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Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of nature, or, in other words, to the regular succession of events." Physics is one of the most fundamental disciplines, and its main goal is to understand how the behaves. "Physics is one of the most fundamental of the sciences. Scientists of all disciplines use the ideas of physics, including chemists who study the structure of molecules, paleontologists who try to reconstruct how dinosaurs walked, and climatologists who study how human activities affect the atmosphere and oceans. Physics is also the foundation of all engineering and technology. No engineer could design a flat-screen TV, an interplanetary spacecraft, or even a better mousetrap without first understanding the basic laws of physics. (...) You will come to see physics as a towering achievement of the human intellect in its quest to understand our world and ourselves." "Physics is an experimental science. Physicists observe the phenomena of nature and try to find patterns that relate these phenomena." "Physics is the study of your world and the world and universe around you." Physics is one of the oldest s and, through its inclusion of , perhaps the oldest. Over much of the past two millennia, physics, , , and certain branches of were a part of , but during the in the 17th century these natural sciences emerged as unique research endeavors in their own right. Physics intersects with many areas of research, such as and , and the boundaries of physics are not . New ideas in physics often explain the fundamental mechanisms studied by other s and suggest new avenues of research in academic disciplines such as mathematics and . Advances in physics often enable advances in new . For example, advances in the understanding of , , and led directly to the development of new products that have dramatically transformed modern-day society, such as , s, s, and s; advances in led to the development of ; and advances in inspired the development of .


History

The word "physics" comes from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), meaning "knowledge of nature"., ,


Ancient astronomy

is one of the oldest s. Early civilizations dating back before 3000 BCE, such as the ians, ians, and the , had a predictive knowledge and a basic awareness of the motions of the Sun, Moon, and stars. The stars and planets, believed to represent gods, were often worshipped. While the explanations for the observed positions of the stars were often unscientific and lacking in evidence, these early observations laid the foundation for later astronomy, as the stars were found to traverse s across the sky, which however did not explain the positions of the s. According to , the origins of astronomy can be found in , and all Western efforts in the s are descended from late . left monuments showing knowledge of the constellations and the motions of the celestial bodies, while Greek poet wrote of various celestial objects in his ' and '; later provided names, which are still used today, for most constellations visible from the .


Natural philosophy

has its origins in during the (650 BCE – 480 BCE), when like rejected explanations for natural phenomena and proclaimed that every event had a natural cause. They proposed ideas verified by reason and observation, and many of their hypotheses proved successful in experiment; for example, was found to be correct approximately 2000 years after it was proposed by and his pupil .


Medieval European and Islamic

The fell in the fifth century, and this resulted in a decline in intellectual pursuits in the western part of Europe. By contrast, the (also known as the ) resisted the attacks from the barbarians, and continued to advance various fields of learning, including physics. In the sixth century, Isidore of Miletus created an important compilation of Archimedes' works that are copied in the . In sixth-century Europe , a Byzantine scholar, questioned 's teaching of physics and noted its flaws. He introduced the . Aristotle's physics was not scrutinized until Philoponus appeared; unlike Aristotle, who based his physics on verbal argument, Philoponus relied on observation. On Aristotle's physics Philoponus wrote:
But this is completely erroneous, and our view may be corroborated by actual observation more effectively than by any sort of verbal argument. For if you let fall from the same height two weights of which one is many times as heavy as the other, you will see that the ratio of the times required for the motion does not depend on the ratio of the weights, but that the difference in time is a very small one. And so, if the difference in the weights is not considerable, that is, of one is, let us say, double the other, there will be no difference, or else an imperceptible difference, in time, though the difference in weight is by no means negligible, with one body weighing twice as much as the other
Philoponus' criticism of Aristotelian principles of physics served as an inspiration for ten centuries later, during the . Galileo cited Philoponus substantially in his works when arguing that Aristotelian physics was flawed. In the 1300s , a teacher in the faculty of arts at the University of Paris, developed the concept of impetus. It was a step toward the modern ideas of inertia and momentum. inherited from the Greeks and during the developed it further, especially placing emphasis on observation and ''a priori'' reasoning, developing early forms of the . The most notable innovations were in the field of optics and vision, which came from the works of many scientists like , , , and . The most notable work was ' (also known as Kitāb al-Manāẓir), written by Ibn al-Haytham, in which he conclusively disproved the ancient Greek idea about vision, but also came up with a new theory. In the book, he presented a study of the phenomenon of the (his thousand-year-old version of the ) 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. He asserted that the light ray is focused, but the actual explanation of how light projected to the back of the eye had to wait until 1604. His ''Treatise on Light'' explained the camera obscura, hundreds of years before the modern development of photography. The seven-volume ''Book of Optics'' (''Kitab al-Manathir'') hugely influenced thinking across disciplines from the theory of visual to the nature of in medieval art, in both the East and the West, for more than 600 years. Many later European scholars and fellow polymaths, from and to , and , 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 devices that replicated those Ibn al-Haytham had built, and understand the way light works. From this, important inventions such as eyeglasses, magnifying glasses, telescopes, and cameras were developed.


Classical

Physics became a separate science when ans used experimental and quantitative methods to discover what are now considered to be the . Major developments in this period include the replacement of the of the with the heliocentric , the (determined by Kepler between 1609 and 1619), Galileo's pioneering work on s and in the 16th and 17th Centuries, and Newton's discovery and unification of the and (that would come to bear his name). Newton also developed , the mathematical study of change, which provided new mathematical methods for solving physical problems. The discovery of new laws in , , and resulted from greater research efforts during the as energy needs increased. 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 and the simplify to their classical equivalents at such scales. However, inaccuracies in for very small objects and very high velocities led to the development of modern physics in the 20th century.


Modern

began in the early 20th century with the work of in quantum theory and 's theory of relativity. Both of these theories came about due to inaccuracies in classical mechanics in certain situations. predicted a varying , which could not be resolved with the constant speed predicted by of electromagnetism; this discrepancy was corrected by Einstein's theory of , which replaced classical mechanics for fast-moving bodies and allowed for a constant speed of light. provided another problem for classical physics, which was corrected when Planck proposed that the excitation of material oscillators is possible only in discrete steps proportional to their frequency; this, along with the and a complete theory predicting discrete of , led to the theory of quantum mechanics taking over from classical physics at very small scales. Quantum mechanics would come to be pioneered by , and . From this early work, and work in related fields, the was derived. Following the discovery of a particle with properties consistent with the at in 2012, all predicted by the standard model, and no others, appear to exist; however, , with theories such as , is an active area of research. Areas of in general are important to this field, such as the study of and .


Philosophy

In many ways, physics stems from . From Thales' first attempt to characterize matter, to Democritus' deduction that matter ought to reduce to an invariant state, the of a crystalline , and Aristotle's book ' (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. 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 on and its "scientific method" to advance our knowledge of the physical world. The scientific method employs ' as well as ' reasoning and the use of to measure the validity of a given theory. 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 and , , and metaphysical outlooks such as , and . Many physicists have written about the philosophical implications of their work, for instance , who championed , and Schrödinger, who wrote on quantum mechanics. The mathematical physicist had been called a by , "I think that Roger is a Platonist at heart but he must answer for himself." a view Penrose discusses in his book, '. Hawking referred to himself as an "unashamed reductionist" and took issue with Penrose's views.


Core theories

Though physics deals with a wide variety of systems, certain theories are used by all physicists. Each of these theories was experimentally tested numerous times and found to be an adequate approximation of nature. For instance, the theory of mechanics accurately describes the motion of objects, provided they are much larger than s and moving at much less than the speed of light. These theories continue to be areas of active research today. , a remarkable aspect of classical mechanics, was discovered in the 20th century, three centuries after the original formulation of classical mechanics by 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 , , and special relativity.


Classical

Classical physics includes the traditional branches and topics that were recognised and well-developed before the beginning of the 20th century—classical mechanics, , , thermodynamics, and electromagnetism. Classical mechanics is concerned with bodies acted on by s and bodies in and may be divided into (study of the forces on a body or bodies not subject to an acceleration), (study of motion without regard to its causes), and (study of motion and the forces that affect it); mechanics may also be divided into and (known together as ), the latter include such branches as , , , and . Acoustics is the study of how sound is produced, controlled, transmitted and received. Important modern branches of acoustics include , the study of sound waves of very high frequency beyond the range of human hearing; , the physics of animal calls and hearing, and , the manipulation of audible sound waves using electronics. Optics, the study of , is concerned not only with but also with and , which exhibit all of the phenomena of visible light except visibility, e.g., reflection, refraction, interference, diffraction, dispersion, and polarization of light. is a form of , the internal energy possessed by the particles of which a substance is composed; thermodynamics deals with the relationships between heat and other forms of energy. and have been studied as a single branch of physics since the intimate connection between them was discovered in the early 19th century; an gives rise to a , and a changing magnetic field induces an electric current. deals with s at rest, with moving charges, and with magnetic poles at rest.


Modern

Classical physics is generally concerned with matter and energy on the normal scale of observation, while much of modern physics is concerned with the behavior of matter and energy under extreme conditions or on a very large or very small scale. For example, and study matter on the smallest scale at which s can be identified. The is on an even smaller scale since it is concerned with the most basic units of matter; this branch of physics is also known as high-energy physics because of the extremely high energies necessary to produce many types of particles in s. On this scale, ordinary, commonsensical notions of space, time, matter, and energy are no longer valid. The two chief theories of modern physics present a different picture of the concepts of space, time, and matter from that presented by classical physics. Classical mechanics approximates nature as continuous, while quantum theory is concerned with the discrete nature of many phenomena at the atomic and subatomic level and with the complementary aspects of particles and waves in the description of such phenomena. The theory of relativity is concerned with the description of phenomena that take place in a that is in motion with respect to an observer; the special theory of relativity is concerned with motion in the absence of gravitational fields and the with motion and its connection with . Both quantum theory and the theory of relativity find applications in all areas of modern physics.


Fundamental concepts in modern physics

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Difference

While physics aims to discover universal laws, its theories lie in explicit domains of applicability. Loosely speaking, the laws of classical physics accurately describe systems whose important length scales are greater than the atomic scale and whose motions are much slower than the speed of light. Outside of this domain, observations do not match predictions provided by classical mechanics. Einstein contributed the framework of special relativity, which replaced notions of with and allowed an accurate description of systems whose components have speeds approaching the speed of light. Planck, Schrödinger, and others introduced quantum mechanics, a probabilistic notion of particles and interactions that allowed an accurate description of atomic and subatomic scales. Later, unified quantum mechanics and special relativity. General relativity allowed for a dynamical, curved spacetime, with which highly massive systems and the large-scale structure of the universe can be well-described. General relativity has not yet been unified with the other fundamental descriptions; several candidate theories of are being developed.


Relation to other fields


Prerequisites

Mathematics provides a compact and exact language used to describe the order in nature. This was noted and advocated by , , "Although usually remembered today as a philosopher, Plato was also one of ancient Greece's most important patrons of mathematics. Inspired by Pythagoras, he founded his Academy in Athens in 387 BC, where he stressed mathematics as a way of understanding more about reality. In particular, he was convinced that geometry was the key to unlocking the secrets of the universe. The sign above the Academy entrance read: 'Let no-one ignorant of geometry enter here.'" Galileo, 'Philosophy is written in that great book which ever lies before our eyes. I mean the universe, but we cannot understand it if we do not first learn the language and grasp the symbols in which it is written. This book is written in the mathematical language, and the symbols are triangles, circles, and other geometrical figures, without whose help it is humanly impossible to comprehend a single word of it, and without which one wanders in vain through a dark labyrinth.' – Galileo (1623), '" and Newton. Physics uses mathematics to organise and formulate experimental results. From those results, or solutions are obtained, or quantitative results, from which new predictions can be made and experimentally confirmed or negated. The results from physics experiments are numerical data, with their and estimates of the errors in the measurements. Technologies based on mathematics, like have made an active area of research. is a prerequisite for physics, but not for mathematics. It means physics is ultimately concerned with descriptions of the real world, while mathematics is concerned with abstract patterns, even beyond the real world. Thus physics statements are synthetic, while mathematical statements are analytic. Mathematics contains hypotheses, while physics contains theories. Mathematics statements have to be only logically true, while predictions of physics statements must match observed and experimental data. The distinction is clear-cut, but not always obvious. For example, is the application of mathematics in physics. Its methods are mathematical, but its subject is physical. The problems in this field start with a "" (system) and a "mathematical description of a physical law" that will be applied to that system. Every mathematical statement used for solving has a hard-to-find physical meaning. The final mathematical solution has an easier-to-find meaning, because it is what the solver is looking for. Pure physics is a branch of (also called basic science). Physics is also called "''the'' fundamental science" because all branches of natural science like chemistry, astronomy, geology, and biology are constrained by laws of physics.; see also and Similarly, chemistry is often called because of its role in linking the physical sciences. For example, chemistry studies properties, structures, and of matter (chemistry's focus on the molecular and atomic scale ). Structures are formed because particles exert electrical forces on each other, properties include physical characteristics of given substances, and reactions are bound by laws of physics, like , , and . Physics is applied in industries like engineering and medicine.


Application and influence

is a general term for physics research, which is intended for a particular use. An applied physics curriculum usually contains a few classes in an applied discipline, like geology or electrical engineering. It usually differs from in that an applied physicist may not be designing something in particular, but rather is using physics or conducting physics research with the aim of developing new technologies or solving a problem. The approach is similar to that of . Applied physicists use physics in scientific research. For instance, people working on might seek to build better s for research in theoretical physics. Physics is used heavily in engineering. For example, statics, a subfield of , is used in the building of s and other static structures. The understanding and use of acoustics results in sound control and better concert halls; similarly, the use of optics creates better optical devices. An understanding of physics makes for more realistic s, s, and movies, and is often critical in investigations. With the that the of physics are universal and do not change with time, physics can be used to study things that would ordinarily be mired in . For example, in the , one can reasonably model earth's mass, temperature, and rate of rotation, as a function of time allowing one to extrapolate forward or backward in time and so predict future or prior events. It also allows for simulations in engineering that drastically speed up the development of a new technology. But there is also considerable , so many other important fields are influenced by physics (e.g., the fields of and ).


Research


Scientific method

Physicists use the scientific method to test the validity of a . By using a methodical approach to compare the implications of a theory with the conclusions drawn from its related s and observations, physicists are better able to test the validity of a theory in a logical, unbiased, and repeatable way. To that end, experiments are performed and observations are made in order to determine the validity or invalidity of the theory. A scientific law is a concise verbal or mathematical statement of a relation that expresses a fundamental principle of some theory, such as Newton's law of universal gravitation.


Theory and experiment

Theorists seek to develop s that both agree with existing experiments and successfully predict future experimental results, while devise and perform experiments to test theoretical predictions and explore new phenomena. Although and experiment are developed separately, they strongly affect and depend upon each other. Progress in physics frequently comes about when experimental results defy explanation by existing theories, prompting intense focus on applicable modelling, and when new theories generate experimentally testable s, which inspire the development of new experiments (and often related equipment). s who work at the interplay of theory and experiment are called , who study complex phenomena observed in experiment and work to relate them to a . Theoretical physics has historically taken inspiration from philosophy; electromagnetism was unified this way. Beyond the known universe, the field of theoretical physics also deals with hypothetical issues, such as , a , and s. Theorists invoke these ideas in hopes of solving particular problems with existing theories; they then explore the consequences of these ideas and work toward making testable predictions. Experimental physics expands, and is expanded by, engineering and . Experimental physicists who are involved in , design and perform experiments with equipment such as particle accelerators and s, whereas those involved in often work in industry, developing technologies such as (MRI) and s. has noted that experimentalists may seek areas that have not been explored well by theorists. "In fact experimenters have a certain individual character. They ... very often do their experiments in a region in which people know the theorist has not made any guesses."


Scope and aims

Physics covers a wide range of , from s (such as quarks, neutrinos, and electrons) to the largest s of galaxies. Included in these phenomena are the most basic objects composing all other things. Therefore, physics is sometimes called the "fundamental science". Physics aims to describe the various phenomena that occur in nature in terms of simpler phenomena. Thus, physics aims to both connect the things observable to humans to s, and then connect these causes together. For example, the observed that certain rocks ( and ) were attracted to one another by an invisible force. This effect was later called magnetism, which was first rigorously studied in the 17th century. But even before the Chinese discovered magnetism, the knew of other objects such as , that when rubbed with fur would cause a similar invisible attraction between the two. This was also first studied rigorously in the 17th century and came to be called electricity. Thus, physics had come to understand two observations of nature in terms of some root cause (electricity and magnetism). However, further work in the 19th century revealed that these two forces were just two different aspects of one force—electromagnetism. This process of "unifying" forces continues today, and electromagnetism and the are now considered to be two aspects of the . Physics hopes to find an ultimate reason (theory of everything) for why nature is as it is (see section ' below for more information).


Research fields

Contemporary research in physics can be broadly divided into nuclear and particle physics; ; ; ; and applied physics. Some physics departments also support and . Since the 20th century, the individual fields of physics have become increasingly specialised, and today most physicists work in a single field for their entire careers. "Universalists" such as Einstein (1879–1955) and (1908–1968), who worked in multiple fields of physics, are now very rare. The major fields of physics, along with their subfields and the theories and concepts they employ, are shown in the following table.


Nuclear and particle

Particle physics is the study of the elementary constituents of and energy and the between them. In addition, particle physicists design and develop the high-energy accelerators, detectors, and necessary for this research. The field is also called "high-energy physics" because many elementary particles do not occur naturally but are created only during high-energy s of other particles. Currently, the interactions of elementary particles and are described by the . The model accounts for the 12 known particles of matter (s and s) that interact via the , weak, and electromagnetic s. Dynamics are described in terms of matter particles exchanging s (s, , and s, respectively). The Standard Model also predicts a particle known as the Higgs boson. In July 2012 CERN, the European laboratory for particle physics, announced the detection of a particle consistent with the Higgs boson, an integral part of the . Nuclear physics is the field of physics that studies the constituents and interactions of . The most commonly known applications of nuclear physics are generation and technology, but the research has provided application in many fields, including those in and magnetic resonance imaging, in , and in and .


Atomic, molecular, and optical

Atomic, , and optical physics (AMO) is the study of matter–matter and light–matter interactions on the scale of single atoms and molecules. The three areas are grouped together because of their interrelationships, the similarity of methods used, and the commonality of their relevant energy scales. All three areas include both classical, semi-classical and treatments; they can treat their subject from a microscopic view (in contrast to a macroscopic view). Atomic physics studies the s of atoms. Current research focuses on activities in quantum control, cooling and trapping of atoms and ions, low-temperature collision dynamics and the effects of electron correlation on structure and dynamics. Atomic physics is influenced by the (see ), but intra-nuclear phenomena such as and are considered part of nuclear physics. focuses on multi-atomic structures and their internal and external interactions with matter and light. is distinct from optics in that it tends to focus not on the control of classical light fields by macroscopic objects but on the fundamental properties of s and their interactions with matter in the microscopic realm.


Condensed matter

Condensed matter physics is the field of physics that deals with the macroscopic physical properties of matter. In particular, it is concerned with the "condensed" that appear whenever the number of particles in a system is extremely large and the interactions between them are strong. The most familiar examples of condensed phases are and s, which arise from the bonding by way of the between atoms. More exotic condensed phases include the and the found in certain atomic systems at very low temperature, the phase exhibited by s in certain materials, and the ic and ic phases of on . Condensed matter physics is the largest field of contemporary physics. Historically, condensed matter physics grew out of solid-state physics, which is now considered one of its main subfields. The term ''condensed matter physics'' was apparently coined by when he renamed his research group—previously ''solid-state theory''—in 1967. In 1978, the Division of Solid State Physics of the was renamed as the Division of Condensed Matter Physics. Condensed matter physics has a large overlap with chemistry, , and engineering.


Astrophysics

Astrophysics and astronomy are the application of the theories and methods of physics to the study of , , the origin of the Solar System, and related problems of . Because astrophysics is a broad subject, astrophysicists typically apply many disciplines of physics, including mechanics, electromagnetism, statistical mechanics, thermodynamics, quantum mechanics, relativity, nuclear and particle physics, and atomic and molecular physics. The discovery by in 1931 that radio signals were emitted by celestial bodies initiated the science of . Most recently, the frontiers of astronomy have been expanded by space exploration. Perturbations and interference from the earth's atmosphere make space-based observations necessary for , , , and . Physical cosmology is the study of the formation and evolution of the universe on its largest scales. Albert Einstein's theory of relativity plays a central role in all modern cosmological theories. In the early 20th century, 's discovery that the universe is expanding, as shown by the , prompted rival explanations known as the universe and the . The Big Bang was confirmed by the success of and the discovery of the in 1964. The Big Bang model rests on two theoretical pillars: Albert Einstein's general relativity and the . Cosmologists have recently established the of the evolution of the universe, which includes , , and . Numerous possibilities and discoveries are anticipated to emerge from new data from the over the upcoming decade and vastly revise or clarify existing models of the universe. In particular, the potential for a tremendous discovery surrounding dark matter is possible over the next several years. Fermi will search for evidence that dark matter is composed of , complementing similar experiments with the and other underground detectors. is already yielding new discoveries: "No one knows what is creating the ribbon" along the of the , "but everyone agrees that it means the textbook picture of the —in which the Solar System's enveloping pocket filled with the solar wind's charged particles is plowing through the onrushing 'galactic wind' of the interstellar medium in the shape of a comet—is wrong."


Current research

Research in physics is continually progressing on a large number of fronts. In condensed matter physics, an important unsolved theoretical problem is that of . Many condensed matter experiments are aiming to fabricate workable and s. In particle physics, the first pieces of experimental evidence for physics beyond the Standard Model have begun to appear. Foremost among these are indications that s have non-zero . These experimental results appear to have solved the long-standing , and the physics of massive neutrinos remains an area of active theoretical and experimental research. The Large Hadron Collider has already found the Higgs boson, but future research aims to prove or disprove the supersymmetry, which extends the Standard Model of particle physics. Research on the nature of the major mysteries of dark matter and dark energy is also currently ongoing. Although much progress has been made in high-energy, , and astronomical physics, many everyday phenomena involving , chaos, or are still poorly understood. Complex problems that seem like they could be solved by a clever application of dynamics and mechanics remain unsolved; examples include the formation of sandpiles, nodes in trickling water, the shape of water droplets, mechanisms of , and self-sorting in shaken heterogeneous collections.
These complex phenomena have received growing attention since the 1970s for several reasons, including the availability of modern mathematical methods and computers, which enabled complex systems to be modeled in new ways. Complex physics has become part of increasingly research, as exemplified by the study of turbulence in aerodynamics and the observation of in biological systems. In the 1932 ''Annual Review of Fluid Mechanics'', said:


See also

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Notes


References


Sources

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External links


PhysicsCentral
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Usenet Physics FAQ
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