Physics' two great theories contradict each other, and no one can unite them

The problem

General relativity describes gravity as smoothly curving spacetime; quantum mechanics describes matter as jumpy and probabilistic. Each works superbly in its own domain, but applied together they break down: naive attempts to quantize gravity produce incurable mathematical infinities, proven rigorously at 'two loops' by Goroff and Sagnotti in 1985. Candidate fixes — string theory, loop quantum gravity, causal sets, asymptotic safety — have flourished for decades without a single confirming experiment, because quantum-gravity effects only become strong at energies far beyond any accelerator. The tantalizing new hope is tabletop experiments testing whether gravity itself can entangle two quantum masses.

Why it matters

Without quantum gravity we cannot say what happens at the center of a black hole or in the universe's first instant — the two places our current physics visibly fails. Whatever resolves the conflict will rewrite our understanding of space, time, and information.

Progress so far

• 1930Léon Rosenfeld and others make the first attempts to apply quantum theory to the gravitational field
• 1985Goroff and Sagnotti prove quantized general relativity is non-renormalizable at two loops, showing the naive approach fails
• 1986Abhay Ashtekar's reformulation of general relativity launches loop quantum gravity, alongside string theory's rise
• 2000squantum-gravity phenomenology emerges: physicists begin designing indirect, feasible experimental tests
• 2025a Nature paper shows some classical gravity theories can also generate entanglement, sharpening the debate over what tabletop 'gravitationally induced entanglement' experiments would actually prove

References

1. Perimeter Institute — why relativity and quantum theory must be united, and the main competing approaches
2. Stanford Encyclopedia of Philosophy — scholarly survey of the problem's history and conceptual stakes
3. Wikipedia — overview including the 1985 two-loop non-renormalizability result and candidate theories