Compton wavelength

The Compton wavelength is a quantum mechanical property of a particle. It was introduced by Arthur Compton in his explanation of the scattering of photons by electrons (a process known as Compton scattering). The Compton wavelength of a particle is equivalent to the wavelength of a photon whose energy is the same as the rest-mass energy of the particle.

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

$\lambda = \frac{h}{m c} \$

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 2010 value for the Compton wavelength of the electron is 2.4263102389(16)×10⁻¹² m. Other particles have different Compton wavelengths.

Reduced Compton wavelength

When the Compton wavelength is divided by ${2 \pi}$, one obtains a smaller or “reduced” Compton wavelength ${\bar \lambda}$:

${\bar \lambda} = \frac{\lambda}{2 \pi} = \frac{\hbar}{m c}$

Role in equations for massive particles

The relation between properties of mass and their associated physical constants. Every massive object is believed to exhibit all five properties. However, due to extremely large or extremely small constants, it is generally impossible to verify more than two or three properties for any object.

• The Schwarzschild radius (r_s) represents the ability of mass to cause curvature in space and time.
• The standard gravitational parameter (μ) represents the ability of a massive body to exert Newtonian gravitational forces on other bodies.
• Inertial mass (m) represents the Newtonian response of mass to forces.
• Rest energy (E₀) represents the ability of mass to be converted into other forms of energy.
• The Compton wavelength (λ) represents the quantum response of mass to local geometry.

The 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:

$\mathbf{\nabla}^2\psi-\frac{1}{c^2}\frac{\partial^2}{\partial t^2}\psi = \left(\frac{m c}{\hbar} \right)^2 \psi.$

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

$-i \gamma^\mu \partial_\mu \psi + \left( \frac{m c}{\hbar} \right) \psi = 0.$

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:

$i\hbar\frac{\partial}{\partial t}\psi=-\frac{\hbar^2}{2m}\nabla^2\psi -\frac{1}{4 \pi \epsilon_0} \frac{Ze^2}{r} \psi.$

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

$\frac{i}{c}\frac{\partial}{\partial t}\psi=-\frac{1}{2} \left(\frac{\hbar}{m c} \right) \nabla^2\psi - \frac{\alpha Z}{r} \psi.$

Relationship between the reduced and non-reduced Compton wavelength

The reduced Compton wavelength is a natural representation for mass on the quantum scale. Equations that pertain to mass in the form of mass, like Klein-Gordon and Schrödinger's, use the reduced Compton wavelength. 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 rest mass m has a rest energy of E = mc². 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

$E = h f = \frac{h c}{\lambda} = m c^2 \$

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

Limitation on measurement

The Compton wavelength expresses a fundamental limitation on measuring the position of a particle, taking into account quantum mechanics and special relativity.

This limitation depends on the rest 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 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. 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.

We can make the above argument a bit more precise as follows. Suppose we wish to measure the position of a particle to within an accuracy Δx. Then the uncertainty relation for position and momentum says that

$\Delta x\,\Delta p\ge \frac{\hbar}{2},$

so the uncertainty in the particle's momentum satisfies

$\Delta p \ge \frac{\hbar}{2\Delta x}.$

Using the relativistic relation between momentum and energy E² = (pc)² + (mc²)², when Δp exceeds mc then the uncertainty in energy is greater than mc², which is enough energy to create another particle of the same type. But we must exclude this. It follows that there is a fundamental limitation on Δx:

$\Delta x \ge \frac{1}{2} \left(\frac{\hbar}{mc} \right).$

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.

Relationship to other constants

Typical atomic lengths, wave numbers, and areas in physics can be related to the reduced Compton wavelength for the electron ($\bar{\lambda}_e \equiv \tfrac{\lambda_e}{2\pi}\simeq 386~\textrm{fm}$) and the electromagnetic fine structure constant ($\alpha\simeq\tfrac{1}{137}$)

The Bohr radius is related to the Compton wavelength by:

$a_0 = \frac{1}{\alpha}\left(\frac{\lambda_e}{2\pi}\right)\simeq 137\times\bar{\lambda}_e\simeq 5.29\times 10^4~\textrm{fm}$

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

$r_e = \alpha\left(\frac{\lambda_e}{2\pi}\right)\simeq\frac{\bar{\lambda}_e}{137}\simeq 2.82~\textrm{fm}$

The Rydberg constant is written:

$R_\infty=\frac{\alpha^2}{2\lambda_e}$

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

$\sigma_T = \frac{8\pi}{3}\alpha^2\bar{\lambda}_e^2 \simeq 66.5~\textrm{fm}^2$

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 rest mass, electromagnetism has infinite range.

Typical lengths and areas in gravitational physics can be related to the Compton wavelength and the gravitational coupling constant ($\alpha_G$ which is the gravitational analog of the fine structure constant):

The Planck mass is special because the Compton wavelength and the Schwarzschild radius for this mass are equal. Their common value is called the Planck length ($\ell_P$). This is a simple case of dimensional analysis: the Schwarzschild radius is proportional to the mass, whereas the Compton wavelength is proportional to the inverse of the mass. The Planck length is written:

$\ell_P=\lambda_e\,\frac{\sqrt{\alpha_G}}{2\pi}$

External links

• Length Scales in Physics: the Compton Wavelengthde:Compton-Effekt#Compton-Wellenlänge