Making Carbon

The heavy elements did not get fabricated straightforwardly. According to the big-bang theory, only hydrogen, deuterium (the isotope of hydrogen consisting of one proton and one neutron), helium, and lithium were formed in the early universe. Carbon, nitrogen, oxygen, iron, and the other elements of the chemical periodic table were not produced until billions of years later. These billions of years were needed for stars to form and, near the end of their lives, assemble the heavier elements out of neutrons and protons. When the more massive stars expended their hydrogen fuel, they exploded as supernovae, spraying the manufactured elements into space. Once in space, these elements cooled, and gravity formed them into planets.

Billions of additional years were needed for our home star, the sun, to provide a stable output of energy so at least one of its planets could develop life. But if the gravitational attraction between protons in stars had not been many orders of magnitude weaker than the electric repulsion, as represented by the very large value of N1, stars would have collapsed and burned out long before nuclear processes could build up the periodic table from the original hydrogen and deuterium. The formation of chemical complexity is likely only in a universe of great age.

Great age is not all. The element-synthesizing processes in stars depend sensitively on the properties and abundances of deuterium and helium produced in the early universe. Deuterium would not exist if the difference between the masses of a neutron and a proton were just slightly displaced from its actual value. The relative abundances of hydrogen and helium also depend strongly on this parameter. They, too, require a delicate balance of the relative strengths of gravity and the weak force—the force responsible for nuclear beta decay. A slightly stronger weak force, and the universe would be 100 percent hydrogen; all the neutrons in the early universe would have decayed, leaving none around to be saved in deuterium nuclei for later use in the synthesizing elements in stars. A slightly weaker weak force, and few neutrons would have decayed, leaving about the same numbers of protons and neutrons; then all the protons and neutrons would have been bound up in helium nuclei, with two protons and two neutrons in each. This would have led to a universe that was 100 percent helium, with no hydrogen to fuel the fusion processes in stars. Neither of these extremes would have allowed for the existence of stars and life as we know it based on carbon chemistry (Livro et al. 1989).

The electron also enters into the tightrope act needed to produce the heavier elements. Because the mass of the electron is less than the neutronproton mass difference, a free neutron can decay into a proton, an electron, and an anti-neutrino. If the mass of the electron were just a bit larger, the neutron would be stable, and most of the protons and electrons in the early universe would have combined to form neutrons, leaving little hydrogen to act as the main component and fuel of stars. The neutron must also be heavier than the proton, but not so much heavier that neutrons cannot be bound in nuclei.

In 1952, astronomer Fred Hoyle (1954) used anthropic arguments to predict that an excited carbon nucleus has an excited energy level at around 7.7 megaelectronvolts (MeV). The success of this prediction gave credibility to anthropic reasoning, so let me discuss this example in detail, because it is the only successful prediction of this line of inference so far.

I have already noted that a delicate balance of physical constants was necessary for carbon and other chemical elements beyond lithium in the periodic table to be cooked in stars. Hoyle looked closely at the nuclear mechanisms involved and found that they appeared to be inadequate.

The basic mechanism for the manufacture of carbon is the fusion of three helium nuclei into a single carbon nucleus:

(The superscripts give the number of nucleons—that is, protons and neutrons in each nucleus—which is specified by its chemical symbol. The total number of nucleons is conserved—that is, remains constant—in a nuclear reaction.) The probability of three bodies coming together simultaneously is very low, however, and some catalytic process in which only two bodies interact at a time must be assisting. An intermediate process in which two helium nuclei first fuse into a beryllium nucleus, which then interacts with the third helium nucleus to give the desired carbon nucleus, gives the desired result:

Hoyle (1954) showed that this still was not sufficient unless the carbon nucleus had a resonant excited state at 7.7 MeV to provide for a high reaction probability. A laboratory experiment was undertaken, and sure enough: a previously unknown excited state of carbon was found at 7.66 MeV (Hoyle et al. 1953).

Nothing can gain you more respect in science than the successful prediction of an unexpected new phenomenon. Here, Hoyle used standard nuclear theory. But his reasoning contained another element whose significance is still hotly debated. Without the 7.7-MeV nuclear state of carbon, our form of life based on carbon would not have existed.

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