Fermi Paradox. What am I missing?

[septimus]

@rat_avatar

Even with a container with 2 or 3 molecules the entropy remains the same or goes down. Another way to consider it is that those 2 or 3 molecules will have a greater difference in momentum in the start, but assuming a ideal container without gravitational red-shift or heat transfer those molecules momentum will get closer to each other with each interaction. As there is less difference in their momentum there is less of a differential to do “work”. Thinking of entropy as that loss in differences of energy levels over time also helps.

Suppose an atom of hydrogen and an atom of tritium travelling at the same speed collide head-on; assume perfect elasticity.

The tritium will come to a standstill, while the hydrogen rebounds at twice its initial speed. Correct so far?

We’re not talking about statistical averages involving large numbers of particles and large numbers of collisions. Neither the Law of Large Numbers nor the empirical fact that we see lower entropy in the past is in dispute. The question involves the Arrow of Causality, if any, at the most microscopic level.