Physics of the Common Sense Lemming World

The real physics of the lemming world is embodied in the Psygnosis program for playing the machine side of lemming games. Presumably there is a general lemming mover, and the designers of particular games build structures with pictures glued onto them. Let's call it the lemming interpreter, since it also interprets the actions of players. These actions consist almost entirely of designating particular lemmings as having powers selected from the list of 9 possibilities.

However, the human player doesn't know the lemming interpreter, and it wouldn't do him much good if he did. What the human knows is the common sense physics of the lemming world. Our goal is to find an epistemologically adequate [McCarthy and Hayes, 1969] way of expressing this physics. Epistemological adequacy requires that the facts that are actually observable be expressible and that the general facts giving the effects of actions be expressed in a way that makes the observations useful.

Here are some aspects of the common sense physics of the lemming world.

1. Lemmings move to the right or left and sometimes climb. I suppose lemmings cannot stand still except for blockers and have only one possible speed. Players use this fact in order to time explosions. If a lemming is triggered while at place x to explode 5 seconds later at place y, triggering at x will always result in an explosion at y.
2. Except for blockers, lemmings don't interact with each other, e.g. they interpenetrate rather than collide.
3. The lemming world has kinematics but apparently not dynamics. Thus the program uses velocities but not forces and accelerations. Our own projections of plans is rarely even kinematic but merely geometric. By non-kinematic I mean that the velocities of the lemmings are not used in projection.
4. Lemmings fall at constant velocity but are destroyed if they fall from too great a height.
5. Bridges are infinitely strong and don't break under the weight of any number of lemmings.
6. Explosions kill only the lemming that has been told to explode. Others are unharmed even if they are within the explosion zone. Structures that seem to have been supported by structure that is destroyed do not collapse but ``hang in the air''.
7. There seems to be no random element in lemming games except that randomness coming from the human player. As is usual in sports, this permits the human to learn precise procedures from repeated plays of a game. Thus he can learn the best place to start a bridge.
8. Lemming time is sometimes important. For example, a digger needs a certain amount of lemming time before the hole is deep enough to trap other lemmings. Before that, a walker falling into the hole will climb on out of it. On a larger scale, the total lemming time available for a game sometimes affects strategy.