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Comment by sliken | Hacker News Reader

No need to perish, current theories explain that the innermost region of a spinning black hole (all real world black holes spin) has a low gravity region where arbitrary navigation is possible. You can't escape, but you also don't have a death date with the singularity.

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I always wondered if you can orbit inside the event horizon of a large black hole and wait until its hawking radiation shrinks the event horizon so that you find yourself outside. You would have to wait a long time I guess.

Won't this "long time" be only for the outside observer? Won't the person orbitting experience much less time and actually could get to watch the black hole evaporate?

"long time" here means several times longer than it will take for all stars to go out, so that's kinda an understatement

My layperson understanding is that collision with the singularity (if that even exists) is mathematically inevitable for an object that has crossed the event horizon. I think your scenario of hanging out within the event horizon and safely away from the singularity for indefinite time would require infinite fuel to counteract the gravitational gradient, or for even more fundamental reasons.

> or for even more fundamental reasons.

Even more fundamental: you'd need infinite fuel to hang out forever just outside the event horizon — once you're inside, the direction of the singularity is "future" not "forwards", so you can't resist getting there with any form of propulsion any more than you can resist getting to next Thursday with any form of propulsion.

This may be hopelessly naive but isn't the idea of GR that with sufficient fuel (and I suppose breaking some laws of physics) I can effectively postpone my experiencing next Thursday (here on earth) by moving away from earth at one lightsecond/second?

Other way around, the more you accelerate, the less time you experience between now and then.

And to emphasize the above point, it doesn't matter which "direction" you accelerate towards; the singularity is in the future, and you are approaching the future faster the more you accelerate.

You can orbit a black hole like any other gravity source, without spending fuel.

I don't know how near the event horizon a safe orbit can be?

That's kind of the definition of the event horizon. You cannot be 'safe' once you're inside. All paths lead to the event horizon. No matter which direction you point, you're pointed at it.

I am not too knowledgeable about black hole physics, but it was my understanding that there's nothing locally interesting about event horizon: it's just the point of no return that doesn't change much for the local observer. Your definition of the event horizon make it sound more locally important.

In fact, I know that as a local observer falling into a black hole you can still see some of the outside world after falling into the event horizon (by looking "behind you"), you just can't send anything back. This also seem to contradict the statement that all paths point inside (or I may misunderstanding something).

Edit again: I did some research and it looks like that while parent's comment may be true for simplified model of a black hole, it is conjured to be possible for rotating black holes where you can stay inside. Also Google "penrose diagram kerr black hole" for some weird physic if you want to follow this rabbit hole. Keep in mind that I'm not a physicist and this is my understanding after 40 minutes of watching YouTube and Wikipedia.

All paths point singularity-way but a cone will also see a false image from in falling photons. That cone will get smaller the further in you fall. That doesn't mean you aren't pointed at the singularity, only that some photons are going faster than you. It's like saying that because some traffic on the highway is passing you, you're actually going backward.

All paths inside the event horizon lead to singularity. Full stop. This is reinforced by the Penrose diagrams you mention.

GP asked about (presumably free-falling) orbits, they are unstable before reaching the event horizon.

The innermost stable circular orbit is 3 radii for a non-rotating black hole, less the faster it spins.

https://en.m.wikipedia.org/wiki/Innermost_stable_circular_or...

Thanks!

No stable orbits exist within 1.5 x the event horizon radius, so just outside that should suffice, just watch for anything else falling in.

For non-spinning blackholes (much like spherical cows that physicts use to simplify things) that's true.

For spinning blackholes not to much. The singularity turns into a ring, the center of which the gravity cancels out.

Uh... But how do you get there? My understanding as a lay-person is that you can't get past the event horizon without getting spaghettified, and cooked by high-energy blue-shifted radiation.

Getting past the event horizon of a small blackhole is tough, the gravitational gradient causes spaghetification. However larger blackholes lessen the gradient at the event horizon, so it's not a problem.

Not sure about the inner horizon, just saw a discussion of the paper for a spinning black hole recently, it described three distinct regions.

Most lay-person discussion of black holes just ignores the spaghettification and radiation problems. Those don't really have anything to do with space-time or information propagation or cosmology or such, those are just limitations of material strength and biology.

The discussion is more like, if we had infinitely resilient materials or biology, what could they observe and experience.

> what could they observe and experience

Observe? Nothing, once you're inside the event horizon, right? The event horizon isn't a solid wall, it's just the point at which light can only move further inward, never outward. So even inside the event horizon, we still can't observe anything further in.

“Spaghettification” only happens to large enough objects near small enough gravitational sources. If the Moon got too close to the Earth, closer than the Roche Limit, then it would break up into a ring of debris. But a communications satellite can exist at that same distance with no ill effects.

The same is true for black holes. A rocket or a human diving into a stellar mass non–spinning black hole would be “spaghettified”; they would be broken up into a thin stream of debris as they crossed the Roche Limit before they crossed the event horizon. But they could cross the event horizon of a much larger black hole, such as a supermassive black hole at the center of a galaxy.

In fact, if the black hole were massive enough then the gravitational field near the event horizon would be so mild as to be Earth–like. If you were to stuff all of the mass of three or four Milky Way–type galaxies into one black hole, you could build an actively–stabilized structure around the black hole to create a livable environment of truly insane proportions with Earth–normal gravity. Look up Birch Worlds sometime.

> you could build an actively–stabilized structure around the black hole to create a livable environment of truly insane proportions with Earth–normal gravity

Which was also a rocketship into the future, moving you super fast towards the heat death of the universe?

Not exactly. Time does run more slowly near a strong gravitational source, and if you are near the event horizon of a stellar–mass black hole this effect can be extreme. However, the larger the black hole is, the flatter the space around it. Furthermore, “near” is relative. A Birch world would be built around a black hole which is approximately a light–year in diameter. The structure would be “near” the black hole’s event horizon in relative terms but in absolute terms it would still be pretty far away, perhaps a quarter of a light year. Expect a time dilation of just 2:1, meaning that for every year on the Birch world two years pass for the rest of the universe.

It might seem like this costs you a lot, since it halves the amount of time you can live near your black hole. However, the lifetime of that black hole will be somewhere between 10¹⁰⁰ and 10¹⁰⁶ years, which is pretty insane even if you only get to use half of them. Furthermore, this is many orders of magnitude longer than the lifetime of a galaxy, so your civilization could potentially outlive everything else in the universe. Large stars burn out the quickest, but with black holes it is the other way around: small black holes evaporate the soonest. You might think that storing hydrogen in brown dwarf planets for use in fusion reactors would power a civilization for a long time, but fusion reactors are surprisingly inefficient. A civilization built around a rotating supermassive black hole can take advantage of the Penrose process to extract more usable energy from the same mass than the fusion reactors would.

I can't find anything about these low gravity regions (Google redirects me to your post) but they sound interesting. Can you share some reading material?

The entry point is probably looking at how Penrose Diagrams describe black holes.

Recent Veritasium video: https://www.youtube.com/watch?v=6akmv1bsz1M

Wiki: https://en.wikipedia.org/wiki/Penrose_diagram

Non-spinning and non-charged blackholes are called Schwarzschild or static black hole is a simple beast. You have a event horizon, a mass, and a slightly strange area near the event horizon where orbits are unstable, I believe within 1.5 times the event horizon distance.

Spinning blackholes (Kerr) of are quite complicated in comparison. They have an outer ergosphere, inner ergosphere, outer event horizon, and an inner event horizon. Also the singularity is no longer a point, but a ring.

A quote from the Ring_singularity link below:

  An observer crossing the event horizon of a non-rotating and uncharged black hole (a Schwarzschild black hole) cannot avoid the central singularity

  This is not necessarily true with a Kerr black hole. An observer falling into a Kerr black hole may be able to avoid the central singularity by making clever use of the inner event horizon associated with this class of black hole.

This is also explained in the Veritasium video at 1610 seconds: https://www.youtube.com/watch?v=6akmv1bsz1M&t=1610s

More info at:

  https://en.wikipedia.org/wiki/Rotating_black_hole
  https://en.wikipedia.org/wiki/Ergosphere
  https://en.wikipedia.org/wiki/Ring_singularity

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