Naturalness: a useful principle, but not an absolute rule
There are two classes of problems that physicists often talk about that might leave you thinking, “geez, a physicist can never be satisfied, can they?” The first is called a coincidence problem: where things that have no apparent reason for being close together actually are close together. Examples include: the normal matter and dark matter densities, the cosmic matter density and the dark energy density, the total energy of the Hoyle state of carbon-12 being almost exactly equal to the rest mass energy of three helium-4 nuclei, or Dirac’s large number coincidence, where the strength of gravity is approximately equal to the inverse of the age of the Universe since the Big Bang. The second is the opposite type of problem, known as a hierarchy problem: where it seems like there is a large, unnatural, inexplicable gap between things that — if everything were equal — you might expect to be close together. Examples of that include: the huge differences between the electroweak scale and the Planck scale, the huge difference in the strength between …
