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The kilogram, the basic unit of mass, is now defined by a quantum quantity known as the Planck constant. That value, an immutable constant of nature, is the same everywhere in space and time.

The four units would, starting in 2019, be defined by "relationships to fundamental constants, rather than abstract or arbitrary definitions" according to Nature. The problem is one of use.

Aston University. "Living Model Of Basic Units Of Human Brain Created." ScienceDaily. ScienceDaily, 22 March 2009. <www.sciencedaily.com / releases / 2009 / 03 / 090317095326.htm>.