this post was submitted on 09 Aug 2025
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I feel like the first step would be a specialized instrument to detect them. We are not just going to stumble on one.
That is way over my head, and I’m trying to think how that would even work… maybe a wide field telescope and a software stack on the ground specifically built to catalog the “wobble” of stars and find invisible binary partners? It would double as an exoplanet detector, and IIRC there are already systems doing this.
I don’t even know how we'd detect solo black holes. According to this:
https://svs.gsfc.nasa.gov/10545
Accretion disks seem to have peaks around 7KeV, so maybe a very specialized x-ray telescope? The sensitivity/resolution would have to be mad.
This has indeed already been done! In fact, the closest known black hole to earth was discovered by GAIA, a space telescope that collects this kind of data.
We've also done what are called 'microlensing surveys' that look for the effect of spacetime distortion on background stars rather than the wobble of binary partners. Some of these have already found candidate objects over the years, however the new Vera Rubin observatory that's just come online is expected to be really good at this sort of thing so we should spot many more over the next few years.
We've done this too! The Chandra space telescope has discovered hundreds of thousands of x-ray sources throughout the universe, including many, many black holes. Most of those are supermassive black holes at the centers of other galaxies, but hundreds of "local" objects have been found as well.
Awesome! Apologies, I didn't look around much, and was going of documentaries floating around in my head, though I did remember Chandra and GAIA.
It just goes to show we need to continue! Specifically, I was thinking something with more resolution than Chandra (and maybe a narrower FoV?) could pick up much dimmer accretion disks, though I have no idea how long they last or if these even exist around "quiet" stellar mass black holes.
I will have to look into Vera Rubin!
I may be wrong, but I suspect that any nearby black holes (i.e. within a few dozen light-years) with active accretion disks would already be visible to us in visible light and would also be bright enough in x-ray emissions that prior searches would have uncovered them.
In my limited googling, the smallest active black hole I could find was A0620-00A, which is about 6 solar masses. Its accretion disk is visible in x-rays from 3000 light-years away, so I assume any small black holes accreting matter anywhere near us would also be visible.
So more sensitive x-ray instruments would be useful for finding more distant SMBHs, but not necessary for finding any small, nearby black holes that we could actually stand a chance of reaching with a spacecraft. Most likely there just aren't any active black holes in our neighborhood — only quiet ones we can't see in x-rays.
Yeah. I thought small accretion disks might persist... But probably not.
Outside of invisible binary partners (which higher resolution "wobble" detectors could catch), it's hard to imagine detecting quiet black holes. Even theoretically... like even radiation or particles they perturb would be impossibly faint from so far.