Part Two: The Universe Set to Specification

The constants of physics are not derived from more fundamental principles. They are measured. Each could, as far as physics can determine, have had any value. And yet the values sit in extraordinarily narrow windows that permit the existence of stars, atoms, chemistry, and life. The entries in this section document specific constants and what happens when you move their values outside the ranges they occupy.

Chapter 17 — The Constant Nobody Explains

The cosmological constant—Einstein's Λ (lambda)—represents the energy density of empty space. Quantum field theory predicts a value for this constant based on the vacuum energy of quantum fields. The observed value—measured from the universe's expansion rate—is approximately 10120 times smaller than the quantum field theory prediction. This is the largest discrepancy between theory and measurement in the history of physics.

The implications run in both directions. If the constant were as large as quantum field theory predicts, the universe would have expanded too rapidly for any structure—galaxies, stars, planets—to form. If it were large and negative, the universe would have recollapsed before chemistry was possible. The observed value sits in a window approximately 10120 times smaller than the prediction, precisely where it must be for the universe to contain structure at all.

Physicist Steven Weinberg analyzed the cosmological constant problem in a 1989 review paper that remains the standard reference. He noted that no known physical principle can explain why the value is so small relative to theoretical expectations. The fine-tuning required is the most extreme of any constant in physics.

Weinberg, S. "The Cosmological Constant Problem." Reviews of Modern Physics 61, no. 1 (1989): 1–23.

Chapter 18 — The Force That Holds Atoms Together

The strong nuclear force holds protons and neutrons together inside atomic nuclei against the electrostatic repulsion of positively charged protons. Its precise value relative to the electromagnetic force determines which atomic nuclei can exist and therefore what elements are possible.

If the strong nuclear force were approximately two percent stronger, diprotons—nuclei of two protons and no neutrons—would be stable. Stable diprotons would cause hydrogen to burn into helium at far greater rates in the early universe, depleting hydrogen before Sun-type stars could form. With little hydrogen remaining there would be no water, no hydrogen-containing organic chemistry, and no long-lived main sequence stars.

If the strong nuclear force were approximately two percent weaker, deuterium—the nucleus required as the first step in stellar nucleosynthesis of heavier elements—would not bind. Without deuterium, the chain of reactions that produces carbon, oxygen, nitrogen, and all the other elements of biology cannot proceed. The periodic table of elements would effectively consist only of hydrogen.

Barrow, J.D. and Tipler, F.J. The Anthropic Cosmological Principle. Oxford: Clarendon Press, 1986. Collins, R. "The Fine-Tuning of the Cosmos." In The Blackwell Companion to Natural Theology, ed. Craig and Moreland. Wiley-Blackwell, 2009.