Section 18 — The Second Law and the Wound-Up Universe

The Christian Statement

The Second Law of Thermodynamics states that in any closed system, entropy — disorder — increases over time. Energy becomes less available for work. Order decreases. The universe is, in this sense, running down.

This has a simple but profound implication: the universe cannot have existed eternally. An eternally existing universe would have reached maximum entropy — heat death, complete disorder, no available energy — an infinite time ago. The fact that the universe has not reached heat death, the fact that stars still burn and structure still exists, means the universe had a beginning. It was wound up. Something wound it.

This converges with the cosmological argument from a different direction. The universe is not a perpetual motion machine. It is a system running down from an initial state of extraordinarily low entropy. That initial state — the remarkably ordered, low-entropy condition at the Big Bang — requires explanation. Physicist Roger Penrose has calculated that the probability of the universe beginning in its actual low-entropy state is 1 in 10 to the power of 10 to the power of 123 — a number so large that if you wrote a zero on every particle in the observable universe, you would not have enough zeros to write it.

✖ Atheist: Maybe the universe has always existed in some form, cycling through expansions and contractions.

✔ How to respond: Cyclic models of the universe are interesting but face the same problem. Each cycle, on the standard models, adds entropy. Run the cycles backward far enough and you reach a cycle of zero entropy — which is the Big Bang under a different name. The thermodynamic argument against an eternal universe applies to cyclic models as well. And even if a cyclic model worked, it simply relocates the question: what is the system within which the cycles occur, and what caused that system?