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

Physics (from the Greek, φύσις (phúsis), "nature" and φυσικός (phusikós), "natural"), the most fundamental natural science, is concerned with the basic principles of the Universe. Physics deals with the elementary constituents of the natural world (e.g. matter, energy, space, and time) and their interactions, as well as the analysis of systems which are best understood in terms of these principles. Physics is both an empirical systematic study of Nature and the body of knowledge obtained as a result of this process. The foremost task of the physicist is to find patterns in the workings of the natural world and deduce those elementary laws that govern the cosmos in the most precise and simplest formulation.

Introduction

Discoveries in physics find applications throughout the other natural sciences as they regard the basic constituents of the Universe. Some of the phenomena studied in physics, such as the phenomenon of conservation of energy, are common to all material systems. These are often referred to as laws of physics. Others, such as superconductivity, stem from these laws, but are not laws themselves because they only appear in some systems. Physics is often said to be the "fundamental science" (chemistry is sometimes included), because each of the other weaker sciences (biology, chemistry, geology, material science, engineering, medicine etc.) deals with particular types of 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 (called in this case quantum chemistry), thermodynamics, and electromagnetism. Refer to Branches of physics

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 is concerned with abstract logical patterns not limited by those observed in the real world (because the real world is limited in the number of dimensions and in many other ways it does not have to correspond to richer mathematical structures). The distinction, however, is not always clear-cut. There is a large area of research intermediate between physics and mathematics, known as mathematical physics.

Physics attempts to describe the natural world by the application of the scientific method. Natural philosophy, its counterpart, is the study of the changing world by philosophy which has been also called "physics" since classical times to at least up to its separation from philosophy as a positive science in the 19th century. Mixed questions, of which solutions can be attempted through the applications of both disciplines (e.g. the divisibility of the atom) can involve natural philosophy in physics the science and vice versa.

Etymology

The word Physics comes from the Greek word φύσις (phúsis) meaning "nature" or from its adjectival form φυσικός (phusikós) meaning "natural".

Subdisciplines of Physics

Physicists study a wide range of physical phenomena, from quarks to black holes, from individual atoms to the many-body systems of superconductors. The subject matter of Physics is broadly classified into Pure Physics and Applied Physics. Based on the chronological development of Physics, Pure Physics is again classified into Classical Physics and Modern Physics. Based on the method adopted for study, Physics is classified into Theoretical physics and Experimental physics.

Classical Physics and Modern physics

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Further information: Classical physics, Quantum physics, Modern physics, Semiclassical

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

Classical physics generally covers the developments of physics up to the end of 19th century. In the domain of classical physics, Nature consists of a continuous distribution of matter and energy in space and time. By the end of the 19th century, however, a series of experimental and theoretical developments provided evidence that some aspects of Nature are non-continuous (e.g., discrete atomic energy levels), and that space and time are interrelated (e.g., the constancy of the speed of light). During a period of about twenty years, whose beginning is often marked by the publication of two seminal papers by Albert Einstein in 1905, physicists discovered the theory of relativity and quantum mechanics. The advances in physics based on quantum physics and relativity during the last century are collectively referred to as modern physics.

Classical Physics

Classical physics has five distinct branches such as Mechanics, Acoustics, Thermodynamics, Electrodynamics and Optics.

Mechanics

Mechanics deals with fundamental concepts like matter, force, motion, energy, work, power, etc.

Acoustics

Acoustics is a branch of Physics that studies sound, namely mechanical waves in gases, liquids, and solids. A physicist who works in the field of acoustics is an acoustician.

Thermodynamics

Thermodynamics is the science of Heat. It deals with heat and temperature. Macroscopic systems are generally analysed by applying statistical principles to their microscopic constituents. The statistical tools used to describe thermodynamic system constitute what is called Statistical mechanics, an auxiliary branch of Thermodynamics. The statistical distribution used in classical thermodynamics is the Maxwell-Boltzmann distribution.

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Electromagnetics

Electromagnetics deals with Electricity, Magnetism, Electromagnetism, Electromagnetic waves etc. The classical electromagnetics was developed over the course of the 19th century, most prominently by James Clerk Maxwell.

Optics

Optics is that branch of physics which describes the behavior and properties of light and the interaction of light with matter. Classical optics is generally divided into two: Ray Optics and Wave Optics.

Modern Physics

Since relativity and quantum mechanics provide the most complete known description of fundamental interactions, and because the changes brought by these two frameworks to the physicist's world view were revolutionary, the term modern physics is used to describe physics which relies on these two theories. Colloquially, modern physics can be described as the physics of extremes: from systems at the extremely small (atoms, nuclei, fundamental particles) to the extremely large (the Universe) and of the extremely fast (relativity).

Relativistic Mechanics

Relativistic mechanics deals with Einstein's Special theory of relativity and General theory of relativity.

Quantum Mechanics

Quantum mechanics

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Quantum Thermodynamics

Quantum thermodynamics is the study of heat and work dynamics in quantum systems. Approximately, quantum thermodynamics attempts to combine thermodynamics and quantum mechanics into a coherent whole. A central objective in quantum thermodynamics is the quantitative and qualitative determination of the laws of thermodynamics at the quantum level in which uncertainty and probability begin to take effect. In quantum thermodynamics, the statistical distributions used are Bose-Einstein Statistics and Fermi-Dirac Statistics.

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Quantum Electrodynamics

Quantum electrodynamics

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Quantum Optics

Quantum optics (Modern Optics) is a branch of Modern physics, dealing with the application of quantum mechanical principles to phenomena involving light and its interactions with matter.

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Applied Physics

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 utilization 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. [1]

Theoretical physics and experimental physics

The culture of physics research differs from the other sciences in the separation of theory and experiment. Since the 20th 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, though 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 on each other. However, theoretical research in physics may further be considered to draw from mathematical physics and computational physics in addition to experimentation. 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.