**The strong and electromagnetic forces**

The strong force is the force that keeps the nucleons—that is, the protons and neutrons—together in an atom. The effect on the stability of the atom of decreasing the strong force is straightforward since the stability of elements depends on the strong force being strong enough to overcome the electromagnetic repulsion between the protons in a nucleus. A 50 percent decrease in the strength of the strong force, for instance, would undercut

the stability of all elements essential for carbon-based life, with a slightly larger decrease eliminating all elements except hydrogen.

Another effect of decreasing the strength of the strong force is that it would throw off the balance between the rates of production of carbon and oxygen in a star. This would have severe life-inhibiting consequences. Although various life-inhibiting effects are claimed for increasing the strength of the strong force, the arguments are not nearly as strong or well developed, except for the one below involving the existence of carbon and oxygen. At present, therefore, we have a solid argument that a decrease in the strength of the strong force would be life-forbidding, along with a significant and well-developed argument for two-sided fine-tuning based on the joint production of carbon and oxygen in stars. Now the forces in nature can be thought of as spanning a range of G0 to 1040G0, at least in one widely used dimensionless measure of the strengths of these forces. (Here, G0 denotes the strength of gravity, with 1040G0 being the strength of the strong force.) If we let the theoretically possible range R of force strengths in nature be the total range of force strengths, then it follows that the degree of one-sided fine-tuning of the strong force is insignificant, being about one part in two by the formula given in note 5. Of course, one might think that it is likely that the theoretically possible range is much larger than given above, hence making the one-sided fine-tuning much more significant. Finally, around a fourteen-fold increase in the electromagnetic force would have the same effect on the stability of elements as a 50 percent decrease in the strong force. Now in the dimensionless units, the strength of the electromagnetic force is considered to have a value of approximately 1037G0, and hence the upper bound of the life-permitting region is approximately 14×1037G0. Consequently, this yields a one-sided fine-tuning of approximately one part in a hundred or less.