Not, she says, General Relativity, the photoelectric effect, or slices of his brain but the“thought experiment,” for example:
But the maybe most influential of his thought experiments was one that he came up with to illustrate that quantum mechanics must be wrong. In this thought experiment, he explored one of the most peculiar effects of quantum mechanics: entanglement. He did this together with Boris Podolsky and Nathan Rosen, so today this is known as the Einstein-Podolsky-Rosen or just EPR experiment.
How does it work? Entangled particles have some measureable property, for example spin, that is correlated between particles even though the value for each single particle is not determined as long as the particles were not measured. If you have a pair of particles, you can know for example that if one particle has spin up, then the other one has spin down, or the other way round, but you may still not know which is which. The consequence is that if one of these particles is measured, the state of the other one seems to change – instantaneously.
Einstein, Podolsky and Rosen suggested this experiment because Einstein believed this instantaneous ‘spooky’ action at a distance is nonsense. You see, Einstein had a problem with it because it seems to conflict with the speed of light limit in Special Relativity. We know today that this is not the case, quantum mechanics does not conflict with Special Relativity because no useful information can be sent between entangled particles. But Einstein didn’t know that. Today, the EPR experiment is no longer a thought experiment. It can, and has been done, and we now know beyond doubt that quantum entanglement is real.Sabine Hossenfelder, “Einstein’s Greatest Legacy: Thought Experiments” at BackRe(Action)
She notes that one needs evidence from real experiments to demonstrate that the outcome of a thought experiment is real. But it is significant that the human mind is capable of developing the basis for momentous discoveries even before we commit to stuff that requires a budget.
See also: Sabine Hossenfelder asks: Do we need a theory of everything? Hossenfelder: So this whole idea of a theory of everything is based on an unscientific premise. Some people would like the laws of nature to be pretty in a very specific way… This is simply not a good strategy to develop scientific theories, and no, it is most certainly not standard methodology.