Chapter 27 — The Slight Asymmetry That Permits Matter

The neutron is slightly more massive than the proton by approximately 0.14 percent—a mass difference of 1.293 MeV/c2. This tiny asymmetry is not derived from any deeper physical principle. It is measured. And its precise value is required for the universe to contain matter at all.

If the neutron were lighter than the proton by a comparable margin, free protons would be unstable, decaying into neutrons. The universe would consist of neutron matter and possess virtually no hydrogen—no water, no long-lived main sequence stars, no organic chemistry. If the mass difference were significantly larger in the observed direction, neutrons would be too unstable to survive inside atomic nuclei, and the periodic table of elements beyond hydrogen would not exist. The observed value sits in a narrow window that allows protons to be stable as the nucleus of hydrogen, the most abundant element, while permitting neutrons to exist stably inside heavier nuclei, making all elements beyond hydrogen possible.

In 2015 physicist Szabolcs Borsanyi and colleagues published the first ab initio calculation of the neutron-proton mass difference using lattice quantum chromodynamics—deriving the value from first principles of particle physics rather than measurement alone. Their calculated value matched the measured value to within experimental precision. The mass difference arises from a delicate interplay of quark masses and electromagnetic effects. That interplay produces exactly the asymmetry required for a universe with atoms. That it does so is not explained by any more fundamental principle.

Borsanyi, S. et al. "Ab initio calculation of the neutron-proton mass difference." Science 347, no. 6229 (2015): 1452–1455. Barrow, J.D. and Tipler, F.J. The Anthropic Cosmological Principle. Clarendon Press, 1986.