# How long can you be trapped orbiting around a black hole for?

I have a story I wanna write but I want to be sure it's not completely scientifically irrelevant.

I know there is black hole modelisation called the Kerr black holes, in which there is a limit around black holes called the event horizon. It's a spatial limit ; no light ray crossing this limit will ever be able to escape the gravity of the black hole. That's the definition of event horizon.

By extension, I guess any physical object (such as a spaceship) crossing the event horizon will be doomed to orbiting around the black hole until it gets destroyed. My question is about the time you can spend between the moment you cross the event horizon (= you are doomed) and the moment you actually die (for example by the tide effect, which basically destroys bodies because of the difference of attraction force between the feet and the head of a human body)? Could this moment last... 100 years ? 1000 years ? (for my story the more the better)

I know black holes studies are very theoretical, I'd just like to avoid any huge scientific plot holes.

I am quite unskilled in that domain of astronomy, about time dilatation, etc. So if anyone has any idea of the order of magnitude of the time a object can spend beyond a black hole event horizon before it gets destroyed ?

• If it was me, I'd work with a super-massive black hole not a stellar mass one, cause then you're working with a radius in light hours, perhaps light-days if you go to other galaxies and the tidal forces aren't likely to tear the ship appart, where a stellar mass one, the radius is a fraction of a light second and scientifically it's a mess. I'd also make use of the photonsphere, where light is bent around the black hole but not inside the even thorizon. The ship would need to maintain constant acceleration just to stay inside the photonsphere, but escape would still be possible. Commented May 20, 2016 at 2:58
• Hi, thanks for your complete answer ! I've looked up what a photonsphere is on wikipedia and... I can't seem to really grasp the difference with the event horizon. Aren't those both regions surrounding a black hole from wich light can not escape ? Commented May 20, 2016 at 14:32
• Ok I misread your comment "inside the photonsphere escape would still be possible" (I read "not possible"). So yeah that's not what I am looking for in my story ; a super massive black hole's event horizon is perfect. Thank you for your lights Commented May 23, 2016 at 12:06

You're right to mention time dilation, because the point of view is important. The standard metaphor is that of an astronaut (A) falling towards the event horizon, wearing a watch, while another astronaut (B) watches from a distance. Astronaut A will simply fall through the horizon and hit the singularity, but B will never actually see A cross the event horizon, instead they will see A's watch run slower and slower.

In terms of survival time, bigger is better. Getting too close to a really small black hole could rip you apart with tidal forces without even having to cross the event horizon (just as moons and planets exert tidal forces on each other) - but with a supermassive one you'd not even notice you'd crossed it.

For a relatively "standard" black hole of about 30 solar masses the infalling astronaut will have around 0.0001s between crossing the EH and hitting the singularity (from their point of view....see above, B will never see them cross the EH).

• This is not completely true. You have no clue as to how long it will take to reach the singularity after crossing the EH. Your angle of attack to the EH will weigh greatly into the matter. It is very possible to be set in an orbit around the singularity for thousands of years worth of time if you hit it at a specific angle. The EH only guarantees that you cannot escape the area that is within the EH. It does not imply anything about the amount of time it will take to reach the EH (even if we are talking about the time from the point of view of Astronaut A)... Commented May 18, 2016 at 19:22
• Yes, good point, I'm talking about a direct "fall straight down", orbital mechanics still apply. Commented May 18, 2016 at 23:09
• Also to add to it, reaching the singularity is also hypothetical. If the black hole is rotating (more than likely, it is), there are theories that suppose that nothing can fall into the singularity as the concept of time falls just before it. This is the point when relativity fails. Simply put, though logically it makes sense that the singularity is reachable, making the math do that isn't possible right now and won't be until a theory of quantum gravity can be discovered. Commented May 18, 2016 at 23:44
• @blahfunk so, there is a lot of matter in limbo until we come up with that theory. What if we never do? It just hangs there?
– user11722
Commented May 19, 2016 at 3:58
• @nocomprende if we never do, then we will never know, it is all in the realm of pretend bcz until we have a solid theory of quantum gravity, what happens at the quantum level with that much matter is totally a guess at best. Commented May 19, 2016 at 4:02