1 Overview of physics research

1.1 Theoretical and experimental physics

The culture of physics research differs from the other sciences in the separation of theoryIn mathematics, a theory is a set of statements closed under logical implication. In mathematical logic, "theory" is the term for a set of well-formed formulae consisting of certain axioms and all theorems provable from said axioms. Godel's incompleteness and experimentIn the scientific method, an experiment is a set of actions and observations, performed to verify or falsify a hypothesis or research a causal relationship between phenomena. The experiment is a cornerstone in empirical approach to knowledge. See the list. Since the 20th century, most individual physicists have specialized in either theoretical physics or experimental physics, and very few physicists have been successful in both forms of research. In contrast, almost all the successful theorists in biology and chemistry have also been experimentalists.

Roughly speaking, theorists seek to develop theories that can explain existing experimental results and successfully predict future results, while experimentalists devise and perform experiments to test theoretical predictions. Although theory and experiment are developed separately, they are strongly dependent on 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. In the absence of experiment, theoretical research frequently goes in the wrong direction; this is one of the criticisms that have been levelled against M-theory, a popular theory in high-energy physics for which no practical experimental test has ever been devised.

1.2 Central theories of physics

While physics deals with an extremely wide variety of systems, there are certain theories that are used by physics as a whole, and not by any single field. Each of these theories is believed to be basically correct, within a certain domain of validity. For instance, the theory of classical mechanics accurately describes the motion of objects, provided they are much larger than atoms and moving at much less than the speed of light. These theories continue to be areas of active research; for instance, a remarkable aspect of classical mechanics known as chaos was discovered in the 20th century, three centuries after its formulation by Isaac Newton. However, few physicists expect any of them to prove fundamentally misguided. Therefore, they are used as the basis for research into more specialized topics, and any contemporary student of physics, regardless of his or her specialization, is generally expected to be well-versed in all of them.

Theory	Major subtopics	Concepts
Classical mechanics	Newton's laws of motion Lagrangian mechanics Hamiltonian mechanics Chaos theory Fluid dynamics Continuum mechanics	Dimension Space Time Motion Length Velocity Mass Momentum Force Energy Angular momentum Torque Conservation law Harmonic oscillator Wave Work Power
Electromagnetism	Electrostatics Electricity Magnetism Maxwell's equations	Electric charge Current Electric field Magnetic field Electromagnetic field Electromagnetic radiation Magnetic monopole
Thermodynamics and Statistical mechanics	Heat engine Kinetic theory	Boltzmann's constant Entropy Free energy Heat Partition function Temperature
Quantum mechanics	Path integral formulation Schrödinger equation Quantum field theory	Hamiltonian Identical particles Planck's constant Quantum entanglement Quantum harmonic oscillator Wavefunction Zero-point energy
Theory of relativity	Special relativity General relativity	Equivalence principle Four-momentum Reference frame Spacetime Speed of light

1.3 Major fields of physics

Contemporary research in physics is divided into several distinct fields that study different aspects of the material world. Condensed matter physics, by most estimates the largest single field of 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. The field of atomic, molecular, and optical physics deals with the behavior of individual atoms and molecules, and in particular the ways in which they absorb and emit light. The field of particle physics, also known as "high-energy physics", is concerned with the properties of submicroscopic particles much smaller than atoms, including the elementary particles from which all other units of matter are constructed. Finally, the field of astrophysics applies the laws of physics to explain astronomical phenomena, ranging from the Sun and the other objects in the solar system to the universe as a whole.

Field	Subfields	Major theories	Concepts
Astrophysics	Cosmology, Planetology, Plasma physics	Big Bang Cosmic inflation General relativity Law of universal gravitation	Black hole Cosmic background radiation Galaxy Gravity Gravitational radiation Planet Solar system Star
Atomic, molecular, and optical physics	Atomic physics, Molecular physics, Optics, Photonics	Quantum optics	Diffraction Electromagnetic radiation Laser Polarization Spectral line
Particle physics	Accelerator physics, Nuclear physics	Standard Model Grand unification theory Loop quantum gravity M-theory	Fundamental force ( gravitational, electromagnetic, weak, strong) Elementary particle Antimatter Spin Spontaneous symmetry breaking Theory of everything Vacuum energy
Condensed matter physics	Solid state physics, Materials 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

1.4 Related fields

There are many areas of research that mix physics with other disciplines. For example, the wide-ranging field of biophysics is devoted to the role that physical principles play in biological systems, and the field of quantum chemistry studies how the theory of quantum mechanics gives rise to the chemical behavior of atoms and molecules. Some of these are listed below.

Acoustics - Astronomy - Biophysics - Computational physics - Electronics - Engineering - Geophysics - Materials science - Mathematical physics - Medical physics - Physical chemistry - Physics of computation - Vehicle dynamics

1.5 Fringe theories

Cold fusion - Dynamic theory of gravity - Luminiferous aether - Orgone energy - Steady state theory

2 History

Main article: History of physics. See also Famous physicists and Nobel Prize in Physics.

Since antiquity, people have tried to understand the behavior of matter: why unsupported objects drop to the ground, why different materials have different properties, and so forth. Also a mystery was the character of the universe, such as the form of the Earth and the behavior of celestial objects such as the Sun and the Moon. Several theories were proposed, most of which were wrong. These theories were largely couched in philosophical terms, and never verified by systematic experimental testing as is popular today. There were exceptions and there are anachronisms: for example, the Greek thinker Archimedes derived many correct quantitative descriptions of mechanics and hydrostatics.

During the early 17th century, Galileo pioneered the use of experiment to validate physical theories, which is the key idea in the scientific method. Galileo formulated and successfully tested several results in dynamics, in particular the Law of Inertia. In 1687, Newton published the Principia Mathematica, 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. Classical mechanics was exhaustively extended by Lagrange, Hamilton, and others, who produced new formulations, principles, and results. The Law of Gravitation initiated the field of astrophysics, which describes astronomical phenomena using physical theories.

From the 18th century onwards, thermodynamics was developed by 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.

The behavior of electricity and magnetism was studied by Faraday, Ohm, and others. In 1855, Maxwell unified the two phenomena into a single theory of electromagnetism, described by Maxwell's equations. A prediction of this theory was that light is an electromagnetic wave.

In 1895, Roentgen discovered X-rays, which turned out to be high-frequency electromagnetic radiation. Radioactivity was discovered in 1896 by Henri Becquerel, and further studied by Marie Curie, Pierre Curie, and others. This initiated the field of nuclear physics.

In 1897, 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. (The existence of the atom had been proposed in 1808 by Dalton.)

In 1905, Albert Einstein formulated the theory of special relativity, unifying 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. In 1915, Einstein extended special relativity to explain gravity with the general theory of relativity, which replaces Newton's law of gravitation. In the regime of low masses and energies, the two theories agree.

In 1911, 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.

Beginning in 1900, Planck, Einstein, Bohr, and others developed quantum theories to explain various anomalous experimental results by introducing discrete energy levels. In 1925, Heisenberg and 1926, Schrödinger and Dirac formulated quantum mechanics, which explained the preceding 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.

Quantum mechanics also provided the theoretical tools for condensed matter physics, which studies the physical behavior of solids and liquids, including phenomena such as crystal structures, semiconductivity, and superconductivity. The pioneers of condensed matter physics include Bloch, who created a quantum mechanical description of the behavior of electrons in crystal structures in 1928.

During World War II, 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.

Quantum field theory was formulated in order to extend quantum mechanics to be consistent with special relativity. It achieved its modern form in the late 1940s with work by Feynman, Schwinger, Tomonaga, and Dyson. They formulated the theory of quantum electrodynamics, which describes the electromagnetic interaction.

Quantum field theory provided the framework for modern particle physics, which studies fundamental forces and elementary particles. In 1954, Yang and Mills developed a class of gauge theories, which provided the framework for the Standard Model. The Standard Model, which was completed in the 1970s, successfully describes almost all elementary particles observed to date.

The United Nations have declared the year 2005 as the World Year of Physics [1].

2.1 Future directions

Main article: unsolved problems in physics.

As of 2004, research in physics is progressing on a large number of fronts.

In condensed matter physics, the biggest 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 this 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, has yet to bear fruit. The current leading candidates are M-theory and loop quantum gravity.

Many astronomical phenomena have yet to be explained, including the existence of ultra-high energy cosmic rays and the anomalous rotation rates of galaxies. Theories that have been proposed to resolve these problems include doubly-special relativity, modified Newtonian dynamics, and the existence of dark matter. In addition, the cosmological predictions of the last several decades have been contradicted by recent evidence that the expansion of the universe is accelerating.

In the rush to solve high-energy, quantum, and astronomical physics, quite a bit of everyday physics (sometimes called quotidian physics by persons not working on such problems) was left behind between circa 1930 and 1970. Complex problems that seem like they could be solved by a clever application of dynamics and mechanics, like the formation of sandpiles, nodes in trickling water, the shape of water droplets, mechanisms of surface tension catastrophes, or self-sorting in shaken heterogeneous collections, still remain insufficiently characterized, and more importantly, poorly understood.

These complex phenomena have received growing attention since the 1970s for a variety of reasons, not least of which has been the availability of modern mathematical methods and computers which enabled complex systems to be modelled in new ways. The interdisciplinary relevance of complex physics has also increased, as exemplified by the study of turbulence in aerodynamics or the observation of pattern formation in biological systems. A quote famous for its prophetic accuracy is due to Horace Lamb (1932): "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."

3 Links and references

3.1 Suggested readings

Wikibooks provides a collection of electronic books on .

• Feynman, The Character of Physical Law, Random House (Modern Library), 1994, hardcover, 192 pages, BooksEnthsiast.com
• Feynman, Leighton, Sands, The Feynman Lectures on Physics, Addison-Wesley 1970, 3 volumes, paperback, BooksEnthsiast.com. Hardcover commemorative edition, 1989, BooksEnthsiast.com
• Landau, et. al., Course of Theoretical Physics, Butterworth-Heinemann, 1976, 10 volumes, paperback, BooksEnthsiast.com
• Walker, The Flying Circus of Physics, Wiley, 1977, paperback, 312 pages, BooksEnthsiast.com
• Anthony Leggett, The Problems of Physics, Oxford University Press, 1988, BooksEnthsiast.com

3.2 External links

Wikiversity , is developing courses on .

• sci.physics The Usenet general physics newsgroup.
• Usenet Physics FAQ. An FAQ compiled by sci.physics and other physics newsgroups.
• World of Physics. An online encyclopedic dictionary of physics.
• HyperPhysics. An online "exploration environment" for physics.
• The Nobel Prize in Physics 1901-2000. Website of the Nobel Prize in Physics.
• Physics.org. Web portal run by the Institute of Physics.
• AIP.org Website of the American Institute of Physics .

3.3 See also

• List of physics topics

General subfields within physics

Classical mechanics | Condensed matter physics | Continuum mechanics | Electromagnetism | General relativity | Particle physics | Quantum field theory | Quantum mechanics | Solid state physics | Special relativity | Statistical mechanics | Thermodynamics

Physics

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