The classical electron radius is a combination of fundamental physical quantities that define a length scale for problems involving an electron interacting with electromagnetic radiation. It links the classical electrostatic self-interaction energy of a homogeneous charge distribution to the electron's relativistic mass-energy. According to modern understanding, the electron is a point particle with a point charge and no spatial extent. Nevertheless, it is useful to define a length that characterizes electron interactions in atomic-scale problems. The classical electron radius is given as
where is the elementary charge, is the electron mass, is the speed of light, and is the permittivity of free space.^{[1]} This numerical value is several times larger than the radius of the proton.
In cgs units, the permittivity factor and do not enter, but the classical electron radius has the same value.
The classical electron radius is sometimes known as the Lorentz radius or the Thomson scattering length. It is one of a trio of related scales of length, the other two being the Bohr radius and the reduced Compton wavelength of the electron ƛ_{e}. Any one of these three length scales can be written in terms of any other using the fine-structure constant :
The classical electron radius length scale can be motivated by considering the energy necessary to assemble an amount of charge into a sphere of a given radius .^{[2]} The electrostatic potential at a distance from a charge is
To bring an additional amount of charge from infinity necessitates putting energy into the system, , by an amount
If the sphere is assumed to have constant charge density, , then
Integrating for from zero to the final radius yields the expression for the total energy , necessary to assemble the total charge into a uniform sphere of radius :
This is called the electrostatic self-energy of the object. The charge is now interpreted as the electron charge, , and the energy is set equal to the relativistic mass–energy of the electron, , and the numerical factor 3/5 is ignored as being specific to the special case of a uniform charge density. The radius is then defined to be the classical electron radius, , and one arrives at the expression given above.
Note that this derivation does not say that is the actual radius of an electron. It only establishes a dimensional link between electrostatic self energy and the mass–energy scale of the electron.
The classical electron radius appears in the classical limit of modern theories as well, including non-relativistic Thomson scattering and the relativistic Klein–Nishina formula. Also, is roughly the length scale at which renormalization becomes important in quantum electrodynamics. That is, at short-enough distances, quantum fluctuations within the vacuum of space surrounding an electron begin to have calculable effects that have measurable consequences in atomic and particle physics.
Based on the assumption of a simple mechanical model, attempts to model the electron as a non-point particle have been described by some as ill-conceived and counter-pedagogic.^{[3]}