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I have written a book in which a planet orbits a star that orbits a black hole. I would like to know if this concept is completely impossible, or if it could, in theory, occur. I realize this all depends on the size of the black hole and many other details, but what sort of circumstances would there need to be for this to theoretically work without ticking off the science community too much?

I would also like to know what the black hole might look like from the surface of the planet, if it would even be visible through the planet's atmosphere.

Lastly, I know that anything close to a black hole is pretty well doomed--but what's the timeline for that sort of thing? Let's say life is possible, and a person lives on this planet (even if this is out of the question impossible). Would the planet be likely to enter the event horizon in a person's life time if the planet was already close enough to see the presence of the black hole from surface of the planet? Does that take thousands of years? Or more like hundreds? Less? More? Obviously the planet has to go in at some point, so I fear that question may be somewhat unanswerable. I want to know if it seems plausible that someone on this planet, who is aware of the black hole, would be actively concerned about it crossing into the event horizon.

Here are some details about the planet's conditions to potentially help with answers:

  • The days are 36 hours long
  • There are sixteen weeks of complete darkness as the sun passes the black hole. My theory is that the black hole would suck the light away from the planet, plummeting it into darkness for at least half the year. It is cold and makes the planet difficult to inhabit, causing food shortages resulting from the lack of light and warmth.
  • There are approximately 6 weeks of twilight both in the spring and fall as the sun moves away/toward the black hole in its orbital pattern. - As the planet passes the black hole, and the sun is at its furthest from the black hole, there are 16 weeks of bright light as the light and heat from the sun is sucked toward the black hole. During this time, the planet is between the sun and the black hole, so it practically bakes for 16 weeks.
  • The majority of the planet is covered in water, but it can hardly be traversed due to massive and chaotic waves caused by the black hole's gravity, though it isn't as extreme as, let's say, the wave on Miller's planet from the movie Interstellar. I assume that would only happen if the planet was too close to the event horizon?
  • There are no visible stars in the night sky, as I have gathered that most things would get absorbed by the black hole (including any light from "nearby" stars that might reach the eyes of someone on the surface of this planet).
  • This book has some weird time travel-esque stuff going on as a result of the black hole. I know that, inside a black hole, current physicist theorize that the laws of physics may break down completely, so it seemed like a good opportunity as far as a sci-fi/fantasy portal travel goes. Feel free to make suggestions about possible theories that might be of interest or relate to this, even if they would have to rely on fantasy elements. I'm sure your knowledge of physics exceeds mine at this point.
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    $\begingroup$ Please edit the question to limit it to a specific problem with enough detail to identify an adequate answer. $\endgroup$
    – Community Bot
    Mar 31 at 3:33
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    $\begingroup$ Earth is a planet that orbits a star that orbits a black hole far outside its event horizon... $\endgroup$
    – AlexP
    Mar 31 at 3:47
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    $\begingroup$ Almost every aspect of this question is simply not how black holes work, to the point that it's difficult to know where to begin. $\endgroup$
    – Cadence
    Mar 31 at 4:09
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    $\begingroup$ Perhaps begin by getting rid of all (or most) of the bold text as it makes the question difficult to read. $\endgroup$ Mar 31 at 5:17

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what sort of circumstances would there need to be for this to theoretically work without ticking off the science community too much?

There is nothing inherently impossible to what you propose. We know of Star-Black Hole binaries. Happens all the time. We have to address a sampling bias here. A Black Hole does not emit any detectable radiation, you can only spot them through the effects on their environment, such as gravitational lensing or X-ray emissions. Almost all black holes we know of are those with massive accretion disks pumping out more Gamma ray emissions than stars in their entire life per day. We simply don't see the presumably many more inactive black holes. So it is entirely possible for a big black hole to not do much of anything and have a star system orbiting it. Sagittarius A*, while not inactive does have a couple of stars orbiting it.

I would also like to know what the black hole might look like from the surface of the planet, if it would even be visible through the planet's atmosphere.

There is a general rule here, if you can see a black hole you are too close. There is no such thing as an inactive black hole. And especially with another star orbiting it, there would be a very thin accretion disk. I.e. matter orbiting the black hole. This disk would not produce a lot of radiation in a galactic context, but for a Stellar one, it would be a death ray. The best you could probably do is something as bright as Venus in the sky.

In essence, if this is how your sky looks;

enter image description here

Your cancerns cancer will get stage 3 terminal cancer within a femtosecond. This problem gets worse the smaller the black hole is by the way. Large black holes have a physically bigger Event Horizon for matter to get spread out along. So there is less friction. Hence why really small black holes can become really angry.

Lastly, I know that anything close to a black hole is pretty well doomed--but what's the timeline for that sort of thing?

Dosn't have to be. Black Holes are not magic, and you can orbit them like any other massive object. Your orbits would be a bit fast, but other than that there is no particular reason you have to worry about your black hole any more than we do the sun.

There are sixteen weeks of complete darkness as the sun passes the black hole. My theory is that the black hole would suck the light away from the planet,

Not if you want anything living on the planet. As I said above, realistically speaking it would take 100s of years for any developing civilization to even notice they were orbiting something sussy. Or realsie that the bright spot in the sky is not a planet.

The majority of the planet is covered in water, but it can hardly be traversed due to massive and chaotic waves caused by the black hole's gravity, t

Nah, not any more than Venus effects tides on Earth.

There are no visible stars in the night sky,

If anything, it would be the opposite. The Black Hole would act as a gravitational lens and focus light onto the planet. So some small portion of the sky would look noticeably brighter.

Feel free to make suggestions about possible theories that might be of interest or relate to this,

This is extremely open ended. Most physicists are of the opinion black holes are not actual singularities. If I had to take a personal guess, I would bet on there being some "Quantum Degeneracy Pressure" that prevents matter from collapsing into it self past a certain density. So all black holes are just spheres of maximum density.

Especially with Rotating black holes you will often hear discussions about time travel, white holes and so on. It is important to note that all of that is almost certainly bs. Yes, the math of the Kerr Metric does allow for time travel. But the Kerr Metric (Describing Rotating black holes) is almost certainly just not correct inside the horizon anyways. Because the inside geometry (A one-dimensional ring) is based on absolutely no actual evidence, it's just a convenient energy distribution to work with.

That's the beauty of black hole physics, anything below the Horizon dosn't matter. Hence we why can use most certainly incorrect mathematical descriptions to study the physics, because they are correct outside the horizon and what is inside dosn't matter. Not like it can effect the outside world.

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A person would not be concerned with crossing the event horizon. The event horizon is a line where not even light can escape. Long before that, you deal with spaghettification which is 100% fatal for all living things. Crossing the event horizon might be a religious moment in time, but practically speaking, nothing will be living on the planet at that time (and, in fact, the planet will be torn into dust by that time).

The length of the day is immaterial. If you want it to be 36 hours, it can be 36 hours. Of all the forces involved, that is one which depends on the black hole the least.

The black hole will not plunge the planet into darkness. If you are close enough to have the gravitational bending of light meaningfully decrease the brightness at the planet, you're already within that region where life is doomed.

If the gravity of the black hole is the dominant forces in the tides, then the planet is not orbiting the sun, it's orbiting the black hole. It may be a major player in tides, but you're not going to see rogue waves.

Stars would be visible, with one exception. Black holes tend to have an accretion disk, containing the dust and particles that it is consuming. This gets hot and glows and might obscure the stars. Of course, being stuck in it is also really unhealthy for... everything. I'd encourage your planet be an extremely unusual case which comes in at a funny angle so that it only has to cross the accretion disk every now and then.

As for time travel, we don't know. We simply don't. But time travel inside the event horizon has very limited potential for affecting life outside the event horizon. We are not aware of any way something inside the event horizon affects the outside world, other than perhaps Hawking radiation (a topic that is beyond my physics understanding)

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Yes, if you start from a well-separated binary star system where the other star is three solar masses or more, the other star may become a black hole. If the orbit of the star is in the same plane as the black hole's accretion disk, but it is far enough away not to be part of it, then it ought to be stable. There are plenty of pictures of black holes on the net. Remember you will probably be looking from a point in the plane of the accretion disc, so this will be a narrow line, like looking at Saturn's rings edge-on.

If you are wanting a massive black hole such as the one at the centre of our galaxy, this is much less stable, with stars swooping in and out, and getting pulled to bits in a few Earth-years.. Maybe your technology can surf this chaos, but it does not look very friendly.

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It seems that your fundamental misunderstanding here is that you think of black holes as vacuum cleaners for light, which is not true: black holes do not "suck light in".

If you have a black hole and a star of equal mass, then as far as gravity is concerned, as long as you're in the region outside of the surface of the star, then you cannot tell the difference between the two. This is a consequence of Birkhoff's theorem.

As a result a black hole will only deflect light as much as a star of equal mass, the only difference being if the light gets so close to the black holes that it falls into the event horizon; a stellar mass black hole actually has a very small event horizon, so the vast majority of the time the black hole will curve the trajectories of light the same way a star would. In other words, you would still see all the stars in the sky.

This also causes a problem for your idea of the black hole blocking the sun for 16 weeks: during the time that the black hole might block some small portion of the light that would reach the planet, namely those rays that go directly into the event horizon, but most of the light will actually be redirected towards the planet -- you will have rays of light that would otherwise have missed being focused onto the surface, an effect known as gravitational lensing. What you will see is a black disc surrounded by a bright ring of light (from all the light being lensed onto the planet), known as an Einstein ring.

Finally, you talk about weird time travel-esque stuff because you claim that current physicist theorize that the laws of physics may break down completely, which is also not correct. Our theories of black holes make sense right up to the singularity in the middle, where the mass density/spacetime curvature becomes infinite. This is likely mollified by quantum gravity, whereby quantum effects cause the singularity to "spread out". This is especially clear in string theory, where the singularity is essentially a highly excited string. As you can see, this does not mean that all laws of physics are thrown out the window, we just have a slight modification making the infinitely dense object finite instead.

There are speculations that would cause more dramatic changes, such as the firewall hypothesis, but this only changes things inside the event horizon, and again it isn't that all laws of physics fall apart, it's just that the inside of the event horizon isn't actually there anymore. Still no room for time travel.

Noe, it is true that spinning black holes (Kerr metric) contain closed timelike curves: portals to the past! But these are likely just mathematical artefacts, like the infinitely dense singularity. I remember reading a paper that showed that any CTC must take a ridiculously huge amount of energy to approach from the outside, such that nothing could actually pass into one. It is generally accepted that quantum effects will remove these pesky paradoxes.

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