Two days ago, there was a post on backreaction about the naturalness in science, which is considered by many as a fundamental concept. The point of view expressed in the post is quite easy to grasp : naturalness is not a justified argument in any sense (edit : see the comment of the autor below).

No need to say that I know a bunch of people who would feel quite offended by these kind of statement. As a response, this post in the reference frame is quite enlightning, and I must admit that although I find him quite rude in his manner, it happens I often agree with Luboš Motl concerning physics. And I recommend everyone who wants to get the point to read his post till the end.

Now, why am I writing this post ? Well after my readings, I felt quite confused by all the arguments about bears and trees, and this led me to the conclusion that the idea of naturalness is maybe not that easy to get.

Is the problem really about why this or that quantity should have this or that value ? absolutly not. Though it often appears that people arguing about naturalness are trying to answer that specific question. Instead, I think the good question to ask is : why in a given system, two objects or quantities, that are related or interacting in some manner have so different values ? To be a bit clearer,  it is not the fact that the trees are the same heights in average, or the bears the same size that matters.

The real question would be why the average size of the living beings doesn't exceed a specific value ?

As you know, the tallest dinosaur was not higher that 20 meters, and nowadays, a more natural value is more about 2 meters for living beings. I'm not an expert in biology, and won't try to answer this question here, however it seems obvious to me that this has to do with optimal size in a given ecosystem. The trees have to find an equilibrium between the highest height that allow them to receive enough light, and not too high height or else gravity would forbid the sap to make its way to the higher branches.  And the size of dinosaurs would have certainly been, at some point limited by the height of trees...

The point here is that when considering a given system, you have natural values for the quantities that characterize it. And that's the way it goes on particle physics for example. The hierarchy problem is not simply about why the Planck scale is much higher than the electroweak scale, but why the hell given these two scales and all the phenomenons happening between the two, do they tend to be so different ?

The real problem arising in the standard model is that the Higgs sector, being coupled to many kind of particles is not naturally living at the electroweak scale, but we need it the be there for phenomenological reasons, thus the introduction of some symmetries that could kill its tendancy to go higher.

It is usually the strong argument for supersymmetry in particle physics (dark matter candidate and other stuffs are only side effects !).