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For other uses, seePhysics (disambiguation).
Not to be confused withPhysis.

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Physicsis the scientific study ofmatter,itsfundamental constituents,itsmotionand behavior throughspace and time,and the related entities ofenergyandforce.[1]Physics is one of the most fundamentalscientificdisciplines.[2][3][4]A scientist who specializes in the field of physics is called aphysicist.

Physics is one of the oldestacademic disciplinesand, through its inclusion ofastronomy,perhapstheoldest.[5]Over much of the past two millennia, physics,chemistry,biology,and certain branches of mathematics were a part ofnatural philosophy,but during theScientific Revolutionin the 17th century, these natural sciences branched into separate research endeavors.[a]Physics intersects with manyinterdisciplinaryareas of research, such asbiophysicsandquantum chemistry,and the boundaries of physics are not rigidly defined. New ideas in physics often explain the fundamental mechanisms studied by other sciences[2]and suggest new avenues of research in these and other academic disciplines such as mathematics and philosophy.

Advances in physics often enable newtechnologies.For example, advances in the understanding ofelectromagnetism,solid-state physics,andnuclear physicsled directly to the development of new products that have dramatically transformed modern-day society, such as television, computers,domestic appliances,andnuclear weapons;[2]advances inthermodynamicsled to the development of industrialization; and advances inmechanicsinspired the development ofcalculus.

The expansion of the universe according to theBig Bangtheory in physics

History

Main article:History of physics

The wordphysicscomes from theLatinphysica('study of nature'), which itself is a borrowing of theGreekφυσική(phusikḗ'natural science'), a term derived fromφύσις(phúsis'origin, nature, property').[7][8][9]

Ancient astronomy

Main article:History of astronomy
AncientEgyptian astronomyis evident in monuments like theceiling of Senemut's tombfrom theEighteenth Dynasty of Egypt.

Astronomyis one of the oldestnatural sciences.Early civilizations dating before 3000 BCE, such as theSumerians,ancient Egyptians,and theIndus Valley Civilisation,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 traversegreat circlesacross the sky,[5]which could not explain the positions of theplanets.

According toAsger Aaboe,the origins of Western astronomy can be found inMesopotamia,and all Western efforts in theexact sciencesare descended from lateBabylonian astronomy.[10]Egyptian astronomersleft monuments showing knowledge of the constellations and the motions of the celestial bodies,[11]while Greek poetHomerwrote of various celestial objects in hisIliadandOdyssey;laterGreek astronomersprovided names, which are still used today, for most constellations visible from theNorthern Hemisphere.[12]

Natural philosophy

Main article:Natural philosophy

Natural philosophyhas its origins inGreeceduring theArchaic period(650 BCE – 480 BCE), whenpre-Socratic philosopherslikeThalesrejectednon-naturalisticexplanations 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,atomismwas found to be correct approximately 2000 years after it was proposed byLeucippusand his pupilDemocritus.[15]

Aristotle and Hellenistic Physics

Aristotle
(384–322BCE)

During theclassical periodin Greece (6th, 5th and 4th centuries BCE) and inHellenistic times,natural philosophydeveloped along many lines of inquiry.Aristotle(Greek:Ἀριστοτέλης,Aristotélēs) (384–322 BCE), a student ofPlato, wrote on many subjects, including a substantial treatise on "Physics"– in the 4th century BC.Aristotelian physicswas influential for about two millennia. His approach mixed some limited observation with logical deductive arguments, but did not rely on experimental verification of deduced statements. Aristotle's foundational work in Physics, though very imperfect, formed a framework against which later thinkers further developed the field. His approach is entirely superseded today.

He explained ideas such asmotion(andgravity) with the theory offour elements. Aristotle believed that each of the fourclassical elements(air, fire, water, earth) had its own natural place.[16]Because of their differing densities, each element will revert to its own specific place in the atmosphere.[17]So, because of their weights, fire would be at the top, air underneath fire, then water, then lastly earth. He also stated that when a small amount of one element enters the natural place of another, the less abundant element will automatically go towards its own natural place. For example, if there is a fire on the ground, the flames go up into the air in an attempt to go back into its natural place where it belongs. His laws of motion included 1) heavier objects will fall faster, the speed being proportional to the weight and 2) the speed of the object that is falling depends inversely on the density object it is falling through (e.g. density of air).[18]He also stated that, when it comes to violent motion (motion of an object when a force is applied to it by a second object) that the speed that object moves, will only be as fast or strong as the measure of force applied to it.[18]The problem of motion and its causes was studied carefully, leading to the philosophical notion of a "primer mover"as the ultimate source of all motion in the world (Book 8 of his treatisePhysics).

Medieval European and Islamic

Main articles:European science in the Middle AgesandPhysics in the medieval Islamic world

TheWestern Roman Empirefell to invaders and internal decay in the fifth century, resulting in a decline in intellectual pursuits in western Europe. By contrast, the Eastern Roman Empire (usually known as theByzantine Empire) resisted the attacks from invaders and continued to advance various fields of learning, including physics.[19]

In the sixth century,Isidore of Miletuscreated an important compilation ofArchimedes' works that are copied in theArchimedes Palimpsest.

In sixth-century EuropeJohn Philoponus,a Byzantine scholar, questionedAristotle's teaching of physics and noted its flaws. He introduced thetheory of impetus.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[20]

Philoponus' criticism of Aristotelian principles of physics served as an inspiration forGalileo Galileiten centuries later,[21]during theScientific Revolution.Galileo cited Philoponus substantially in his works when arguing that Aristotelian physics was flawed.[22][23]In the 1300sJean Buridan,a teacher in the faculty of arts at theUniversity of Paris,developed the concept of impetus. It was a step toward the modern ideas of inertia and momentum.[24]

Islamic scholarshipinheritedAristotelian physicsfrom the Greeks and during theIslamic Golden Agedeveloped it further, especially placing emphasis on observation anda priorireasoning, developing early forms of thescientific method.

Ibn Al-Haytham (Alhazen) drawing
Ibn al-Haytham(c. 965– c. 1040) wrote of hiscamera obscuraexperiments in theBook of Optics.[25]

The most notable innovations under Islamic scholarship were in the field ofopticsand vision,[26]which came from the works of many scientists likeIbn Sahl,Al-Kindi,Ibn al-Haytham,Al-FarisiandAvicenna.The most notable work wasThe Book of Optics(also known as Kitāb al-Manāẓir), written by Ibn al-Haytham, in which he presented the alternative to the ancient Greek idea about vision.[27]In hisTreatise on Lightas well as in hisKitāb al-Manāẓir,he presented a study of the phenomenon of thecamera obscura(his thousand-year-old version of thepinhole camera) and delved further into the way the eye itself works. Using the knowledge of previous scholars, he began 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. HisTreatise on Lightexplained the camera obscura, hundreds of years before the modern development of photography.[28]

The basic way a pinhole camera works

The seven-volumeBook of Optics(Kitab al-Manathir) influenced thinking[29]across disciplines from the theory of visualperceptionto the nature ofperspectivein medieval art, in both the East and the West, for more than 600 years. This included later European scholars and fellow polymaths, fromRobert GrossetesteandLeonardo da VincitoJohannes Kepler.

The translation ofThe Book of Opticshad an 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 vision works.

Galileo Galilei(1564–1642) related mathematics, theoretical physics, and experimental physics.

Classical

Main article:Classical physics
Isaac Newtondiscovered thelaws of motionanduniversal gravitation

Physics became a separate science whenearly modern Europeansused experimental and quantitative methods to discover what are now considered to be thelaws of physics.[30][page needed]

Major developments in this period include the replacement of thegeocentric modelof theSolar Systemwith the heliocentricCopernican model,thelaws governing the motion of planetary bodies(determined by Kepler between 1609 and 1619), Galileo's pioneering work ontelescopesandobservational astronomyin the 16th and 17th centuries, andIsaac Newton's discovery and unification of thelaws of motionanduniversal gravitation(that would come to bear his name).[31]Newton also developedcalculus,[b]the mathematical study of continuous change, which provided new mathematical methods for solving physical problems.[32]

The discovery of laws inthermodynamics,chemistry,andelectromagneticsresulted from research efforts during theIndustrial Revolutionas energy needs increased.[33]The laws comprising classical physics remain widely used for objects on everyday scales travelling at non-relativistic speeds, since they provide a close approximation in such situations, and theories such asquantum mechanicsand thetheory of relativitysimplify to their classical equivalents at such scales. Inaccuracies inclassical mechanicsfor very small objects and very high velocities led to the development of modern physics in the 20th century.

Modern

Main article:Modern physics
See also:History of special relativityandHistory of quantum mechanics
Max Planck(1858–1947), the originator of the theory ofquantum mechanics
Albert Einstein(1879–1955), discovered thephotoelectric effectandtheory of relativity.

Modern physicsbegan in the early 20th century with the work ofMax Planckin quantum theory andAlbert Einstein's theory of relativity. Both of these theories came about due to inaccuracies in classical mechanics in certain situations.Classical mechanicspredicted that thespeed of lightdepends on the motion of the observer, which could not be resolved with the constant speed predicted byMaxwell's equationsof electromagnetism. This discrepancy was corrected by Einstein's theory ofspecial relativity,which replaced classical mechanics for fast-moving bodies and allowed for a constant speed of light.[34]Black-body radiationprovided 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 thephotoelectric effectand a complete theory predicting discreteenergy levelsofelectron orbitals,led to the theory of quantum mechanics improving on classical physics at very small scales.[35]

Quantum mechanics would come to be pioneered byWerner Heisenberg,Erwin SchrödingerandPaul Dirac.[35]From this early work, and work in related fields, theStandard Model of particle physicswas derived.[36]Following the discovery of a particle with properties consistent with theHiggs bosonatCERNin 2012,[37]allfundamental particlespredicted by the standard model, and no others, appear to exist; however,physics beyond the Standard Model,with theories such assupersymmetry,is an active area of research.[38]Areas of mathematics in general are important to this field, such as the study ofprobabilitiesandgroups.

Philosophy

Main article:Philosophy of physics

In many ways, physics stems fromancient Greek philosophy.FromThales' first attempt to characterize matter, toDemocritus' deduction that matter ought to reduce to an invariant state to thePtolemaic astronomyof a crystallinefirmament,and Aristotle's bookPhysics(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.[c]

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 thephilosophy of scienceand its "scientific method"to advance knowledge of the physical world.[40]The scientific method employsa priori and a posteriorireasoning as well as the use ofBayesian inferenceto measure the validity of a given theory.[41]

The development of physics has answered many questions of early philosophers, but has also raised new questions. Study of the philosophical issues surrounding physics, thephilosophy of physics,involves issues such as the nature ofspace and time,determinism,andmetaphysicaloutlooks such asempiricism,naturalism,andrealism.[42]

Many physicists have written about the philosophical implications of their work, for instanceLaplace,who championedcausal determinism,[43]andErwin Schrödinger,who wrote on quantum mechanics.[44][45]The mathematical physicistRoger Penrosehas been called aPlatonistbyStephen Hawking,[46]a view Penrose discusses in his book,The Road to Reality.[47]Hawking referred to himself as an "unashamed reductionist" and took issue with Penrose's views.[48]

Core theories

Further information:Branches of physicsandOutline of physics

Physics deals with a wide variety of systems, although 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 ofclassical mechanicsaccurately describes the motion of objects, provided they are much larger thanatomsand moving at a speed much less than the speed of light. These theories continue to be areas of active research today.Chaos theory,an 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 specialized topics, and any physicist, regardless of their specialization, is expected to be literate in them. These include classical mechanics, quantum mechanics, thermodynamics andstatistical mechanics,electromagnetism,and special relativity.

Classical

Main article:Classical physics

Classical physics includes the traditional branches and topics that were recognized and well-developed before the beginning of the 20th century—classical mechanics,acoustics,optics,thermodynamics, and electromagnetism. Classical mechanics is concerned with bodies acted on byforcesand bodies inmotionand may be divided intostatics(study of the forces on a body or bodies not subject to an acceleration),kinematics(study of motion without regard to its causes), anddynamics(study of motion and the forces that affect it); mechanics may also be divided intosolid mechanicsandfluid mechanics(known together ascontinuum mechanics), the latter include such branches ashydrostatics,hydrodynamicsandpneumatics.Acoustics is the study of how sound is produced, controlled, transmitted and received.[49]Important modern branches of acoustics includeultrasonics,the study of sound waves of very high frequency beyond the range of human hearing;bioacoustics,the physics of animal calls and hearing,[50]andelectroacoustics,the manipulation of audible sound waves using electronics.[51]

Optics, the study of light, is concerned not only withvisible lightbut also withinfraredandultraviolet radiation,which exhibit all of the phenomena of visible light except visibility, e.g., reflection, refraction, interference, diffraction, dispersion, and polarization of light. Heat is a form of energy, 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. Electricity andmagnetismhave been studied as a single branch of physics since the intimate connection between them was discovered in the early 19th century; anelectric currentgives rise to amagnetic field,and a changing magnetic field induces an electric current.Electrostaticsdeals withelectric chargesat rest,electrodynamicswith moving charges, andmagnetostaticswith magnetic poles at rest.

Modern

Main article:Modern physics
Modern physics

SchrödingerandEinstein field equations
Founders
Concepts
Branches
Scientists
Categories

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,atomicandnuclear physicsstudy matter on the smallest scale at whichchemical elementscan be identified. Thephysics of elementary particlesis 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 inparticle accelerators.On this scale, ordinary, commonsensical notions of space, time, matter, and energy are no longer valid.[52]

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 aframe of referencethat is in motion with respect to an observer; the special theory of relativity is concerned with motion in the absence of gravitational fields and thegeneral theory of relativitywith motion and its connection withgravitation.Both quantum theory and the theory of relativity find applications in many areas of modern physics.[53]

Fundamental concepts in modern physics

Difference

The basic domains of physics

While physics itself aims to discover universal laws, its theories lie in explicit domains of applicability.

Solvay Conferenceof 1927, with prominent physicists such asAlbert Einstein,Werner Heisenberg,Max Planck,Hendrik Lorentz,Niels Bohr,Marie Curie,Erwin SchrödingerandPaul Dirac

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 ofabsolute time and spacewithspacetimeand 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,quantum field theoryunified 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 ofquantum gravityare being developed.

Relation to other fields

Thisparabola-shapedlava flowillustrates the application of mathematics in physics—in this case, Galileo'slaw of falling bodies.
Mathematics and ontology are used in physics. Physics is used in chemistry andcosmology.

Prerequisites

Mathematics provides a compact and exact language used to describe the order in nature. This was noted and advocated byPythagoras,[54]Plato,[55]Galileo,[56]and Newton. Some theorists, likeHilary PutnamandPenelope Maddy,hold that logical truths, and therefore mathematical reasoning, depend on theempiricalworld. This is usually combined with the claim that the laws of logic express universal regularities found in the structural features of the world, which may explain the peculiar relation between these fields.

Physics uses mathematics[57]to organise and formulate experimental results. From those results,preciseorestimatedsolutions 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 theirunits of measureand estimates of the errors in the measurements. Technologies based on mathematics, likecomputationhave madecomputational physicsan active area of research.

The distinction between mathematics and physics is clear-cut, but not always obvious, especially in mathematical physics.

Ontologyis 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,mathematical physicsis the application of mathematics in physics. Its methods are mathematical, but its subject is physical.[58]The problems in this field start with a "mathematical model of a physical situation"(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.[clarification needed]

Pure physics is a branch offundamental science(also called basic science). Physics is also called "thefundamental science "because all branches of natural science like chemistry, astronomy, geology, and biology are constrained by laws of physics.[59]Similarly, chemistry is often calledthe central sciencebecause of its role in linking the physical sciences. For example, chemistry studies properties, structures, andreactionsof matter (chemistry's focus on the molecular and atomic scaledistinguishes it from physics). 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, likeconservation of energy,mass,andcharge.Physics is applied in industries like engineering and medicine.

Application and influence

Main article:Applied physics
Classical physics implemented in anacoustic engineeringmodel of sound reflecting from an acoustic diffuser
Archimedes' screw,asimple machinefor lifting

Applied physics 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 engineering 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 ofapplied mathematics.Applied physicists use physics in scientific research. For instance, people working onaccelerator physicsmight seek to build betterparticle detectorsfor research in theoretical physics.

Physics is used heavily in engineering. For example, statics, a subfield ofmechanics,is used in the building of bridges 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 realisticflight simulators,video games, and movies, and is often critical inforensicinvestigations.

Experiment using alaser

With thestandard consensusthat thelawsof physics are universal and do not change with time, physics can be used to study things that would ordinarily be mired inuncertainty.For example, in the study of the origin of the Earth, a physicist can reasonably model Earth's mass, temperature, and rate of rotation, as a function of time allowing the extrapolation forward or backward in time and so predict future or prior events. It also allows for simulations in engineering that speed up the development of a new technology.

There is also considerableinterdisciplinarity,so many other important fields are influenced by physics (e.g., the fields ofeconophysicsandsociophysics).

Research

Scientific method

Physicists use the scientific method to test the validity of aphysical theory.By using a methodical approach to compare the implications of a theory with the conclusions drawn from its related experiments 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 a theory.[60]

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.[61]

Theory and experiment

Main articles:Theoretical physicsandExperimental physics
Theastronautand Earth are both infree fall.(Pictured: Astronaut Bruce McCandless.)
Lightningis anelectric current.

Theorists seek to developmathematical modelsthat both agree with existing experiments and successfully predict future experimental results, whileexperimentalistsdevise and perform experiments to test theoretical predictions and explore new phenomena. Althoughtheoryand 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 testablepredictions,which inspire the development of new experiments (and often related equipment).[62]

Physicistswho work at the interplay of theory and experiment are calledphenomenologists,who study complex phenomena observed in experiment and work to relate them to afundamental theory.[63]

Theoretical physics has historically taken inspiration from philosophy; electromagnetism was unified this way.[d]Beyond the known universe, the field of theoretical physics also deals with hypothetical issues,[e]such asparallel universes,amultiverse,andhigher dimensions.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 technology. Experimental physicists who are involved inbasic researchdesign and perform experiments with equipment such as particle accelerators andlasers,whereas those involved inapplied researchoften work in industry, developing technologies such asmagnetic resonance imaging(MRI) andtransistors.Feynmanhas noted that experimentalists may seek areas that have not been explored well by theorists.[64]

Scope and aims

Physics involves modeling the natural world with theory, usually quantitative. Here, the path of a particle is modeled with the mathematics ofcalculusto explain its behavior: the purview of the branch of physics known asmechanics.

Physics covers a wide range ofphenomena,fromelementary particles(such asquarks,neutrinos,andelectrons) to the largestsuperclustersof galaxies. Included in these phenomena are the most basic objects composing all other things. Therefore, physics is sometimes called the "fundamental science".[59]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 root causes, and then connect these causes together.

For example, theancient Chineseobserved that certain rocks (lodestoneandmagnetite) 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, theancient Greeksknew of other objects such asamber,that when rubbed with fur would cause a similar invisible attraction between the two.[65]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 theweak nuclear forceare now considered to be two aspects of theelectroweak interaction.Physics hopes to find an ultimate reason (theory of everything) for why nature is as it is (see sectionCurrent researchbelow for more information).[66]

Research fields

Contemporary research in physics can be broadly divided intonuclearandparticle physics;condensed matter physics;atomic, molecular, and optical physics;astrophysics;and applied physics. Some physics departments also supportphysics education researchandphysics outreach.[67]

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) andLev Landau(1908–1968), who worked in multiple fields of physics, are now very rare.[f]

The major fields of physics, along with their subfields and the theories and concepts they employ, are shown in the following table.

Field Subfields Major theories Concepts
Nuclearandparticle physics Nuclear physics,Nuclear astrophysics,Particle physics,Astroparticle physics,Particle physics phenomenology Standard Model,Quantum field theory,Quantum electrodynamics,Quantum chromodynamics,Electroweak theory,Effective field theory,Lattice field theory,Gauge theory,Supersymmetry,Grand Unified Theory,Superstring theory,M-theory,AdS/CFT correspondence Fundamental interaction(gravitational,electromagnetic,weak,strong),Elementary particle,Spin,Antimatter,Spontaneous symmetry breaking,Neutrino oscillation,Seesaw mechanism,Brane,String,Quantum gravity,Theory of everything,Vacuum energy
Atomic, molecular, and optical physics Atomic physics,Molecular physics,Atomic and molecular astrophysics,Chemical physics,Optics,Photonics Quantum optics,Quantum chemistry,Quantum information science Photon,Atom,Molecule,Diffraction,Electromagnetic radiation,Laser,Polarization (waves),Spectral line,Casimir effect
Condensed matter physics Solid-state physics,High-pressure physics,Low-temperature physics,Surface physics,Nanoscale and mesoscopic physics,Polymer physics BCS theory,Bloch's theorem,Density functional theory,Fermi gas,Fermi liquid theory,Many-body theory,Statistical mechanics Phases(gas,liquid,solid),Bose–Einstein condensate,Electrical conduction,Phonon,Magnetism,Self-organization,Semiconductor,superconductor,superfluidity,Spin
Astrophysics Astronomy,Astrometry,Cosmology,Gravitation physics,High-energy astrophysics,Planetary astrophysics,Plasma physics,Solar physics,Space physics,Stellar astrophysics Big Bang,Cosmic inflation,General relativity,Newton's law of universal gravitation,Lambda-CDM model,Magnetohydrodynamics Black hole,Cosmic background radiation,Cosmic string,Cosmos,Dark energy,Dark matter,Galaxy,Gravity,Gravitational radiation,Gravitational singularity,Planet,Solar System,Star,Supernova,Universe
Applied physics Accelerator physics,Acoustics,Agrophysics,Atmospheric physics,Biophysics,Chemical physics,Communication physics,Econophysics,Engineering physics,Fluid dynamics,Geophysics,Laser physics,Materials physics,Medical physics,Nanotechnology,Optics,Optoelectronics,Photonics,Photovoltaics,Physical chemistry,Physical oceanography,Physics of computation,Plasma physics,Solid-state devices,Quantum chemistry,Quantum electronics,Quantum information science,Vehicle dynamics

Nuclear and particle

Main articles:Particle physicsandNuclear physics
A simulated event in the CMS detector of theLarge Hadron Collider,featuring a possible appearance of theHiggs boson

Particle physics is the study of the elementary constituents ofmatterand energy and theinteractionsbetween them.[68]In addition, particle physicists design and develop the high-energy accelerators,[69]detectors,[70]andcomputer programs[71]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-energycollisionsof other particles.[72]

Currently, the interactions of elementary particles andfieldsare described by theStandard Model.[73]The model accounts for the 12 known particles of matter (quarksandleptons) that interact via thestrong,weak, and electromagneticfundamental forces.[73]Dynamics are described in terms of matter particles exchanginggauge bosons(gluons,W and Z bosons,andphotons,respectively).[74]The Standard Model also predicts a particle known as the Higgs boson.[73]In July 2012 CERN, the European laboratory for particle physics, announced the detection of a particle consistent with the Higgs boson,[75]an integral part of theHiggs mechanism.

Nuclear physics is the field of physics that studies the constituents and interactions ofatomic nuclei.The most commonly known applications of nuclear physics arenuclear powergeneration andnuclear weaponstechnology, but the research has provided application in many fields, including those innuclear medicineand magnetic resonance imaging,ion implantationinmaterials engineering,andradiocarbon datingin geology andarchaeology.

Atomic, molecular, and optical

Main article:Atomic, molecular, and optical physics

Atomic,molecular,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 andquantumtreatments; they can treat their subject from a microscopic view (in contrast to a macroscopic view).

Atomic physics studies theelectron shellsof atoms. Current research focuses on activities in quantum control, cooling and trapping of atoms and ions,[76][77][78]low-temperature collision dynamics and the effects of electron correlation on structure and dynamics. Atomic physics is influenced by thenucleus(seehyperfine splitting), but intra-nuclear phenomena such asfissionandfusionare considered part of nuclear physics.

Molecular physicsfocuses on multi-atomic structures and their internal and external interactions with matter and light.Optical physicsis 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 ofoptical fieldsand their interactions with matter in the microscopic realm.

Condensed matter

Main article:Condensed matter physics
Velocity-distribution data of a gas ofrubidiumatoms, confirming the discovery of a new phase of matter, theBose–Einstein condensate

Condensed matter physics is the field of physics that deals with the macroscopic physical properties of matter.[79][80]In particular, it is concerned with the "condensed"phasesthat appear whenever the number of particles in a system is extremely large and the interactions between them are strong.[81]

The most familiar examples of condensed phases aresolidsand liquids, which arise from the bonding by way of theelectromagnetic forcebetween atoms.[82]More exotic condensed phases include thesuperfluid[83]and theBose–Einstein condensate[84]found in certain atomic systems at very low temperature, thesuperconductingphase exhibited byconduction electronsin certain materials,[85]and theferromagneticandantiferromagneticphases ofspinsonatomic lattices.[86]

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.[87]The termcondensed matter physicswas apparently coined byPhilip Andersonwhen he renamed his research group—previouslysolid-state theory—in 1967.[88]In 1978, the Division of Solid State Physics of theAmerican Physical Societywas renamed as the Division of Condensed Matter Physics.[87]Condensed matter physics has a large overlap with chemistry,materials science,nanotechnologyand engineering.[81]

Astrophysics

Main articles:AstrophysicsandPhysical cosmology
The deepest visible-light image of theuniverse,theHubble Ultra-Deep Field.The vast majority of objects seen above are distant galaxies.

Astrophysics and astronomy are the application of the theories and methods of physics to the study ofstellar structure,stellar evolution,the origin of the Solar System, and related problems of cosmology. 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.[89]

The discovery byKarl Janskyin 1931 that radio signals were emitted by celestial bodies initiated the science ofradio astronomy.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 forinfrared,ultraviolet,gamma-ray,andX-ray astronomy.

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,Hubble's discovery that the universe is expanding, as shown by theHubble diagram,prompted rival explanations known as thesteady stateuniverse and theBig Bang.

The Big Bang was confirmed by the success ofBig Bang nucleosynthesisand the discovery of thecosmic microwave backgroundin 1964. The Big Bang model rests on two theoretical pillars: Albert Einstein's general relativity and thecosmological principle.Cosmologists have recently established theΛCDM modelof the evolution of the universe, which includescosmic inflation,dark energy,anddark matter.

Numerous possibilities and discoveries are anticipated to emerge from new data from theFermi Gamma-ray Space Telescopeover the upcoming decade and vastly revise or clarify existing models of the universe.[90][91]In particular, the potential for a tremendous discovery surrounding dark matter is possible over the next several years.[92]Fermi will search for evidence that dark matter is composed ofweakly interacting massive particles,complementing similar experiments with theLarge Hadron Colliderand other underground detectors.

IBEXis already yielding newastrophysicaldiscoveries: "No one knows what is creating theENA (energetic neutral atoms)ribbon "along thetermination shockof thesolar wind,"but everyone agrees that it means the textbook picture of theheliosphere—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. "[93]

Current research

Further information:List of unsolved problems in physics
Feynman diagramsigned byR. P. Feynman
A typical phenomenon described by physics: amagnetlevitating above asuperconductordemonstrates theMeissner effect.

Research in physics is continually progressing on a large number of fronts.

In condensed matter physics, an important unsolved theoretical problem is that ofhigh-temperature superconductivity.[94]Many condensed matter experiments are aiming to fabricate workablespintronicsandquantum computers.[81][95]

In particle physics, the first pieces of experimental evidence for physics beyond the Standard Model have begun to appear. Foremost among these are indications thatneutrinoshave non-zeromass.These experimental results appear to have solved the long-standingsolar neutrino problem,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 thesupersymmetry,which extends the Standard Model of particle physics. Research on the nature of the major mysteries of dark matter anddark energyis also currently ongoing.[96]

Although much progress has been made in high-energy,quantum,and astronomical physics, many everyday phenomena involvingcomplexity,[97]chaos,[98]orturbulence[99]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 ofsurface tensioncatastrophes,and self-sorting in shaken heterogeneous collections.[g][100]

These complex phenomena have received growing attention since the 1970s for several reasons, including the availability of modern mathematical methods and computers, which enabledcomplex systemsto be modeled in new ways. Complex physics has become part of increasingly interdisciplinary research, as exemplified by the study of turbulence in aerodynamics and the observation ofpattern formationin biological systems. In the 1932Annual Review of Fluid Mechanics,Horace Lambsaid:[101]

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.

Education

This section is an excerpt fromPhysics education.[edit]
Physics educationor physics teaching refers to theeducationmethods currently used toteachphysics. The occupation is called physics educator or physics teacher.Physics education researchrefers to an area of pedagogical research that seeks to improve those methods. Historically, physics has been taught at the high school and college level primarily by the lecture method together with laboratory exercises aimed at verifying concepts taught in the lectures. These concepts are better understood when lectures are accompanied with demonstration, hand-on experiments, and questions that require students to ponder what will happen in an experiment and why. Students who participate inactive learningfor example with hands-on experiments learn through self-discovery. By trial and error they learn to change their preconceptions about phenomena in physics and discover the underlying concepts. Physics education is part of the broader area ofscience education.

Career

This section is an excerpt fromPhysicist.[edit]

Aphysicistis ascientistwho specializes in the field of physics, which encompasses the interactions of matter and energy at all length and time scales in the physical universe.[102][103]Physicists generally are interested in the root or ultimate causes ofphenomena,and usually frame their understanding in mathematical terms. They work across a wide range ofresearch fields,spanning all length scales: fromsub-atomicandparticle physics,throughbiological physics,tocosmologicallength scales encompassing theuniverseas a whole. The field generally includes two types of physicists:experimental physicistswho specialize in the observation of natural phenomena and the development and analysis of experiments, andtheoretical physicistswho specialize in mathematical modeling of physical systems to rationalize, explain and predict natural phenomena.[102]

Physicists can apply their knowledge towards solving practical problems or to developing new technologies (also known asapplied physicsorengineering physics).[104][105][106]

See also

Lists

Notes

  1. ^Francis Bacon's 1620Novum Organumwas critical in thedevelopment of scientific method.[6]
  2. ^Calculus was independently developed at around the same time byGottfried Wilhelm Leibniz;while Leibniz was the first to publish his work and develop much of the notation used for calculus today, Newton was the first to develop calculus and apply it to physical problems. See alsoLeibniz–Newton calculus controversy
  3. ^Noll notes that some universities still use this title.[39]
  4. ^See, for example, the influence ofKantandRitteronØrsted.
  5. ^Concepts which are denotedhypotheticalcan change with time. For example, theatomof nineteenth-century physics was denigrated by some, includingErnst Mach's critique ofLudwig Boltzmann's formulation ofstatistical mechanics.By the end of World War II, the atom was no longer deemed hypothetical.
  6. ^Yet, universalism is encouraged in the culture of physics. For example, theWorld Wide Web,which was innovated atCERNbyTim Berners-Lee,was created in service to the computer infrastructure of CERN, and was/is intended for use by physicists worldwide. The same might be said forarXiv.org
  7. ^See the work ofIlya Prigogine,on 'systems far from equilibrium', and others.

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