This article is about the field of science.
For other uses, see Physics (disambiguation).
Not to be confused with Physical science.
Physics is one of the most fundamental scientific disciplines, and its main goal is to understand how the universe behaves.
Over much of the past two millennia, physics, chemistry, biology, and certain branches of mathematics were a part of natural philosophy, but during the Scientific Revolution in the 17th century these natural sciences emerged as unique research endeavors in their own right.
New ideas in physics often explain the fundamental mechanisms studied by other sciences and suggest new avenues of research in academic disciplines such as mathematics and philosophy.
Advances in physics often enable advances in new technologies.
For example, advances in the understanding of electromagnetism, solid-state physics, and 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; advances in thermodynamics led to the development of industrialization; and advances in mechanics inspired the development of calculus.
Main article: History of physics
Main article: History of astronomy
Early civilizations dating back before 3000 BCE, such as the Sumerians, ancient Egyptians, and the Indus Valley Civilisation, had a predictive knowledge and a basic understanding 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 great circles across the sky, which however did not explain the positions of the planets.
Egyptian astronomers left monuments showing knowledge of the constellations and the motions of the celestial bodies, 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.
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.
They proposed ideas verified by reason and observation, and many of their hypotheses proved successful in experiment; for example, atomism was found to be correct approximately 2000 years after it was proposed by Leucippus and his pupil Democritus.
Physics in the medieval European and Islamic world
The Western Roman Empire fell in the fifth century, and this resulted in a decline in intellectual pursuits in the western part of Europe.
In the sixth century Isidore of Miletus created an important compilation of Archimedes' works that are copied in the Archimedes Palimpsest.
He introduced the theory 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:
Galileo cited Philoponus substantially in his works when arguing that Aristotelian physics was flawed.
In the 1300s Jean Buridan, 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.
Islamic scholarship inherited Aristotelian physics from the Greeks and during the Islamic Golden Age developed it further, especially placing emphasis on observation and a priori reasoning, developing early forms of the scientific method.
The most notable work was The Book of Optics (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.
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 perception to the nature of perspective in medieval art, in both the East and the West, for more than 600 years.
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, such important things as eyeglasses, magnifying glasses, telescopes, and cameras were developed.
Main article: Classical physics
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 Kepler between 1609 and 1619, pioneering work on telescopes and observational astronomy by Galileo in the 16th and 17th Centuries, and Newton's discovery and unification of the laws of motion and universal gravitation that would come to bear his name.
Newton also developed calculus, the mathematical study of change, which provided new mathematical methods for solving physical problems.
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.
Main article: Modern physics
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 by Maxwell'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.
Black-body radiation 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 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.
From this early work, and work in related fields, the Standard Model of particle physics was derived.
Following the discovery of a particle with properties consistent with the Higgs boson at CERN in 2012, all fundamental 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.
Main article: Philosophy of physics
In many ways, physics stems from ancient Greek philosophy.
From Thales' first attempt to characterise 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.
By the 19th century, physics was realised as a discipline distinct from philosophy and the other sciences.
Physics, as with the rest of science, relies on philosophy of science and its "scientific method" to advance our knowledge of the physical world.
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 as empiricism, naturalism and realism.
Hawking referred to himself as an "unashamed reductionist" and took issue with Penrose's views.
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.
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 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.
Main article: Classical physics
Classical physics includes the traditional branches and topics that were recognised 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 by forces and bodies in motion and may be divided into statics (study of the forces on a body or bodies not subject to an acceleration), kinematics (study of motion without regard to its causes), and dynamics (study of motion and the forces that affect it); mechanics may also be divided into solid mechanics and fluid mechanics (known together as continuum mechanics), the latter include such branches as hydrostatics, hydrodynamics, aerodynamics, and pneumatics.
Acoustics is the study of how sound is produced, controlled, transmitted and received.
Important modern branches of acoustics include ultrasonics, the study of sound waves of very high frequency beyond the range of human hearing; bioacoustics, the physics of animal calls and hearing, and electroacoustics, the manipulation of audible sound waves using electronics.
Optics, the study of light, is concerned not only with visible light but also with infrared and ultraviolet radiation, which exhibit all of the phenomena of visible light except visibility, e.g., reflection, refraction, interference, diffraction, dispersion, and polarization of light.
Electricity and magnetism have been studied as a single branch of physics since the intimate connection between them was discovered in the early 19th century; an electric current gives rise to a magnetic field, and a changing magnetic field induces an electric current.
Main article: Modern physics
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.
The physics of elementary particles 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 particle accelerators.
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 frame of reference 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 general theory of relativity with motion and its connection with gravitation.
Both quantum theory and the theory of relativity find applications in all areas of modern physics.
Difference between classical and modern physics
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 absolute time and space with spacetime 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, quantum field theory 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 quantum gravity are being developed.
Relation to other fields
Mathematics provides a compact and exact language used to describe the order in nature.
Physics uses mathematics to organise and formulate experimental results.
The results from physics experiments are numerical data, with their units of measure and estimates of the errors in the measurements.
Ontology 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, mathematical physics is the application of mathematics in physics.
Its methods are mathematical, but its subject is physical.
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.
Pure physics is a branch of fundamental science (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.
Similarly, chemistry is often called the central science because of its role in linking the physical sciences.
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 conservation of energy, mass, and charge.
Physics is applied in industries like engineering and medicine.
Application and influence
Main article: Applied physics
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 of applied mathematics.
Applied physicists use physics in scientific research.
Physics is used heavily in engineering.
The understanding and use of acoustics results in sound control and better concert halls; similarly, the use of optics creates better optical devices.
For example, in the study of the origin of the earth, 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.
Physicists use the scientific method to test the validity of a physical 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 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 mathematical models that both agree with existing experiments and successfully predict future experimental results, while experimentalists devise and perform experiments to test theoretical predictions and explore new phenomena.
Although theory 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 predictions, which inspire the development of new experiments (and often related equipment).
Theoretical physics has historically taken inspiration from philosophy; electromagnetism was unified this way.
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 in basic research, design and perform experiments with equipment such as particle accelerators and lasers, whereas those involved in applied research often work in industry, developing technologies such as magnetic resonance imaging (MRI) and transistors.
Feynman has noted that experimentalists may seek areas that have not been explored well by theorists.
Scope and aims
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 root causes, and then connect these causes together.
This effect was later called magnetism, which was first rigorously studied in the 17th century.
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.
Physics hopes to find an ultimate reason (theory of everything) for why nature is as it is (see section Current research below for more information).
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 Lev Landau (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 physics
In addition, particle physicists design and develop the high-energy accelerators, detectors, and computer programs 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 collisions of other particles.
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 a Higgs mechanism.
Nuclear physics is the field of physics that studies the constituents and interactions of atomic nuclei.
The most commonly known applications of nuclear physics are nuclear power generation and nuclear weapons technology, but the research has provided application in many fields, including those in nuclear medicine and magnetic resonance imaging, ion implantation in materials engineering, and radiocarbon dating in geology and archaeology.
Atomic, molecular, and optical physics
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 and quantum treatments; they can treat their subject from a microscopic view (in contrast to a macroscopic view).
Atomic physics studies the electron shells 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.
Molecular physics focuses on multi-atomic structures and their internal and external interactions with matter and light.
Optical physics 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 optical fields and their interactions with matter in the microscopic realm.
Condensed matter physics
Main article: Condensed matter physics
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" phases that appear whenever the number of particles in a system is extremely large and the interactions between them are strong.
More exotic condensed phases include the superfluid and the Bose–Einstein condensate found in certain atomic systems at very low temperature, the superconducting phase exhibited by conduction electrons in certain materials, and the ferromagnetic and antiferromagnetic phases of spins on atomic lattices.
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 Philip Anderson when he renamed his research group—previously solid-state theory—in 1967.
In 1978, the Division of Solid State Physics of the American Physical Society was renamed as the Division of Condensed Matter Physics.
Astrophysics and astronomy are the application of the theories and methods of physics to the study of stellar 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.
Most recently, the frontiers of astronomy have been expanded by space exploration.
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.
The Big Bang model rests on two theoretical pillars: Albert Einstein's general relativity and the cosmological principle.
Numerous possibilities and discoveries are anticipated to emerge from new data from the Fermi Gamma-ray Space Telescope 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.
IBEX is already yielding new astrophysical discoveries: "No one knows what is creating the ENA (energetic neutral atoms) ribbon" along the termination shock of the solar wind, "but everyone agrees that it means the textbook picture of the heliosphere—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."
Further information: List of unsolved problems in physics
Research in physics is continually progressing on a large number of fronts.
In condensed matter physics, an important unsolved theoretical problem is that of high-temperature superconductivity.
In particle physics, the first pieces of experimental evidence for physics beyond the Standard Model have begun to appear.
These experimental results appear to have solved the long-standing solar 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 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.
Theoretical attempts to unify quantum mechanics and general relativity into a single theory of quantum gravity, a program ongoing for over half a century, have not yet been decisively resolved.
Many astronomical and cosmological phenomena have yet to be satisfactorily explained, including the origin of ultra-high-energy cosmic rays, the baryon asymmetry, the accelerating expansion of the universe and the anomalous rotation rates of galaxies.
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 surface tension catastrophes, 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.
In the 1932 Annual Review of Fluid Mechanics, Horace Lamb said:
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