An uncontained gas would never cool

[EastUmpqua]

@MikeS

“Adiabatically” usually means that the temperature does change. Specifically, the gas doesn’t absorb any heat from its environment, which means that if it expands (and does work on its surroundings), that energy must come from the internal thermal energy of the gas, and thus the temperature decreases.

In thermodynamics, an adiabatic process is one that occurs without transfer of heat or mass of substances between a thermodynamic system and its surroundings. In an adiabatic process, energy is transferred to the surroundings only as work.

[EastUmpqua]

@rat_avatar

There is no friction, that was disproved by experiment and was the basis of the superseded luminous ether theory of light.

If there was “friction” the speed of light wouldn’t be the same for all observers, which would mean that there is some frame that is more privileged than others.

I still don’t get why ‘friction’ would slow down a photon instead of just changing its energy.

[Chronos]

If we’re talking about photons, then yes, you could (in principle) have some sort of phenomenon which redshifted them while still being frame-invariant (though no good evidence has ever been found for this). But for massive particles, it wouldn’t work.