For a monolayer sheet to migrate cohesively, it has long been suspected that each constituent cell must exert physical forces not only upon its extracellular matrix but also upon neighboring cells. I will present the first comprehensive maps of these distinct force components. These maps reveal an unexpectedly rich physical picture in which the distribution of physical forces is dominated by heterogeneity, both in space and in time, which emerges spontaneously, propagates over great distances, and cooperates over the span of many cell bodies. Both in epithelial and endothelial cell sheets, these heterogeneous forces are mechanically linked to cell velocities through a newly discovered emergent mechanism of innately collective cell guidance - plithotaxis.