Just in time for Christmas, I’ve had a paper accepted by the Journal of Cosmology and Astroparticle Physics. It’s called “Binding the Diproton in Stars: Anthropic Limits on the Strength of Gravity“. Here’s the short version.
Diproton Disaster?
In 1971, Freeman Dyson discussed a seemingly fortunate fact about nuclear physics in our universe. Because two protons won’t stick to each other, when they collide inside stars, nothing much happens. Very rarely, however, in the course of the collision the weak nuclear force will turn a proton into a neutron, and the resulting deuterium nucleus (proton + neutron) is stable. These the star can combine into helium, releasing energy.
If a super-villain boasted of a device that could bind the diproton (proton + proton) in the Sun, then we’d better listen. The Sun, subject to such a change in nuclear physics, would burn through the entirety of its fuel in about a second. Ouch.
A very small change in the strength of the strong force or the masses of the fundamental particles would bind the diproton. This looks like an outstanding case of find-tuning for life: a very small change in the fundamental constants of nature would produce a decidedly life-destroying outcome.
Asking the Right Question
However, this is not the right conclusion. The question of fine-tuning is this: how would the universe have been different if the constants of nature had different values? In the example above, we took our universe and abruptly changed the constants half-way through its life. The Sun would explode, but would a bound-diproton universe create stars that explode? (more…)
Letters to Nature 