Which fundamental force is hardest to unify with the others?

Three of the four fundamental forces are described within the Standard Model of particle physics, but gravity remains outside that framework. While general relativity explains gravity very successfully on large scales, it is not yet reconciled with quantum mechanics. That is why theories such as string theory and loop quantum gravity are studied as possible paths to unification. The challenge becomes especially important in extreme environments like the Big Bang and black holes. A full theory of quantum gravity remains one of physics’ biggest open problems.