: Persistent spin textures in semiconductor nanostructures

Author(s): John SchliemannSpintronics continues to be a field of fast evolution where new concepts and devices are continuously being developed. One of the limiting factors in spintronics is the decoh … - Nonlinear optical signals and spectroscopy with quantum light

Author(s): Konstantin E. Dorfman, Frank Schlawin, and Shaul MukamelBy variation of the photon statistics and the entanglement between time and frequency components, quantum states of light feature spe … - Particle and nuclear physics instrumentation and its broad connections

Author(s): M. Demarteau, R. Lipton, H. Nicholson, and I. ShipseyProgress in the physical sciences depends as much on innovations in instrumentation that enable more refined experiments as on new theor … - Colloquium: Herbertsmithite and the search for the quantum spin liquid

Author(s): M. R. NormanQuantum spin liquids form a novel class of matter where, despite the existence of strong exchange interactions, spins do not order down to the lowest measured temperature. Typic … - Active particles in complex and crowded environments

Author(s): Clemens Bechinger, Roberto Di Leonardo, Hartmut Löwen, Charles Reichhardt, Giorgio Volpe, and Giovanni VolpeThis article reviews both experimental and theoretical advances in the field of a … - Quantum memories at finite temperature

Author(s): Benjamin J. Brown, Daniel Loss, Jiannis K. Pachos, Chris N. Self, and James R. WoottonWhile the typical scenario for quantum error correction involves active intervention there are advantag … - Time-dependent density-functional description of nuclear dynamics

Author(s): Takashi Nakatsukasa, Kenichi Matsuyanagi, Masayuki Matsuo, and Kazuhiro YabanaMany excitation modes of atomic nuclei and their reactions can be described as time-dependent processes, in whi … - Materials perspective on Casimir and van der Waals interactions

Author(s): L. M. Woods, D. A. R. Dalvit, A. Tkatchenko, P. Rodriguez-Lopez, A. W. Rodriguez, and R. PodgornikElectromagnetic fluctuation-induced interactions known as van der Waals, Casimir, and Casim … - $CP$ violation in the ${B}_{s}^{0}$ system

Author(s): Marina Artuso, Guennadi Borissov, and Alexander LenzSources of CP violation beyond those of the standard model of particle physics are needed to explain the dominance of matter over antimat … - Colloquium: Protecting quantum information against environmental noise

Author(s): Dieter Suter and Gonzalo A. ÁlvarezQuantum-mechanical systems retain their properties so long as the phase of quantum superpositions evolve stably over time. Contact with an environment can … - Alternatives to an elementary Higgs

Author(s): Csaba Csáki, Christophe Grojean, and John TerningExperiments point to the Higgs particle being the excitation of an elementary scalar field, as originally proposed in the standard model. To … - CODATA recommended values of the fundamental physical constants: 2014

Author(s): Peter J. Mohr, David B. Newell, and Barry N. TaylorThis review article contains the 2014 self-consistent set of values of the constants and conversion factors of physics and chemistry recom … - Leptonic and semileptonic decays of $B$ mesons

Author(s): Jochen Dingfelder and Thomas MannelIn the standard model of particle physics, the strong and weak interaction eigenstates of the three generations of quarks differ and are related by the un … - Publisher’s Note: Google matrix analysis of directed networks [Rev. Mod. Phys. 87, 1261 (2015)]

Author(s): Leonardo Ermann, Klaus M. Frahm, and Dima L. Shepelyansky[Rev. Mod. Phys. 88, 039905] Published Tue Sep 20, 2016 - Sap flow and sugar transport in plants

Author(s): K. H. Jensen, K. Berg-Sørensen, H. Bruus, N. M. Holbrook, J. Liesche, A. Schulz, M. A. Zwieniecki, and T. BohrGreen plants harvest the energy of the Sun in the leaves by converting light en … - Control principles of complex systems

Author(s): Yang-Yu Liu and Albert-László BarabásiComplex networks range from subcellular biological networks to the Internet. Our ability to control these systems deeply challenges our understanding. … - Classification of topological quantum matter with symmetries

Author(s): Ching-Kai Chiu, Jeffrey C. Y. Teo, Andreas P. Schnyder, and Shinsei RyuIn recent years an increasing amount of attention has been devoted to quantum materials with topological characteristi … - Strangeness in nuclear physics

Author(s): A. Gal, E. V. Hungerford, and D. J. MillenerEveryday matter is made of the lightest up and down quarks. The strange quark is the third lightest of all quarks. Strangeness, a property of par … - Erratum: Ultrafast optical manipulation of magnetic order [Rev. Mod. Phys. 82, 2731 (2010)]

Author(s): Andrei Kirilyuk, Alexey V. Kimel, and Theo Rasing[Rev. Mod. Phys. 88, 039904] Published Thu Aug 11, 2016 - Statistical mechanics of ecological systems: Neutral theory and beyond

Author(s): Sandro Azaele, Samir Suweis, Jacopo Grilli, Igor Volkov, Jayanth R. Banavar, and Amos MaritanIt is of societal importance to advance the understanding of emerging patterns of biodiversity f … - Nonlinear waves in $\mathcal{PT}$-symmetric systems

Author(s): Vladimir V. Konotop, Jianke Yang, and Dmitry A. ZezyulinThe concept of parity-time symmetric systems is rooted in non-Hermitian quantum mechanics where complex potentials obeying this symme … - Erratum: Hidden variables and the two theorems of John Bell [Rev. Mod. Phys. 65, 803 (1993)]

Author(s): N. David Mermin[Rev. Mod. Phys. 88, 039902] Published Fri Jul 15, 2016 - Erratum: Modeling semiflexible polymer networks [Rev. Mod. Phys. 86, 995 (2014)]

Author(s): C. P. Broedersz and F. C. MacKintosh[Rev. Mod. Phys. 88, 039903] Published Fri Jul 15, 2016 - Fermion path integrals and topological phases

Author(s): Edward WittenSymmetry-protected phases of matter have been at the forefront of condensed matter physics in recent years. Bosonic symmetry-protected phases have been interpreted in terms of … - Publisher’s Note: Metallic quantum ferromagnets [Rev. Mod. Phys. 88, 25006 (2016)]

Author(s): M. Brando, D. Belitz, F. M. Grosche, and T. R. Kirkpatrick[Rev. Mod. Phys. 88, 039901] Published Mon Jul 11, 2016 - Nobel Lecture: Discovery of atmospheric neutrino oscillations

Author(s): Takaaki KajitaThe 2015 Nobel Prize for Physics was shared by Takaaki Kajita and Arthur B. McDonald. These papers are the text of the address given in conjunction with the award.[Rev. Mod. P ... - Nobel Lecture: The Sudbury Neutrino Observatory: Observation of flavor change for solar neutrinos

Author(s): Arthur B. McDonaldThe 2015 Nobel Prize for Physics was shared by Takaaki Kajita and Arthur B. McDonald. These papers are the text of the address given in conjunction with the award.[Rev. Mo ... - Colloquium: Topological band theory

Author(s): A. Bansil, Hsin Lin, and Tanmoy DasFirst-principles band theory, properly augmented by topological considerations, has provided a remarkably successful framework for predicting new classes ... - Metallic quantum ferromagnets

Author(s): M. Brando, D. Belitz, F. M. Grosche, and T. R. KirkpatrickA full understanding of long range ferromagnetic order in metallic systems reflects a variety of phenomena which are best understoo ... - Physical properties of low-dimensional ${sp}^{2}$-based carbon nanostructures

Author(s): V. Meunier, A. G. Souza Filho, E. B. Barros, and M. S. DresselhausThis review focuses on the fundamental physical properties of low-dimensional carbon nanostructures (graphene, graphene nan ... - Scaled plane-wave Born cross sections for atoms and molecules

Author(s): H. Tanaka, M. J. Brunger, L. Campbell, H. Kato, M. Hoshino, and A. R. P. RauElectron-atom and electron-molecule collisional cross sections are needed in the modeling and understanding of ph ... - Coulomb drag

Author(s): B. N. Narozhny and A. LevchenkoCoulomb drag, a term coined in analogy to phonon drag, refers to the effect of mutual friction between nonequilibrium conduction electrons belonging to two el ... - Colloquium

Author(s): W. Ubachs, J. Bagdonaite, E. J. Salumbides, M. T. Murphy, and L. KaperLooking back into 10-12 billion years of cosmic history this Colloquium paper summarizes what is presently known about ... - The physics of epigenetics

Author(s): Ruggero Cortini, Maria Barbi, Bertrand R. Caré, Christophe Lavelle, Annick Lesne, Julien Mozziconacci, and Jean-Marc VictorEpigenetics is essential in understanding the development, from a ... - Colloquium: Non-Markovian dynamics in open quantum systems

Author(s): Heinz-Peter Breuer, Elsi-Mari Laine, Jyrki Piilo, and Bassano VacchiniAn ongoing theme in quantum physics is the interaction of small quantum systems with an environment. If that environmen ... - Colloquium: Measuring the neutron star equation of state using x-ray timing

Author(s): Anna L. Watts, Nils Andersson, Deepto Chakrabarty, Marco Feroci, Kai Hebeler, Gianluca Israel, Frederick K. Lamb, M. Coleman Miller, Sharon Morsink, Feryal Özel, Alessandro Patruno, Juri Po ... - Phase diagram of QCD in a magnetic field

Author(s): Jens O. Andersen, William R. Naylor, and Anders TranbergThis review addresses the current theoretical understanding of hadronic matter at large magnetic fields. Applications include heavy-i ... - Physics of Alfvén waves and energetic particles in burning plasmas

Author(s): Liu Chen and Fulvio ZoncaIn magnetic fusion reactors relying on the burning of deuterium and tritium, sufficient confinement of the alpha particles produced in the nuclear reactions is cruc ... - The physics of x-ray free-electron lasers

Author(s): C. Pellegrini, A. Marinelli, and S. ReicheThe advent of x-ray free electron lasers has made possible the study of matter at the characteristic space and time scales of atomic and molecular ... - Linac Coherent Light Source: The first five years

Author(s): Christoph Bostedt, Sébastien Boutet, David M. Fritz, Zhirong Huang, Hae Ja Lee, Henrik T. Lemke, Aymeric Robert, William F. Schlotter, Joshua J. Turner, and Garth J. WilliamsIn the five yea ... - Delayed-choice gedanken experiments and their realizations

Author(s): Xiao-song Ma, Johannes Kofler, and Anton ZeilingerWave-particle duality lies at the root of quantum mechanics and is central in the description of interferences observed with elementary obj ... - Big bang nucleosynthesis: Present status

Author(s): Richard H. Cyburt, Brian D. Fields, Keith A. Olive, and Tsung-Han YehHow do we understand the production of the lightest nuclides from H to Li during the first seconds of cosmic time? This ... - Colloquium: Water’s controversial glass transitions

Author(s): Katrin Amann-Winkel, Roland Böhmer, Franz Fujara, Catalin Gainaru, Burkhard Geil, and Thomas LoertingBesides being fundamental for life on Earth, water is one of the complex materials in na ... - Experimental tests of particle flow calorimetry

Author(s): Felix Sefkow, Andy White, Kiyotomo Kawagoe, Roman Pöschl, and José RepondMethods of measuring the energies of jets have relied on detection of photons, charged hadrons, and neutral hadrons ... - Jerusalem lectures on black holes and quantum information

Author(s): D. HarlowThe quantum mechanics of black holes, and particularly the information paradox, have been a crucial arena for testing theories of quantum gravity. This review covers the quantum ph ... - Colloquium: Nanoplasmas generated by intense radiation

Author(s): Kostya (Ken) Ostrikov, Farhat Beg, and Andrew NgPhysical phenomena at the nanoscale can be considerably different from the behavior in the macroscopic world. This rule is also true for plas ... - Lattice tests of beyond standard model dynamics

Author(s): Thomas DeGrandOriginally developed for quantum chromodynamics at strong coupling, lattice approximations provide an essential tool applicable to any strongly coupled gauge theory. The resol ... - Symmetry violations in nuclear and neutron $β$ decay

Author(s): K. K. Vos, H. W. Wilschut, and R. G. E. TimmermansExperimental searches for new physics beyond the standard model of particle physics are not constrained to the high energy sector alone. Th ... - Ion-trap measurements of electric-field noise near surfaces

Author(s): M. Brownnutt, M. Kumph, P. Rabl, and R. BlattHow can the electric noise in the vicinity of a metallic body be measured and understood? Trapped ions, known as unique tools for metrology and ... - Cavity-based quantum networks with single atoms and optical photons

Author(s): Andreas Reiserer and Gerhard RempeA vision has formed in recent years of the components necessary for a large-scale quantum network. Single trapped atoms can serve as the nodes of this netw ... - Spinodal nanodecomposition in semiconductors doped with transition metals

Author(s): T. Dietl, K. Sato, T. Fukushima, A. Bonanni, M. Jamet, A. Barski, S. Kuroda, M. Tanaka, Pham Nam Hai, and H. Katayama-YoshidaThe occurrence of high-temperature ferromagnetism in transition ... - Google matrix analysis of directed networks

Author(s): Leonardo Ermann, Klaus M. Frahm, and Dima L. ShepelyanskyHow can information from communication and social networks in modern societies be processed, classified, and retrieved? For this new ... - Spin Hall effects

Author(s): Jairo Sinova, Sergio O. Valenzuela, J. Wunderlich, C. H. Back, and T. JungwirthIn solid-state materials with strong relativistic spin-orbit coupling, charge currents generate transverse spi ... - Nobel Lecture: Single molecules, cells, and super-resolution optics

Author(s): Eric BetzigThe 2014 Nobel Prize for Chemistry was shared by Eric Betzig, Stefan W. Hell, and William E. Moerner. These papers are the text of the address given in conjunction with the award ... - Nobel Lecture: Nanoscopy with freely propagating light

Author(s): Stefan W. HellThe 2014 Nobel Prize for Chemistry was shared by Eric Betzig, Stefan W. Hell, and William E. Moerner. These papers are the text of the address given in conjunction with the aw ... - Nobel Lecture: Single-molecule spectroscopy, imaging, and photocontrol: Foundations for super-resolution microscopy

Author(s): W. E. (William E.) MoernerThe 2014 Nobel Prize for Chemistry was shared by Eric Betzig, Stefan W. Hell, and William E. Moerner. These papers are the text of the address given in conjunction ... - Nobel Lecture: Fascinated journeys into blue light

Author(s): Isamu Akasaki[Rev. Mod. Phys. 87, 1119] Published Mon Oct 05, 2015 - Nobel Lecture: Growth of GaN on sapphire via low-temperature deposited buffer layer and realization of $p$-type GaN by Mg doping followed by low-energy electron beam irradiation

Author(s): Hiroshi AmanoThis is a personal history of one of the Japanese researchers engaged in developing a method for growing GaN on a sapphire substrate, paving the way for the realization of smar … - Nobel Lecture: Background story of the invention of efficient blue InGaN light emitting diodes

Author(s): Shuji Nakamura[Rev. Mod. Phys. 87, 1139] Published Mon Oct 05, 2015 - Quantum Monte Carlo methods for nuclear physics

Author(s): J. Carlson, S. Gandolfi, F. Pederiva, Steven C. Pieper, R. Schiavilla, K. E. Schmidt, and R. B. WiringaQuantum Monte Carlo techniques aim at providing a description of complex quantum syste … - Random-matrix theory of Majorana fermions and topological superconductors

Author(s): C. W. J. BeenakkerRandom-matrix theory has a long history of applications, from nuclear physics to electron localization to quantum dots. This review discusses yet another application of th … - Epidemic processes in complex networks

Author(s): Romualdo Pastor-Satorras, Claudio Castellano, Piet Van Mieghem, and Alessandro VespignaniComplex networks arise in a wide range of biological and sociotechnical systems. Epidemic spreading … - Surface nanobubbles and nanodroplets

Author(s): Detlef Lohse and Xuehua Zhang (张雪花)Surface nanobubbles are nanoscopic gaseous domains on immersed substrates which can survive for days. They were first speculated to exist about 20 years a …