In our earlier work, we showed that the deep “stuff” beneath Einstein’s geometry behaves like a single, smooth field — more like a frictionless fluid than a collection of particles. That result narrows the possibilities dramatically: if spacetime emerges from one underlying field, the natural next question is what that field actually is. Not every candidate makes sense. Particle count doesn’t work in a universe where particles are routinely created and destroyed. Electric charge only applies to some things, not all light and gravity. Even energy, while essential, is slippery as a label for the field: it depends on your point of view and doesn’t by itself give the arrow from past to future.
What does pass every test is entropy — the measure of how information and disorder spread. Entropy is universal: every physical process, from stars shining to matter falling into black holes, produces or exchanges it. It survives coarse-graining when the fine details are blurred, it gives time a direction (toward more, not less), and it’s a true scalar — the same for every observer. That’s why we identify the hidden field with entropy flow rather than with particles, charge, or energy.
There’s more than philosophy behind that choice. Read thermodynamically, the single conserved “current” in this simple field theory lines up with entropy and with nothing else. The identification also behaves exactly as a real temperature-and-entropy picture should: it matches standard thermodynamic relations, it’s consistent with how near-equilibrium systems behave, and it can be tested — in the patterns of the cosmic microwave background, in the subtle damping of gravitational waves, and in the way black-hole horizons account for their area. In short: after showing the universe’s geometry emerges from one scalar field, we now show that entropy is the only option that fits the bill.