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Comment by squrky | Hacker News Reader

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.

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> 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.

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.

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.

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