from wikipedia - "The Bohr radius is a physical constant, approximately equal to the most probable distance between the proton and electron in a hydrogen atom in its ground state." in planck units bohr radius (a0) = (mp/me).(1/[itex]\alpha[/itex]).(lp) the 1st 2 terms of the RHS of the equation are dimensionless. now , there are 2 different inertial observers one at rest and the other moving at some uniform velocity relative a hydrogen atom whose radius they are measuring.... would or wouldn't the 2 measurements differ ? if they differ, wouldn't it mean a change in the term '(lp)' of the above equation ,since the other terms of the RHS of the equation are dimensionless? if they don't, why? it has the dimensions of length after all? does this mean that an absolute length scale is possible?