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Black holes were first seriously theorized in the early 20th century, but it took many decades before one could be observed; depending on your definition of "observe," this may have occurred as early as the 1998 research theorizing that only a black hole could explain the orbits of objects around Sagittarius A*, or as late as the 2019 direct imaging of M87*. And these observations were achieved by astronomers laboring under the consensus that black holes probably exist.

Assuming that scientific/engineering research progressed at about the same pace (up until this discovery, which would fork its development significantly), what are the closest and/or furthest locations where a black hole could be located such that it will be observed before it would be theorized in the 20th century, and how early would that observation happen? "Observe" being defined as "collect observations sufficient to convince astronomers that an object defying understood physics exists at the specific location of a real black hole," so something like the Sag A* research would count.

Both the upper and lower limit on distance are interesting questions - a black hole as nearby as the moon would need to be small and lack an incandescent accretion disk in order to not disrupt life on Earth, which might make its optical lensing effect too small to observe with pre-20th century telescopes. Black holes further away can be larger and have incandescent accretion disks without cooking Earth, but they're harder to observe and you would need to be able to tell it apart from a normal star based on historical observation - just being bright isn't sufficient.

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  • $\begingroup$ There are theories that some meteor impacts are actually effects of micro black holes passing through Earth. Would this mean that lower limit is 0? $\endgroup$
    – Alexander
    Commented Nov 10, 2022 at 21:40
  • $\begingroup$ Don't forget Cygnus X-1. You could consider it an observation from 1964. $\endgroup$ Commented Nov 10, 2022 at 23:34
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    $\begingroup$ Didn't one of Newton's contemporaries posit that a sufficiently dense mass might have gravity so high even light couldn't escape? $\endgroup$
    – Zeiss Ikon
    Commented Nov 11, 2022 at 16:42
  • $\begingroup$ I wonder if there were a black hole as nearby as the moon, wouldn't there always (unless eclipsed) be a path around it that bent the light from the sun toward our eye? I feel like it might deliver the same energy as the sun, i.e. have the same apparent brightness. $\endgroup$
    – Wyck
    Commented Nov 11, 2022 at 19:25
  • $\begingroup$ There would always be a path to bring sunlight, but we'd see a crescent or ring (depending on orbital position) and it would be nowhere near as bright as the sun. $\endgroup$
    – Zeiss Ikon
    Commented Nov 11, 2022 at 19:41

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Although philosophers had suggested such things earlier, it wasn't until 1915 that Einstein provided us with a mathematical basis to bring the concept out of the realm of angels into the world of physics. Anything before then would then qualify.

Not Actually Neptune

Anything within our solar system would be visible as an occasional flare when something passed too close. Neptune was spotted in 1846 based on its gravitational effects on the other planets. It could very well have been a black hole for all the effect it had on history.

Proxima Centauri is another of those that had to be spotted with a large telescope, but would have been a fun mystery if they looked and saw nothing or, even better, an accretion disk.

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    $\begingroup$ Good catch that Neptune was found through gravitational inference! Would have certainly been a shocker for everyone to get all excited about the search for the the first mathematically predicted planet only to find a black hole. $\endgroup$
    – Willa
    Commented Nov 11, 2022 at 5:38
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A binary star system plus a black hole.

It is pretty easy to see stars which are fairly close. Your star system has a main solar system and a fairly close star which orbits with it. In that nearby star system there's a black hole pretty close to the star.

Fairly simple newtonian calculations should reveal to an astronomer that the gravity in the star near their sun is acting in a really funky way, that can only be explained by a large gravitational mass acting on the star and planets.

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  • $\begingroup$ That could be a description for dark matter and dark energy as well. Also the same explanation was once tried for Mercury's messed up precession. $\endgroup$ Commented Nov 11, 2022 at 1:00
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    $\begingroup$ @TheSquare-CubeLaw At least with our current understanding this black hole scenario can be easily distinguished from dark matter. The black hole points to something that has gravitational effects that correspond to a mass similar to that of a sun but concentrated on a volume much smaller than a sun. Currently observed dark matter effects only happen on much much larger scales and correspond to a mass density that is neglible. $\endgroup$
    – quarague
    Commented Nov 11, 2022 at 9:51
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    $\begingroup$ From what I've read yeah the estimate is we have a bit less than a Ceres of mass worth of dark matter in our solar system. Black holes are a lot more dense. Dark matter has a big impact because there's lots of empty space between solar systems, not because it is a large percentage of mass in an average solar system. $\endgroup$
    – Nepene Nep
    Commented Nov 11, 2022 at 9:58
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    $\begingroup$ @NepeneNep most of the dark matter is also concentrated in a halo around the edge of the galaxy, rather than within the main bulk of the galaxy where the baryonic matter lives $\endgroup$
    – Tristan
    Commented Nov 11, 2022 at 10:54
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I don't think this can be answered properly.

Observe" being defined as "collect observations sufficient to convince astronomers that an object defying understood physics exists at the specific location of a real black hole (...)

This is the bread and butter of scientists. When we run out of phenomena to explain they will all be out o job other than infotainment consultants.

As or black holes specifically: before relativity was a thing, there was a theory of dark stars, which will also be the name of metal band someday. This dates back to the 1783. So as soon as newtonian physics become a thing, people will be on the lookout.

And they will find a lot of stuff. Whenever something is amiss, someone will shout dark star/black hole. Even today, people are looking for a planet in the solar system beyond Pluto (the infamous Planet X), and some... unorthodox people are claiming it is actually a small black hole.

An earlier example: Mercury precedes differently than what would be expected under purely newtonian physics. Relativity resolves this, but before it became mainstream, there were still astronomers looking for a planet or something else orbiting really close to the sun and messing up with Mercury. I'm fairly sure dark stars were considered by some.

I think that for us to look at something and say "yep, this can only be either the gravitational effect of an invisible yet extremely massive body or otherwise the work of Satan" - and be right about it - without knowing what a black hole is, the thing would to be so close to our face that the Earth probably wouldn't last long.

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