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Why is gravity difficult to include in a unified theory of fundamental forces?

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subatomic particle The Large Hadron Collider (LHC), the world's most powerful particle accelerator. At the LHC, located underground in Switzerland, physicists study subatomic particles.

Gravity is difficult to include in a unified theory because it is described by general relativity, which treats gravity as the curvature of space-time, whereas other fundamental forces are described by quantum mechanics as the exchange of elementary particles.123

These two frameworks are fundamentally incompatible. In quantum theory, forces arise from particle interactions, not the geometry of space-time. Furthermore, attempting to apply quantum mechanics to gravity leads to "nonphysical" infinite quantities in calculations. Additionally, quantum effects are thought to severely distort space-time at the Planck length, suggesting that a quantum description of gravity requires understanding space-time at scales far smaller than elementary particles.12

While the graviton—a hypothetical massless particle with a spin of 2—is proposed to mediate gravity in a quantum framework, it has not been observed. Current research into supergravity and superstring theory seeks to resolve these inconsistencies by incorporating extra dimensions and particle symmetries, though these remain untested hypotheses.4231