Professor George Smoot, University of California Berkeley: "Einstein was able to predict, WITHOUT ANY ADJUSTMENTS WHATSOEVER, that the orbit of Mercury should precess by an extra 43 seconds of arc per century should the General Theory of Relativity be correct." http://aether.lbl.gov/www/classes/p10/gr/PrecessionperihelionMercury.htm
"In his presentations of the complete version of General Relativity in late 1915 and the overview paper in 1916, Einstein presented his calculations of the three experimental tests. These were the rate of precession of the perihelion of the planet Mercury, light bending close to the Sun and the gravitational redshift. He showed the Mercury orbit correction was in excellent agreement with observation, solving a long-standing problem. He repeatedly emphasized that there was no freedom in his theory to adjust the predictions (no free parameters)..." https://royalsocietypublishing.org/doi/10.1098/rsnr.2020.0040
Here Michel Janssen describes countless ad hoc adjustments made again and again until "excellent agreement with observation" was reached:
"But - as we know from a letter to his friend Conrad Habicht of December 24, 1907 - one of the goals that Einstein set himself early on, was to use his new theory of gravity, whatever it might turn out to be, to explain the discrepancy between the observed motion of the perihelion of the planet Mercury and the motion predicted on the basis of Newtonian gravitational theory. [...] The Einstein-Grossmann theory - also known as the "Entwurf" ("outline") theory after the title of Einstein and Grossmann's paper - is, in fact, already very close to the version of general relativity published in November 1915 and constitutes an enormous advance over Einstein's first attempt at a generalized theory of relativity and theory of gravitation published in 1912. The crucial breakthrough had been that Einstein had recognized that the gravitational field - or, as we would now say, the inertio-gravitational field - should not be described by a variable speed of light as he had attempted in 1912, but by the so-called metric tensor field. The metric tensor is a mathematical object of 16 components, 10 of which independent, that characterizes the geometry of space and time. In this way, gravity is no longer a force in space and time, but part of the fabric of space and time itself: gravity is part of the inertio-gravitational field. Einstein had turned to Grossmann for help with the difficult and unfamiliar mathematics needed to formulate a theory along these lines. [...] Einstein did not give up the Einstein-Grossmann theory once he had established that it could not fully explain the Mercury anomaly. He continued to work on the theory and never even mentioned the disappointing result of his work with Besso in print. So Einstein did not do what the influential philosopher Sir Karl Popper claimed all good scientists do: once they have found an empirical refutation of their theory, they abandon that theory and go back to the drawing board. [...] On November 4, 1915, he presented a paper to the Berlin Academy officially retracting the Einstein-Grossmann equations and replacing them with new ones. On November 11, a short addendum to this paper followed, once again changing his field equations. A week later, on November 18, Einstein presented the paper containing his celebrated explanation of the perihelion motion of Mercury on the basis of this new theory. Another week later he changed the field equations once more. These are the equations still used today. This last change did not affect the result for the perihelion of Mercury. Besso is not acknowledged in Einstein's paper on the perihelion problem. Apparently, Besso's help with this technical problem had not been as valuable to Einstein as his role as sounding board that had earned Besso the famous acknowledgment in the special relativity paper of 1905. Still, an acknowledgment would have been appropriate. After all, what Einstein had done that week in November, was simply to redo the calculation he had done with Besso in June 1913, using his new field equations instead of the Einstein-Grossmann equations. It is not hard to imagine Einstein's excitement when he inserted the numbers for Mercury into the new expression he found and the result was 43", in excellent agreement with observation." Janssen, M. (2002) The Einstein-Besso Manuscript: A Glimpse Behind the Curtain of the Wizard. In The Collected Papers of Albert Einstein (Vols. 1-10, pp. 1987-2006). Princeton, NJ: Princeton University Press.
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