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Why Neutrinos Aren't Part Of Ordinary Matter
- Neutrinos are fundamental leptons that rarely form normal matter because they only feel the weak force and gravity.
- Their weak interactions make them effectively invisible to everyday matter despite being abundant across the universe.
Supernova Timing Proves Neutrino Mass
- We know neutrinos have mass because higher-energy neutrinos from SN1987A arrived earlier than lower-energy ones.
- Massless particles travel at light speed, so a spread in neutrino velocities proves nonzero mass.
Big Bang Sets Tight Upper Limit
- Cosmological data constrains the sum of neutrino masses by how neutrinos affected early-universe oscillations and expansion.
- That limit implies the three neutrino masses sum to under ~0.1 eV, far below electron or proton masses.