Why is gravity especially difficult to combine with quantum mechanics?

Combining gravity with quantum mechanics is difficult because the two frameworks describe the universe using fundamentally different principles.1

General relativity describes gravity as a consequence of the curvature of space-time, whereas quantum theory explains forces through the exchange of elementary particles. To reconcile these, gravity would need to be treated as a quantum field theory, requiring a messenger particle called the graviton. However, this approach faces significant mathematical hurdles, including the emergence of inconsistent "infinities" in calculations.123

Furthermore, quantum effects are believed to cause severe distortions of space-time at the Planck length, a scale far smaller than elementary particles, suggesting that a quantum description of gravity requires understanding space-time at extreme, currently inaccessible scales. String theory attempts to resolve these issues by modeling particles as vibrating strings rather than points, offering a potential framework to unify gravity with the other fundamental forces.14