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Physics (Greek: Template:Polytonic (phúsis), "nature" and Template:Polytonic (phusiké), "knowledge of nature") is the branch of science concerned with discovering and characterizing universal laws that govern such things as matter, energy, space, and time. 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 depend on the laws of physics.

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. Theoretical physicists may also rely on numerical analysis and computer simulations, which play an ever richer role in the formulation of physical models. The fields of mathematical and computational physics are active areas of research. Theoretical physics sometimes relates to philosophy and metaphysics when it deals with speculative ideas like multidimensional spaces and parallel universes.

The emergence of 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. Grounded in observations and experiments and supported by deep, far-reaching theories, physics has made a multitude of contributions to science, technology, and philosophy.

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 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.
• Relativity is a generalization of classical mechanics that describes fast-moving or very massive systems. It includes special and general relativity.
  • Special relativity, or the "special theory of relativity", is based on 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. It asserts an equivalence of mass and energy and a change in mass, dimension, and time with increased velocity.
  • General relativity, or the "general theory of relativity", extends special relativity to include transformations between non-inertial frames. It is formulated using differential geometry and interprets gravity as a distortion of spacetime caused by the presence of mass or energy.
• Quantum mechanics describes the physics of atomic and subatomic scales. It is based on the observation that all forms of energy are released in discrete units or bundles called quanta. 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.

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.

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.

• Physics
• Natural sciences
• Physical sciences