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Observer (physics)

The term observer in special relativity refers most commonly to an inertial reference frame. In such cases an inertial reference frame may be called an "inertial observer" to avoid ambiguity. Note that these uses differ significantly from the ord ...

Observer (quantum physics)

In quantum mechanics, "observation" is synonymous with quantum measurement and "observer" with a measurement apparatus and "observable" with what can be measured. Thus the quantum mechanical observer does not have to necessarily present or solve ...

Observer effect (physics)

In physics, the observer effect is the theory that the mere observation of a phenomenon inevitably changes that phenomenon. This is often the result of instruments that, by necessity, alter the state of what they measure in some manner. A common ...

One-electron universe

The one-electron universe postulate, proposed by John Wheeler in a telephone call to Richard Feynman in the spring of 1940, is the hypothesis that all electrons and positrons are actually manifestations of a single entity moving backwards and for ...

Onsager reciprocal relations

In thermodynamics, the Onsager reciprocal relations express the equality of certain ratios between flows and forces in thermodynamic systems out of equilibrium, but where a notion of local equilibrium exists. "Reciprocal relations" occur between ...

Optogalvanic effect

The Optogalvanic effect is the change in the conductivity of a gas discharge induced by a light source. This effect has found many applications in atomic spectroscopy and laser stabilization.

Palatini identity

In general relativity and tensor calculus, the Palatini identity is: δ R σ ν = ∇ ρ δ Γ ν σ ρ − ∇ ν δ Γ ρ σ ρ, {\displaystyle \delta R_{\sigma \nu }=\nabla _{\rho }\delta \Gamma _{\nu \sigma }^{\rho }-\nabla _{\nu }\delta \Gamma _{\rho \sigma }^{\ ...

Paramagnetism

Paramagnetism is a form of magnetism whereby some materials are weakly attracted by an externally applied magnetic field, and form internal, induced magnetic fields in the direction of the applied magnetic field. In contrast with this behavior, d ...

Parity (physics)

In quantum mechanics, a parity transformation is the flip in the sign of one spatial coordinate. In three dimensions, it can also refer to the simultaneous flip in the sign of all three spatial coordinates: P: x y z ↦ − x − y − z. {\displaystyle ...

Partial differential equation

In mathematics, a partial differential equation is a differential equation that contains unknown multivariable functions and their partial derivatives. PDEs are used to formulate problems involving functions of several variables, and are either s ...

Partition function (statistical mechanics)

In physics, a partition function describes the statistical properties of a system in thermodynamic equilibrium. Partition functions are functions of the thermodynamic state variables, such as the temperature and volume. Most of the aggregate ther ...

Path integral formulation

The path integral formulation of quantum mechanics is a description of quantum theory that generalizes the action principle of classical mechanics. It replaces the classical notion of a single, unique classical trajectory for a system with a sum, ...

Pauli exclusion principle

The Pauli exclusion principle is the quantum mechanical principle which states that two or more identical fermions cannot occupy the same quantum state within a quantum system simultaneously. This principle was formulated by Austrian physicist Wo ...

Permittivity

In electromagnetism, the absolute permittivity, often simply called permittivity and denoted by the Greek letter ε, is a measure of the electric polarizability of a dielectric. A material with high permittivity polarizes more in response to an ap ...

Perturbation theory

Perturbation theory comprises mathematical methods for finding an approximate solution to a problem, by starting from the exact solution of a related, simpler problem. A critical feature of the technique is a middle step that breaks the problem i ...

Phase (matter)

In the physical sciences, a phase is a region of space, throughout which all physical properties of a material are essentially uniform. Examples of physical properties include density, index of refraction, magnetization and chemical composition. ...

Phase space

In dynamical system theory, a phase space is a space in which all possible states of a system are represented, with each possible state corresponding to one unique point in the phase space. For mechanical systems, the phase space usually consists ...

Phase transition

The term phase transition is most commonly used to describe transitions between solid, liquid, and gaseous states of matter, as well as plasma in rare cases. A phase of a thermodynamic system and the states of matter have uniform physical propert ...

Photon polarization

Photon polarization is the quantum mechanical description of the classical polarized sinusoidal plane electromagnetic wave. An individual photon can be described as having right or left circular polarization, or a superposition of the two. Equiva ...

Physical information

Physical information is a form of information. In physics, it refers to the information of a physical system. Physical information is an important concept used in a number of fields of study in physics. For example, in quantum mechanics, the form ...

Physical theories modified by general relativity

This article will use the Einstein summation convention. The theory of general relativity required the adaptation of existing theories of physical, electromagnetic, and quantum effects to account for non-Euclidean geometries. These physical theor ...

Physics applications of asymptotically safe gravity

The asymptotic safety approach to quantum gravity provides a nonperturbative notion of renormalization in order to find a consistent and predictive quantum field theory of the gravitational interaction and spacetime geometry. It is based upon a n ...

Point particle

A point particle is an idealization of particles heavily used in physics. Its defining feature is that it lacks spatial extension: being zero-dimensional, it does not take up space. A point particle is an appropriate representation of any object ...

Poisson bracket

In mathematics and classical mechanics, the Poisson bracket is an important binary operation in Hamiltonian mechanics, playing a central role in Hamiltons equations of motion, which govern the time evolution of a Hamiltonian dynamical system. The ...

Polynomial Wigner–Ville distribution

In signal processing, the polynomial Wigner–Ville distribution is a quasiprobability distribution that generalizes the Wigner distribution function. It was proposed by Boualem Boashash and Peter OShea in 1994.

Potential gradient

In physics, chemistry and biology, a potential gradient is the local rate of change of the potential with respect to displacement, i.e. spatial derivative, or gradient. This quantity frequently occurs in equations of physical processes because it ...

Potentiometric surface

A potentiometric surface is the imaginary plane where a given reservoir of fluid will "equalize out to" if allowed to flow. A potentiometric surface is based on hydraulic principles. For example, we know that two connected storage tanks with one ...

Power (physics)

In physics, power is the rate of doing work or of transferring heat, i.e. the amount of energy transferred or converted per unit time. In the International System of Units, the unit of power is the watt, equal to one joule per second. The equatio ...

Pressure

Pressure is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure is the pressure relative to the ambient pressure. Various units are used to express pressure. Some of these ...

Covariance principle

In physics, the principle of covariance emphasizes the formulation of physical laws using only those physical quantities the measurements of which the observers in different frames of reference could unambiguously correlate. Mathematically, the p ...

Probability amplitude

In quantum mechanics, a probability amplitude is a complex number used in describing the behaviour of systems. The modulus squared of this quantity represents a probability or probability density. Probability amplitudes provide a relationship bet ...

Probability density function

In probability theory, a probability density function, or density of a continuous random variable, is a function whose value at any given sample in the sample space can be interpreted as providing a relative likelihood that the value of the rando ...

Probability of occupation

In condensed matter physics, the probability of occupation shows how likely it is for a given energy level to be occupied. Fermions such as electrons follow a Fermi–Dirac distribution and bosons such as phonons and photons follow a Bose–Einstein ...

Pulse (physics)

In physics, a pulse is a generic term describing a single disturbance that moves through a transmission medium. This medium may be vacuum or matter, and may be indefinitely large or finite.

Quantization (physics)

In physics, quantization is the process of transition from a classical understanding of physical phenomena to a newer understanding known as quantum mechanics. This is a generalization of the procedure for building quantum mechanics from classica ...

Quantum chromodynamics

In theoretical physics, quantum chromodynamics is the theory of the strong interaction between quarks and gluons, the fundamental particles that make up composite hadrons such as the proton, neutron and pion. QCD is a type of quantum field theory ...

Quantum chromodynamics binding energy

Quantum chromodynamic binding energy, gluon binding energy or chromodynamic binding energy is the energy binding quarks together into hadrons. It is the energy of the field of the strong force, which is mediated by gluons. Motion-energy and inter ...

Quantum electrodynamics

In particle physics, quantum electrodynamics is the relativistic quantum field theory of electrodynamics. In essence, it describes how light and matter interact and is the first theory where full agreement between quantum mechanics and special re ...

Quantum field theory

In theoretical physics, quantum field theory is a theoretical framework that combines classical field theory, special relativity, and quantum mechanics and is used to construct physical models of subatomic particles and quasiparticles. QFT treats ...

Quantum hadrodynamics

Quantum hadrodynamics is an effective field theory pertaining to interactions between hadrons, that is, hadron-hadron interactions or the inter-hadron force. It is "a framework for describing the nuclear many-body problem as a relativistic system ...

Quantum mechanics

Quantum mechanics, including quantum field theory, is a fundamental theory in physics which describes nature at the smallest – including atomic and subatomic – scales. Classical physics, the description of physics existing before the formulation ...

Quantum non-equilibrium

In the Copenhagen interpretation, that is, the most widely used interpretation of quantum mechanics, the Born rule ρ x, t = | ψ x, t | 2 {\displaystyle \rho \mathbf {x},t=|\psi \mathbf {x},t|^{2}} defines that ρ {\displaystyle \rho }, the probabi ...

Quantum number

Quantum numbers describe values of conserved quantities in the dynamics of a quantum system. In the case of electrons, the quantum numbers can be defined as "the sets of numerical values which give acceptable solutions to the Schrodinger wave equ ...

Quantum potential

The quantum potential or quantum potentiality is a central concept of the de Broglie–Bohm formulation of quantum mechanics, introduced by David Bohm in 1952. Initially presented under the name quantum-mechanical potential, subsequently quantum po ...

Quantum pseudo-telepathy

Quantum pseudo-telepathy is the fact that in certain Bayesian games with asymmetric information, players who have access to a shared physical system in an entangled quantum state, and who are able to execute strategies that are contingent upon me ...

Quantum triviality

In a quantum field theory, charge screening can restrict the value of the observable "renormalized" charge of a classical theory. If the only resulting value of the renormalized charge is zero, the theory is said to be "trivial" or noninteracting ...

Quasiparticle

In physics, quasiparticles and collective excitations are emergent phenomena that occur when a microscopically complicated system such as a solid behaves as if it contained different weakly interacting particles in free space. For example, as an ...

Radiation

In physics, radiation is the emission or transmission of energy in the form of waves or particles through space or through a material medium. This includes: acoustic radiation, such as ultrasound, sound, and seismic waves dependent on a physical ...

Reflection (physics)

Reflection is the change in direction of a wavefront at an interface between two different media so that the wavefront returns into the medium from which it originated. Common examples include the reflection of light, sound and water waves. The l ...

Regenerative capacitor memory

Regenerative capacitor memory is a type of computer memory that uses the electrical property of capacitance to store the bits of data. Because the stored charge slowly leaks away, these memories must be periodically regenerated to prevent data lo ...