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Recently I came across a video were the origin of inertia was attributed to Sciama’s paper (1953). I have seen only a couple of questions regarding this topic on Stack Physics. Both of them are showing some level of confusion, e.g., it is not clear why Sciama used G=1 at times, but not at other times, or that it predicts a critical density that is out of a factor of two (I am not sure if this is too much, given the conditions of predictions Sciama was working with).

However, despite being seminal, Sciama’s work is still largely unknown and somehow a fringe theory. The confusion around the questions on Stack, or the rarity of questions linked to it, is even more convincing to me that Sciama’s paper (with only 300 citations from ADS) is underrated or has flaws I can’t see.

Why has Sciama’s theory not been experimentally tested? And what would be a possible test for it?

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    $\begingroup$ What experimental result motivates this theory? $\endgroup$
    – John Doty
    Commented Mar 13 at 13:53
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    $\begingroup$ Inertia is a concept that unifies models of a wide range of observations. The observations are its foundation. But if you had a theoretical concept for its origin, then what would it stand on? You'd need another theoretical concept to support it. "Turtles all the way down." $\endgroup$
    – John Doty
    Commented Mar 15 at 0:00
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    $\begingroup$ “All models are wrong, some are useful.” -- George Box. Unless you have observations motivating a concept, it isn't physics. It may be math (nothing wrong with math), but it has no physical significance. $\endgroup$
    – John Doty
    Commented Mar 15 at 15:09
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    $\begingroup$ This isn't a theory of gravity, it's a few rough estimates of the size of gravitomagnetic-like effects, which are also present in GR. Claiming this replaces GR is like saying a verbal version of Lenz's law replaces Maxwell's equations. In the present day, GR has been tested quite precisely, both in the regime where its corrections are small and in the strong-field regime, so any theory of modified gravity has to start by matching GR and then introducing small corrections. So the answer is that there is no theory here, and if it was extended into one, it'd probably be already ruled out. $\endgroup$
    – knzhou
    Commented Apr 10 at 18:02
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    $\begingroup$ Also, in the future I would strongly advise against trying to learn anything from Unzicker's YouTube channel, which is entirely worthless. He makes content by taking isolated phrases from 75 year old papers totally out of context, and using that to claim that all of modern physics is wrong. You know astrophysics, so how would you react to a tremendously popular channel that says pulsars don't exist, because Jocelyn Bell's supervisor said that the first pulsar signal was probably noise in the fall of 1967? Or that there's no CMB, because some astrophysicists dismissed the idea in the 1950s? $\endgroup$
    – knzhou
    Commented Apr 10 at 18:05

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There is a discussion of Sciama's paper in Williams and Inan's paper Maxwellian mirages in general relativity, New J. Phys. 23 (2021) 053019:

https://iopscience.iop.org/article/10.1088/1367-2630/abf322/pdf

Sciama's proposal was a linear theory that could only approximate general relativity in some approximations under suitable circumstances in a weak gravitational field. Sciama's proposal for explaining inertia by gravitational induction has not been successfully reproduced in general relativity or in any of the alternatives currently under consideration although attempts were made to do so.

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