this is relativistic length contraction by the looks of it ... there's a term which goes something like gamma = 1/{root[1-v^2/c^2]} which is applied to one of the lengths (the one in the rest frame, possibly) to obtain the relativistic contraction in the moving frame. If it were lmoving=lrestxgamma then 0.5=gammax1 so gamma = 0.5 and then there's a load of algebra.
Problem is I can't remember the way round these things going off the top of my head ...
