Inspired by Spaceship orbiting a black hole under the event horizon - a prison where no one escapes, I was wondering if it is possible to make a space elevator that goes down into a black hole.

In particular, I imagine it would have the following parts:

  • The space prison, where the prisoners can play Yahtzee for the rest of their lives. It is below the event horizon slightly.
  • The space anchor, which is connected to the space prison. This is basically a giant counter weight orbiting the black hole, outside of and far away from the event horizon. It is tethered to the space prison. Also, the wardens are here.
  • Elevators: The elevators take new prisoners to the space prison. When the elevator goes into the black hole, it can't get out, so its just ejected into the singularity. That means you need a new elevator each time you go down into the black hole.

My question is, is such a prison even physically possible?

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    $\begingroup$ "the rest of their lives" will be brief. Once inside an event horizon you inevitably fall into the singularity, regardless of what direction you try to travel in or how much energy you try to use doing that. So they're on death row, not in prison. The entire idea is deeply flawed and you need to learn more about black holes to see why. Try this video. $\endgroup$ Commented Feb 12, 2018 at 5:13
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    $\begingroup$ Why would you put wardens to a place not even light can escape from? $\endgroup$
    – L.Dutch
    Commented Feb 12, 2018 at 5:34
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    $\begingroup$ @L.Dutch escort prisoners to the elevator. Hold them before they go on the elevator (if none are available the time). Send down new editions of Yahtzee. $\endgroup$ Commented Feb 12, 2018 at 5:54
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    $\begingroup$ I don't understand what the tether buys you, and I don't understand how the tether is tethered at it's "down" end. $\endgroup$
    – AlexP
    Commented Feb 12, 2018 at 6:06
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    $\begingroup$ As I have explained, your ideas about black holes are incorrect. As you want hard science there is simply no way to do any of of the things you are describing. The tether is just ridiculous, as there is no way to construct such a thing. From the outside the tether never crosses the event horizon. Any tether inside would instantly start falling to the singularity and there is no way to support it from outside or inside to prevent this. Read about real black holes and you'll see why this whole idea is nonsense. $\endgroup$ Commented Feb 12, 2018 at 15:14

3 Answers 3


The space prison, where the prisoners can play Yahtzee for the rest of their lives. It is below the event horizon slightly.

No, it isn't. It is below the event horizon significantly. If you want your prison to remain at a roughly constant distance from the central singularity, it will have to be inside a rotating and/or charged Kerr-Newman black hole, and positioned inside the inner horizon. If it's not a Kerr-Newman black hole, the prison will inevitably fall into the singularity in a very short time--a matter of hours, even for the very largest of hypermassive galactic black holes. If it's not below the inner horizon, it will inevitably fall through the inter-horizon space in a Very Short Time.

That also means that it is not possible to string a static tether between the prison and an orbiting anchor station outside the black hole. Could you build an anchor station with a tether going down to the event horizon? Possibly. The black hole would have to be rotating ridiculously quickly to put the innermost stable circular orbit at a low enough altitude to permit the construction of a cable that could reach the event horizon without breaking under its own weight, but it's not physically prohibited. And while the control systems would be ridiculously sensitive, if you can build an orbital station, you could even probably build an orbital ring which could support an arbitrary mass of tethers and prison facilities arbitrarily close to the black hole.

But the exterior anchor station will always be separated from the interior prison by the inter-horizon space. And any piece of tether that crosses the outer horizon will be inevitably transported through that space, as inevitably as it moves forwards in time, until it crosses the inner horizon. This is true even if the black hole is massive enough to make tidal effects (and therefore spaghettification) utterly negligible at the outer event horizon. Tidal effects have nothing to do with it. Bits of the tether that are below the outer event horizon are moving in a different direction through time than bits that are outside, and they cannot remain connected.

So, you cannot have a static tether. You could, maybe, continuously feed tether material down into the black hole, but what would be the point of that? In that sort of extreme environment, there is no practical difference between that, and just firing a stream of individual atoms of tether material into the black hole. It gains you nothing over just dropping an "elevator car" freely, with rockets to ensure that it can (theoretically--after all, you'll never know if it actually made it or not, once it crosses the event horizon) navigate to and dock with the prison on the other side.

  • $\begingroup$ Ah, okay. I guess the photons that communicate the internal bonds of the tether can't escape the black hole, right? $\endgroup$ Commented Feb 13, 2018 at 18:51
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    $\begingroup$ @PyRulez Nothing escapes a black hole. $\endgroup$ Commented Feb 13, 2018 at 18:52

No, the differing gravitational forces exerted upon an object as it moves closer to a black hole's event horizon tear the object in half. As an object moves towards the black hole it will be torn in half fractally down to the sub-atomic elemental level. IE, you get torn in half, then those pieces get torn in half, then those pieces get torn in half etc etc down to your constituent quantum particles. There are also relativistic effects to the person entering the event horizon they would experience the above obliteration nearly instantly, while to the outside observer watching them they would appear to slow in infinitely smaller increments until they seemed to come to a complete stop and be frozen in stasis.

Its kinda mind-boggling to describe, but I will use a scenario to describe it: If you were to toss a digital stop-watch into a black hole you would see it keeping track of time slower and slower. The second counter would begin counting in periods of minutes, then hours. eventually it would stop seemingly frozen. Then the light from the screen would begin to red shift down the spectrum until it was in the radio-wave length of electromagnetic energy. You would need a radio telescope and several years to watch the second counter count only one second, then decades, then centuries. Eventually it would slow down so much you would be stuck waiting billions of years in between individual photons emitted by the screen to reach you.

So nothing can actually enter a black hole from the point of view of an outside observer, it can only get infinitely closer to it in infinitely longer and infinitely smaller increments without ever touching the event horizon. Meanwhile to the object entering its just immediate obliteration.

  • $\begingroup$ I don't think spaghettification happens as soon as you hit the event horizon. See physics.stackexchange.com/questions/187917/… $\endgroup$ Commented Feb 12, 2018 at 4:47
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    $\begingroup$ Tidal forces this strong only happen near the event horizon in some cases. For instance, by a supermassive black hole, you'd (counterintuitively) be just fine. $\endgroup$
    – HDE 226868
    Commented Feb 12, 2018 at 5:45
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    $\begingroup$ From a local point of view, the event horizon is not special in any way. That is, an observer who can perform only local measurements cannot tell whether they passed the event horizon or not. $\endgroup$
    – AlexP
    Commented Feb 12, 2018 at 6:05
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    $\begingroup$ Aside from spaghettification which would assuredly give you a bad day, to your local senses, time travels normal, however the outside universe would proceed at an exponential rate. By the time you reached the actual black hole, the universe would have ended. So no time for a game of Yahtzee. $\endgroup$
    – Neil
    Commented Feb 12, 2018 at 9:44
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    $\begingroup$ @ArtisticPhoenix These ideas about black holes are not correct. When dealing with concepts like cross the event horizon you need to understand the very significant difference between what a distant observer sees and what the object crossing the event horizon sees. $\endgroup$ Commented Feb 12, 2018 at 18:48

If it is a large (massive) black hole, the event horizon should be far enough out to prevent tidal sheering of small objects like people.

However, unless you have FTL tech, you can't get anyone back from past the event horizon. Encyclopedia Britannica has a very good definition of the Event Horizon.

Event horizon, boundary marking the limits of a black hole. At the event horizon, the escape velocity is equal to the speed of light. Since general relativity states that nothing can travel faster than the speed of light, nothing inside the event horizon can ever cross the boundary and escape beyond it, including light. Thus, nothing that enters a black hole can get out or can be observed from outside the event horizon.

So, even if the tether can survive the tension forces, it would be a one way trip. The elevator could never come back up unless it is an FTL elevator.

Even if it is possible to build an FTL elevator in your story, that seems to be an awful lot of work to make a prison.

Another issue is time dilation. The rest of the universe would be running much faster than the prison. You would get the same effects by putting the prisoners into cryo and then thawing them out at a later time.


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