public void startPhysics() {
        if (isRunning) {
            return;
        }

        switch (threadingType) {
            case PARALLEL:
                boolean success = startPhysicsOnExecutor();
                assert success;
                assert pSpace != null;
                break;

            case SEQUENTIAL:
                pSpace = new PhysicsSpace(worldMin, worldMax, broadphaseType);
                pSpace.addTickListener(this);
                break;

            default:
                throw new IllegalStateException(threadingType.toString());
        }

        isRunning = true;
    }
Allocate a physics space and start physics.
<p>
Physics starts automatically after the state is attached. To start it
sooner, invoke this method.
jme3-jbullet/src/main/java/com/jme3/bullet/BulletAppState.java