public Vector3f getGravity(Vector3f gravity) {
        javax.vecmath.Vector3f tempVec = new javax.vecmath.Vector3f();
        dynamicsWorld.getGravity(tempVec);
        return Converter.convert(tempVec, gravity);
    }
Gets the gravity of the PhysicsSpace

@param gravity storage for the result (modified if not null)
@return the acceleration vector (in physics-space coordinates, either
gravity or a new instance)
jme3-jbullet/src/main/java/com/jme3/bullet/PhysicsSpace.java