Long time no chat... I was trying to use AI to help me reconcile the two claims I commonly hear, but without success. Maybe someone here can help me better:

claim 1: If you lower a rope below event horizon you are not able to pull it back

claim 2: You can cross the event horizon of a supermassive black hole without even noticing.

The two seem incompatible to me, but LLMs do not seem to be bothered.

7 minutes ago, Danijel Gorupec said:

Long time no chat... I was trying to use AI to help me reconcile the two claims I commonly hear, but without success. Maybe someone here can help me better:

claim 1: If you lower a rope below event horizon you are not able to pull it back

claim 2: You can cross the event horizon of a supermassive black hole without even noticing.

The two seem incompatible to me, but LLMs do not seem to be bothered.

If you lower a rope below the event horizon you will experience a humongous gravitational field. You are standing "still" against one of the strongenst gravitational forces in Nature.

If you cross the event horizon in free fall, you don't "see" any gravitational field at all. It feels like empty space.

It's a matter of frames of reference.

18 minutes ago, joigus said:

If you lower a rope below the event horizon you will experience a humongous gravitational field. You are standing "still" against one of the strongenst gravitational forces in Nature.

I am imagining orbiting a supermassive BH just above its event horizon, then lowering a rope (or better a rod) - it does not have to be a very long rope/rod... I don't understand why I should be dealing with strong forces here?

10 minutes ago, Danijel Gorupec said:

I am imagining orbiting a supermassive BH

Free fall. Orbiting is falling.

Remember Newton and his famous apple.

Ok. I see what you mean.

You wouldn't be able to pull back the rope. But that's not the same case as falling through the horizon.

Why is that a contradiction?

41 minutes ago, joigus said:

Why is that a contradiction?

Because as I am falling through the horizon, at one moment my legs are below horizon and my head is above it. Does it mean that my blood cannot circulate any more - the blood from my legs cannot return to my heart? If so, I would definitely notice I crossed the event horizon (disproving the claim 2). Or, my blood can continue to circulate, in which case also the rope can be dipped and removed in/out of event horizon (disproving the claim 1).

Geodesics continue through the event horizon.
In free fall you are moving along a geodesic and experience no forces.
Even partially crossing the EH ( assuming the BH is large enough that tidals are trivial ) you will experience no forces.

If you are pulling a rope up against any gravitational field you are doing work and expending energy.
In effect, you are keeping the rope from travelling along the geodesic and changing its momentum wrt time; the rope will feel a force.

4 minutes ago, Danijel Gorupec said:

Because as I am falling through the horizon, at one moment my legs are below horizon and my head is above it. Does it mean that my blood cannot circulate any more - the blood from my legs cannot return to my heart? If so, I would definitely notice I crossed the event horizon (disproving the claim 2). Or, my blood can continue to circulate, in which case also the rope can be dipped and removed in/out of event horizon (disproving the claim 1).

Just an aside: You also must assume a very big BH in comparison to your body size... Anyway.

But your heart keeps falling towards the horizon. It's not like it's kept outside it, like I understood you meant when I pictured an enormous radial acceleration. So your blood would keep circulating as far as I know. Yes. If the radius is big enough you wouldn't be able to feel any noticeable effect, if the theory is correct. That's just the equivalence principle at work.

Now, if there are forces involved... the problem suddenly becomes more involved, IMO. Forces other than gravitation do not transform in any simple way I know of. My guess is that you would be unable to pull the rope and, quite unaccountably to you, it would have turned "rigid" (again, assuming the BH is really big and tidal forces are not noticeable).

1 hour ago, joigus said:

Just an aside: You also must assume a very big BH in comparison to your body size... Anyway.

But your heart keeps falling towards the horizon. It's not like it's kept outside it, like I understood you meant when I pictured an enormous radial acceleration. So your blood would keep circulating as far as I know. Yes. If the radius is big enough you wouldn't be able to feel any noticeable effect, if the theory is correct. That's just the equivalence principle at work.

Now, if there are forces involved... the problem suddenly becomes more involved, IMO. Forces other than gravitation do not transform in any simple way I know of. My guess is that you would be unable to pull the rope and, quite unaccountably to you, it would have turned "rigid" (again, assuming the BH is really big and tidal forces are not noticeable).

I remember reading that it is a myth that you would be spaghettified. I think.

6 minutes ago, exchemist said:

I remember reading that it is a myth that you would be spaghettified. I think.

Depends on the radius. Honestly, I don't know what the real thing would be like, but if the black hole is small enough in comparison to the body's dimensions, the tidal forces would be enormous. The spaghettification conjecture comes from the difference in the direction of the forces corresponding to different points of the big object in the picture below, which represent the tidal forces.

Whether this results in ordinary matter turning into a spaguetti of quarks and gluons, as I've heard said, I don't know.

The picture is very exaggerated, of course, in order to illustrate the effect clearly.

8 hours ago, Danijel Gorupec said:

Does it mean that my blood cannot circulate any more

The crossing of the horizon would happen in a tiny fraction of a second, orders of magnitude shorter than even the electrochemical signals in your nerves could propagate. So no, you wouldn’t notice anything special.

8 hours ago, Danijel Gorupec said:

in which case also the rope can be dipped and removed in/out of event horizon (disproving the claim 1).

Below the horizon there are no stationary frames, so if you lower a rope through it, it would simply break. In practice it would in fact break quite a bit before it even reaches the horizon, since at the horizon itself only massless particles could remain (at least in principle) radially stationary under the right conditions, though this would not be an equilibrium.

So no, you can’t pull it back out. What you pull back towards you will be the broken, shortened end that hadn’t crossed the horizon yet.

8 hours ago, Danijel Gorupec said:

Because as I am falling through the horizon, at one moment my legs are below horizon and my head is above it.

Actually, your head and legs won’t share a common notion of simultaneity in this situation, so it’s rather more complicated than you think. What’s more, for your head the horizon is below, whereas for your legs the horizon is in the past.

PS. One more thing to consider is that for an outside stationary observer lowering a rope, its loose end would never appear to reach the horizon, even if he lets go and allows it to free-fall. The loose end would just appear to move downwards more and more slowly, while getting dimmer and dimmer, until it eventually fades out. Actually hitting the horizon would take infinitely long on his own clock. For a clock moving downwards alongside the rope, on the other hand, the (freely falling) rope would cross the horizon in finite time.

So nothing about this seemingly simple scenario is quite straightforward.

On 8/31/2025 at 1:38 PM, joigus said:

If you lower a rope below the event horizon you will experience a humongous gravitational field. You are standing "still" against one of the strongenst gravitational forces in Nature.

Perhaps counterintuitively, the gravitational acceleration at the event horizon can be small

r = 2GM/c^2

g = GM/r^2 (Newtonian) so g = c^4/4GM

If the mass is really big, g can be quite small.

And the tidal forces would be small, so small g and no spaghettification, hence the “not noticing”

On 9/2/2025 at 2:53 AM, swansont said:

Perhaps counterintuitively, the gravitational acceleration at the event horizon can be small

r = 2GM/c^2

g = GM/r^2 (Newtonian) so g = c^4/4GM

If the mass is really big, g can be quite small.

And the tidal forces would be small, so small g and no spaghettification, hence the “not noticing”

Yes, good point. The question about the dimensions of the infalling object are still relevant though, IMO. Imagine a humongous space station with parts wildly separated in comparison to the Schwarzschild radius. Arguably, the tidal forces can be made relevant by making objects extense enough. After all, the equivalence principle is only exact locally.

(Sorry I didn't follow up before; I was on a short vacation).