In the limit of low Reynolds number, the mobility μ is the inverse of the drag coefficient ζ ζ . A damping constant γ γ = ζ ζ / m is frequently used for the inverse momentum relaxation time (time needed for the inertia momentum to become negligible compared to the random momenta) of the diffusive object. For spherical particles of radius r, Stokes' law gives ζ ζ = 6 π π η η r , where η η is the viscosity of the medium. Thus the Einstein–Smoluchowski relation results into the Stokes–Einstein–Sutherland relation D = k B T 6 π π η η r . This has been applied for many years to estimating the self-diffusion coefficient in liquids, and a version consistent with isomorph theory has been confirmed by computer simulations of the Lennard-Jones system.