The **Compton wavelength** is a quantum mechanical property of a particle. The Compton wavelength of a particle is equal to the wavelength of a photon whose energy is the same as the mass of that particle (see mass–energy equivalence). It was introduced by Arthur Compton in his explanation of the scattering of photons by electrons (a process known as Compton scattering).

- Reduced Compton wavelength
- Role in equations for massive particles
- Distinction between reduced and non-reduced
- Limitation on measurement
- Relationship to other constants
- References
- External links

The standard Compton wavelength, λ, of a particle is given by,

while its frequency is given by,

where *h* is the Planck constant, *m* is the particle's rest mass, and *c* is the speed of light. The significance of this formula is shown in the derivation of the Compton shift formula.

The CODATA 2018 value for the Compton wavelength of the electron is 2.42631023867(73)×10^{−12} m.^{ [1] } Other particles have different Compton wavelengths.

When the Compton wavelength is divided by 2*π*, one obtains the "reduced" Compton wavelength ƛ (barred lambda), i.e. the Compton wavelength for 1 radian instead of 2*π* radians:

- ƛ = λ/2
*π*=*ħ*/*mc*,

where *ħ* is the "reduced" Planck constant.

The inverse reduced Compton wavelength is a natural representation for mass on the quantum scale, and as such, it appears in many of the fundamental equations of quantum mechanics. The reduced Compton wavelength appears in the relativistic Klein–Gordon equation for a free particle:

It appears in the Dirac equation (the following is an explicitly covariant form employing the Einstein summation convention):

The reduced Compton wavelength also appears in Schrödinger's equation, although its presence is obscured in traditional representations of the equation. The following is the traditional representation of Schrödinger's equation for an electron in a hydrogen-like atom:

Dividing through by , and rewriting in terms of the fine structure constant, one obtains:

The reduced Compton wavelength is a natural representation of mass on the quantum scale. Equations that pertain to inertial mass like Klein-Gordon and Schrödinger's, use the reduced Compton wavelength.^{ [2] }^{:18–22} The non-reduced Compton wavelength is a natural representation for mass that has been converted into energy. Equations that pertain to the conversion of mass into energy, or to the wavelengths of photons interacting with mass, use the non-reduced Compton wavelength.

A particle of mass *m* has a rest energy of *E* = *mc*^{2}. The non-reduced Compton wavelength for this particle is the wavelength of a photon of the same energy. For photons of frequency *f*, energy is given by

which yields the non-reduced or standard Compton wavelength formula if solved for λ.

The Compton wavelength expresses a fundamental limitation on measuring the position of a particle, taking into account quantum mechanics and special relativity.^{ [3] }

This limitation depends on the mass *m* of the particle. To see how, note that we can measure the position of a particle by bouncing light off it – but measuring the position accurately requires light of short wavelength. Light with a short wavelength consists of photons of high energy. If the energy of these photons exceeds *mc*^{2}, when one hits the particle whose position is being measured the collision may yield enough energy to create a new particle of the same type.^{[ citation needed ]} This renders moot the question of the original particle's location.

This argument also shows that the reduced Compton wavelength is the cutoff below which quantum field theory – which can describe particle creation and annihilation – becomes important. The above argument can be made a bit more precise as follows. Suppose we wish to measure the position of a particle to within an accuracy Δ*x*. Then the uncertainty relation for position and momentum says that

so the uncertainty in the particle's momentum satisfies

Using the relativistic relation between momentum and energy *E*^{2} = (*pc*)^{2} + (*mc*^{2})^{2}, when Δ*p* exceeds *mc* then the uncertainty in energy is greater than *mc*^{2}, which is enough energy to create another particle of the same type. But we must exclude this. In particular the minimum uncertainty is when the scattered photon has limit energy equal to the incident observing energy. It follows that there is a fundamental minimum for Δ*x*:

Thus the uncertainty in position must be greater than half of the reduced Compton wavelength *ħ*/*mc*.

The Compton wavelength can be contrasted with the de Broglie wavelength, which depends on the momentum of a particle and determines the cutoff between particle and wave behavior in quantum mechanics. Notably, de Broglie's derivation of the de Broglie wavelength is based on the assumption that an observed particle is associated with a periodic phenomenon of the particle's Compton frequency.

Typical atomic lengths, wave numbers, and areas in physics can be related to the reduced Compton wavelength for the electron () and the electromagnetic fine structure constant ().

The Bohr radius is related to the Compton wavelength by:

The classical electron radius is about 3 times larger than the proton radius, and is written:

The Rydberg constant, having dimensions of linear wavenumber, is written:

This yields the sequence:

- .

For fermions, the reduced Compton wavelength sets the cross-section of interactions. For example, the cross-section for Thomson scattering of a photon from an electron is equal to^{[ clarification needed ]}

which is roughly the same as the cross-sectional area of an iron-56 nucleus. For gauge bosons, the Compton wavelength sets the effective range of the Yukawa interaction: since the photon has no mass, electromagnetism has infinite range.

The Planck mass is the order of mass for which the Compton wavelength and the Schwarzschild radius are the same, when their value is close to the Planck length (). The Schwarzschild radius is proportional to the mass, whereas the Compton wavelength is proportional to the inverse of the mass. The Planck mass and length are defined by:

In quantum mechanics, the **particle in a box** model describes a particle free to move in a small space surrounded by impenetrable barriers. The model is mainly used as a hypothetical example to illustrate the differences between classical and quantum systems. In classical systems, for example, a particle trapped inside a large box can move at any speed within the box and it is no more likely to be found at one position than another. However, when the well becomes very narrow, quantum effects become important. The particle may only occupy certain positive energy levels. Likewise, it can never have zero energy, meaning that the particle can never "sit still". Additionally, it is more likely to be found at certain positions than at others, depending on its energy level. The particle may never be detected at certain positions, known as spatial nodes.

In particle physics, the **Dirac equation** is a relativistic wave equation derived by British physicist Paul Dirac in 1928. In its free form, or including electromagnetic interactions, it describes all spin-½ massive particles such as electrons and quarks for which parity is a symmetry. It is consistent with both the principles of quantum mechanics and the theory of special relativity, and was the first theory to account fully for special relativity in the context of quantum mechanics. It was validated by accounting for the fine details of the hydrogen spectrum in a completely rigorous way.

The **Schrödinger equation** is a linear partial differential equation that governs the wave function of a quantum-mechanical system. It is a key result in quantum mechanics, and its discovery was a significant landmark in the development of the subject. The equation is named after Erwin Schrödinger, who postulated the equation in 1925, and published it in 1926, forming the basis for the work that resulted in his Nobel Prize in Physics in 1933.

The **Bohr radius** (*a*_{0}) is a physical constant, equal to the most probable distance between the nucleus and the electron in a hydrogen atom in its ground state. It is named after Niels Bohr, due to its role in the Bohr model of an atom. Its value is 5.29177210903(80)×10^{−11} m.

In spectroscopy, the **Rydberg constant**, symbol for heavy atoms or for hydrogen, named after the Swedish physicist Johannes Rydberg, is a physical constant relating to the electromagnetic spectra of an atom. The constant first arose as an empirical fitting parameter in the Rydberg formula for the hydrogen spectral series, but Niels Bohr later showed that its value could be calculated from more fundamental constants via his Bohr model. As of 2018, and electron spin *g*-factor are the most accurately measured physical constants.

In physics, a **coupling constant** or **gauge coupling parameter**, is a number that determines the strength of the force exerted in an interaction. Originally, the coupling constant related the force acting between two static bodies to the "charges" of the bodies divided by the distance squared, , between the bodies: for Newton's gravity and for electrostatic. This description remains valid in modern physics for linear theories with static bodies and massless force carriers.

In quantum physics, a **bound state** is a quantum state of a particle subject to a potential such that the particle has a tendency to remain localized in one or more regions of space. The potential may be external or it may be the result of the presence of another particle; in the latter case, one can equivalently define a bound state as a state representing two or more particles whose interaction energy exceeds the total energy of each separate particle. One consequence is that, given a potential vanishing at infinity, negative-energy states must be bound. In general, the energy spectrum of the set of bound states is discrete, unlike free particles, which have a continuous spectrum.

**Creation and annihilation operators** are mathematical operators that have widespread applications in quantum mechanics, notably in the study of quantum harmonic oscillators and many-particle systems. An annihilation operator lowers the number of particles in a given state by one. A creation operator increases the number of particles in a given state by one, and it is the adjoint of the annihilation operator. In many subfields of physics and chemistry, the use of these operators instead of wavefunctions is known as second quantization.

In quantum field theory, and specifically quantum electrodynamics, **vacuum polarization** describes a process in which a background electromagnetic field produces virtual electron–positron pairs that change the distribution of charges and currents that generated the original electromagnetic field. It is also sometimes referred to as the **self-energy** of the gauge boson (photon).

In quantum mechanics, the **Hellmann–Feynman theorem** relates the derivative of the total energy with respect to a parameter, to the expectation value of the derivative of the Hamiltonian with respect to that same parameter. According to the theorem, once the spatial distribution of the electrons has been determined by solving the Schrödinger equation, all the forces in the system can be calculated using classical electrostatics.

The **finite potential well** is a concept from quantum mechanics. It is an extension of the infinite potential well, in which a particle is confined to a "box", but one which has finite potential "walls". Unlike the infinite potential well, there is a probability associated with the particle being found outside the box. The quantum mechanical interpretation is unlike the classical interpretation, where if the total energy of the particle is less than the potential energy barrier of the walls it cannot be found outside the box. In the quantum interpretation, there is a non-zero probability of the particle being outside the box even when the energy of the particle is less than the potential energy barrier of the walls.

In quantum mechanics the **delta potential** is a potential well mathematically described by the Dirac delta function - a generalized function. Qualitatively, it corresponds to a potential which is zero everywhere, except at a single point, where it takes an infinite value. This can be used to simulate situations where a particle is free to move in two regions of space with a barrier between the two regions. For example, an electron can move almost freely in a conducting material, but if two conducting surfaces are put close together, the interface between them acts as a barrier for the electron that can be approximated by a delta potential.

**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. Equivalently, a photon can be described as having horizontal or vertical linear polarization, or a superposition of the two.

The **theoretical and experimental justification for the Schrödinger equation** motivates the discovery of the Schrödinger equation, the equation that describes the dynamics of nonrelativistic particles. The motivation uses photons, which are relativistic particles with dynamics described by Maxwell's equations, as an analogue for all types of particles.

The **Gamow factor** or **Gamow–Sommerfeld factor**, named after its discoverer George Gamow, is a probability factor for two nuclear particles' chance of overcoming the Coulomb barrier in order to undergo nuclear reactions, for example in nuclear fusion. By classical physics, there is almost no possibility for protons to fuse by crossing each other's Coulomb barrier at temperatures commonly observed to cause fusion, such as those found in the sun. When George Gamow instead applied quantum mechanics to the problem, he found that there was a significant chance for the fusion due to tunneling.

The **Kapitza–Dirac effect** is a quantum mechanical effect consisting of the diffraction of matter by a standing wave of light. The effect was first predicted as the diffraction of electrons from a standing wave of light by Paul Dirac and Pyotr Kapitsa in 1933. The effect relies on the wave–particle duality of matter as stated by the de Broglie hypothesis in 1924.

The **Planck constant**, or **Planck's constant**, is the quantum of electromagnetic action that relates a photon's energy to its frequency. The Planck constant multiplied by a photon's frequency is equal to a photon's energy. The Planck constant is a fundamental physical constant denoted as , and of fundamental importance in quantum mechanics. In metrology it is used to define the kilogram in SI units.

An LC circuit can be quantized using the same methods as for the quantum harmonic oscillator. An **LC circuit** is a variety of resonant circuit, and consists of an inductor, represented by the letter L, and a capacitor, represented by the letter C. When connected together, an electric current can alternate between them at the circuit's resonant frequency:

The **Monte Carlo method for electron transport ** is a semiclassical Monte Carlo(MC) approach of modeling semiconductor transport. Assuming the carrier motion consists of free flights interrupted by scattering mechanisms, a computer is utilized to simulate the trajectories of particles as they move across the device under the influence of an electric field using classical mechanics. The scattering events and the duration of particle flight is determined through the use of random numbers.

In **quantum mechanics**, **universality** is the observation that there are properties for a large class of systems that are independent of the exact structural details of the system. The notion of universality is familiar in the study and application of statistical mechanics to various physical systems since its introduction in a very precise fashion by Leo Kadanoff. Although not quite the same, it is closely related to universality as applied to quantum systems. This concept links to the essence of renormalization and scaling in many problems. Renormalization is based on the notion that a measurement device of wavelength is insensitive to details of structure at distances much smaller than . An important consequence of universality is that one can mimic the *real* short-structure distance of the measurement device and the system to be measured by *simple* short-distance structure. Even though it is seen that scaling, universality and renormalization are closely related, they are not to be used interchangeably.

- ↑ CODATA 2018 value for Compton wavelength for the electron from NIST
- ↑ Greiner, W.,
*Relativistic Quantum Mechanics: Wave Equations*(Berlin/Heidelberg: Springer, 1990), pp. 18–22. - ↑ Garay, Luis J. (1995). "Quantum Gravity And Minimum Length".
*International Journal of Modern Physics A*.**10**(2): 145–65. arXiv: gr-qc/9403008 . Bibcode:1995IJMPA..10..145G. doi:10.1142/S0217751X95000085.

- Length Scales in Physics: the Compton Wavelength
- B.G. Sidharth, Planck scale to Compton scale, International Institute for Applicable Mathematics, Hyderabad (India) & Udine (Italy), Aug 2006.

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