Chapter 25 — The Improbability of Order

The Second Law of Thermodynamics states that entropy—disorder—increases in any closed system over time. The universe is a closed system. Its entropy is increasing. But it began at extraordinarily low entropy. The question this raises is not minor: why did it begin ordered?

Roger Penrose of Oxford calculated the probability of the universe's observed low-entropy initial state. He compared the volume of phase space corresponding to our universe's initial conditions to the total available phase-space volume. His result: the probability is 1 in 10 raised to the power 10 raised to the power 123. Writing this number out would require more zeros than there are atoms in the observable universe.

No physical law requires a low-entropy beginning. The laws of physics are consistent with a high-entropy initial state and simply describe what happens after whatever conditions are present. A random initial state for the universe would overwhelmingly likely be high-entropy—already disordered, already featureless. The universe we observe began in a state of extraordinary order and has been running down ever since. Why it began ordered is not a question physics answers. It is a question physics poses and cannot escape. The only known cause of specified low-entropy initial conditions is a mind.

Penrose, R. The Road to Reality: A Complete Guide to the Laws of the Universe. London: Jonathan Cape, 2004. Chapter 27. Penrose, R. The Emperor's New Mind. Oxford University Press, 1989.

Chapter 26 — The Gravity That Holds Worlds

The gravitational constant G equals 6.674 × 10-11 N⋅m²/kg². It is measured with high precision and is not derived from any other principle. Like all the constants in this section, it is simply what it is.

If the gravitational constant were significantly stronger, stars would be more massive, burn faster, and exhaust their nuclear fuel in millions rather than billions of years. Sun-type stars with the stable, long-duration output required for complex life to develop would not exist. If the constant were significantly weaker, clouds of gas and dust would not collapse under gravity to form stars at all. Without stars there is no stellar nucleosynthesis—no production of carbon, oxygen, or any element heavier than hydrogen and helium. No rocky planets, no chemistry of life.

The gravitational constant must sit in the window between the failure mode of no star formation and the failure mode of stars too short-lived for planetary life. It does. That it does is not explained by physics. Physics describes the consequences of the value. It does not explain the selection of the value.

Rees, M.J. Just Six Numbers. Basic Books, 2000. Chapter 3. Collins, R. in Blackwell Companion to Natural Theology, ed. Craig and Moreland. Wiley-Blackwell, 2009.