Arbitrage:

Does the theory need to be verified experimentally as describing our world? String theory already provides a theory of quantum gravity at least if you're ok with a theory only defined perturbatively.

@TomBouley Yes. Verified experimentally.

The theory of relativity was published in 1915 by Sir Einstein. Quantum mechanics theories started blooming in the mid-1920s by Professors Niels Bohr, Erwin Schrodinger, Werner Heisenberg, Max Born, Paul Dirac, and others. Fast forward to this, in 2023 the Nobel Prize was awarded to Bawendi, Brus, and Ekimov for their work with quantum dots. They have invented such fluids which emit light solely dependent on the size of the particles. So, in the 19th century, the idea of quantum was related to very minute particles. But in the 20th century, the relation between the nano world and microworld could be found out. Considering the progress of physics inventions I bet that a theory of everything (a theory that unites relativity and quantum mechanics) be found in physics by 2040.