Physics

Physics (Greek: physis – φύσις) is the science of matter^[1] and its motion.^[2] It is the science that seeks to understand very basic concepts such as force, energy, mass, and charge. More completely, it is the general analysis of nature, conducted in order to understand how the world around us and, more broadly, the universe, behaves.^[3][4] Note that the term 'universe' is defined as everything that physically exists: the entirety of space and time, all forms of matter, energy and momentum, and the physical laws and constants that govern them. However, the term 'universe' may also be used in slightly different contextual senses, denoting concepts such as the cosmos, the world, and nature.

In one form or another, physics is one of the oldest academic disciplines, perhaps the oldest through its inclusion of astronomy.^[5] Over the last two millennia, physics had been considered synonymous with philosophy, chemistry, and certain branches of mathematics and biology, but during the Scientific Revolution in the 16th century, it emerged to become a unique modern science in its own right.^[6] However, in some subject areas such as in mathematical physics and quantum chemistry, the boundaries and the borderlines of physics remain difficult to distinguish.

Physics is both significant and influential, in part because advances in its understanding have often translated into new technologies, but also because new ideas in physics often resonate with the other sciences, mathematics and philosophy. For example, advances in the understanding of electromagnetism led directly to the development of new products which have dramatically transformed modern-day society (e.g., television, computers, and domestic appliances); advances in thermodynamics led to the development of motorized transport; and advances in mechanics inspired the development of the calculus, quantum chemistry, and the use of instruments like the electron microscope in microbiology.

Today, physics is both a broad and very deep subject that, in practical/fundamental terms, can be split into several subfields. It can also be divided into two conceptually different branches: Theoretical physics and experimental physics. The former deals with the inquiry and foundation of new theories while the latter deals with the experimental testing of these new, or existing, theories. Even though significant progress and important discoveries have been made in the field of physics during the last four centuries, many significant questions about nature and the universe still remain unanswered. In many areas of physics, it is still a continuing effort to try to gain a clearer understanding to the unknown and the outskirts of physics.

Contents 1 Etymology 2 Introduction 2.1 Scope and goals 2.2 The scientific method 2.3 Data collection and theory development 2.4 Physics is quantitative 2.5 Relation to mathematics and the other sciences 2.6 Philosophical implications 3 History 3.1 Ancient times 3.2 Middle Ages 3.3 The Scientific Revolution 3.4 1895 to present 4 Principles and concepts 4.1 Major topics of physics 5 Current research 6 Applications and influence 7 See also 8 References 9 External links

Etymology

Further information: Physis

The word 'physics' comes from the Greek words, φύσις (phúsis), meaning "nature", and φυσικός (phusikós), meaning "natural".

Introduction

See also: List of basic physics topics and List of basic science topics

Scope and goals

Physics is the discipline devoted to understanding nature in a very general sense: the fundamental characteristic of physics is that it aims to gain knowledge, and hopefully understanding, of the general properties of the world around us. As an example, we can consider asking the following question on the nature of the Universe itself: how many dimensions do we need? Given that we know the Universe to consist of four dimensions (three space dimensions, and one time dimension), we can also ask why the universe picked those particular numbers: why not have four space dimensions? The fact that a choice was made out of a possibility of many means that questions like these fall under the scope of physics. Other general properties of nature include the existence of mass (as in Newton's laws of motion), charge (as in Maxwell's equations), and spin (in Quantum mechanics), amongst others.

However, whilst physics studies the general properties of nature, it will often also study the properties of certain objects within nature. Thus it is also physics whose job it is to describe what happens to, for example, planets whose motion is affected by nearby stars. Generally, the study of the specific objects in nature are shared between the three sciences: biology is roughly responsible for the living organisms, chemistry for the study of the chemical elements and molecules, and physics is given responsibility over all that remains (See the section Relation to mathematics and the other sciences for further information).

The fact that physics is delegated all objects besides those covered by biology and chemistry means that it is responsible for the study of a wide range objects and phenomena, from the smallest sub-atomic particles, to the largest galaxies. Included in this are the very most basic objects from which all other things are composed of, and therefore physics is sometimes said to be the "fundamental science".

Generalities aside, physics aims to describe the various phenomena that occur in nature in terms of simpler phenomena: that is, to find the mechanisms for why nature behaves the way it does. Thus, physics aims to both connect the things we see around us to a root cause, and then to try to connect these root causes together in the hope of finding an ultimate reason for why nature is as it is. For example, the ancient Chinese observed that certain rocks (lodestone) were attracted to one another by some invisible force. This effect was later called magnetism, and was first rigorously studied in the 17th century. A little earlier than the Chinese, the ancient Greeks knew of other objects (amber) 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 (see section Current research for more information).

The scientific method

Physics uses the scientific method to test the validity of a physical theory, using a methodical approach to compare the implications of the theory in question with the associated conclusions drawn from experiments and observations conducted to test it. Experiments and observations are to be collected and matched with the predictions and hypotheses made by a theory, thus aiding in the determination or the validity/invalidity of the theory.

Theories which are very well supported by data and which are regarded in observation as being almost certainly a naturally-occurring fact have been called scientific laws, or natural laws. Of course, all theories, including those called scientific laws, can always be replaced by more accurate, generalized statements if a disagreement of theory with observed data was to be found.^[7]

Data collection and theory development

There are many approaches to studying physics, and many different kinds of activities in physics. There are two main types of activities in physics; the collection of data and the development of theories.

The data in some subfields of physics is amenable to experiment. For example, condensed matter physics and nuclear physics benefit from the ability to perform experiments. Sometimes experiments are done to explore nature, and in other cases experiments are performed to produce data to compare with the predictions of theories.

Some other fields in physics like astrophysics and geophysics are primarily observational sciences because most their data has to be collected passively instead of through experimentation. Nevertheless, observational programs in these fields uses many of the same tools and technology that are used in the experimental subfields of physics. The accumulated body of knowledge in some area of physics through experiment and observation is known as phenomenology.

Theoretical physics often uses quantitative approaches to develop the theories that attempt to explain the data. In this way, theoretical physics often relies heavily on tools from mathematics and computational technologies (particularly in the subfield known as computational physics). Theoretical physics often involves creating quantitative predictions of physical theories, and comparing these predictions quantitatively with data. Theoretical physics sometimes creates models of physical systems before data are available to test and validate these models.

These two main activities in physics, data collection and theory production and testing, draw on many different skills. This has lead to a lot of specialization in physics, and the introduction, development and use of tools from other fields. For example, theoretical physicists apply mathematics and numerical analysis and statistics and probability and computers and computer software in their work. Experimental physicists develop instruments and techniques for collecting data, drawing on engineering and computer technology and many other fields of technology. Often the tools from these other areas are not quite appropriate for the needs of physics, and need to be adapted or more advanced versions have to be produced.

The culture of physics research differs from the other sciences in the separation of theory from data collection through experiment and observation. Since the 20th century, most individual physicists have specialized in either theoretical physics or experimental physics. The great Italian physicist Enrico Fermi (1901–1954), who made fundamental contributions to both theory and experimentation in nuclear physics, was a notable exception. In contrast, almost all the successful theorists in biology and chemistry (e.g. American quantum chemist and biochemist Linus Pauling) have also been experimentalists, though this is changing as of late.

Although theory and experiment are usually performed by separate groups, they are strongly dependent on each other. Progress in physics frequently comes about when experimentalists make a discovery that existing theories cannot account for, necessitating the formulation of new theories. Likewise, ideas arising from theory often inspire new experiments. In the absence of experiment, theoretical research can go in the wrong direction; this is one of the criticisms that has been leveled against M-theory, a popular theory in high-energy physics for which no practical experimental test has ever been devised.

Physics is quantitative

Orders of magnitude
area
angular velocity
charge
currency
data
density
energy
frequency
length
magnetic field
mass
numbers
power
pressure
specific energy density
specific heat capacity
speed
temperature
time
volume
Conversion of units
physical unit
SI
SI base unit
SI derived unit
SI prefix
Planck units

Physics is more quantitative than most other sciences. That is, many of the observations experimental results in physics are numerical measurements. Most of the theories in physics use mathematics to express their principles. Most of the predictions from these theories are numerical. This is because of the areas which physics has addressed are more amenable to quantitative approaches than other areas. Physical definitions, models and theories can often be expressed using mathematical relations, as early as 1638, when Galileo published the law of falling bodies in his Two New Sciences.

A key difference between physics and mathematics is that because physics is ultimately concerned with descriptions of the material world, it tests its theories by comparing the predictions of its theories with data procured from observations and experimentation, whereas mathematics is concerned with abstract logical patterns not limited by those observed in the real world (because the real world is limited in the number of dimensions and in many other ways it does not have to correspond to richer mathematical structures). The distinction, however, is not always clear-cut. There is a large area of research intermediate between physics and mathematics, known as mathematical physics.

Relation to mathematics and the other sciences

Physics relies on mathematics^[8] to provide the logical framework in which physical laws can be precisely formulated and their predictions quantified. Physical definitions, models and theories can be succinctly expressed using mathematical relations.

Whenever analytic solutions are not feasible, numerical analysis and simulations can be utilized. Thus, scientific computation is an integral part of physics, and the field of computational physics is an active area of research.

Beyond the known universe, the field of theoretical physics also deals with hypothetical issues^[9], such as parallel universes, a multiverse, or whether the universe could have expanded as predominantly antimatter rather than matter.

In the Assayer (1622), Galileo noted that mathematics is the language in which Nature expresses laws, to be discovered by physicists. Physics is also intimately related to many other sciences,^[10] as well as applied fields like engineering and medicine. The principles of physics find applications throughout the other natural sciences as they depend on the interactions of the fundamental constituents of the natural world. Some of the phenomena studied in physics, such as the phenomenon of conservation of energy, are common to all material systems. These are often referred to as laws of physics. Others, such as superconductivity, stem from these laws, but are not laws themselves because they only appear in some systems. Physics is often said to be the "fundamental science" (chemistry is sometimes included), because each of the other disciplines (biology, chemistry, geology, material science, engineering, medicine etc.) deals with particular types of material systems that obey the laws of physics. For example, chemistry is the science of collections of matter (such as gases and liquids formed of atoms and molecules) and the processes known as chemical reactions that result in the change of chemical substances. The structure, reactivity, and properties of a chemical compound are determined by the properties of the underlying molecules, which can be described by areas of physics such as quantum mechanics (called in this case quantum chemistry), thermodynamics, and electromagnetism.

Philosophical implications

Physics in many ways stemmed from ancient Greek philosophy. From Thales' first attempt to characterize matter, to Democritus' deduction that matter ought to reduce to an invariant state, to the Ptolemaic astronomy of a crystalline firmament upon which the stars rested, our view of the universe seemed static. By the twentieth century, this picture became less certain, and now a static universe is only one possibility in an array of possible universes.

Aristotle's early observations in natural history, and natural philosophy usually did not involve much fact checking or detailed observation, which allowed errors to come to rest in our knowledge of the world. When closer investigation overturned this picture of the world, philosophers came to study other possible forms of reasoning. The use of a priori reasoning found a natural place in scientific method as well as the use of experiments and a posteriori reasoning came to be used in Bayesian inference^[11]. By the 19th century physics was realized as a positive science and a distinct discipline separate from philosophy and the other sciences.

Study of the philosophical issues surrounding physics, the philosophy of physics can be encapsulated as empiricism, naturalism, and for some, realism^[12]. The mathematical physicist Roger Penrose has been called a Platonist by Stephen Hawking^[13], while Penrose continues to eschew quantum mechanics as a final theory about reality^[14].

Ørsted (1811) noted that physicists readily make deductions about nature, based on their closer familiarity with experiments about nature,^[15] whereas the mathematicians and philosophers must make do with fewer positive statements about nature.

There are certain statements, such as Newton's Third Law of Motion,^[16] that can be generalized into the Principle of Equivalence. This principle is the logical basis for general relativity, whose solutions give metrics for spacetime. The success of general relativity influenced Einstein to eschew quantum theory, to which he made seminal contributions, and to eventually believe that all physical theory ought to be independent of observation.^[17][18]

History

Main article: History of physics

See also: Famous physicists and Nobel Prize in physics

Ancient times

Since antiquity, people have tried to understand the behavior of matter: why unsupported objects drop to the ground, why different materials in science have different properties, and so forth. Another mystery was the character of the universe, such as the form of the Earth and the behavior of celestial objects such as the Sun and the Moon. Several theories were proposed, the majority of which were disproved. These theories were largely couched in philosophical terms, and never verified by systematic experimental testing as is popular today. On the other hand, the commonly accepted works of Ptolemy and Aristotle are not always found to match everyday observations. There were exceptions and there are anachronisms: for example, Indian philosophers and astronomers gave many correct descriptions in atomism and astronomy, and the Greek thinker Archimedes derived many correct quantitative descriptions of mechanics and hydrostatics.

Middle Ages

The willingness to question previously held truths and search for new answers eventually resulted in a period of major scientific advancements, now known as the Scientific Revolution of the late 17th century. The precursors to the scientific revolution can be traced back to the important developments made in India and Persia, including the elliptical model of planetary orbits based on the heliocentric solar system of gravitation developed by Indian mathematician-astronomer Aryabhata; the basic ideas of atomic theory developed by Hindu and Jaina philosophers; the theory of light being equivalent to energy particles developed by the Indian Buddhist scholars Dignāga and Dharmakirti; the optical theory of light developed by Arab scientist Alhazen; the Astrolabe invented by the Persian Mohammad al-Fazari; and the significant flaws in the Ptolemaic system pointed out by Persian scientist Nasir al-Din al-Tusi. As the influence of the Islamic Caliphate expanded to Europe, the works of Aristotle preserved by the Arabs, and the works of the Indians and Persians, became known in Europe by the 12th and 13th centuries.

The Middle Ages saw the emergence of experimental physics with the development of an early scientific method emphasizing the role of experimentation and mathematics. Ibn al-Haytham (Alhazen, 965–1039) is considered a central figure in this shift in physics from a philosophical activity to an experimental one. In his Book of Optics (1021), he developed an early scientific method in order to prove the intromission theory of vision and discredit the emission theory of vision previously supported by Euclid and Ptolemy.^[19][20][21] His most famous experiments involve his development and use of the camera obscura in order to test several hypotheses on light, such as light travelling in straight lines and whether different lights can mix in the air.^[22] This experimental tradition in optics established by Ibn al-Haytham continued among his successors in both the Islamic world, with the likes of Qutb al-Din al-Shirazi, Kamāl al-Dīn al-Fārisī and Taqi al-Din, and in Europe, with the likes of Robert Grosseteste, Roger Bacon, Witelo, John Pecham, Theodoric of Freiberg, Johannes Kepler, Willebrord Snellius, René Descartes and Christiaan Huygens.

The Scientific Revolution

The Scientific Revolution is held by most historians (e.g., Howard Margolis) to have begun in 1543, when the first printed copy of Nicolaus Copernicus's De Revolutionibus (most of which had been written years prior but whose publication had been delayed) was brought from Nuremberg to the astronomer, who died soon after receiving the copy.

Further significant advances were made over the following century by Galileo Galilei, Christiaan Huygens, Johannes Kepler, and Blaise Pascal. During the early 17th century, Galileo pioneered the use of experimentation to validate physical theories, which is the key idea in modern scientific method. Galileo formulated and successfully tested several results in dynamics, in particular the Law of Inertia. In 1687, Newton published the Principia, detailing two comprehensive and successful physical theories: Newton's laws of motion, from which arise classical mechanics; and Newton's Law of Gravitation, which describes the fundamental force of gravity. Both theories agreed well with experiment. The Principia also included several theories in fluid dynamics. Classical mechanics was re-formulated and extended by Leonhard Euler, French mathematician Joseph-Louis Comte de Lagrange, Irish mathematical physicist William Rowan Hamilton, and others, who produced new results in mathematical physics. The law of universal gravitation initiated the field of astrophysics, which describes astronomical phenomena using physical theories.

After Newton defined classical mechanics, the next great field of inquiry within physics was the nature of electricity. Observations in the 17th and 18th century by scientists such as Robert Boyle, Stephen Gray, and Benjamin Franklin created a foundation for later work. These observations also established our basic understanding of electrical charge and electric current.

In 1821, the English physicist and chemist Michael Faraday integrated the study of magnetism with the study of electricity. This was done by demonstrating that a moving magnet induced an electric current in a conductor. Faraday also formulated a physical conception of electromagnetic fields. James Clerk Maxwell built upon this conception, in 1864, with an interlinked set of 20 equations that explained the interactions between electric and magnetic fields. These 20 equations were later reduced, using vector calculus, to a set of four equations, namely Maxwell's equations, by Oliver Heaviside.

In addition to other electromagnetic phenomena, Maxwell's equations also can be used to describe light. Confirmation of this observation was made with the 1888 discovery of radio by Heinrich Hertz and in 1895 when Wilhelm Roentgen detected X rays. The ability to describe light in electromagnetic terms helped serve as a springboard for Albert Einstein's publication of the theory of special relativity in 1905. This theory combined classical mechanics with Maxwell's equations.

The theory of special relativity unifies space and time into a single entity, spacetime. Relativity prescribes a different transformation between reference frames than classical mechanics; this necessitated the development of relativistic mechanics as a replacement for classical mechanics. In the regime of low (relative) velocities, the two theories agree. Einstein built further on the special theory by including gravity into his calculations, and published his theory of general relativity in 1915.

One part of the theory of general relativity is Einstein's field equation. This describes how the stress-energy tensor creates curvature of spacetime and forms the basis of general relativity. Further work on Einstein's field equation produced results which predicted the Big Bang, black holes, and the expanding universe. Einstein believed in a static universe and tried (and failed) to fix his equation to allow for this. However, by 1929 Edwin Hubble's astronomical observations suggested that the universe is expanding. Thus, the universe must have been smaller and therefore hotter in the past. In 1933 Karl Jansky at Bell Labs discovered the radio emission from the Milky Way, and thereby initiated the science of radio astronomy. By the 1940s, researchers like George Gamow proposed the Big Bang theory,^[23] evidence for which was discovered in 1964;^[24] Enrico Fermi and Fred Hoyle were among the doubters in the 1940s and 1950s. Hoyle had dubbed Gamow's theory the Big Bang in order to debunk it. Today, it is one of the principal tenents of physical cosmology.

From the late 17th century onwards, thermodynamics was developed by physicist and chemist Robert Boyle, Thomas Young, and many others. In 1733, Daniel Bernoulli used statistical arguments with classical mechanics to derive thermodynamic results, initiating the field of statistical mechanics. In 1798, Benjamin Thompson demonstrated the conversion of mechanical work into heat, and in 1847 James Joule stated the law of conservation of energy, in the form of heat as well as mechanical energy. Ludwig Boltzmann, in the 19th century, is responsible for the modern form of statistical mechanics.

1895 to present

In 1895, Wilhelm Röntgen discovered X-rays, which turned out to be high-frequency electromagnetic radiation. Radioactivity was discovered in 1896 by Henri Becquerel, and further studied by Marie Curie, Pierre Curie, and others. This initiated the field of nuclear physics.

In 1897, Joseph J. Thomson discovered the electron, the elementary particle which carries electrical current in circuits. In 1904, he proposed the first model of the atom, known as the plum pudding model. (The existence of the atom had been proposed in 1808 by John Dalton.)

These discoveries revealed that the assumption of many physicists that atoms were the basic unit of matter was flawed, and prompted further study into the structure of atoms.

In 1911, Ernest Rutherford deduced from scattering experiments the existence of a compact atomic nucleus, with positively charged constituents dubbed protons. Neutrons, the neutral nuclear constituents, were discovered in 1932 by Chadwick. The equivalence of mass and energy (Einstein, 1905) was spectacularly demonstrated during World War II, as research was conducted by each side into nuclear physics, for the purpose of creating a nuclear bomb. The German effort, led by Heisenberg, did not succeed, but the Allied Manhattan Project reached its goal. In America, a team led by Fermi achieved the first man-made nuclear chain reaction in 1942, and in 1945 the world's first nuclear explosive was detonated at Trinity site, near Alamogordo, New Mexico.

In 1900, Max Planck published his explanation^[25] of blackbody radiation. This equation assumed that radiators are quantized in nature, which proved to be the opening argument in the edifice that would become quantum mechanics.

Beginning in 1900, Planck, Einstein, Niels Bohr, and others developed quantum theories to explain various anomalous experimental results by introducing discrete energy levels. In 1925, Heisenberg and 1926, Schrödinger and Paul Dirac formulated quantum mechanics, which explained the preceding heuristic quantum theories. In quantum mechanics, the outcomes of physical measurements are inherently probabilistic; the theory describes the calculation of these probabilities. It successfully describes the behavior of matter at small distance scales. During the 1920s Erwin Schrödinger, Werner Heisenberg, and Max Born were able to formulate a consistent picture of the chemical behavior of matter, a complete theory of the electronic structure of the atom, as a byproduct of the quantum theory.

Quantum field theory was formulated in order to extend quantum mechanics to be consistent with special relativity. It was devised in the late 1940s with work by Richard Feynman, Julian Schwinger, Sin-Itiro Tomonaga, and Freeman Dyson. They formulated the theory of quantum electrodynamics, which describes the electromagnetic interaction, and successfully explained the "Lamb shift". Quantum field theory provided the framework for modern particle physics, which studies fundamental forces and elementary particles.

Chen Ning Yang and Tsung-Dao Lee, in the 1950s, discovered an unexpected asymmetry^[26] in the decay of a subatomic particle. In 1954, Yang and Robert Mills then developed a class of gauge theories^[27] which provided the framework for understanding the nuclear forces. The theory for the strong nuclear force was first proposed by Murray Gell-Mann. The electroweak force, the unification of the weak nuclear force with electromagnetism, was proposed by Sheldon Lee Glashow, Abdus Salam and Steven Weinberg and confirmed in 1964 by James Watson Cronin and Val Fitch. This led to the so-called Standard Model of particle physics in the 1970s, which successfully describes all the elementary particles observed to date.

Quantum mechanics also provided the theoretical tools for condensed matter physics, whose largest branch is solid state physics. It studies the physical behavior of solids and liquids, including phenomena such as crystal structures, semiconductivity, and superconductivity. The pioneers of condensed matter physics include Bloch, who created a quantum mechanical description of the behavior of electrons in crystal structures in 1928. The transistor was developed by physicists John Bardeen, Walter Houser Brattain and William Bradford Shockley in 1947 at Bell Telephone Laboratories.

The two themes of the 20th century, general relativity and quantum mechanics, appear inconsistent with each other^[28]. General relativity describes the universe on the scale of planets and solar systems while quantum mechanics operates on sub-atomic scales. This challenge is being attacked by string theory, which treats spacetime as composed, not of points, but of one-dimensional objects, strings. Strings have properties like a common string (e.g., tension and vibration). The theories yield promising, but not yet testable results. The search for experimental verification of string theory is in progress.

The United Nations declared the year 2005, the centenary of Einstein's annus mirabilis, as the World Year of Physics and 2009 the International Year of Astronomy.

Principles and concepts

Further information: Classical physics, Modern physics, Topic outline of physics#Essence of physics

Topics in Physics Mechanics Classical mechanics Quantum mechanics Relativity Electromagnetism Thermodynamics Statistical Mechanics Fluid Mechanics Mathematical Physics Bulk matter

The search for regularities in nature serves to motivate our search for principles of physics. Thus Kepler's discovery of the inscribed Platonic solid model of the solar system seemed to him his greatest achievement. Kepler's laws, which he derived over a period of twenty years, were the mathematical relations which Newton was able to incorporate into his system of the world.

Matter is a mass noun which can refer to ensembles of atoms and molecules as well as their constituent subatomic particles. Einstein believed that fields were more fundamental than particles, which illustrates that matter is not the simple topic it appears to be. Newton treated matter as points endowed with mass. This allowed mechanics to be reduced to geometry, as illustrated in Galileo's Two New Sciences.

Feynman started out the Feynman Lectures on Physics with the existence of atoms, which he considered to be the most compact statement of physics, from which the science could be rebuilt, were we to lose all our knowledge but that. By modeling matter as collections of hard spheres, much like Galileo's bronze ball, with which the law of falling bodies was measured, it is possible to describe statistical mechanics.

Statistical mechanics, and the assumption that gases can be modeled by the collisions of hard spheres, can be used to derive the laws of thermodynamics. Liouville's theorem for statistical and Hamiltonian mechanics is a classical nineteenth century result which describes the behavior of the phase space distribution function^[29]. Liouville's theorem has a suggestive formulation, the Poisson bracket, which encodes Hamilton's equations of classical mechanics. The Poisson bracket is in form much like the commutator of quantum mechanics. The laws of nature appear to follow the postulates of quantum mechanics, and the theories that follow these postulates are said to have been quantized.

The special theory of relativity enjoys a relationship with electromagnetism and mechanics; that is, the principle of relativity and the principle of stationary action in mechanics can be used to derive Maxwell's equations^[30],^[31], and vice versa.

Relativity and quantum mechanics can describe the physics of the extremely small (atoms, nuclei, fundamental particles), the extremely large (the Universe), and the extremely fast (relativity). But no complete theory yet exists. The Schrödinger picture of quantum mechanics and the Heisenberg picture can be connected by the Ehrenfest theorem, the analog of Liouville's theorem noted above.

While physics deals with a wide variety of systems, there are certain theories that are used by all physicists. Each of these theories were experimentally tested numerous times and found correct as an approximation of Nature (within a certain domain of validity). For instance, the theory of classical mechanics accurately describes the motion of objects, provided they are much larger than atoms and moving at much less than the speed of light. These theories continue to be areas of active research; for instance, a remarkable aspect of classical mechanics known as chaos was discovered in the 20th century, three centuries after the original formulation of classical mechanics by Isaac Newton (1642–1727). These "central theories" are important tools for research into more specialized topics, and any physicist, regardless of his or her specialization, is expected to be literate in them.

Theory	Major subtopics	Concepts
Classical mechanics	Newton's laws of motion, Lagrangian mechanics, Hamiltonian mechanics, Kinematics, Statics, Dynamics, Chaos theory, Acoustics, Fluid dynamics, Continuum mechanics	Density, Dimension, Gravity, Space, Time, Motion, Length, Position, Velocity, Acceleration, Mass, Momentum, Force, Energy, Angular momentum, Torque, Conservation law, Harmonic oscillator, Wave, Work, Power, Harmonic oscillator
Electromagnetism	Electrostatics, Electrodynamics, Electricity, Magnetism, Maxwell's equations, Optics	Capacitance, Electric charge, Current, Electrical conductivity, Electric field, Electric permittivity, Electrical resistance, Electromagnetic field, Electromagnetic induction, Electromagnetic radiation, Gaussian surface, Magnetic field, Magnetic flux, Magnetic monopole, Magnetic permeability
Thermodynamics and Statistical mechanics	Heat engine, Kinetic theory	Boltzmann's constant, Conjugate variables, Enthalpy, Entropy, Equation of state, Equipartition theorem, First Law of Thermodynamics, Free energy, Heat, Ideal gas law, Internal energy, Irreversible process, Partition function, Pressure, Reversible process, Second Law of Thermodynamics, Spontaneous process, State function, Statistical ensemble, Temperature, Thermodynamic equilibrium, Thermodynamic potential, Thermodynamic processes, Thermodynamic state, Thermodynamic system, Third Law of Thermodynamics, Viscosity, Zeroth Law of Thermodynamics
Quantum mechanics	Path integral formulation, Scattering theory, Schrödinger equation, Quantum field theory, Quantum statistical mechanics	Adiabatic approximation, Correspondence principle, Free particle, Hamiltonian, Hilbert space, Identical particles, Matrix Mechanics, Planck's constant, Operators, Quanta, Quantization, Quantum entanglement, Quantum harmonic oscillator, Quantum number, Quantum tunneling, Schrödinger's cat, Dirac equation, Spin, Wavefunction, Wave mechanics, Wave-particle duality, Zero-point energy, Pauli Exclusion Principle, Heisenberg Uncertainty Principle
Theory of relativity	Special relativity, General relativity, Einstein field equations	Covariance, Einstein manifold, Equivalence principle, Four-momentum, Four-vector, General principle of relativity, Geodesic motion, Gravity, Gravitoelectromagnetism, Inertial frame of reference, Invariance, Length contraction, Lorentzian manifold, Lorentz transformation, Metric, Minkowski diagram, Minkowski space, Principle of Relativity, Proper length, Proper time, Reference frame, Rest energy, Rest mass, Relativity of simultaneity, Spacetime, Special principle of relativity, Speed of light, Stress-energy tensor, Time dilation, Twin paradox, World line

Major topics of physics

Contemporary research in physics is divided into several distinct fields that study different aspects of the material world. Condensed matter physics, by most estimates the largest single field of physics, is concerned with how the properties of bulk matter, such as the ordinary solids and liquids we encounter in everyday life, arise from the properties and mutual interactions of the constituent atoms. The field of atomic, molecular, and optical physics deals with the behavior of individual atoms and molecules, and in particular the ways in which they absorb and emit light. The field of particle physics, also known as "high-energy physics", is concerned with the properties of submicroscopic particles much smaller than atoms, including the elementary particles from which all other units of matter are constructed. Finally, the field of astrophysics applies the laws of physics to explain celestial phenomena, ranging from the Sun and the other objects in the solar system to the universe as a whole.

Since the 20th century, the individual fields of physics have become increasingly specialized, and nowadays it is not uncommon for physicists to work in a single field for their entire careers. "Universalists" like Albert Einstein (1879–1955) and Lev Landau (1908–1968), who were comfortable working in multiple fields of physics, are now very rare.

Field	Subfields	Major theories	Concepts
Astrophysics	Cosmology, Gravitation physics, High-energy astrophysics, Planetary astrophysics, Plasma physics, Space physics, Stellar astrophysics	Big Bang, Lambda-CDM model, Cosmic inflation, General relativity, Law of universal gravitation	Black hole, Cosmic background radiation, Cosmic string, Cosmos, Dark energy, Dark matter, Galaxy, Gravity, Gravitational radiation, Gravitational singularity, Planet, Solar system, Star, Supernova, Universe
Atomic, molecular, and optical physics	Atomic physics, Molecular physics, Atomic and Molecular astrophysics, Chemical physics, Optics, Photonics	Quantum optics, Quantum chemistry, Quantum information science	Atom, Molecule, Diffraction, Electromagnetic radiation, Laser, Polarization, Spectral line, Casimir effect
Particle physics	Accelerator physics, Nuclear physics, Nuclear astrophysics, Particle astrophysics, Particle physics phenomenology	Standard Model, Quantum field theory, Quantum chromodynamics, Electroweak theory, Effective field theory, Lattice field theory, Lattice gauge theory, Gauge theory, Supersymmetry, Grand unification theory, Superstring theory, M-theory	Fundamental force (gravitational, electromagnetic, weak, strong), Elementary particle, Spin, Antimatter, Spontaneous symmetry breaking, Brane, String, Quantum gravity, Theory of everything, Vacuum energy
Condensed matter physics	Solid state physics, High pressure physics, Low-temperature physics, Nanoscale and mesoscopic physics, Polymer physics	BCS theory, Bloch wave, Fermi gas, Fermi liquid, Many-body theory	Phases (gas, liquid, solid, Bose-Einstein condensate, superconductor, superfluid), Electrical conduction, Magnetism, Self-organization, Spin, Spontaneous symmetry breaking

Current research

Further information: problems in physics, more concepts

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. Many condensed matter experiments are aiming to fabricate workable spintronics and quantum computers.

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 neutrinos have non-zero mass. 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. In the next several years, particle accelerators will begin probing energy scales in the TeV range, in which experimentalists are hoping to find evidence^[32] for the Higgs boson and supersymmetric particles.

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. The current leading candidates are M-theory, superstring theory and loop quantum gravity.

Many astronomical and cosmological phenomena have yet to be satisfactorily explained, including the existence of ultra-high energy cosmic rays, the baryon asymmetry, the acceleration of the universe and the anomalous rotation rates of galaxies.

Although much progress has been made in high-energy, quantum, and astronomical physics, many everyday phenomena involving complexity, chaos, or turbulence 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 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. Complex physics has become part of increasingly interdisciplinary research, as exemplified by the study of turbulence in aerodynamics and the observation of pattern formation in biological systems. In 1932, Horace Lamb correctly prophesized:

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.^[33]

Applications and influence

Applied physics is a general term for physics which is intended for a particular use. An applied physics curriculum usually contains a few classes from the applied disciplines, like chemistry, computer science, 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 can also be interested the use of physics for scientific research. For instance, people working on accelerator physics might seek to build better particle detectors for research in theoretical physics.

Physics is used heavily in engineering. Statics, a subfield of mechanics, is used in the building of bridges or other structures; the simple machines such as the lever and the ramp had to be discovered before they could be used; today, they can be taught to schoolchildren. The understanding and use of acoustics will result in better concert halls; similarly, the use of optics creates better optical devices. An understanding of physics makes for more realistic flight simulators, video games, and movies, as well as in forensic investigations (what do we know and when do we know it; what did the subject know and when did the subject know it).

Because of its historical relationship to the development of scientific method, physics reasoning can handle items which would ordinarily be mired in conundrums or uncertainty. For example, in the study of the origin of the Earth, one can reasonably model Earth's mass, temperature, and rate of rotation, over time. From these values, the chemical composition of Earth at differing epochs can be posited. Even if a precise linear timeline might be problematic, qualitative statements can then be made about the history of Earth, which are still founded in the laws of physics.

There are many fields of physics which have strong applied branches, as well as many related and overlapping fields from other disciplines that are closely related to applied physics.

• Acoustics, the study of sound waves, is used everywhere we wish to hear, for example in music, speech, and audible alarms
• Agrophysics, the study of the physics in agronomy
• Biophysics, the interface of biology and physics
• Chemical physics, the study of the structure and dynamics of ions, free radicals, polymers, clusters, and molecules, using both classical and quantum mechanical viewpoints
• Communications has used physics extensively (e.g., in the Bell Laboratories). The first communications satellite appeared 300 years after Newton.
• Econophysics, the interface of economics and physics
• Engineering, the utilization of physics in service of technology rather than of science
• Engineering physics, specializing in optics, nanotechnology, control theory, aerodynamics, and solid-state physics
• Fluid mechanics, the study of fluids (liquids and gases) at rest and in motion. The Navier-Stokes equations are used in supercomputers to model Earth's weather patterns.
• Geophysics, the study of the physics of Earth
• Lasers and radar were developed in the laboratories, used by the military, and now have extensive peacetime uses. Quantum electronics includes the study of lasers.
• Materials science, the systematic study of the properties of materials
• Medical physics includes the standards for radiation exposure and infrastructure for radiology.
• Nanotechnology, the study of the creation of machines less than a micrometer in size
• Optics has existed as a science for over 1000 years. Like acoustics, it has its own journals, practitioners, and university departments, as well as industries which utilize those graduates.
• Optoelectronics, the study of the creation of devices which use light (such as Kerr cells or Pockels cells) rather than current.
• Photovoltaics, the study of solar cells that generate current from light
• Physical chemistry
• Physics of computation must rely upon the state of the art. For example, the accuracy of a computation depends on the ability to manufacture to tolerance, which was the undoing of Babbage's difference engine. Babbage's design worked when built with twentieth century technology.
• Plasma physics, the physics of an ionized gas
• Quantum chemistry, the modeling of matter using quantum mechanics
• Quantum information science, the application of qubit devices
• Solid state physics, the study of the material properties of semiconductor devices and integrated circuits
• Vehicle dynamics, a form of kinematics

See also

General

• Classical physics
• Glossary of classical physics
• List of basic physics topics
• List of physics topics
• Perfection in physics and chemistry
• Philosophy of physics
• Physics (Aristotle) – an early book on physics, which attempted to analyze and define motion from a philosophical point of view
• Unsolved problems in physics

Related fields

• Astronomy
• Chemistry
• Engineering
• Mathematics

References

External links

• A large number of textbooks, popular books, and webpages about physics are available for further reading.
• Important publications in physics

General

• Physics, trans. by R. P. Hardie and R. K. Gaye
• Physics and Math Textbooks – Numerous online textbooks on Physics and Mathematics
• Usenet Physics FAQ – A FAQ compiled by sci.physics and other physics newsgroups
• Physics.org – Web portal run by the Institute of Physics
• World of Physics – An online encyclopedic dictionary of physics
• Website of the Nobel Prize in physics
• HyperPhysics website – HyperPhysics, a physics and astronomy mind-map from Georgia State University
• Physics Today – Your daily physics news and research source
• The Skeptic's Guide to Physics
• PlanetPhysics – Online physics
• Physics 2005 – Website of the World Year of Physics 2005
• Physicsweb.org
• Contemporary Physics

Organizations

• AIP.org – Website of the American Institute of Physics
• IOP.org – Website of the Institute of Physics
• APS.org – Website of the American Physical Society
• SPS National – Website of the Society of Physics Students
• Physics & Math Forums
• Royal Society – Although not exclusively a physics institution, it has a strong history of physics

The Four Fundamental Interactions of Physics Strong interaction · Electromagnetism · Weak interaction · Gravitation

General subfields within physics Astrophysics · Atomic, molecular, and optical physics · Condensed matter physics · Dynamics (Fluid dynamics · Thermodynamics)  · Electromagnetism (Optics) · High energy physics · Mechanics (Classical mechanics · Quantum mechanics · Statistical mechanics) · Relativity (Special relativity · General relativity) · Quantum field theory · Statics (Fluid statics)

Fields within the natural sciences Astronomy · Biology · Chemistry · Earth science · Physics