<P> The number enclosed in parentheses is the measurement uncertainty on the last two digits . The value of μ is known to about 0.1 parts per billion . </P> <P> μ is an important fundamental physical constant because: </P> <Ul> <Li> Nearly all of science deals with baryonic matter and how the fundamental interactions affect such matter . Baryonic matter consists of quarks and particles made from quarks, like protons and neutrons . Free neutrons have a half life of 613.9 seconds . Electrons and protons appear to be stable, to the best of current knowledge . (Theories of proton decay predict that the proton has a half life on the order of at least 10 years . To date, there is no experimental evidence of proton decay .); </Li> <Li> The proton is the most important baryon, while the electron is the most important lepton; </Li> <Li> μ and the fine structure constant α are the two dimensionless quantities emerging in elementary physics, and two of the three dimensionless quantities discussed in Barrow (2002); </Li> <Li> The proton mass m is composed primarily of gluons, and of the quarks (the up quark and down quark) making up the proton . Hence m, and therefore the ratio μ, are easily measurable consequences of the strong force . In fact, in the chiral limit, m is proportional to the QCD energy scale, Λ . At a given energy scale, the strong coupling constant α is related to the QCD scale (and thus μ) as </Li> </Ul> <Li> Nearly all of science deals with baryonic matter and how the fundamental interactions affect such matter . Baryonic matter consists of quarks and particles made from quarks, like protons and neutrons . Free neutrons have a half life of 613.9 seconds . Electrons and protons appear to be stable, to the best of current knowledge . (Theories of proton decay predict that the proton has a half life on the order of at least 10 years . To date, there is no experimental evidence of proton decay .); </Li>

The mass of an electron is about equal to the mass of a proton