The Compton wavelength is a quantum mechanical property of a particle, defined as the wavelength of a photon whose energy is the same as the rest energy of that particle (see mass–energy equivalence).
It was introduced by Arthur Compton in 1923 in his explanation of the scattering of photons by electrons (a process known as Compton scattering).
The standard Compton wavelength λ of a particle of mass
The reduced Compton wavelength ƛ (barred lambda, denoted below by
) is defined as the Compton wavelength divided by 2π: 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.
[citation needed] 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 is also present in Schrödinger's equation, although this is not readily apparent in traditional representations of the equation.
The following is the traditional representation of Schrödinger's equation for an electron in a hydrogen-like atom:
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 and is used in equations that pertain to inertial mass, such as the Klein–Gordon and Schrödinger's equations.
which yields the 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.
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 mc2, 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.
Suppose we wish to measure the position of a particle to within an accuracy Δx.
In particular the minimum uncertainty is when the scattered photon has limit energy equal to the incident observing energy.
Thus the uncertainty in position must be greater than half of the reduced Compton wavelength ħ/mc.
Typical atomic lengths, wave numbers, and areas in physics can be related to the reduced Compton wavelength for the electron (
The Bohr radius is related to the Compton wavelength by:
The Rydberg constant, having dimensions of linear wavenumber, is written:
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]
For gauge bosons, the Compton wavelength sets the effective range of the Yukawa interaction: since the photon has no mass, electromagnetism has infinite range.
A geometrical origin of the Compton wavelength has been demonstrated using semiclassical equations describing the motion of a wavepacket.
[4] In this case, the Compton wavelength is equal to the square root of the quantum metric, a metric describing the quantum space: