Also, as with GR, there’s an experimental observation that doesn’t fit with the simpler model. For GR, it was the Michelson-Morley experiment, which showed that the speed of light is constant, for this, it’s “the anomalous behaviour of its thermodynamic response functions upon cooling, the most famous being the density maximum at ambient pressure” (https://www.nature.com/articles/s41567-022-01698-6.pdf)
So, basically, there’s a reigning theory that correctly describes lot of observations but then, a loose end is discovered, and theorists try to find the simplest theory that fixes that loose end.
I think string theory is different. There are no observations that show the existence of loose ends, only a desire to have a quantum theory of gravity, or even a Grand Unified Theory (https://en.wikipedia.org/wiki/Grand_Unified_Theory)
> For GR, it was the Michelson-Morley experiment, which showed that the speed of light is constant
The Michelson-Morley experiment did not show that the speed of light is constant, that was known for some time before; and it and was the basis for Special Relativity, not General Relativity. The Michelson-Morley experiment showed that there is no "frame dragging" - that, if there exists a "luminiferous aether", it is not itself moved by the movement of the Earth through it, as would be expected for any normal substance.
General Relativity was necessary as a theory to bring back gravity (and describe acceleration) in the context of SR (since SR requires the speed of light to be constant and finite, but Newtonian gravity is instant). It was experimentally confirmed 42 years after the Michelson-Morley experiment, by confirming the gravitational lens effect in a solar eclipse.
> I think string theory is different. There are no observations that show the existence of loose ends, only a desire to have a quantum theory of gravity, or even a Grand Unified Theory
The lack of a quantum theory of gravity is a huge gap in our understanding of the world, not a simple desire. Quantum mechanics makes obviously wrong predictions when adding gravitational effects (even worse if trying to account for curved space-time). General relativity makes wrong predictions about the behavior of elementary particles. So, we are left today without a model of how the world works - and the precise missing piece is "quantum gravity".
Of course, it may well turn out that in fact we need a completely new theory that would replace both QM and GR - but what we know for sure is that QM and GR as they are right now are not correct, and the most obvious incongruity is quantum gravity.
You're right about a GUT though - that would be nice to have from a mathematical beauty point of view, but it's not in any way required, unlike quantum gravity.
So, basically, there’s a reigning theory that correctly describes lot of observations but then, a loose end is discovered, and theorists try to find the simplest theory that fixes that loose end.
I think string theory is different. There are no observations that show the existence of loose ends, only a desire to have a quantum theory of gravity, or even a Grand Unified Theory (https://en.wikipedia.org/wiki/Grand_Unified_Theory)