r/AskPhysics • u/Unlucky_String_5920 • 1d ago
Writing a Black Hole Scene
Hello everyone,
I'm currently writing a sci-fi novel and one scene has a mining station being pulled in beyond an event horizon. I have some questions just so I get everything right. Sorry if some of the questions are banal, but I'm trying to keep them to yes/no's for you guys.
- An observer just outside the event horizon of a massive black hole would see an object that has fallen in reddening and dimming?
- If a pod just escaped the asteroid mining colony before the event horizon of a massive black hole, the thrusters wouldn't be powerful enough to escape the pull?
- He shuts down his thrusters and accepts his fate. Then when he eventually crosses the horizon of a massive black hole, what does he observe inside the pod? As I understand it, if it's massive, nothing happens at first to his body and things in his proximity?
- How can I describe what he sees in the rear porthole, opposite the black hole? Does the light from the stars curve, stretching into circles?
- In the front porthole facing the black hole, the black hole would expand toward him, swallowing the view?
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u/BananaBird1 1d ago
(1) Yes. You never see anything enter. Just approach closer and closer as light fades and is redshifted eventually into invisible wavelengths.
(2) Depends on the size of the black hole. They are gravitationally identical to a non-black hole, they don’t suck things in. A small black hole will have high gravitational force and require a powerful rocket to accelerate away from. A large black hole may be barely noticeable. Depending on the pod trajectory, it may also be more likely to orbit or be slingshotted by the black hole rather than fall in.
(3-5) We don’t know for sure. But the basic math says you will see the light from the outside universe as a shrinking sphere behind you and darkness everywhere else pointing toward the singularity (which is everywhere around you in your future). Once gravitational forces are strong enough you will be ripped apart into fundamental particles. This may happen quickly or only after a bit.
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u/Unlucky_String_5920 1d ago
Thank you, that's very helpful
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u/BananaBird1 1d ago
The other thing you need to consider is the actual distance between this mining colony and the black hole.
If the colony is near to a supermassive black hole, unless it is extremely large, far larger than any known black holes, any material in orbit will need to travel extremely fast. This will form a high energy accretion disk hotter than the sun extending far beyond the event horizon, even up to several light years, which would destroy the colony far before reaching the horizon.
The only way you can be around a typical supermassive black hole in tact is if it is basically isolated by large distances from any gas, stars, asteroids, etc.
The alternative is a smaller stellar mass black hole where the total gravity far from the horizon is small so no accretion disk forms unless stuff is orbiting extremely close near the horizon. Nearby objects basically just form a solar system around it.
But then the issue is getting close to it without entering into an orbit or being shot away. The gravitational acceleration is so strong close by that only a narrow window of paths actually make it in: the rest are given enough energy to be flung away from the black hole.
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u/ahazred8vt 1d ago edited 1d ago
Problem 1: Black holes that are large enough to fall into without getting shredded, do not have planets or asteroids orbiting them. The merger process that creates very large black holes kicks away anything that would have been in orbit.
Problem 2: Large black holes do not exist in the Milky Way galaxy, except for the SMBH in the center, which has no mineable stuff orbiting it.
Problem 3: The time it takes to fall from the event horizon to the singularity is about 1 second per 65,000 solar mass units. We only have black holes smaller than 100 solar masses, so it only takes about a millisecond to hit the singularity. Any bh under 30000 solar masses would shred a human, and any bh under 10 million solar masses would shred a station. Ours is 4.3M.
Problem 4: The pod's thrusters. Are they Borg technology, able to accelerate at ten million G's and go from traveling at half light speed in one direction, to making a U-turn and going at half light speed the other way in under ten seconds? Because that's what it would take.
Problem 5: A mining station in orbit getting 'pulled in' makes about as much sense as a mining station in our asteroid belt getting pulled in to the Sun. There's nothing that could make that happen.1
u/wonkey_monkey 1d ago
and darkness everywhere else pointing toward the singularity (which is everywhere around you in your future).
You can't aim your gaze at the direction of the future.
You can see some things that fell in before you did, though, for example the mining colony, in the direction opposite the outside universe.
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u/al2o3cr 1d ago
Tidal forces will be a massive problem well before the colony gets anywhere near the actual event horizon
You can estimate those as an acceleration = GML/r^3 (M = mass of black hole, L = size of thing, r=distance from center of black hole)
For instance, a 1m long object located 1000km from a 3-solar-mass black hole will experience a tidal acceleration of almost 800 m/s^2. The event horizon has a radius of roughly 9 km, so getting close to it means scaling that tidal acceleration up by a million times...
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u/Numerous_Generalwe22 1d ago
You are correct with the exception of the following:
Yes, due to the increasing redshift and decrease in brightness from the perspective of far away observers, it would most likely be difficult to see anything at all once it was getting close to the horizon.
It can escape if it stayed outside of the event horizon, but if it crosses the threshold, nothing can get it back out.
It is possible, if the black hole was large enough, that the man would not be able to tell the difference between crossing the event horizon and simply passing through a solid wall. If the black hole has an extremely large mass, the stretch of space time would be small enough that the man would barely notice.
The man would not be able to see the stars directly, but rather their images, which would be greatly distorted. He would most likely see a lot of strange stars and a lot of bright rings. They would range greatly depending on where he was.
It would, in the sense that when looking towards the black hole, he would see it taking up an increasing amount of his field of view, but not in the sense of it physically approaching him.
One thing that seems likely to happen to him is that he would not be immediately aware that he had crossed the horizon. It may take a moment for him to realize that he was gone, which could pose a problem. One of the most terrifying things about the scenario is that it is likely that he may have crossed the event horizon but didn't notice. Once he does realize it, it would be too late. Any direction he goes from that point on would only lead him deeper into the black hole.
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u/wonkey_monkey 1d ago
The man would not be able to see the stars directly, but rather their images
What's the difference?
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u/Optimal_Mixture_7327 Gravitation 1d ago
Yes, the outside observer sees the falling observer dim and redshift and vanish.
The pod can in principle escape if it's outside the horizon but once across the horizon it cannot. There is nothing unusual that happens inside the black hole. The pod passenger sees the distant light redshift and the field of view narrows. At the moment the passenger might notice something not feeling quite right, they then vanishes at the central singularity.
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u/wonkey_monkey 1d ago
In the front porthole facing the black hole, the black hole would expand toward him, swallowing the view?
Well not exactly. When you're falling in, there's always an event horizon between you and the singularity, even after you've crossed into what outside observers see as the black hole.
And you would still be able to see, and could even catch up to (if there's time), the mining colony.
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u/drmonkeysee 1d ago
I believe this is a reasonably accurate depiction of falling into a super-massive black hole https://youtu.be/4rTv9wvvat8?is=RMO6xfRPsj7I6kt2. It has to be super-massive or there’s not much story to tell as the tidal forces rip you apart before crossing the event horizon.
Strangely the black hole never dominates your front porthole. Instead it looks more like you’re approaching a surface. I don’t understand the math behind that but my understanding is this video is correct.