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#4239
Ed Lantz
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Hi Ursula – Carbon is formed in stars through a “triple alpha” process which was previously unknown and thought to be improbable. Hoyle recognized that there had to be a way for carbon to form and hypothesized a quirky resonance  that would allow it.  Some use this as evidence that the anthropic principle has predictive power. This from Wikipedia (http://en.wikipedia.org/wiki/Triple-alpha_process):
 

Triple-alpha process

Discovery

The triple alpha process is highly dependent on carbon-12 and beryllium-8 having resonances with the same energy as helium-4, and before 1952, no such energy levels were known. The astrophysicist Fred Hoyle used the fact that carbon-12 is abundant in the universe as evidence for the existence of a carbon-12 resonance. This could be considered to be an example of the application of the anthropic principle: we are here, and we are made of carbon, thus the carbon must have been produced somehow. The only physically conceivable way is through a triple alpha process that requires the existence of a resonance in a given very specific location in the spectra of carbon-12 nuclei.

 

Hoyle went boldly into nuclear physicist William Alfred Fowler‘s lab at Caltech and said that there had to be a resonance of 7.69 MeV in the carbon-12 nucleus, and that all of the physicists in the world had missed it. Fred Hoyle’s audacity in doing this is remarkable, and initially all the nuclear physicists in the lab were skeptical to say the least. But he was persistent and kept coming back to the lab and talked to every assistant and associate individually. Finally, a junior physicist, Ward Whaling, fresh from Rice University, who was looking for a project started believing Hoyle, and decided to look for the resonance. Fowler gave Ward permission to use an old Van de Graaff generator that no one else was using, and everyone joined in with suggestions for Ward. The experiment took 6 months, and Hoyle was back in Cambridge when his outrageous prediction was verified. They put Hoyle as first author on a paper delivered by Ward Whaling at the Summer meeting of the American Physical Society. A long and fruitful collaboration between Hoyle and Fowler soon followed, with Fowler even coming to Cambridge.[6] By 1952, Fowler had discovered the beryllium-8 resonance, and Edwin Salpeter calculated the reaction rate taking this resonance into account.[7][8]

 

This helped to explain the rate of the process, but the rate calculated by Salpeter was still somewhat too low. A few years later, after a project by his research group at the Kellogg Radiation Laboratory at the California Institute of Technology, Fowler discovered a carbon-12 resonance near 7.65 MeV. This eliminated the final discrepancy between the nuclear theory and the theory of stellar evolution.
The final reaction product lies in a 0+ state. Since the Hoyle State was predicted to be either a 0+ or a 2+ state, electron–positron pairs or gamma rays were expected to be seen. However, when experiments were carried out, the gamma emission reaction channel was not observed, and this meant the state must be a 0+ state. This state completely suppresses single gamma emission, since single gamma emission must carry away at least 1 unit of angular momentumPair production from an excited 0+ state is possible because their combined spins (0) can couple to a reaction that has a change in angular momentum of 0.[9]

 

Improbability and fine-tuning

Main article: Fine-tuned universe
 

Carbon is a vital component of human biology. 12C, a stable isotope of carbon, is abundantly produced in stars due to three factors:

 

  1. The decay lifetime of a 8Be nucleus is four orders of magnitude larger than the time for two 4He nuclei (alpha particles) to scatter.[10]
  2. An excited state of the 12C nucleus exists just above the energy level 8Be + 4He. This is necessary because the ground state of 12C is 7.3367 MeV below the energy of 8Be + 4He. Therefore a 8Be nucleus and a 4He nucleus cannot reasonably fuse directly into a ground-state 12C nucleus. The excited Hoyle state of 12C is 7.656 MeV above the ground state of 12C. This allows 8Be and 4He to use the kinetic energy of their collision to fuse into the excited 12C, which can then transition to its stable ground state. According to one calculation, the energy level of this excited state must be between about 7.3 and 7.9 MeV to produce sufficient carbon for life to exist, and must be further “fine-tuned” to between 7.596 MeV and 7.716 MeV in order to produce the abundant level of 12C observed in nature.[11]
  3. Conversion of 12C + 4He to 16O is much more difficult than the production of carbon; no resonance exists for this reaction. Were this not true, insufficient carbon would exist in nature; it would almost all have converted to oxygen.[10]

 

The 7.656 MeV Hoyle resonance, in particular, has been cited by physicists arguing for the existence of a multiverse where different regions of a vast multiverse have different fundamental constants. According to this controversial fine-tuning hypothesis, life can only evolve in rare patches of the multiverse where the fundamental constants are fine-tuned to support the existence of life.