Triple Alpha Process
From SkepticWiki
Contents |
[edit] Definition
The triple alpha process is a nuclear reaction which takes place in stars late in their lifetimes, and which produces carbon, an element crucial to life. It has been claimed that this process is an example of fine tuning in nature, and hence evidence of design.
The triple alpha process has also been claimed as an example of a testable prediction made by the Anthropic Principle
[edit] How the triple alpha process works
- First, two helium nuclei (also known as "alpha particles") fuse, making a beryllium-8 nucleus and releasing energy to the tune of 0.09 MeV.
- A beryllium-8 nucleus is unstable and very short-lived -- its half-life is less than a millionth of a nanosecond[1] -- but during its brief existence, it may be struck by a third alpha particle, forming a 8Be-4He association.
- Now this association of a beryllium and a helium nucleus is also unstable, and in most cases, this unstable association will decay. However, it may instead turn into a carbon-12 nucleus at an excited energy level. This reaction, unlike most nuclear reactions going on in stars, is endothermic --- it requires energy, to the tune of 0.37 MeV, to get up from the ground state of the beryllium and helium nuclei to this excited state of the carbon nucleus. We shall refer to this amount of energy from here on in as the required energy.
- Once this has happened, the carbon nucleus will fall from its excited state to its ground state, releasing 7.65 MeV of energy. And a carbon nucleus is born. [2]
[edit] The Fine Tuning Argument
We have remarked that the second transition, from beryllium and helium to carbon is rare: it only happens about four times in every ten thousand. We have also pointed out that this transition requires energy. These two statements are intimately linked: it can be shown in theory that the greater the required energy, the less likely is the leap to the excited state of carbon (and conversely, if the required energy was smaller, carbon formation would become more probable).[3]
And here comes the Fine-Tuning Argument. The difference between the ground state and the excited state of the carbon nucleus is, as we've said, 7.65 MeV. Now, if we could somehow change this figure --- if we could hold the ground state constant, and increase the energy difference by only 1% (i.e. 0.07 MeV), then, it is argued, the required energy to get from the ground state of the beryllium and helium nuclei to this excited state of carbon would also be increased by 0.07 MeV, making this transition virtually impossible. Hence, it is argued, the difference between the ground state and the excited state of carbon-12 is "fine tuned" to within 1% for the production of carbon, which is essential not only for the chemistry of life in itself, but also for the nuclear synthesis of heavier elements such as oxygen.
[edit] Critique of the Fine Tuning Argument
However, it has been argued that this is the wrong sum to do. The possibility of the reaction does not depend on the fact that the release in energy from the excited state to the ground state of the carbon nucleus is precisely of such-and-such a size, but on the fact that the required energy to get from the beryllium and helium nuclei to the excited carbon nucleus is fairly small. It is, therefore, this number that we should look at for evidence of fine-tuning, as has been argued by Nobel Prize-winning physicist Steve Weinberg:
- Another question is about the fine-tuning. I, as I said in my talk, am not terribly impressed by the examples of fine-tuning of constants of nature that have been presented. To be a little bit more precise about the case of carbon, the energy levels of carbon, which is the most notorious example that’s always cited, there is an energy level that is 7.65 MeV above the ground state of carbon. If it was .06 of an MeV higher, then carbon production would be greatly diminished and there would be much less chance of life forming. That looks like a 1% fine-tuning of the constants of nature ... However, as has been realized subsequently after this “fine-tuning” was pointed out, you should really measure the energy level not above the ground state of carbon but above the state of the nucleus Beryllium 8 (8Be) plus a helium nucleus ... In other words, the fine-tuning is not 1% but it’s something like 25%. So, it’s not very impressive fine-tuning at all. [4] [5]
His point may be clarified by analogy: imagine a man standing on top of Mount Everest looking at a giraffe. "Such exquisite precision!" he exclaims. "Do you realise that if its head was just 1% higher above sea level, its neck would be nearly 100 meters long and it would snap under its own weight?" But the "fine-tuning" of the giraffe must be measured on the scale of the giraffe, not the mountain, and in the same way, the fine-tuning of the required energy should be measured on the scale of the required energy, not the purely arbitrary scale of the energy above the ground energy of the excited state of carbon.
[edit] The Anthropic Principle
When the triple alpha process was first proposed by Fred Hoyle, the existence of this crucial energy level of beryllium was not known to scientists. Hoyle postulated it to make the process work; only subsequently was it detected in the laboratory.
Controversially, Fred Hoyle claimed this triumph as a successful prediction made by the Anthropic Principle. According to Hoyle, the line of reasoning goes like this: Here am I, observing the Universe. My biochemistry is based on carbon. If stars didn't make carbon, I wouldn't be here; hence, if beryllium lacks this resonance, I wouldn't be here. Therefore I can deduce the existence of this resonance from the fact that I'm observing the Universe.
We find this line of reasoning unconvincing. In the first place, it applies equally well to lime jello, as in the following chain of reasoning: Here is some lime Jello. Its biochemistry is based on carbon. If stars didn't make carbon, it wouldn't be here; hence, if beryllium lacks this resonance, it wouldn't be here. Therefore I can deduce the existence of this resonance from the existence of lime Jello.
In this chain of reasoning, as with Hoyle's, it is not the existence of lime jello or of Fred Hoyle which is the crucial fact, but the existence of carbon. It is from this that we actually deduce the existence of the crucial energy level; and we would be able to make the same deduction from the existence of carbon even if it was not crucial to biochemistry (or to lime Jello) : that fact is irrelevant to the reasoning.
In the second place, it is possible to conceive of a universe with alternate physics and chemistry which contains intelligent observers but not carbon. Unless such universes can be ruled out as impossible (which Hoyle has not done) then it is not possible to reason from "Here am I, observing the Universe" to the existence of carbon, let alone the beryllium resonance.
