General Relativity is a very successful theorem. It explains many things that other theories cannot – the precession of the planet Mercury, which Enstein himself explained as the first test of his theory – and makes many predictions – the Big Bang, the bending of light paths in the presence of masses among them that have been shown to be real – but it must be remembered that General Relativity is only one in a long line of theories of space, time and gravity that have been superceded by other theories – that the Earth was carried on the back of an elephant, Euclidean space, that the universe orbited the Earth at the centre, the Copernican picture and Newton's Theory of Gravitation - notwithstanding the God like status of Einstein. These theories have become every better approximations, but each has in turn turned out to be wrong. In fact we have long known that General Relativity is an incomplete theory. It cannot deal with the singularity inside a black hole, or with the infinities – of infinite matter and energy density - at the birth of the Universe, nor does it explain the properties of space on the quantum scale, and cannot be married with quantum mechanics to produce a bigger theory of space, time and matter. In fact General Relativity can be thought of as an approximation to a true theory of the structure of space, in the same way as Newton's Theory of Gravity is an approximation to the General Theory of Relativity. It explains the large scale structure of spacetime as a consequence of the distribution of matter and energy but it is in conception a theorem of space as a continuum, whereas space is seething with virtual particles, and there is a mysterious limit to any length we can measure – the Planck length,
The Planck length is a property of quantum space, fundamentally different from the idea of spacetime in General Relativity as a continuum.
It is important to realise that most quantum mechanical calculation only use the principles and equations of quantum mechanics. Gravity is not taken into account because gravity is so weak compared to the other fundamental forces. Only in extreme situations such as in the interior of a neutron star, do we need to include gravity in some way because we must balance the neutron degeneracy which stops the star from collapsing with the force of gravity which stops the star exploding. In these situations the calculations are extremely complex and not the result of a single physical theory, but an approximation forced by the use of two fundamentally incomplete theories to model phenomena outside the range they may be used to model – scientists may be using extrapolation to expand the theories beyond the range at which they are valid. While not good practice, the limitations of the theories we have make this the best option.