If you are familiar with Newton's bucket, you may skip to 6:10.
Until recently, I had not realized the flash of genius of Dennis Sciama who linked inertia and gravity in a Machian way already in 1953.
Fay's paper: https://relativemotionorg.files.wordp...
Sciama's paper: https://academic.oup.com/mnras/articl...
See also: https://www.amazon.com/Einsteins-Lost...
Thanks to Jonathan Fay for pointing my attention to it: http://www.jonathanfay.com

Mind also my backup channel:
https://odysee.com/@TheMachian:c
My books: www.amazon.com/Alexander-Unzicker/e/B00DQCRYYY/