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Physics (Greek: Template:Polytonic (phúsis), "nature" and Template:Polytonic (phusiké), "knowledge of nature") is the branch of science concerned with the discovery and characterization of universal laws which govern matter, energy, space, and time. The role of physics, then, is to provide a logically ordered picture of nature in agreement with experience.

Introduction

Since antiquity, natural philosophers have sought to explain physical phenomena such as the movement of the planets and the nature of matter, and this pursuit was formerly the study known as "physics" (once spelled physike, in imitation of Aristotle). The emergence of modern physics as a science distinct from natural philosophy began with the scientific revolution of the 16th and 17th centuries and continued through the dawn of modern physics in the early 20th century. The field has continued to expand, with a growing body of research leading to discoveries such as the Standard Model of fundamental particles and a detailed history of the universe, along with revolutionary new technologies like nuclear weapons and semiconductors. Research today progresses on a vast array of topics, including high-temperature superconductivity, quantum computing, the search for the Higgs boson, and the attempt to develop a theory of quantum gravity. Firmly grounded in observations and experiments, with a rich set of theories expressed in elegant mathematics, physics has made a multitude of contributions to science, technology, and philosophy.

Discoveries in physics resonate throughout the natural sciences, and physics has been described as the "fundamental science" because other fields such as chemistry and biology investigate systems whose properties are based upon the laws of physics. Chemistry, for example, is the science of substances formed by atoms and molecules in bulk, but the properties of chemical compounds are determined by the physical properties of their underlying molecules.

Experimental physics is closely related to engineering and technology. Physicists involved in basic research design and perform experiments with equipment such as particle accelerators and lasers, whereas physicists involved in applied research invent technologies such as magnetic resonance imaging (MRI) and transistors.

Theoretical physics is closely related to mathematics, which provides the language of physical theories. While it is true that the first task of theory is to disclose relationships in a world-picture which agrees with experience, an equally important part of its work is to formulate these relationships mathematically. The use of mathematics in theory represents a rationalization of thought, in that the process of deriving important conclusions from the initial hypotheses runs in the channels of formal rules of calculation. However, the position of mathematics in the field of theoretical physics implies that it is not the problem of the theoretical physicist to devise mathematical proofs and, indeed, the strict requirements of mathematics often contradict the physical facts. Theoretical physicists also often rely on numerical analysis and computer simulations, and, thus, computers and computer programming have an ever richer role in the formulation of physical models. In fact, the fields of mathematical and computational physics are active areas of research. Theoretical physics often relates to philosophy and metaphysics when it deals with speculative ideas like multidimensional spaces and parallel universes.

Theories

Although physicists study a wide variety of phenomena, there are certain theories that are used by all physicists. Each of these theories has been tested in numerous experiments and proven to be a correct approximation of nature within its domain of validity. For example, the theory of classical mechanics accurately describes the motion of objects, provided that they are much larger than atoms and move much slower than the speed of light. While these theories have long been well-understood, they continue to be areas of active research—for example, a remarkable aspect of classical mechanics known as chaos was discovered in the 20th century, three centuries after its original formulation by Isaac Newton (1642–1727). The "central theories" are important tools for research into more specialized topics, and all physicists are expected to be literate in them.

• Classical mechanics is a model of the physics of forces acting upon bodies. It is often referred to as "Newtonian mechanics" after Newton and his laws of motion. Classical mechanics is subdivided into statics, which models objects at rest, kinematics, which models objects in motion, and dynamics, which models objects subjected to forces. It is superseded by relativistic mechanics for systems moving at large velocities near the speed of light, quantum mechanics for systems at small distance scales, and relativistic quantum field theory for systems with both properties. Nevertheless, classical mechanics is still very useful, because it is much simpler and easier to apply than these other theories, and it has a very large range of approximate validity. Classical mechanics can be used to describe the motion of human-sized objects (such as tops and baseballs), many astronomical objects (such as planets and galaxies), and certain microscopic objects (such as organic molecules.)
• Electromagnetism -- called also Electromagnetics -- is the physics of the electromagnetic field, a field that results from the presence and motion of charged particles and exerts forces on them. The sub-discipline of electrodynamics describes the behavior of moving charged particles interacting with electromagnetic fields. Electromagnetism encompasses various real-world electromagnetic phenomena. In fact, light is an oscillating electromagnetic field that is radiated from accelerating charged particles. Aside from gravity, most of the forces in everyday experience are ultimately a result of electromagnetism.
• Thermodynamics is the branch of physics that deals with the action of heat and the conversions from one to another of various forms of energy. Thermodynamics is particularly concerned with how these affect temperature, pressure, volume, mechanical action, entropy, and work. Statistical mechanics, a related theory, is the branch of physics that analyzes macroscopic systems by applying statistical principles to their microscopic constituents. It can be applied to calculate the thermodynamic properties of bulk materials from the properties of individual molecules, which is the basis of statistical thermodynamics.
• The theory of relativity is:
  • a physical theory which is based on the two postulates: (1) that the speed of light in a vacuum is constant and independent of the source or observer and (2) that the mathematical forms of the laws of physics are invariant in all inertial systems-- called also special relativity, special theory of relativity. It leads to the assertion of the equivalence of mass and energy and of change in mass,dimension, and time with increased velocity.
  • an extension of special relativity to include transformations between non-inertial frames-- also called general relativity or the general theory of relativity. It is formulated using differential geometry and interprets gravity as a distortion of spacetime caused by the presence of mass or energy.
• Quantum mechanics is the branch of mathematical physics treating atomic and subatomic systems and their interaction with radiation in terms of observable quantities. It is based on the observation that all forms of energy are released in discrete units or bundles called quanta. Quantum mechanics provides a physical theory of matter that is based on the concept of the possession of wave properties by elementary particles, affords a mathematical interpretation of the structure and interactions of matter on the basis of these properties, and incorporates within it quantum theory and the uncertainty principle -- called also wave mechanics. Remarkably, quantum theory typically permits only probable or statistical calculation of the observed features of subatomic particles, understood in terms of wavefunctions. The discovery of quantum mechanics in the early 20th century revolutionized physics, and quantum mechanics is fundamental to most areas of current research.

Theories and concepts

The table below lists many physical theories and the concepts they employ.

Research

Contemporary research in physics is divided into several distinct fields.

• Condensed matter physics is concerned with how the properties of bulk matter, such as the ordinary solids and liquids we encounter in everyday life, arise from the properties and mutual interactions of the constituent atoms. A topic of current interest is high-temperature superconductivity.

• Atomic, molecular, and optical physics deals with small numbers of atoms and molecules, particularly with how they interact with light. A topic of current interest is the behavior of Bose-Einstein condensates.

• Particle physics, also known as "high-energy physics", is concerned with the properties of submicroscopic particles much smaller than atoms, including elementary particles such as electrons, photons, and quarks. A topic of current interest is the search for the Higgs boson.

• Astrophysics and cosmology apply the laws of physics to explain celestial phenomena, including stellar dynamics, black holes, galaxies, and the big bang. A topic of current interest is determining the nature of dark matter and dark energy.

Since the twentieth century, the individual fields of physics have become increasingly specialized, and today most physicists work in a single field for their entire careers. "Universalists" such as Albert Einstein (1879–1955) and Lev Landau (1908–1968), who worked in multiple fields of physics, are now very rare.

Theory and experiment, pure and applied

The culture of physics research differs from most sciences in the separation of theory and experiment. Since the twentieth century, most individual physicists have specialized in either theoretical physics or experimental physics. The great Italian physicist Enrico Fermi (1901–1954), who made fundamental contributions to both theory and experimentation in nuclear physics, was a notable exception. In contrast, almost all the successful theorists in biology and chemistry (e.g. American quantum chemist and biochemist Linus Pauling) have also been experimentalists, although this is changing as of late.

Roughly speaking, theorists seek to develop through abstractions and mathematical models theories that can both describe and interpret existing experimental results, and successfully predict future results, while experimentalists devise and perform experiments to explore new phenomena and test theoretical predictions. Although theory and experiment are developed separately, they are strongly dependent upon each other. Progress in physics frequently comes about when experimentalists make a discovery that existing theories cannot account for, necessitating the formulation of new theories. Likewise, ideas arising from theory often inspire new experiments. In the absence of experiment, theoretical research can go in the wrong direction; this is one of the criticisms that has been leveled against M-theory, a popular theory in high-energy physics for which no practical experimental test has ever been devised. Theorists working closely with experimentalists frequently employ phenomenology.

Applied physics is physics that is intended for a particular technological or practical use, as for example in engineering, as opposed to basic research. This approach is similar to that of applied mathematics. Applied physics is rooted in the fundamental truths and basic concepts of the physical sciences, but is concerned with the use of scientific principles in practical devices and systems, and in the application of physics in other areas of science. "Applied" is distinguished from "pure" by a subtle combination of factors such as the motivation and attitude of researchers and the nature of the relationship to the technology or science that may be affected by the work.

Subfields

The table below lists many of the fields and subfields of physics along with the theories and concepts they employ.

History

Since antiquity, people have tried to understand the workings of Nature and the behavior of matter: why unsupported objects drop to the ground, why different materials have different properties, and so forth. The character of the universe was also a mystery, for instance the earth and the behavior of celestial objects such as the sun and the moon. Several theories were proposed, most of which were incorrect, such as the earth orbiting the moon. These first theories were largely couched in philosophical terms, and never verified by systematic experimental testing, as is popular today. The works of Ptolemy and Aristotle were not always found to match everyday observations. There were exceptions and there are anachronisms - for example, Indian philosophers and astronomers gave many correct descriptions in atomism and astronomy, and the Greek mathematician Archimedes derived many correct quantitative descriptions of mechanics and hydrostatics.

Middle Ages

The willingness to question previously held truths and search for new answers eventually resulted in a period of major scientific advancements, now known as the Scientific Revolution of the late seventeenth century. The precursors to the scientific revolution may be traced back to the important developments made in Indian science and especially Muslim science. Examples of these developments include including the elliptical model of the planets perhaps based on a heliocentric solar system of gravitation developed by Indian mathematician-astronomer Aryabhata; the basic ideas of atomic theory developed by Hindu and Jaina philosophers; the theory of light being equivalent to energy particles developed by the Indian Buddhist scholars Dignāga and Dharmakirti; the optical theory of light developed by the Iraqi scientist Ibn al-Haytham (Alhazen); the Astrolabe invented by the Persian astronomer Muhammad al-Fazari; and the significant flaws in the Ptolemaic system pointed out by Persian scientist Nasir al-Din Tusi.

The most important scientific development during the Middle Ages, however, was the development of the scientific method, which began with the Iraqi Muslim scientist Ibn al-Haytham (Latinized as Alhazen), who pioneered the used of experimentation during his investigations on optics in his Book of Optics.

Scientific Revolution

As the influence of the Arab Empire expanded to Europe, the works of Aristotle, preserved by the Arabs, and the works of the Indians and Persians, became known in medieval Europe by the twelfth and thirteenth centuries through Latin translations of Arabic scientific texts.

This eventually led to the scientific revolution, held by most historians (e.g., Howard Margolis) to have begun in 1543, when the first printed copy of Nicolaus Copernicus's De Revolutionibus was brought to the influential astronomer from Nuremberg (Nürnberg), where it had been printed by Johannes Petreius. Most of its contents had been written years prior, but the publication had been delayed. Copernicus died soon after receiving the copy.

Further significant advances were made over the following century by Galileo Galilei, Christiaan Huygens, Johannes Kepler, and Blaise Pascal. During the early seventeenth century, Galileo made extensive use of experimentation to validate physical theories, which is the key idea in the modern scientific method. Galileo formulated and successfully tested several results in dynamics, in particular the Law of Inertia.

The scientific revolution is considered to have culminated with the publication of the Philosophiae Naturalis Principia Mathematica in 1687 by the mathematician, physicist, alchemist and inventor Sir Isaac Newton (1643-1727).In 1687, Newton published the Principia, detailing two comprehensive and successful physical theories: Newton's laws of motion, from which arise classical mechanics; and Newton's Law of Gravitation, which describes the fundamental force of gravity. Both theories agreed well with experiment. The Principia also included several theories in fluid dynamics.

From the late seventeenth century onward, thermodynamics was developed by physicist and chemist Boyle, Young, and many others. In 1733, Bernoulli used statistical arguments with classical mechanics to derive thermodynamic results, initiating the field of statistical mechanics. In 1798, Thompson demonstrated the conversion of mechanical work into heat, and in 1847 Joule stated the law of conservation of energy, in the form of heat as well as mechanical energy. Ludwig Boltzmann, in the nineteenth century, is responsible for the modern form of statistical mechanics.

Classical mechanics was re-formulated and extended by Leonhard Euler, French mathematician Joseph-Louis Comte de Lagrange, Irish mathematical physicist William Rowan Hamilton, and others, who produced new results in mathematical physics. The law of universal gravitation initiated the field of astrophysics, which describes astronomical phenomena using physical theories.

After Newton defined classical mechanics, the next great field of inquiry within physics was the nature of electricity. Observations in the seventeenth and eighteenth century by scientists such as Robert Boyle, Stephen Gray, and Benjamin Franklin created a foundation for later work. These observations also established our basic understanding of electrical charge and current.

The existence of the atom was proposed in 1808 by John Dalton.

In 1821, the English physicist and chemist Michael Faraday integrated the study of magnetism with the study of electricity. This was done by demonstrating that a moving magnet induced an electric current in a conductor. Faraday also formulated a physical conception of electromagnetic fields. James Clerk Maxwell built upon this conception, in 1864, with an interlinked set of twenty equations that explained the interactions between electric and magnetic fields. These twenty equations were later reduced, using vector calculus, to a set of four equations by Oliver Heaviside.

In addition to other electromagnetic phenomena, Maxwell's equations also can be used to describe light. Confirmation of this observation was made with the 1888 discovery of radio by Heinrich Hertz and in 1895 when Wilhelm Roentgen detected X-rays.

Modern physics

The ability to describe light in electromagnetic terms helped serve as a springboard for Albert Einstein's publication of the theory of special relativity in 1905. This theory combined classical mechanics with Maxwell's equations. The theory of special relativity unifies space and time into a single entity, spacetime. Relativity prescribes a different transformation between reference frames than classical mechanics; this necessitated the development of relativistic mechanics as a replacement for classical mechanics. In the regime of low (relative) velocities, the two theories agree. Einstein built further on the special theory by including gravity into his calculations, and published his theory of general relativity in 1915.

One part of the theory of general relativity is Einstein's field equation. This describes how the stress-energy tensor creates curvature of spacetime and forms the basis of general relativity. Further work on Einstein's field equation produced results which predicted the Big Bang, black holes, and the expanding universe. Einstein believed in a static universe. He tried, and failed, to fix his equation to allow for this. By 1929, however, Edwin Hubble's astronomical observations suggested that the universe is expanding at a possibly exponential rate.

In 1895, Röntgen discovered X-rays, which turned out to be high-frequency electromagnetic radiation.

Radioactivity was discovered in 1896 by Henri Becquerel, and further studied by Maria Sklodowska-Curie, Pierre Curie, and others. This initiated the field of nuclear physics.

In 1897, Joseph J. Thomson discovered the electron, the elementary particle which carries electrical current in circuits. In 1904, he proposed the first model of the atom, known as the plum pudding model. Its existence had been proposed in 1808 by John Dalton.

These discoveries revealed that the assumption of many physicists, that atoms were the basic unit of matter, was flawed, and prompted further study into the structure of atoms.

In 1911, Ernest Rutherford deduced from scattering experiments the existence of a compact atomic nucleus, with positively charged constituents dubbed protons. Neutrons, the neutral nuclear constituents, were discovered in 1932 by Chadwick. The equivalence of mass and energy (Einstein, 1905) was spectacularly demonstrated during World War II, as research was conducted by each side into nuclear physics, for the purpose of creating a nuclear bomb. The German effort, led by Heisenberg, did not succeed, but the Allied Manhattan Project reached its goal. In America, a team led by Fermi achieved the first man-made nuclear chain reaction in 1942, and in 1945 the world's first nuclear explosive was detonated at Trinity site, near Alamogordo, New Mexico.

In 1900, Max Planck published his explanation of blackbody radiation. This equation assumed that radiators are quantized, which proved to be the opening argument in the edifice that would become quantum mechanics. By introducing discrete energy levels, Planck, Einstein, Niels Bohr, and others developed quantum theories to explain various anomalous experimental results.

Quantum mechanics was formulated in 1925 by Heisenberg and in 1926 by Schrödinger and Paul Dirac, in two different ways, that both explained the preceding heuristic quantum theories. In quantum mechanics, the outcomes of physical measurements are inherently probabilistic; the theory describes the calculation of these probabilities. It successfully describes the behavior of matter at small distance scales. During the 1920s Schrödinger, Heisenberg, and Max Born were able to formulate a consistent picture of the chemical behavior of matter, a complete theory of the electronic structure of the atom, as a byproduct of the quantum theory.

Quantum field theory was formulated in order to extend quantum mechanics to be consistent with special relativity. It was devised in the late 1940s with work by Richard Feynman, Julian Schwinger, Sin-Itiro Tomonaga, and Freeman Dyson. They formulated the theory of quantum electrodynamics, which describes the electromagnetic interaction, and successfully explained the Lamb shift. Quantum field theory provided the framework for modern particle physics, which studies fundamental forces and elementary particles.

Chen Ning Yang and Tsung-Dao Lee, in the 1950s, discovered an unexpected asymmetry in the decay of a subatomic particle. In 1954, Yang and Robert Mills then developed a class of gauge theories which provided the framework for understanding the nuclear forces (Yang, Mills 1954). The theory for the strong nuclear force was first proposed by Murray Gell-Mann. The electroweak force, the unification of the weak nuclear force with electromagnetism, was proposed by Sheldon Lee Glashow, Abdus Salam, and Steven Weinberg and confirmed in 1964 by James Watson Cronin and Val Fitch. This led to the so-called Standard Model of particle physics in the 1970s, which successfully describes all the elementary particles observed to date.

Quantum mechanics also provided the theoretical tools for condensed matter physics, whose largest branch is solid state physics. It studies the physical behavior of solids and liquids, including phenomena such as crystal structures, semiconductivity, and superconductivity. The pioneers of condensed matter physics include Felix Bloch, who created a quantum mechanical description of the behavior of electrons in crystal structures in 1928. The transistor was developed by physicists John Bardeen, Walter Houser Brattain, and William Bradford Shockley in 1947 at Bell Laboratories.

The two themes of the twentieth century, general relativity and quantum mechanics, appear inconsistent with each other. General relativity describes the universe on the scale of planets and solar systems, while quantum mechanics operates on sub-atomic scales. This challenge is being attacked by string theory, which treats spacetime as composed, not of points, but of one-dimensional objects, strings. Strings have properties similar to a common string (e.g., tension and vibration). The theories yield promising, but not yet testable, results. The search for experimental verification of string theory is in progress.

The United Nations declared the year 2005, the centenary of Einstein's annus mirabilis, as the World Year of Physics.

Future directions

Research in physics is progressing constantly on a large number of fronts, and is likely to do so for the foreseeable future.

In condensed matter physics, the greatest unsolved theoretical problem is the explanation for high-temperature superconductivity. Strong efforts, largely experimental, are being put into making workable spintronics and quantum computers.

In particle physics, the first pieces of experimental evidence for physics beyond the Standard Model have begun to appear. Foremost amongst these are indications that neutrinos have non-zero mass. These experimental results appear to have solved the long-standing solar neutrino problem in solar physics. The physics of massive neutrinos is currently an area of active theoretical and experimental research. In the next several years, particle accelerators will begin probing energy scales in the TeV range, in which experimentalists are hoping to find evidence for the Higgs boson and supersymmetric particles.

Theoretical attempts to unify quantum mechanics and general relativity into a single theory of quantum gravity, a program ongoing for over half a century, have not yet borne fruit. Currently, the leading candidates are M-theory, superstring theory, and loop quantum gravity.

Many astronomical and cosmological phenomena have yet to be explained satisfactorily, including the existence of ultra-high energy cosmic rays, the baryon asymmetry, the acceleration of the universe, and the anomalous rotation rates of galaxies.

Although much progress has been made in high-energy, quantum, and astronomical physics, many everyday phenomena, involving complexity, chaos, or turbulence remain poorly understood. Complex problems that would appear to be soluble by a clever application of dynamics and mechanics, such as the formation of sand piles, nodes in trickling water, the shape of water droplets, mechanisms of surface tension catastrophes, or self-sorting in shaken heterogeneous collections are unsolved.

These complex phenomena have received growing attention since the 1970s for several reasons, not least of which has been the availability of modern mathematical methods and computers, which enabled complex systems to be modeled in new ways. The interdisciplinary relevance of complex physics also has increased, as exemplified by the study of turbulence in aerodynamics, or the observation of pattern formation in biological systems. In 1932, Horace Lamb correctly prophesied the success of the theory of quantum electrodynamics and the near-stagnant progress in the study of turbulence:

I am an old man now, and when I die and go to heaven there are two matters on which I hope for enlightenment. One is quantum electrodynamics, and the other is the turbulent motion of fluids. And about the former I am rather optimistic.

Notes

1. The Feynman Lectures on Physics Volume I, Chapter III. Feynman, Leighton and Sands. ISBN 0-201-02115-3 For the philosophical issues of whether other sciences can be "reduced" to physics, see reductionism and special sciences.
2. Rosanna Gorini (2003). "Al-Haytham the Man of Experience. First Steps in the Science of Vision", International Society for the History of Islamic Medicine. Institute of Neurosciences, Laboratory of Psychobiology and Psychopharmacology, Rome, Italy:

   "According to the majority of the historians al-Haytham was the pioneer of the modern scientific method. With his book he changed the meaning of the term optics and established experiments as the norm of proof in the field. His investigations are based not on abstract theories, but on experimental evidences and his experiments were systematic and repeatable."

Further reading

• A large number of textbooks, popular books, and webpages about physics are available for further reading.
• Important publications in physics

Organizations

• AIP.org is the website of the American Institute of Physics
• IOP.org is the website of the Institute of Physics
• APS.org is the website of the American Physical Society
• SPS National is the website of the American Society of Physics Students
• CAP.ca is the website of the Canadian Association of Physicists
• EPS.org is the website of the European Physical Society

References

• Alpher, Herman, and Gamow. Nature 162,774 (1948). Wilson's 1978 Nobel lecture

• C.S. Wu's contribution to the overthrow of the conservation of parity

• Yang, Mills 1954 Physical Review 95, 631; Yang, Mills 1954 Physical Review 96, 191.

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