I'm not a scientist, just have a casual interest in science and its history, so please allow for some inaccuracies and feel free to correct me.
It appears that gravity is somewhat different than the other forces in fundamental ways. The other three forces are adequately described using quantum mechanics - gravity not so. Gravity is explained using Einsteins general theory of relativity, and describes gravity as a curvature of the time-space continuum. Modern physics rests on two main pillars: quantum mechanics and relativity. Gravity is the only one of the fundamental forces explained by the relativity column, and so it is harder to incorporate into a unified theory encompassing all the forces.
Of course it doesn't help that quantum mechanics and relativity are by many thought to be incompatible theories: The first works well for sub-atom sizes and weights, the latter works well for planet sizes and weights. This is also one of the reasons black holes are so interesting - sub atom size and planet weight. Both quantum mechanics and relativity must be at work.
It appears that gravity is somewhat different than the other forces in fundamental ways. The other three forces are adequately described using quantum mechanics - gravity not so. Gravity is explained using Einsteins general theory of relativity, and describes gravity as a curvature of the time-space continuum. Modern physics rests on two main pillars: quantum mechanics and relativity. Gravity is the only one of the fundamental forces explained by the relativity column, and so it is harder to incorporate into a unified theory encompassing all the forces.
Of course it doesn't help that quantum mechanics and relativity are by many thought to be incompatible theories: The first works well for sub-atom sizes and weights, the latter works well for planet sizes and weights. This is also one of the reasons black holes are so interesting - sub atom size and planet weight. Both quantum mechanics and relativity must be at work.
Really interesting stuff :-)