The difficulty with merging general relativity and quantum mechanics arises from quantum energy fluctuations. Quantum mechanics says that even the gravitational field is effected by these fluctuations. General relativity states that there is a zero gravitational field in empty space, but quantum mechanics say it averages zero, and fluctuates more and more wildly on a smaller and smaller scale. According to quantum mechanics, if you could magnify empty space enough, you would find that it is not flat at all but tangled, distorted, bubbly, and tumultuous. This frenzy is called quantum foam. The equations of general relativity canšt handle quantum foam, even though quantum foam is only visible when you magnify to smaller than the Planck length or 10 cm. This makes it extremely difficult to unite the general relativity theory with quantum mechanics.