The Strong Nuclear Force as an example of fine tuning for life
The Strong Nuclear Force is fine tuned for life. If the strong force was weaker than it is, the chemical elements needed for life would not be stable, and we would not be here. If it were stronger, all the hydrogen in the universe would have been burned to helium in the Big Bang. As a result, there would be no long-lived stars like the sun, and no water. There would probably be no complicated chemistry in the universe, and we would not be here.
(A slightly longer version of this article, which includes some extended quotations from the source literature, and discusses one important objection to the fine tuning of the strong nuclear force, can be found at www.focus.org.uk/strongforce_long.pdf)
In everyday life we’re only aware of two fundamental forces: gravity and electromagnetism. Physicists know about two more forces, which only work at very short range (inside atoms): the strong nuclear force and the weak nuclear force.
This article is about the strong nuclear force – the force that holds protons and neutrons together in the nucleus of atoms. It is about ten thousand billion billion billion billion times (1040) times more powerful than the force of gravity.
Picture two protons. They are pulled together by the strong nuclear force (as long as they are within range to start with.) But the electromagnetic force pushes them away from each other, because they both have the same positive electric charge.
The electromagnetic repulsion wins over the strong nuclear force attraction, and you can’t get two protons to stick together. So a nucleus made of just two protons (called a ‘diproton’) isn’t stable.
But if you add a neutron the balance of the forces shifts: neutrons feel the strong nuclear force, but they don’t feel the electromagnetic force, because they’re electrically neutral. So adding a neutron is enough to tip the balance: a nucleus made of two protons and one neutron is stable.
So the balance between the strong nuclear force and the electromagnetic force affects the way protons and neutrons can combine to make stable atomic nuclei. This balance has to be fine-tuned for life to be possible.
What would happen if the strong nuclear force were a bit weaker?
If the strong force were a bit weaker, it would not be able to hold atomic nuclei together against the repulsion of the electromagnetic force. According to Barrow and Tipler:
‘A 50% decrease in the strength of the nuclear force… would adversely affect the stability of all the elements essential to living organisms and biological systems.’
A bit more of a decrease, and there wouldn’t be any stable elements except hydrogen.
What would happen if the strong nuclear force were a bit stronger?
If the strong nuclear force was just a bit stronger compared to the electromagnetic force, two protons could stick together in spite of their electromagnetic repulsion (forming a diproton).
If this happened, all the hydrogen in the universe would have been burned to helium in the big bang. It’s very difficult to imagine how a universe with no hydrogen could produce the complicated chemistry needed for life – there would be no water, for a start, and there would be no long-lived stars like the sun. (Stars made from helium burn up much more quickly than stars made from hydrogen.) Barrow and Tipler again:
‘All the hydrogen in the Universe would be burned to He2 during the early stages of the big bang and no hydrogen compounds or long-lived stable stars would exist today.’
If the strong nuclear force was weaker than it is, the chemical elements needed for life would not be stable, and we would not be here. If it were stronger, all the hydrogen in the universe would have been burned to helium in the Big Bang. As a result, there would be no long-lived stars like the sun, and no water. There would probably be no complicated chemistry in the universe, and we would not be here.
David Couchman MA, M.Sc, M.Min, August 2010
 Barrow, J D and Tipler, F J, ‘The Anthropic Cosmological Principle’ Oxford University Press 1986, p. 327
 Barrow and Tipler, p. 322