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.wordpress.com/2024/01/on_sciama_1953.pdf
Sciama's paper: https://academic.oup.com/mnras/article/113/1/34/2602000
See also: https://www.amazon.com/Einsteins-Lost-Key-Overlooked-Century/dp/1519473435/
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/