Later in 1927 Paul Ehrenfest showed that the time, T for a matter wave packet of width Δ Δ x and mass m to spread by a factor of 2 was T ≈ ≈ Δ Δ x m / ℏ ℏ . Since ℏ ℏ is so small, wave packets on the scale of macroscopic objects, with large width and mass, double only at cosmic time scales.