# A Maxwell Universe – 6. Energy ## Energy Density Within the Node, subnodes interact. These interactions are the only primitives. When awareness notices that one transition reliably precedes another, we call the first a “cause” and the second an “effect.” Nothing is “in space.” Nothing “ticks” in time. Only relational influence exists. The speed at which a cause produces an effect is what we call the speed of light. Nothing can exceed it because it is a relational bound, not a moving object’s velocity. As a preview for later chapters, this fundamental cause–effect speed $c_0$ has no requirement to be uniform everywhere [^FundamentalC]. Things may happen faster or slower in different regions of the Node. Maxwell’s equations already show that wave speed depends on energy density, as in a dielectric. Further details come later. [^FundamentalC]: If certain subnodes behave according to Maxwell’s equations, the effective propagation speed depends on local energy density. Written plainly, this is unsurprising: energy alters how fast things happen.