>a big innovation of HL2 was the extensive use of a real physics engine. The door and the guard are both physical objects, both have momentum, they impart an impulse on each other, and although the door hinge is frictionless, the guard's boots have some amount of friction with the floor.

It's been a while since I've played HL2 but this isn't exactly how I remember it. While a lot of things were physics objects I thought the doors would just smoothly rotate towards their target position without any physics at all. You can't bump them shut with another physics object for instance.

You can't move them (apart from the opening and closing animation), but they can move other objects that are in their way. Both need to be physics objects for that to work, even though the door is just kinematic (i.e. it won't react to forces applied to it). Although if I remember correctly, they are not even fully kinematic. I think you could get them stuck halfway closed by cramming something in the door frame that would get the whole thing jammed.

Just did some quick testing - the doors definitely have physics and can get stuck on objects and can impart forces. But unimpeded yes, they smoothly open/close.

I stuck a tire in a door frame and tried to close it, the tire emitted a bunch of dust clouds as the two objects fought before the door finally ejected the tire at high speed.