The book is built on three axioms, each experimentally confirmed: the constancy of the speed of light, the equivalence principle, and the Past Hypothesis. From these it derives the block universe, time dilation, and the relativity of simultaneity, then asks the questions the physics forces: what is choice when the future already exists at its coordinates? What is death when no moment is privileged? Why do we experience flow when the geometry is static?

The diagrams are the arguments. Every chapter derives its conclusion from a picture the reader can verify, and the second half of the book lives inside the conclusion rather than flinching from it. The structure does not care whether you are kind to the people whose worldlines run alongside yours. The caring is the only warmth the geometry contains. We are each other’s refuge.

Worldlines is the first volume of The Indifference Suite. Its companion is Multitudes: The Indifference of Measure (Vol II), which takes quantum mechanics literally the way this book takes relativity literally. The suite’s fiction is the novel Measure.

Related: Epilogue: measure and meaning, the closing post of the quantum-from-scratch series, discusses this book alongside its companions.

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