So a nice discussion on the extra spatial dimensions post has led me to elaborate on what the fuss over gravity is. I have a wide range of audience, and most are probably in the philosophy school due to Philosophers’ Carnival. I think it is sort of important to know the controversy in physics right now, especially as a philosopher pondering the nature of the universe, and with CERN nearing its completion.

It is true that this fuss is over gravity, but it isn’t in an extra-dimensional sense. The four fundamental forces are the strong force, electromagnetic force, weak force, and gravity. The first three fit perfectly together through quantum mechanics. Then gravity has its own theory: general relativity. I actually listed them in order of strength. Gravity is by far the weakest and the strong force is the strongest (hence the name).

The problem is to get gravity into a quantum mechanics theory. The problem has to do with the uncertainty principle. Essentially, gravity is nonrenormalizable. This is terribly challenging to explain, but when you attempt to correct for small errors, those errors have to go to zero, but with gravity the errors build up and actually go to infinity. In a sense gravity “blows up.” Now if we try the other way, to put the really small into general relativity, we get what are known as singularities. This may be possible to overcome, but the postulates of quantum mechanics cannot be carried over to curved space-time (which is the whole point of GR).

So instead of trying to put one into the other people are trying completely new things and trying to get QM and GR as special cases of a more general theory. There are many attempts to do this. There is loop quantum gravity, topological quantum field theory, string theory, etc. It turns out that so far string theory is the only one that can get gravity into it successfully. It also turns out that string theory demands extra dimensions.

Disclaimer: Before some crazy physicist yells at me that other theories have fit gravity into their picture, I am saying this from my own knowledge. I know that there are many many different attempts at this, it is to my knowledge that none have successfully incorporated gravity without severe compromises somewhere else.