Ethan Siegel: "Scientific Theories Never Die, Not Unless Scientists Choose To Let Them. When it comes to science, we like to think that we formulate hypotheses, test them, throw away the ones that fail to match, and continue testing the successful one until only the best ideas are left. But the truth is a lot muddier than that. The actual process of science involves tweaking your initial hypothesis over and over, trying to pull it in line with what we already know. [...] By the addition of enough extra free parameters, caveats, behaviors, or modifications to your theory, you can literally salvage any idea. As long as you're willing to tweak what you've come up with sufficiently, you can never rule anything out."
https://www.forbes.com/sites/startswithabang/2017/11/16/scientific-theories-never-die-not-unless-scientists-choose-to-let-them/
Sabine Hossenfelder (Bee): "The criticism you raise that there are lots of speculative models that have no known relevance for the description of nature has very little to do with string theory but is a general disease of the research area. Lots of theorists produce lots of models that have no chance of ever being tested or ruled out because that's how they earn a living. The smaller the probability of the model being ruled out in their lifetime, the better. It's basic economics. Survival of the 'fittest' resulting in the natural selection of invincible models that can forever be amended."
http://www.math.columbia.edu/~woit/wordpress/?p=9375
Before 1915 theoretical physics was mainly DEDUCTIVE - you cannot tweak (add a fudge factor to) your theory unless the tweak is deducible from initial axioms (postulates). In 1915 Einstein replaced deduction with induction, and unlimited ad hoc tweaking, unrelated to any axioms, was allowed. Here Michel Janssen describes relentless tweaking performed again and again until "excellent agreement with observation" was reached:
"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
In order to be consistent with dark matter, general relativity needs four fudge factors:
"Verlinde's calculations fit the new study's observations without resorting to free parameters – essentially values that can be tweaked at will to make theory and observation match. By contrast, says Brouwer, conventional dark matter models need four free parameters to be adjusted to explain the data."
https://www.newscientist.com/article/2116446-first-test-of-rival-to-einsteins-gravity-kills-off-dark-matter/
How many fudge factors LIGO fakers needed to model the nonexistent gravitational waves is a deep mystery:
"Cornell professors Saul Teukolsky, astrophysics, and Larry Kidder, astronomy, played an instrumental role in the first detection of gravitational waves, a century after Albert Einstein predicted their existence in his theory of general relativity...The LIGO and Virgo group confirmed that these gravitational waves had come from the collision of black holes by comparing their data with a theoretical model developed at Cornell. Teukolsky and the Cornell-founded Simulation of eXtreme Spacetimes collaboration group have been developing this model since 2000, according to the University."
http://cornellsun.com/2016/02/10/cornell-scientists-validate-einsteins-theory-of-relativity/
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