The talk started out on viscosity of fluids, like when a plate is sliding across a fluid spread across a stationary plate. There was a graphic showing how the fluid formed a ‘gradient’ as the top layer of fluid had to move faster to keep up with the top plate, while the bottom fluid did not really want to move, but eventually the top fluid managed to drag it along.
Then this moved into the viscosity of gases, and eventually arrived at Maxwell’s formula for viscosity of dilute gas and how pressure of the gas does not matter. There was a quick aside as the speaker mentioned how Robert Boyle might have conducted an experiment along the same lines, using a pendulum suspended inside of a vacuum, where despite changes in pressure no visible difference in the swing of the pendulum could be detected. Robert Boyle also thought to do other things, like stick a butterfly into the vacuum jar and see what happened…
The reason for the viscosity of dilute gas not being affected by pressure had to do with “mass density” (I think). So even though the gas is more spread out the speeds it is bouncing around at increase. (I think).
This was just the introduction. Then discussion moved onto Quantum Cromo Dynamics, lots of phase diagrams and energy density / c2. And c in this case is something like mass density in the general description of viscosity.