Binding energy — the mass that glue costs

Weigh a nucleus and it comes up LIGHT: the missing mass is the binding energy, E = mc². Sealed: the deuteron (one proton + one neutron) is short 2.224 MeV — the first rung of the strong force — and helium-4 binds at 7.074 MeV per nucleon (in-domain): https://narrowhighway.com/s/8a5e89ab13bc499f39240c64360d90c34a65a291c91c73fcac4fc976131b17c5 . The curve of binding per nucleon rises to IRON-56 and falls after — which is the single fact that shapes the universe's energy economy: fusing light nuclei releases energy, splitting heavy ones releases energy, and iron is the ash of both fires. The curve's shape is measured (every mass in the CODATA/AME tables); its explanation is the strong force.