Physics (from Ancient Greek: φυσική (ἐπιστήμη) phusikḗ (epistḗmē) "knowledge of nature", from φύσις phúsis "nature"[1][2][3]) is the natural science that involves the study of matter[4] and its motion through space and time, along with related concepts such as energy and force.[5] One of the most fundamental scientific disciplines, the main goal of physics is to understand how the universe behaves.[a][6][7][8]
Physics is one of the oldest academic disciplines, perhaps the oldest through its inclusion of astronomy.[9] Over the last two millennia, physics was a part of natural philosophy along with chemistry, biology, and certain branches of mathematics, but during the scientific revolution in the 17th century, the natural sciencesemerged as unique research programs in their own right.[b] Physics intersects with many interdisciplinary areas of research, such as biophysics and quantum chemistry, and the boundaries of physics are not rigidly defined. New ideas in physics often explain the fundamental mechanisms of other sciences[6] while opening new avenues of research in areas such as mathematics and philosophy.
Physics also makes significant contributions through advances in new technologies that arise from theoretical breakthroughs. For example, advances in the understanding of electromagnetism or nuclear physics led directly to the development of new products that have dramatically transformed modern-day society, such as television, computers, domestic appliances, and nuclear weapons;[6] advances in thermodynamics led to the development of industrialization, and advances in mechanics inspired the development of calculus.
Contents
[hide] * 1History * 1.1Ancient astronomy * 1.2Natural philosophy * 1.3Physics in the medieval Islamic world * 1.4Classical physics * 1.5Modern physics * 2Philosophy * 3Core theories * 3.1Classical physics * 3.2Modern physics * 3.3Difference between classical and modern physics * 4Relation to other fields * 4.1Prerequisites * 4.2Application and influence * 5Research * 5.1Scientific method * 5.2Theory and experiment * 5.3Scope and aims * 5.4Research fields * 5.4.1Particle physics * 5.4.2Atomic, molecular, and optical physics * 5.4.3Condensed matter physics * 5.4.4Astrophysics * 6Current research * 7See also * 8Notes * 9References * 10Sources * 11External links
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History
Main article: History of physics
Ancient astronomy
Main article: History of astronomy

Ancient Egyptian astronomy is evident in monuments like the ceiling of Senemut's tomb from the Eighteenth Dynasty of Egypt.
Astronomy is the oldest of the natural sciences. The earliest civilizations dating back to beyond 3000 BCE, such as the Sumerians, ancient Egyptians, and the Indus Valley Civilization, all had a predictive knowledge and a basic understanding of the motions of the Sun, Moon, and stars. The stars and planets were often a target of worship, believed to represent their gods. While the explanations for these phenomena were often unscientific and lacking in evidence, these early observations laid the foundation for later astronomy.[9]
According to Asger Aaboe, the origins of Western astronomy can be found in Mesopotamia, and all Western efforts in the exact sciences are descended from late Babylonian astronomy.[10] Egyptian astronomers left monuments showing knowledge of the constellations and the motions of the celestial bodies,[11] while Greek poet Homer wrote of various celestial objects in his Iliad and Odyssey; later Greek astronomers provided names, which are still used today, for most constellations visible from the northern hemisphere.[12]
Natural philosophy
Main article: Natural philosophy

Natural philosophy has its origins in Greece during the Archaic period, (650 BCE – 480 BCE), when Pre-Socratic philosophers like Thales rejected non-naturalistic explanations for natural phenomena and proclaimed that every event had a natural cause.[13] They proposed ideas verified by reason and observation, and many of their hypotheses proved successful in experiment;[14] for example, atomism was found to be correct approximately 2000 years after it was first proposed by Leucippus and his pupilDemocritus.[15]
Physics in the medieval Islamic world
Main article: Physics in the medieval Islamic world

The basic way a pinhole camera works
Islamic scholarship had inherited Aristotelian physics from the Greeks and during the Islamic Golden Age developed it further, especially placing emphasis on observation anda priori reasoning, developing early forms of the scientific method.
The most notable innovations were in the field of Optics and vision, which came from the works of many scientists like Ibn Sahl, Al-Kindi, Ibn Al-Haitham, Al-Farisi andAvicenna. The most notable work, was The Book of Optics (also known as Kitāb al-Manāẓir) written by Ibn Al-Haitham, in which he was not only the first to disprove the ancient Greek idea about vision, but also came up with a new theory.[16] In the book, he was also the first to study the phenomenon of the pinhole camera[17] and delved further into the way the eye itself works. Using dissections and the knowledge of previous scholars, he was able to begin to explain how light enters the eye, is focused, and is projected to the back of the eye: and built then the world's first obscura camera hundreds of years before the modern development of photography.[18]

Ibn Al Haytham, The Father of Optics
The seven-volume Book of Optics (Kitab al-Manathir) hugely influenced thinking across disciplines from the theory of visual perception to the nature of perspective in medieval art, in both the East and the West, for more than 600 years. Many later European scholars and fellow polymaths, from Robert Grosseteste and Leonardo da Vinci to René Descartes, Johannes Kepler and Isaac Newton, were in his debt. Indeed, the influence of Ibn al-Haytham’s Optics ranks alongside that of Newton’s work of the same title, published 700 years later.
The translation of The Book of Optics had a huge impact on Europe. From it, later European scholars were able to build the same devices as what Ibn Al Haytham did, and understand the way light works. From this, such important things as eyeglasses, magnifying glasses, telescopes, and cameras were developed.
Classical physics
Main article: Classical physics

Sir Isaac Newton (1643–1727), whose laws of motionand universal gravitation were major milestones in classical physics
Physics became a separate science when early modern Europeans used experimental and quantitative methods to discover what are now considered to be the laws of physics.[19]
Major developments in this period include the replacement of the geocentric model of the solar system with the heliocentric Copernican model, the laws governing the motion of planetary bodies determined by Johannes Kepler between 1609 and 1619, pioneering work on telescopes and observational astronomy by Galileo Galilei in the 16th and 17th Centuries, and Isaac Newton's discovery and unification of the laws of motion and universal gravitation that would come to bear his name.[20] Newton also developedcalculus,[c] the mathematical study of change, which provided new mathematical methods for solving physical problems.[21]
The discovery of new laws in thermodynamics, chemistry, and electromagnetics resulted from greater research efforts during the Industrial Revolution as energy needs increased.[22] The laws comprising classical physics remain very widely used for objects on everyday scales travelling at non-relativistic speeds, since they provide a very close approximation in such situations, and theories such as quantum mechanics and the theory of relativity simplify to their classical equivalents at such scales. However, inaccuracies in classical mechanics for very small objects and very high velocities led to the development of modern physics in the 20th century.
Modern physics
Main article: Modern physics
See also: History of special relativity and History of quantum mechanics

Albert Einstein (1879–1955), whose work on thephotoelectric effect and thetheory of relativity led to a revolution in 20th century physics

Max Planck (1858–1947), the originator of the theory ofquantum mechanics
Modern physics began in the early 20th century with the work of Max Planck in quantum theory and Albert Einstein's theory of relativity. Both of these theories came about due to inaccuracies in classical mechanics in certain situations. Classical mechanics predicted a varying speed of light, which could not be resolved with the constant speed predicted byMaxwell's equations of electromagnetism; this discrepancy was corrected by Einstein's theory of special relativity, which replaced classical mechanics for fast-moving bodies and allowed for a constant speed of light.[23] Black body radiation provided another problem for classical physics, which was corrected when Planck proposed that light comes in individual packets known as photons; this, along with the photoelectric effect and a complete theory predicting discrete energy levels of electron orbitals, led to the theory of quantum mechanics taking over from classical physics at very small scales.[24]
Quantum mechanics would come to be pioneered by Werner Heisenberg, Erwin Schrödinger and Paul Dirac.[24] From this early work, and work in related fields, the Standard Model of particle physics was derived.[25] Following the discovery of a particle with properties consistent with the Higgs boson at CERN in 2012,[26] allfundamental particles predicted by the standard model, and no others, appear to exist; however, physics beyond the Standard Model, with theories such as supersymmetry, is an active area of research.[27] Areas of mathematics in general are important to this field, such as study of probabilities.
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Philosophy
Main article: Philosophy of physics
In many ways, physics stems from ancient Greek philosophy. From Thales' first attempt to characterize matter, to Democritus' deduction that matter ought to reduce to an invariant state, the Ptolemaic astronomy of a crystalline firmament, and Aristotle's book Physics (an early book on physics, which attempted to analyze and define motion from a philosophical point of view), various Greek philosophers advanced their own theories of nature. Physics was known as natural philosophy until the late 18th century.[28]
By the 19th century, physics was realized as a discipline distinct from philosophy and the other sciences. Physics, as with the rest of science, relies onphilosophy of science and its "scientific method" to advance our knowledge of the physical world.[29] The scientific method employs a priori reasoning as well as a posteriori reasoning and the use of Bayesian inference to measure the validity of a given theory.[30]
The development of physics has answered many questions of early philosophers, but has also raised new questions. Study of the philosophical issues surrounding physics, the philosophy of physics, involves issues such as the nature of space and time, determinism, and metaphysical outlooks such asempiricism, naturalism and realism.[31]
Many physicists have written about the philosophical implications of their work, for instance Laplace, who championed causal determinism,[32] and Erwin Schrödinger, who wrote on quantum mechanics.[33][34] The mathematical physicist Roger Penrose has been called a Platonist by Stephen Hawking,[35] a view Penrose discusses in his book, The Road to Reality.[36] Hawking refers to himself as an "unashamed reductionist" and takes issue with Penrose's views.[37]
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Core theories
Further information: Branches of physics and Outline of physics
Though physics deals with a wide variety of systems, certain theories are used by all physicists. Each of these theories were experimentally tested numerous times and found to be an adequate approximation of nature. 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 today. Chaos theory, a remarkable aspect of classical mechanics was discovered in the 20th century, three centuries after the original formulation of classical mechanics by Isaac Newton (1642–1727).
These central theories are important tools for research into more specialised topics, and any physicist, regardless of their specialisation, is expected to be literate in them. These include classical mechanics, quantum mechanics, thermodynamics and statistical mechanics, electromagnetism, and special relativity.

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