Why do my hair particles disappear when physics is set to None

Question

As detailed in this question, I'm trying to create a scene with a grassy patch of dirt and I'm having trouble controlling scale and direction.

One of the bizarre relationships I discovered is that the "timestep" setting in the "Physics" tab will change the scale of the rendered particles. So since I'm not currently using physics, I tried turning physics off by setting physics to "None".

And then all my particles simply disappeared. They come back when I set physics back to "Newtonian".

Same problem as in this question, but the only answer given says that physics are needed for hair (then why is there a "None" option?!) and that one should use a "regular particle system" instead of hair. I'm not sure what a "regular particle system" is. Are there two types of particle systems in Blender? I know there is instancing available, but as far as I can tell I can't randomize things well using that.

Can someone explain a) why the timestep affects the particle's scale and b) how I avoid that interaction if I can't set physics to "None"?

Answer 1

Different particle systems: A "regular particle system" is of type Emitter and spawns particles which hava a size, different modes of display (object instancing etc) and move according to the selected form of physics simulation.

A Hair particle system is not identical to that, since Hair particles require a different set of information; they have a root connecting them to the emitter and are path objects, which can be physically simulated as well - this is what the Hair Dynamics option is for. As long as you don't enable Hair Dynamics, your particles shouldn't move or collide, regardless of the selected Physics option.

Hair particles require a velocity. Tweaking it will result in a different size, which should correlate to your observation regarding timesteps: changing the speed by which time passes changes all velocities. A hair particle with a velocity of zero has a length of zero, so setting the Physics type to "None" is a bad idea; it means we are creating particles without speed and therefor without length.