Invariance principle is proved for a random Lorentz-gas particle in 3 dimensions
under the Boltzmann-Grad limit and simultaneous diffusive scaling. That is, for the
trajectory of a point-like particle moving among infinite-mass, hard-core, spherical
scatterers of radius $r$, placed according to a Poisson point process of density
$\varrho$, in the limit $\varrho \to \infty$, $r \to 0$,...