4 ms·

why are there generations at all? why 3?

• ncmncm 5y ago
  Might we just not wield enough energy to evoke a 4th generation?
  • PaulHoule 5y ago
    https://en.wikipedia.org/wiki/Generation_(particle_physics)#Fourth_generation https://en.wikipedia.org/wiki/Generation_(particle_physics)#...
• frutiger 5y ago
  No one knows. For what it's worth, we thought there were only 2 generations until well into the 20th century.
• evanb 5y ago
  We know why SM particles come in "whole generations": a mathematical consistency condition known as anomaly cancellation [0]. We also know that with only 2 generations we cannot get violation of charge-parity symmetry [1] via the Cabibo angle [2] alone; to get CP violation we need at least a 3x3 CKM matrix [2]. CP violation is one of the Sakharov conditions [3] that is required to get matter/antimatter asymmetry from early universe dynamics. This is not an a priori reason, per se, but it does waggle its eyebrows up and down in a sort of anthropic way. [0] https://en.wikipedia.org/wiki/Anomaly_(physics)#Anomaly_cancellation https://en.wikipedia.org/wiki/Anomaly_(physics)#Anomaly_canc... [1] https://en.wikipedia.org/wiki/CP_violation https://en.wikipedia.org/wiki/CP_violation [2] https://en.wikipedia.org/wiki/Cabibbo%E2%80%93Kobayashi%E2%80%93Maskawa_matrix https://en.wikipedia.org/wiki/Cabibbo%E2%80%93Kobayashi%E2%8... [3] https://en.wikipedia.org/wiki/Sakharov_conditions https://en.wikipedia.org/wiki/Sakharov_conditions