My friend, who has no scientific training but is otherwise pretty intelligent, texted me with:

"Why is it when gasses collide they don't slow down at all? Shouldn't they eventually slow to the point that they become liquids?"

I texted back saying that the only way they could slow down was if the kinetic energy was converted into some other form of energy, but I couldn't give a coherent explanation of why that doesn't ever happen - could anyone here help explain it?

In the ideal case the collisions are elastic, so there is no possibility to lose energy other than by colliding with the walls of a container. But the container has energy, too, so if it's maintained at the same temperature nothing will happen, overall.

 

In reality gases radiate electromagnetic energy when they collide, or can cause excitations which then release EM energy, but they absorb it, too, and will eventually reach equilibrium with whatever the background level is. A gas sample in deep space would eventually reach 2.7K, regardless of what state of matter that represented.

Thanks. So would it be fair to say, the particles can loose kinetic energy but are just as likely to pick it up again?