It is shown that the same phenomenological Newtonian model recently proposed , which accounts for the cosmological evolution of the fine structure constant , suggests furthermore an explanation of the unmodelled acceleration a _ { P } \simeq 8.5 \times 10 ^ { -10 } \mbox { m / s } ^ { 2 } of the Pioneer 10/11 spaceships reported by Anderson et al in 1998 .
In the view presented here , the permittivity and permeability of empty space are decreasing adiabatically , and the light is accelerating therefore , as a consequence of the progressive attenuation of the quantum vacuum due to the combined effect of its gravitational interaction with all the expanding universe and the fourth Heisenberg relation .
It is argued that the spaceships might not have any extra acceleration ( but would follow instead the unchanged Newton laws ) , the observed effect being due to an adiabatic acceleration of the light equal to a _ { P } , which has the same observational radio signature as the anomalous acceleration of the Pioneers .