Figure 10.8 — Force measurement from drift velocity Figure 10.8 — Force measurement from drift velocity. Schematic view of the propagator ρ(Δz; z0, Δt), which gives the distribution of the particle position increments Δz from its initial position z0 after a time step Δt. The distribution is Gaussian for sufficiently small t with standard deviation (2D(z0)Δt)0.5. The force can be estimated from the measured drift according to Eq. (10.29).