Neutron Mass Energy

The neutron's rest energy, 939.565 MeV — a sliver above the proton's, and that sliver is fate.

Specifications & Metrics

symbol

mnc²

status

Measured — CODATA 2022 adjustment

value

939.56542194 ± 4.8 × 10⁻⁷ MeV

Relative Scale in Constants

Core Principles & Analysis

• Exceeding the proton's by 1.293 MeV makes free neutrons beta-decay in about 15 minutes.
• Inside nuclei the balance reverses, which is why bound neutrons survive.
• That 0.14% difference decides whether the universe has neutrons left today.