Physics (from the Greek physikos, nature) is the science of nature at its most fundamental form, and is the foundation of the natural sciences. From quarks to galaxies, from individual atoms to macroscopic biological systems, Physicists study a wide range of physical phenomena.
 Key Areas of Physics
Over the course of time, physical phenomena have been grouped together under specific branches. While it is widely believed that the whole of physics can be considered within a single unified theory, including all phenomena both at the level of quantum mechanics and on a scale as as wide as the universe, this has not yet been proven. This is made clear especially by the fact that the current models developed by the different branches of physics give contradictory solutions to the same problem when they are combined. For example, the unification of general relativity and quantum mechanics has so far proven impossible due to creation of infinite values when looking at some properties of systems of subatomic particles. Nevertheless, these are considered as problems "to be solved". The central branches of physics are:
- Classical mechanics is a model of the physics of forces acting upon bodies. As opposed to quantum mechanics, classical mechanics is deterministic. Classical mechanics is usually regarded as a limit of quantum mechanics, although this has not been proven in general. Classical mechanics can be divided into two parts:
- Newtonian, after Newton and his laws of motion
- Relativistic, due to Albert Einstein and his theory of relativity. This includes both special and general relativity.
- Quantum mechanics is the branch of 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 theory typically permits only probable or statistical calculation of the observed features of particles, understood in terms of wave functions. Quantum mechanics itself has several levels of approximation.
- Electromagnetism, or electromagnetic theory, is the physics of the electromagnetic field: a field, encompassing all of space, which exerts a force on those particles that possess the property of electric charge, and is in turn affected by the presence and motion of such particles. Electromagnetism encompasses various real-world electromagnetic phenomena.
- Statistical mechanics and Thermodynamics are the branches of physics that deals with heat, work and entropy. Thermodynamics is particularly concerned macroscopic energy and the effects of temperature, pressure, volume, mechanical action, and work. Statistical mechanic is the branch of physics that analyzes macroscopic systems by applying statistical principles to their microscopic constituents and, thus, connects the macroscopic viewpoint of thermodynamics with the atomic nature of matter described by either classical physics or quantum mechanics.
 Research and fields within physics
Physics can be subdivided in a variety of different manners; for teaching, for historical purposes, or for research purposes. Contemporary research in physics is divided into many distinct subfields. A incomplete listing include:
- Condensed matter physics is the study of the condensed phases; Solids and liquids, and how the properties of these phases arise from the properties and mutual interactions of the constituent atoms. More physicists study condensed matter physics than any other field.
- Particle physics, also known as "high-energy physics". This branch is concerned with the properties of subatomic particles much smaller than atoms, including the elementary particles from which all other units of matter are constructed.
- Atomic, molecular, and optical physics (AMO physics) deals with the behavior of individual atoms and molecules, including the ways in which they absorb and emit light. Molecular physics is sometimes also considered a branch of chemical physics. Laser science may be considered a subfield of AMO or as a separate field.
A number of fields of physics overlap strongly with other sciences: Biophysics, Physical chemistry and Geophysics overlap considerably with biology, chemistry and geography, but the focus is on the application of physics and physical techniques to problems within the other field.
 Classical and quantum physics
The distinction between classical and quantum theories is important in physics. Classical theories are generally valid despite not considering the quantum nature of things, but are ultimately an approximation to a deeper quantized truth; this approximation typically breaks down at extreme scales, particularly the subatomic. Some fundamental classical theories, such as relativity do not yet have full analogous quantum theories.
Both classical and quantum physics are active areas of research. However, there exist problems in physics in which classical and quantum aspects must be combined to attain some approximation or limit that may acquire several forms as the passage from classical to quantum mechanics is often difficult — such problems are termed semiclassical.
 Theoretical and experimental physics
Most individual physicists specialize in either theoretical physics or experimental physics. There have been a few exceptions, such as great Italian physicist Enrico Fermi (1901–1954), who made fundamental contributions to both theory and experimentation.
Roughly speaking, theorists seek to develop theories, through mathematical and computational models, 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 can be 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, or when theorists make predictions that experimentalists test.
 Physics and Other disciplines
Physics finds applications throughout the other natural sciences as they regard the basic principles of nature. Physics is often said to be the "fundamental science", because the other sciences deal with material systems that obey the laws of physics. For example, chemistry is the science of matter (such as atoms and molecules) and the chemical substances that they form in the bulk. The structure, reactivity, and properties of a chemical compound are determined by the properties of the underlying molecules, which can be described by areas of physics such as quantum mechanics (in the applied subfiled of quantum chemistry), thermodynamics, and electromagnetism.
Physics is closely related to mathematics, which provides the logical framework in which physical laws can be precisely formulated and their predictions quantified. Physical definitions, models and theories are invariably expressed using mathematical relations. A key difference between physics and mathematics is that because physics is ultimately concerned with descriptions of the material world, it tests its theories by observations (called experiments), whereas mathematics does not have such requirements. The distinction, however, is not always clear-cut. This large area of research intermediate between physics and mathematics is known as mathematical physics.
Physics is also closely related to engineering and technology. For instance, electrical engineering is the study of the practical application of electromagnetism. Statics, a subfield of mechanics, is responsible for the building of bridges. Further, physicists, or practitioners of physics, invent and design processes and devices, such as the transistor, whether in basic or applied research. Experimental physicists design and perform experiments with particle accelerators, nuclear reactors, telescopes, barometers, synchrotrons, cyclotrons, spectrometers, lasers, and other equipment.
 Current research directions
Research in physics is progressing constantly on a large number of fronts, and is likely to do so for the foreseeable future. Some current directions include:
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 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. The current leading candidates are M-theory, superstring theory and loop quantum gravity.
Many astronomical and cosmological phenomena have yet to be satisfactorily explained, 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 are still poorly understood. Complex problems that seem like they could be solved 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 has also increased, as exemplified by the study of turbulence in aerodynamics or the observation of pattern formation in biological systems.
Two rapidly-growing applied fields to which physics makes contributions are biophysics and nanotechnology.
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