For me, the difference is whether these things a) have an external force being applied to them, or b) are slowing down and are only going on "forever" because t

For me, the difference is whether these things a) have an external force being applied to them, or b) are slowing down and are only going on "forever" because the rest of the universe will collapse before they come to a standstill. For example, even ignoring the fact that the sun is going to explode before the universe does, the earth's rotation around the sun is gradually getting slower and slower. If the sun, and the universe, did last forever then the earth would eventually come to a halt. In the case of these crystals, my impression is that even in that hypothetical never-ending universe, the movement in these crystals would never slow down.