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== History ==
{{Mergeto|History of physics|date=July 2007}}

{{main | History of physics}}

{{further | ], ]}}

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

===Middle Ages===
{{main|Islamic science|History of science in the Middle Ages}}
{{see|Science and technology in ancient India|History of science and technology in China}}
] (Alhazen)]]
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 ] of the late ]. The precursors to the scientific revolution may be traced back to the important developments made in ] and especially ]. Examples of these developments include including the ] model of the planets perhaps based on a ] ] of ] developed by ]-astronomer ]; the basic ideas of ] developed by ] and ] philosophers; the theory of light being equivalent to energy particles developed by the Indian ] scholars ] and ]; the ] theory of ] developed by the ]i scientist ] (Alhazen); the ] invented by the ] ] ]; and the significant flaws in the ] pointed out by Persian scientist ].

The most important scientific development during the ], however, was the development of the ], which began with the ]i ] ] (]ized as ''Alhazen''), who pioneered the used of ]ation during his investigations on ] in his '']''.<ref>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:
{{quote|"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."}}</ref>

===Scientific Revolution===
{{main|Scientific Revolution}}
As the influence of the ] expanded to Europe, the works of Aristotle, preserved by the ]s, and the works of the Indians and Persians, became known in medieval Europe by the ] and ] through ] of ].

] <!-- do not enter nationality claims here--> 1473-1543]]
This eventually led to the '''''scientific revolution''''', held by most historians (e.g., Howard Margolis) to have begun in ], when the first printed copy of ]'s '']'' was brought to the influential <!-- do not enter nationality claims here--> astronomer from ] (Nürnberg), where it had been printed by ]. 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 ], ], ], and ]. During the early ], Galileo made extensive use of experimentation to validate physical theories, which is the key idea in the modern ]. Galileo formulated and successfully tested several results in ], in particular the Law of ].

]]] The scientific revolution is considered to have culminated with the publication of the '']'' in ] by the mathematician, physicist, alchemist and inventor Sir ] (]-]).In ], ] published the '']'', detailing two comprehensive and successful physical theories: ], from which arise ]; and ], which describes the ] of ]. Both theories agreed well with experiment. The Principia also included several theories in ].

From the late ] onward, ] was developed by physicist and chemist ], ], and many others. In ], ] used statistical arguments with classical mechanics to derive thermodynamic results, initiating the field of ]. In ], ] demonstrated the conversion of mechanical work into heat, and in ] ] stated the law of conservation of ], in the form of heat as well as mechanical energy. ], in the nineteenth century, is responsible for the modern form of statistical mechanics.

Classical mechanics was re-formulated and extended by ], French mathematician ], Irish mathematical physicist ], and others, who produced new results in mathematical physics. The law of universal gravitation initiated the field of ], which describes ] phenomena using physical theories.

After Newton defined ], the next great field of inquiry within physics was the nature of ]. Observations in the ] and ] by scientists such as ], ], and ] created a foundation for later work. These observations also established our basic understanding of electrical charge and ].

The existence of the atom was proposed in ] by ].

<!-- Unsourced image removed: ]]] -->
In ], the English physicist and chemist ] integrated the study of ] with the study of electricity. This was done by demonstrating that a moving ] induced an ] in a ]. Faraday also formulated a physical conception of ]s. ] built upon this conception, in ], with an interlinked set of twenty equations that explained the interactions between ] and ]s. These twenty equations were later reduced, using ], to a set of ] by ].

In addition to other electromagnetic phenomena, Maxwell's equations also can be used to describe ]. Confirmation of this observation was made with the ] discovery of ] by ] and in ] when ] detected ]s.

===Modern physics===

]]]
The ability to describe light in electromagnetic terms helped serve as a springboard for ]'s publication of the theory of ] in 1905. This theory combined classical mechanics with Maxwell's equations.
The theory of ] unifies space and time into a single entity, ]. Relativity prescribes a different transformation between ] 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 ] in ].

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

]]]
In ], ] discovered ]s, which turned out to be high-frequency electromagnetic radiation.

] was discovered in ] by ], and further studied by ], ], and others. This initiated the field of ].

In ], ] discovered the ], the elementary particle which carries electrical current in ]. In ], he proposed the first model of the ], known as the ]. Its existence had been proposed in ] by ].

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

]]]
In ], ] deduced from ] the existence of a compact atomic nucleus, with positively charged constituents dubbed ]s. ]s, the neutral nuclear constituents, were discovered in ] by ]. The equivalence of mass and energy (Einstein, 1905) was spectacularly demonstrated during ], as research was conducted by each side into ], for the purpose of creating a ]. The German effort, led by Heisenberg, did not succeed, but the Allied ] reached its goal. In America, a team led by ] achieved the first man-made ] in ], and in ] the world's first ] was detonated at ], near ], ].

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

]]]
Quantum mechanics was formulated in ] by ] and in ] by ] and ], in two different ways, that both explained the preceding heuristic quantum theories. In quantum mechanics, the outcomes of physical measurements are inherently ]; the theory describes the calculation of these probabilities. It successfully describes the behavior of matter at small distance scales. During the ] Schrödinger, Heisenberg, and ] 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.

<!-- Image with unknown copyright status removed: ]]] -->
] was formulated in order to extend quantum mechanics to be consistent with special relativity. It was devised in the late ] with work by ], ], ], and ]. They formulated the theory of ], which describes the electromagnetic interaction, and successfully explained the ]. Quantum field theory provided the framework for modern ], which studies ]s and elementary particles.

] and ], in the ], discovered an unexpected ] in the decay of a ]. In ], Yang and ] then developed a class of ] which provided the framework for understanding the nuclear forces (Yang, Mills 1954). The theory for the ] was first proposed by ]. The ], the unification of the ] with electromagnetism, was proposed by ], ], and ] and confirmed in ] by ] and ]. This led to the so-called ] of particle physics in the ], which successfully describes all the elementary particles observed to date.

Quantum mechanics also provided the theoretical tools for ], whose largest branch is ]. It studies the physical behavior of solids and liquids, including phenomena such as ]s, ], and ]. The pioneers of condensed matter physics include ], who created a quantum mechanical description of the behavior of electrons in crystal structures in ]. The transistor was developed by physicists ], ], and ] in ] at ].

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

]
The ] declared the year ], the centenary of Einstein's ], as the ].

=== Future directions ===

{{main | Unsolved problems in physics}}

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 ]. Strong efforts, largely experimental, are being put into making workable ] and ]s.

In particle physics, the first pieces of experimental evidence for physics beyond the ] have begun to appear. Foremost amongst these are indications that ]s have non-zero ]. These experimental results appear to have solved the long-standing ] in solar physics. The physics of massive neutrinos is currently an area of active theoretical and experimental research. In the next several years, ]s will begin probing energy scales in the ] range, in which experimentalists are hoping to find evidence for the ] and ].
] per ion) ] ions in the ] of the ]; an experiment done in order to investigate the properties of a ] such as the one thought to exist in the ultrahot first few microseconds after the ]]]

Theoretical attempts to unify ] and ] into a single theory of ], a program ongoing for over half a century, have not yet borne fruit. Currently, the leading candidates are ], ], and ].

Many ] and ] phenomena have yet to be explained satisfactorily, including the existence of ], the ], the ], and the ].

Although much progress has been made in high-energy, ], and astronomical physics, many everyday phenomena, involving ], ], or ] 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 ], the shape of water ]s, mechanisms of ] ], 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 ] methods and ], which enabled ] to be modeled in new ways. The ] ] of complex physics also has increased, as exemplified by the study of ] in ], or the ] of ] ] in ] systems. In 1932, ] correctly prophesied the success of the theory of quantum electrodynamics and the near-stagnant progress in the study of turbulence:
<blockquote>''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.''</blockquote>


== Notes == == Notes ==

Revision as of 23:01, 16 July 2007

The first few hydrogen atom electron orbitals shown as cross-sections with color-coded probability density

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.

The deepest visible-light image of the universe, the Hubble Ultra Deep Field

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.

Typical thermodynamic system - heat moves from hot (boiler) to cold (condenser) and work is extracted
  • 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.
  • 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.

Theory Major subtopics Concepts
Classical mechanics Newton's laws of motion, Lagrangian mechanics, Hamiltonian mechanics, Kinematics, Statics, Dynamics, Chaos theory, Acoustics, Fluid dynamics, Continuum mechanics Density, Dimension, Gravity, Space, Time, Motion, Length, Position, Velocity, Acceleration, Galilean invariance, Mass, Momentum, Impulse, Force, Energy, Angular velocity, Angular momentum, Moment of inertia, Torque, Conservation law, Harmonic oscillator, Wave, Work, Power, Lagrangian, Hamiltonian, Tait-Bryan angles, Euler angles
Electromagnetism Electrostatics, Electrodynamics, Electricity, Magnetism, Magnetostatics, Maxwell's equations, Optics Capacitance, Electric charge, Current, Electrical conductivity, Electric field, Electric permittivity, Electric potential, Electrical resistance, Electromagnetic field, Electromagnetic induction, Electromagnetic radiation, Gaussian surface, Magnetic field, Magnetic flux, Magnetic monopole, Magnetic permeability
Thermodynamics and Statistical mechanics Heat engine, Kinetic theory Boltzmann's constant, Conjugate variables, Enthalpy, Entropy, Equation of state, Equipartition theorem, Free energy, Heat, Ideal gas law, Internal energy, Laws of thermodynamics, Maxwell relations, Irreversible process, Ising model, Mechanical action, Partition function, Pressure, Reversible process, Spontaneous process, State function, Statistical ensemble, Temperature, Thermodynamic equilibrium, Thermodynamic potential, Thermodynamic processes, Thermodynamic state, Thermodynamic system, Viscosity, Volume, Work, Granular material
Quantum mechanics Path integral formulation, Scattering theory, Schrödinger equation, Quantum field theory, Quantum statistical mechanics Adiabatic approximation, Blackbody radiation, Correspondence principle, Free particle, Hamiltonian, Hilbert space, Identical particles, Matrix Mechanics, Planck's constant, Observer effect, Operators, Quanta, Quantization, Quantum entanglement, Quantum harmonic oscillator, Quantum number, Quantum tunneling, Schrödinger's cat, Dirac equation, Spin, Wavefunction, Wave mechanics, Wave-particle duality, Zero-point energy, Pauli Exclusion Principle, Heisenberg Uncertainty Principle
Relativity Special relativity, General relativity, Einstein field equations Covariance, Einstein manifold, Equivalence principle, Four-momentum, Four-vector, General principle of relativity, Geodesic motion, Gravity, Gravitoelectromagnetism, Inertial frame of reference, Invariance, Length contraction, Lorentzian manifold, Lorentz transformation, Mass-energy equivalence, Metric, Minkowski diagram, Minkowski space, Principle of Relativity, Proper length, Proper time, Reference frame, Rest energy, Rest mass, Relativity of simultaneity, Spacetime, Special principle of relativity, Speed of light, Stress-energy tensor, Time dilation, Twin paradox, World line

Research

File:Meissner effect.jpg
A magnet levitating above a high-temperature superconductor (with boiling liquid nitrogen underneath), demonstrating the Meissner effect — a phenomenon of importance to the field of condensed matter physics

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.
  • 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.

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.

Field Subfields Major theories Concepts
Astrophysics Cosmology, Gravitation physics, High-energy astrophysics, Planetary astrophysics, Plasma physics, Space physics, Stellar astrophysics Big Bang, Lambda-CDM model, Cosmic inflation, General relativity, Newton's law of universal gravitation Black hole, Cosmic background radiation, Cosmic string, Cosmos, Dark energy, Dark matter, Galaxy, Gravity, Gravitational radiation, Gravitational singularity, Planet, Solar system, Star, Supernova, Universe
Atomic, molecular, and optical physics Atomic physics, Molecular physics, Atomic and Molecular astrophysics, Chemical physics, Optics, Photonics Quantum optics, Quantum chemistry, Quantum information science Photon, Atom, Molecule, Diffraction, Electromagnetic radiation, Laser, Polarization, Spectral line, Casimir effect
Particle physics Nuclear physics, Nuclear astrophysics, Particle astrophysics, Particle physics phenomenology Standard Model, Quantum field theory, Quantum electrodynamics, Quantum chromodynamics, Electroweak theory, Effective field theory, Lattice field theory, Lattice gauge theory, Gauge theory, Supersymmetry, Grand unification theory, Superstring theory, M-theory Fundamental force (gravitational, electromagnetic, weak, strong), Elementary particle, Spin, Antimatter, Spontaneous symmetry breaking, Neutrino oscillation, Seesaw mechanism, Brane, String, Quantum gravity, Theory of everything, Vacuum energy
Condensed matter physics Solid state physics, High pressure physics, Low-temperature physics, Surface Physics,Nanoscale and Mesoscopic physics, Polymer physics BCS theory, Bloch wave, Fermi gas, Fermi liquid, Many-body theory Phases (gas, liquid, solid, Bose-Einstein condensate, superconductor, superfluid), Electrical conduction, Magnetism, Self-organization, Spin, Spontaneous symmetry breaking
Applied Physics Accelerator physics, Acoustics, Agrophysics, Biophysics, Chemical Physics, Communication Physics, Econophysics, Engineering physics, Fluid dynamics, Geophysics, Materials physics, Medical physics, Nanotechnology, Optics, Optoelectronics, Photovoltaics, Physical chemistry, Physics of computation, Plasma physics, Solid-state devices, Quantum chemistry, Quantum electronics, Quantum information science, Vehicle dynamics

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

Organizations

References

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


Major branches of physics
Divisions
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Fundamental interactions of physics
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