Redefinition of SI units, Draft 9th Brochure 16 December 2013
"SI units in the framework of general relativity:
The definitions of the base units of the SI were adopted in a context that takes no account of relativistic effects. When such account is taken, it is clear that the definitions apply only in a small spatial domain sharing the motion of the standards that realise them. These units are known as proper units; they are realised from local experiments in which the relativistic effects that need to be taken into account are those of special relativity. The defining constants are local quantities with their values expressed in proper units.
Physical realisations of the definition of a unit are usually compared locally. For frequency standards, however, it is possible to make such comparisons at a
distance by means of electromagnetic signals. To interpret the results the theory of general relativity is required, since it predicts, among other things, a relative frequency shift between standards of about 1 part in 10 16 per metre of altitude difference at the surface of the earth. Effects of this magnitude cannot be
neglected when comparing the best frequency standards."
Are we now able to measure "length contraction" using light signals?