In what sense was Einstein left out of the mainstream
Not out of the mainstream in general, only out of the useful scientific research.
his (correct, assuming MWI) criticisms of QM
His criticism of QM was useful regardless of MWI. Among other things, he pointed out several issues with objective collapse and hidden variables (with his famous EPR paradox). Even when he was wrong (in his almost as famous debates with Bohr), he did not make any obvious errors, it took Bohr some time to figure why a certain thought experiment did not contradict QM in its shut-up-and-calculate non-interpretation.
Now, what I was referring to is that he was fortunate to get the education the he had, to have a fellow scientist as a fiancee and (apparently) as a sounding board for his ideas during his work on SR, he was fortunate to have had the mathematician Marcel Grossmann as a friend who helped him with the critical piece of differential geometry later on, etc.
Early on he also had a good sense to apply his genius to constructing models based on known but not yet explained experimental data: photoelectric effect, Brownian motion, Michelson-Morley experiment, Maxwell equations, gravity acting like acceleration, and a few others.
This changed some time in 1920s/1930s, when he decided that unifying classical gravity and classical EM is a good idea on general principles (like Occam’s razor and aesthetic considerations), probably because of his understandable dissatisfaction with QM. To be fair, he had quite a bit of success with models not based on experiment, such as predicting Bose-Einstein condensate. He also remained confused about some of the less clear the aspects of GR, like gauge invariance, gravitational waves and stress-energy tensor. And that’s what meant by “went wrong”.
Not out of the mainstream in general, only out of the useful scientific research.
His criticism of QM was useful regardless of MWI. Among other things, he pointed out several issues with objective collapse and hidden variables (with his famous EPR paradox). Even when he was wrong (in his almost as famous debates with Bohr), he did not make any obvious errors, it took Bohr some time to figure why a certain thought experiment did not contradict QM in its shut-up-and-calculate non-interpretation.
Now, what I was referring to is that he was fortunate to get the education the he had, to have a fellow scientist as a fiancee and (apparently) as a sounding board for his ideas during his work on SR, he was fortunate to have had the mathematician Marcel Grossmann as a friend who helped him with the critical piece of differential geometry later on, etc.
Early on he also had a good sense to apply his genius to constructing models based on known but not yet explained experimental data: photoelectric effect, Brownian motion, Michelson-Morley experiment, Maxwell equations, gravity acting like acceleration, and a few others.
This changed some time in 1920s/1930s, when he decided that unifying classical gravity and classical EM is a good idea on general principles (like Occam’s razor and aesthetic considerations), probably because of his understandable dissatisfaction with QM. To be fair, he had quite a bit of success with models not based on experiment, such as predicting Bose-Einstein condensate. He also remained confused about some of the less clear the aspects of GR, like gauge invariance, gravitational waves and stress-energy tensor. And that’s what meant by “went wrong”.