A diproton is a hypothetical type of helium nucleus consisting of two protons and no neutrons. Diprotons are not stable, since the electrostatic repulsion of the two protons' positive charges overcomes the strong nuclear force holding them together.
It has been speculated that if the strength of the strong force had been only 2% greater all protons would have bound together into diprotons in the early stages of the Big Bang. If true, then such a universe would have a different periodic table from ours, possibly precluding life as we know it. The impossibility of diprotons may therefore be an example of an anthropic coincidence.
- John D. Barrow, Frank J. Tipler. The Anthropic Cosmological Principle. Oxford University Press, 1988, p. 322. ISBN 0192821474.