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Comment by bsedlm | Hacker News Reader

why is the absolute overwhelming majority of stuff in the universe (and 100% of things outside of special lab settings) made up from 1st generation matter?

why is it that all stuff made from charm, strange, top, and bottom, quarks decays right away?

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> why is the absolute overwhelming majority of stuff in the universe (and 100% of things outside of special lab settings) made up from 1st generation matter?

The first generation have the lowest mass, and there are interactions between generations. Since physical systems like to explore the local energy space and find the lowest one, it follows that more energetic systems will quickly stabilize to lower energy configurations.

You might rightfully have two follow up questions at this point:

1. Why do the first generation fermions have the lowest mass?

2. Why do physical systems like to find the lowest energy configuration?

I don't believe anyone has the answers to those - so far as we can tell this is just the way the universe is. Maybe some day someone will figure out why.

Does thermodynamics not suffice for (2)? Or does that just restate the question, somehow?

I learned many things about the SM from this presentation I had never before retained.

Wondering now to what degree string theory seems to exactly require all of this, vs. merely be apparently compatible with it, insofar as it can be "solved" at all. Where it is too hard, maybe it is not known whether certain SM features are compatible, and everyone just hopes?

By thermodynamics: energy is conserved globally so when a protein molecule relaxes to a minimum energy configuration the energy gets transferred from the protein molecule to its environment. Total energy stays the same but it is more spread out. There are dramatically more possible ways the energy could be spread out than be bunched together: so if it starts out in a bunched out state it will spread out unless there is something, usually a "conserved quantity" or quantum number that makes it impossible for the state to roam randomly across the phase space.

The first generation ones don't come with a little number "!" on them. They are "first generation" because they're the ones we found first. Why did we find them first? Because they're the most common ones. And why are they the most common? Because they're the ones with lowest mass.

I mean, you could still ask why the different generations have different masses at all, but if they do, it follows that the "first generation" has the lowest mass.

Absolutely. “First” is a synonym for “lowest energy” given that our civilisation (our lineage?) has explored lower energy regimes before higher ones.

PS. Q. Is that a tautology? A. Insufficient data points for a conclusive answer.

Indeed, 1. is a mystery. But, 2. is a consequence of 2nd law of thermo. Specifically, the lighter the mass of a decay product, the larger the number of possible final states in the kinematical phase space (in general). For instance, if there is just enough energy to make a pair of particles, then only one final state is possible: particles sitting at rest. On the other hand if energy is “left over” in addition to rest energy, then there are many possible final states corresponding to the pair flying outward in different directions. Hence, the decay to the lighter products are entropically preferred. This is mathematically encapsulated in Fermi’s Golden Rule.

why are there generations at all? why 3?

Might we just not wield enough energy to evoke a 4th generation?

https://en.wikipedia.org/wiki/Generation_(particle_physics)#...

No one knows. For what it's worth, we thought there were only 2 generations until well into the 20th century.

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_canc...

[1] https://en.wikipedia.org/wiki/CP_violation

[2] https://en.wikipedia.org/wiki/Cabibbo%E2%80%93Kobayashi%E2%8...

[3] https://en.wikipedia.org/wiki/Sakharov_conditions

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