Theoretical physics is a branch of physics that employs mathematical models and abstractions of physical objects and systems to rationalize, explain and predict natural phenomena. This is in contrast to experimental physics, which uses experimental tools to probe these phenomena.
The advancement of science generally depends on the interplay between experimental studies and theory. In some cases, theoretical physics adheres to standards of mathematical rigour while giving little weight to experiments and observations.[a] For example, while developing special relativity, Albert Einstein was concerned with the Lorentz transformation which left Maxwell's equations invariant, but was apparently uninterested in the Michelson–Morley experiment on Earth's drift through a luminiferous aether. Conversely, Einstein was awarded the Nobel Prize for explaining the photoelectric effect, previously an experimental result lacking a theoretical formulation.
A physical theory is a model of physical events. It is judged by the extent to which its predictions agree with empirical observations. The quality of a physical theory is also judged on its ability to make new predictions which can be verified by new observations. A physical theory differs from a mathematical theorem in that while both are based on some form of axioms, judgment of mathematical applicability is not based on agreement with any experimental results. A physical theory similarly differs from a mathematical theory, in the sense that the word "theory" has a different meaning in mathematical terms.[b]
A physical theory involves one or more relationships between various measurable quantities. Archimedes realized that a ship floats by displacing its mass of water, Pythagoras understood the relation between the length of a vibrating string and the musical tone it produces. Other examples include entropy as a measure of the uncertainty regarding the positions and motions of unseen particles and the quantum mechanical idea that (action and) energy are not continuously variable.
Theoretical physics consists of several different approaches. In this regard, theoretical particle physics forms a good example. For instance: "phenomenologists" might employ (semi-) empirical formulas to agree with experimental results, often without deep physical understanding.[c] "Modelers" (also called "model-builders") often appear much like phenomenologists, but try to model speculative theories that have certain desirable features (rather than on experimental data), or apply the techniques of mathematical modeling to physics problems.[d] Some attempt to create approximate theories, called effective theories, because fully developed theories may be regarded as unsolvable or too complicated. Other theorists may try to unify, formalise, reinterpret or generalise extant theories, or create completely new ones altogether.[e] Sometimes the vision provided by pure mathematical systems can provide clues to how a physical system might be modeled;[f] e.g., the notion, due to Riemann and others, that space itself might be curved. Theoretical problems that need computational investigation are often the concern of computational physics.
Theoretical advances may consist in setting aside old, incorrect paradigms (e.g., aether theory of light propagation, caloric theory of heat, burning consisting of evolving phlogiston, or astronomical bodies revolving around the Earth) or may be an alternative model that provides answers that are more accurate or that can be more widely applied. In the latter case, a correspondence principle will be required to recover the previously known result. Sometimes though, advances may proceed along different paths. For example, an essentially correct theory may need some conceptual or factual revisions; atomic theory, first postulated millennia ago (by several thinkers in Greece and India) and the two-fluid theory of electricity are two cases in this point. However, an exception to all the above is the wave–particle duality, a theory combining aspects of different, opposing models via the Bohr complementarity principle.
Physical theories become accepted if they are able to make correct predictions and no (or few) incorrect ones. The theory should have, at least as a secondary objective, a certain economy and elegance (compare to mathematical beauty), a notion sometimes called "Occam's razor" after the 13th-century English philosopher William of Occam (or Ockham), in which the simpler of two theories that describe the same matter just as adequately is preferred (but conceptual simplicity may mean mathematical complexity). They are also more likely to be accepted if they connect a wide range of phenomena. Testing the consequences of a theory is part of the scientific method.
Theoretical physics began at least 2,300 years ago, under the Pre-socratic philosophy, and continued by Plato and Aristotle, whose views held sway for a millennium. During the rise of medieval universities, the only acknowledged intellectual disciplines were the seven liberal arts of the Trivium like grammar, logic, and rhetoric and of the Quadrivium like arithmetic, geometry, music and astronomy. During the Middle Ages and Renaissance, the concept of experimental science, the counterpoint to theory, began with scholars such as Ibn al-Haytham and Francis Bacon. As the Scientific Revolution gathered pace, the concepts of matter, energy, space, time and causality slowly began to acquire the form we know today, and other sciences spun off from the rubric of natural philosophy. Thus began the modern era of theory with the Copernican paradigm shift in astronomy, soon followed by Johannes Kepler's expressions for planetary orbits, which summarized the meticulous observations of Tycho Brahe; the works of these men (alongside Galileo's) can perhaps be considered to constitute the Scientific Revolution.
The great push toward the modern concept of explanation started with Galileo, one of the few physicists who was both a consummate theoretician and a great experimentalist. The analytic geometry and mechanics of Descartes were incorporated into the calculus and mechanics of Isaac Newton, another theoretician/experimentalist of the highest order, writing Principia Mathematica. In it contained a grand synthesis of the work of Copernicus, Galileo and Kepler; as well as Newton's theories of mechanics and gravitation, which held sway as worldviews until the early 20th century. Simultaneously, progress was also made in optics (in particular colour theory and the ancient science of geometrical optics), courtesy of Newton, Descartes and the Dutchmen Snell and Huygens. In the 18th and 19th centuries Joseph-Louis Lagrange, Leonhard Euler and William Rowan Hamilton would extend the theory of classical mechanics considerably. They picked up the interactive intertwining of mathematics and physics begun two millennia earlier by Pythagoras.
Among the great conceptual achievements of the 19th and 20th centuries were the consolidation of the idea of energy (as well as its global conservation) by the inclusion of heat, electricity and magnetism, and then light. The laws of thermodynamics, and most importantly the introduction of the singular concept of entropy began to provide a macroscopic explanation for the properties of matter. Statistical mechanics (followed by statistical physics and Quantum statistical mechanics) emerged as an offshoot of thermodynamics late in the 19th century. Another important event in the 19th century was the discovery of electromagnetic theory, unifying the previously separate phenomena of electricity, magnetism and light.
The pillars of modern physics, and perhaps the most revolutionary theories in the history of physics, have been relativity theory and quantum mechanics. Newtonian mechanics was subsumed under special relativity and Newton's gravity was given a kinematic explanation by general relativity. Quantum mechanics led to an understanding of blackbody radiation (which indeed, was an original motivation for the theory) and of anomalies in the specific heats of solids — and finally to an understanding of the internal structures of atoms and molecules. Quantum mechanics soon gave way to the formulation of quantum field theory (QFT), begun in the late 1920s. In the aftermath of World War 2, more progress brought much renewed interest in QFT, which had since the early efforts, stagnated. The same period also saw fresh attacks on the problems of superconductivity and phase transitions, as well as the first applications of QFT in the area of theoretical condensed matter. The 1960s and 70s saw the formulation of the Standard model of particle physics using QFT and progress in condensed matter physics (theoretical foundations of superconductivity and critical phenomena, among others), in parallel to the applications of relativity to problems in astronomy and cosmology respectively.
All of these achievements depended on the theoretical physics as a moving force both to suggest experiments and to consolidate results — often by ingenious application of existing mathematics, or, as in the case of Descartes and Newton (with Leibniz), by inventing new mathematics. Fourier's studies of heat conduction led to a new branch of mathematics: infinite, orthogonal series.
Modern theoretical physics attempts to unify theories and explain phenomena in further attempts to understand the Universe, from the cosmological to the elementary particle scale. Where experimentation cannot be done, theoretical physics still tries to advance through the use of mathematical models.
Mainstream theories (sometimes referred to as central theories) are the body of knowledge of both factual and scientific views and possess a usual scientific quality of the tests of repeatability, consistency with existing well-established science and experimentation. There do exist mainstream theories that are generally accepted theories based solely upon their effects explaining a wide variety of data, although the detection, explanation, and possible composition are subjects of debate.
The proposed theories of physics are usually relatively new theories which deal with the study of physics which include scientific approaches, means for determining the validity of models and new types of reasoning used to arrive at the theory. However, some proposed theories include theories that have been around for decades and have eluded methods of discovery and testing. Proposed theories can include fringe theories in the process of becoming established (and, sometimes, gaining wider acceptance). Proposed theories usually have not been tested.
Fringe theories include any new area of scientific endeavor in the process of becoming established and some proposed theories. It can include speculative sciences. This includes physics fields and physical theories presented in accordance with known evidence, and a body of associated predictions have been made according to that theory.
Some fringe theories go on to become a widely accepted part of physics. Other fringe theories end up being disproven. Some fringe theories are a form of protoscience and others are a form of pseudoscience. The falsification of the original theory sometimes leads to reformulation of the theory.
"Thought" experiments are situations created in one's mind, asking a question akin to "suppose you are in this situation, assuming such is true, what would follow?". They are usually created to investigate phenomena that are not readily experienced in every-day situations. Famous examples of such thought experiments are Schrödinger's cat, the EPR thought experiment, simple illustrations of time dilation, and so on. These usually lead to real experiments designed to verify that the conclusion (and therefore the assumptions) of the thought experiments are correct. The EPR thought experiment led to the Bell inequalities, which were then tested to various degrees of rigor, leading to the acceptance of the current formulation of quantum mechanics and probabilism as a working hypothesis.
The Course of Theoretical Physics is a ten-volume series of books covering theoretical physics that was initiated by Lev Landau and written in collaboration with his student Evgeny Lifshitz starting in the late 1930s.
It is said that Landau composed much of the series in his head while in an NKVD prison in 1938-1939. However, almost all of the actual writing of the early volumes was done by Lifshitz, giving rise to the witticism, "not a word of Landau and not a thought of Lifshitz". The first eight volumes were finished in the 1950s, written in Russian and translated into English in the late 1950s by John Stewart Bell, together with John Bradbury Sykes, M. J. Kearsley, and W. H. Reid. The last two volumes were written in the early 1980s. Vladimir Berestetskii and Lev Pitaevskii also contributed to the series. The series is often referred to as "Landau and Lifshitz", "Landafshitz" (Russian: "Ландафшиц"), or "Lanlifshitz" (Russian: "Ланлифшиц") in informal settings.Department of Physics, University of Oxford
The Department of Physics is the physics department of the University of Oxford, England, which is part of the university's Mathematical, Physical and Life Sciences Division.
The department has several buildings and sub-departments:
Clarendon LaboratoryAtomic and Laser Physics
Condensed Matter PhysicsDenys Wilkinson BuildingAstrophysics
Particle PhysicsDobson Square, Sherrington RoadAtmospheric, Ocean and Planetary PhysicsBeecroft BuildingTheoretical PhysicsEverything
Everything (or every thing) is all that exists; the opposite of nothing, or its complement. It is the totality of things relevant to some subject matter. Without expressed or implied limits, it may refer to anything. The Universe is everything that exists theoretically, though a multiverse may exist according to theoretical cosmology predictions. It may refer to an anthropocentric worldview, or the sum of human experience, history, and the human condition in general. Every object and entity is a part of everything, including all physical bodies and in some cases all abstract objects.Faculty of Mathematics, University of Cambridge
The Faculty of Mathematics at the University of Cambridge comprises the Department of Pure Mathematics and Mathematical Statistics (DPMMS) and the Department of Applied Mathematics and Theoretical Physics (DAMTP). It is housed in the Centre for Mathematical Sciences site in West Cambridge, alongside the Isaac Newton Institute. Many distinguished mathematicians have been members of the faculty.Heim theory
Heim theory, first proposed by German physicist Burkhard Heim publicly in 1957, is an attempt to develop a theory of everything in theoretical physics. The theory has received little attention in the scientific literature and is regarded as being outside mainstream science but has attracted some interest in popular and fringe media.Heim attempted to resolve incompatibilities between quantum theory and general relativity. To meet that goal, he developed a mathematical approach based on quantizing spacetime. Others have attempted to apply Heim theory to nonconventional space propulsion and faster than light concepts, as well as the origin of dark matter.Heim claimed that his theory yields particle masses directly from fundamental physical constants and that the resulting masses are in agreement with experiment, but this claim has not been confirmed.Heim theory is formulated mathematically in six or more dimensions and uses Heim's own version of difference equations.
Geoffrey A. Landis has compared the story behind the creation of Heim theory with the plot of a science fiction story.International Centre for Theoretical Physics
The Abdus Salam International Centre for Theoretical Physics (ICTP) is an international research institute for physical and mathematical sciences that operates under a tripartite agreement between the Italian Government, United Nations Educational, Scientific and Cultural Organization (UNESCO), and International Atomic Energy Agency (IAEA). It is located near the Miramare Park, about 10 kilometres from the city of Trieste, Italy. The centre was founded in 1964 by Pakistani Nobel Laureate Abdus Salam.
ICTP is part of the Trieste System, a network of national and international scientific institutes in Trieste, promoted by the Italian physicist Paolo Budinich.International Journal of Theoretical Physics
The International Journal of Theoretical Physics is a peer-reviewed scientific journal of physics published by Springer Science+Business Media since 1968. According to the Journal Citation Reports, the journal has a 2012 impact factor of 1.086 and publishes both original research and review articles. The editor-in-chief is Heinrich Saller (Max Planck Institute for Physics).Journal of Experimental and Theoretical Physics
The Journal of Experimental and Theoretical Physics (JETP) [Russian: Журнал Экспериментальной и Теоретической Физики (ЖЭТФ), or Zhurnal Éksperimental’noĭ i Teoreticheskoĭ Fiziki (ZhÉTF)] is a peer-reviewed Russian scientific journal covering all areas of experimental and theoretical physics. For example, coverage includes solid state physics, elementary particles, and cosmology. The journal is published simultaneously in both Russian and English languages.
The Editor-in-Chief is Alexander F. Andreev. In addition, this journal is a continuation of Soviet physics, JETP (1931–1992), which began English translation in 1955.Kavli Institute for Theoretical Physics
The Kavli Institute for Theoretical Physics (KITP) is a research institute of the University of California, Santa Barbara. KITP is one of the most renowned institutes for theoretical physics in the world, and brings theorists in physics and related fields together to work on topics at the forefront of theoretical science. The National Science Foundation has been the principal supporter of the Institute since it was founded as the Institute for Theoretical Physics in 1979. In a 2007 article in the Proceedings of the National Academy of Sciences, KITP was given the highest impact index in a comparison of nonbiomedical research organizations across the U.S.The Directors of the KITP since its beginning have been:
Walter Kohn, 1979–1984 (Nobel Prize in Chemistry, 1998)
Robert Schrieffer, 1984–1989 (Nobel Prize for Physics, 1972)
James S. Langer, 1989–1995
James Hartle, 1995–1997 (Einstein Prize (APS), 2009)
David Gross, 1997–2012 (Nobel Prize in Physics, 2004)
Lars Bildsten, 2012–presentThe Director and the permanent members of the KITP (Leon Balents, David Gross, Alexei Kitaev, and Boris Shraiman) are also on the faculty of the UC Santa Barbara Physics Department. Former permanent members include Physics Nobel Laureate Frank Wilczek and Joseph Polchinski.
In the early 2000s, the institute, formerly known as the Institute for Theoretical Physics, or ITP, was named for the Norwegian-American physicist and businessman Fred Kavli, in recognition of his donation of $7.5 million to the Institute.
Kohn Hall, which houses KITP, is located just beyond the Henley Gate at the East Entrance of the UCSB campus. The building was designed by the architect Michael Graves, and a new wing designed by Graves was added in 2003-2004.Lev Landau
Lev Davidovich Landau (22 January 1908 – 1 April 1968) was a Soviet physicist who made fundamental contributions to many areas of theoretical physics.His accomplishments include the independent co-discovery of the density matrix method in quantum mechanics (alongside John von Neumann), the quantum mechanical theory of diamagnetism, the theory of superfluidity, the theory of second-order phase transitions, the Ginzburg–Landau theory of superconductivity, the theory of Fermi liquid, the explanation of Landau damping in plasma physics, the Landau pole in quantum electrodynamics, the two-component theory of neutrinos, and Landau's equations for S matrix singularities. He received the 1962 Nobel Prize in Physics for his development of a mathematical theory of superfluidity that accounts for the properties of liquid helium II at a temperature below 2.17 K (−270.98 °C).MIT Center for Theoretical Physics
The MIT Center for Theoretical Physics (CTP) is a subdivision of MIT Laboratory for Nuclear Science and Department of Physics. The CTP is a unified research and teaching center focused on fundamental physics. CTP activities range from string theory and cosmology at the highest energies down through unification and beyond-the-standard-model physics, through the standard model, to QCD, hadrons, quark matter, and nuclei at the low energy scale.
Members of the CTP are also currently working on quantum computation and on energy policy. The breadth and depth of research in nuclear, particle, string, and gravitational physics at the CTP makes it a unique environment for researchers in these fields.
In addition to the 15 MIT faculty members working in the CTP, at any one time there are roughly a dozen postdoctoral fellows, and as many, or more, long-term visitors working at the postdoctoral or faculty level. The CTP supports 25-35 MIT graduate students, who work with the faculty and postdocs on problems across the energy spectrum.
Current research areas in the center include particle physics, Cosmology, String theory, Phenomenology beyond the standard model, standard model, quantum field theory, lattice QCD, condensed matter physics, quantum computing, and Energy research. Notable current faculty include Frank Wilczek, Jeffrey Goldstone, Roman Jackiw, Alan Guth, Max Tegmark, Isadore Singer, Peter Shor, Daniel Freedman, Robert Jaffe and Allan AdamsMichael Green (physicist)
Michael Boris Green (born 22 May 1946) is a British physicist and one of the pioneers of string theory. Currently a professor in the Department of Applied Mathematics and Theoretical Physics and a Fellow in Clare Hall, Cambridge in England, he was Lucasian Professor of Mathematics from 2009 to 2015.Niels Bohr Institute
The Niels Bohr Institute (Danish: Niels Bohr Institutet) is a research institute of the University of Copenhagen. The research of the institute spans astronomy, geophysics, nanotechnology, particle physics, quantum mechanics and biophysics.Paul Townsend
Paul Kingsley Townsend FRS () is a British physicist, currently a Professor of Theoretical Physics in Cambridge University's Department of Applied Mathematics and Theoretical Physics. He is notable for his work on string theory.Perimeter Institute for Theoretical Physics
Perimeter Institute for Theoretical Physics (PI, Perimeter, PITP) is an independent research centre in foundational theoretical physics located in Waterloo, Ontario, Canada. It was founded in 1999. The Institute's founding and major benefactor is Canadian entrepreneur and philanthropist Mike Lazaridis.The original building, designed by Saucier + Perrotte, opened in 2004 and was awarded a Governor General's Medal for Architecture in 2006. The Stephen Hawking Centre, designed by Teeple Architects, was opened in 2011 and was LEED Silver certified in 2015.In addition to research, Perimeter also provides scientific training and educational outreach activities to the general public. This is done in part through Perimeter’s Educational Outreach team.Phenomenology (physics)
In physics, phenomenology is the application of theoretical physics to experimental data by making quantitative predictions based upon known theories. It is in contrast to experimentation in the scientific method, in which the goal of the experiment is to test a scientific hypothesis instead of making predictions. Phenomenology is related to the philosophical notion in that these predictions describe anticipated behaviors for the phenomena in reality.
Phenomenology is commonly applied to the field of particle physics, where it forms a bridge between the mathematical models of theoretical physics (such as quantum field theories and theories of the structure of space-time) and the results of the high-energy particle experiments. It is sometimes used in other fields such as in condensed matter physics and plasma physics, when there are no existing theories for the observed experimental data.Progress of Theoretical and Experimental Physics
Progress of Theoretical and Experimental Physics is a monthly peer-reviewed scientific journal published by Oxford University Press on behalf of the Physical Society of Japan. It was established as Progress of Theoretical Physics in July 1946 by Hideki Yukawa and obtained its current name in January 2013.
Progress of Theoretical and Experimental Physics is part of the SCOAP3 initiative.Stanford Institute for Theoretical Physics
The Stanford Institute for Theoretical Physics (SITP) is a research institute within the Physics Department at Stanford University. Led by 16 physics faculty members, the institute conducts research in High Energy and Condensed Matter theoretical physics.
Research within SITP includes a strong focus on fundamental questions about the new physics underlying the Standard Models of particle physics and cosmology, and on the nature and applications of our basic frameworks (quantum field theory and string theory) for attacking these questions.
Central areas of research include:
What governs particle theory beyond the scale of electroweak symmetry breaking?
How do string theory and holography resolve the basic puzzles of general relativity, including the deep issues arising in black hole physics and the study of cosmological horizons?
Which class of models of inflationary cosmology captures the physics of the early universe, and what preceded inflation?
Can physicists develop new techniques in quantum field theory and string theory to shed light on mysterious phases arising in many contexts in condensed matter physics (notably, in the high temperature superconductors)?William I. Fine Theoretical Physics Institute
The William I. Fine Theoretical Physics Institute is a research institute in the University of Minnesota College of Science and Engineering. FTPI was largely the work of physics Professor Emeritus, Stephen Gasiorowicz and University alumnus and Twin-Cities real-estate developer William I. Fine. The Institute officially came into existence in January 1987. FTPI faculty consists of seven permanent members: Alex Kamenev, Keith Olive, Mikhail Shifman, Boris Shklovskii, Arkady Vainshtein, Mikhail Voloshin, and Andrey V. Chubukov as well as postdoctoral and graduate students.The William I. Fine Theoretical Physics Institute has on Oversight Committee consisting of 8 members. The Oversight Committee is essentially a board of directors that make decisions concerning the staffing and budgeting of the Institute.