I put my hand just inside the event horizon of a supermassive black hole...what do I feel?

I’ve googled the various SD threads on black holes, and other sources, but I didn’t see this specific question addressed. Plenty of info on “what happens when you fall into a black hole?” and “why can’t I escape by just moving outwards from the singularity at constant velocity?”, but not this.

So, I fly my space ship towards the monster black hole at the center of Andromeda. From what I’ve read, for sufficiently massive black holes the tidal forces are manageable, so spaghettification isn’t an issue. I also read that I wouldn’t feel anything different at the moment I would cross the event horizon, except that my fate would then be sealed.

Being careful not to cross over, I orbit just at the edge of the EH, and either stick my arm slightly inside, or perhaps a long stick. I obviously can’t pull it back out again, but since the tidal forces aren’t that huge, what would I feel? Could I withdraw my stick/arm, and get back whatever didn’t cross the EH?

What happens?

Black holes have a lot of gravity. That’s it, really. You’d feel the same as you would poking a stick in the direction of the sun.

(There are some other effects, mostly because the gravity attracts particles and creates heated streams of x-rays and other stuff, but that’s nothing special to a black hole, since all forms of massive gravitational pulls would cause the same secondary phenomena.)

Remember, a black hole is the singularity at the center. The stuff out to the event horizon is also popularly called a black hole but is mostly regular matter than can’t escape. We could be inside a universe-sized black hole and not know it. The event horizon isn’t a magical portal to another universe. It’s a distance. If it’s big enough you wouldn’t notice much of anything special about it.

And yes, by definition, if you could exert sufficient force you get back everything outside the horizon.

My WAG is not only is your hand trapped in there but it also HAS to eventually move more towards the center of the hole.

So, either more and more of your body also must enter the hole or if you can keep the rest of your body outside the hole the pull on your hand will keep increasing till it gets ripped off.

Whether this increase in force to bad levels takes seconds or years probably requires some detailed calculations.

Thing is, the event horizon is just the region where you’d have to travel faster than light to escape orbiting the black hole. Juuuuuuuust outside the black hole’s event horizon is a region where you’d have to travel 99.9999999999% of the speed of light to escape orbiting the black hole. So to do the “Look Ma, no hands!” trick and escape, you’d have to have some method of accelerating yourself to 99.999999999% of the speed of light.

You do have such a device? Well, you’re fine then, except your hand is gonna be ripped off.

Realistically, the only thing that will actually escape from juuuuuuuuust outside the event horizon are particles that travel at c. And they are going to be cooled down so much they’ll be low energy radio waves. And yeah, the occasional freak particle that can travel at some arbitrary percentage of c. Your whole body (minus your hand) isn’t gonna be that particle without some sort of magic reactionless drive.

I’ve also wondered about this. Apart from any other effects I would assume that your hand would go numb because your nerves inside the event horizon wouldn’t be able to supply information to your brain.

If your hand is close enough, you won’t feel anything, but when you are at the theoretical event horizon, your whole body is long past the practical event horizon. All the energy in the universe won’t get your hand back out, and almost all the energy in the universe will be necessary to get the rest of your body out, never mind that the space ship you are on is in the exact same pickle.

You can’t orbit “just at the edge” of the event horizon. Even light can’t orbit closer than 1.5 (I think) times the Schwarzschild radius. The only way you’re not going in too is if you accelerate so enormously that you’re pancaked.

I love thought experiments.

I have a hard time thinking you would lose feeling in your hand. I mean…it is right there still connected to your body. You are stationary compared to it…so signals nerve signals should be able to reach you. You should be able to pull it back…again you don’t have to pull it back faster than the speed of light…because you are right there!

I think what this thought experiment shows is that how we view/our visual model of an event horizon is inaccurate.

Wouldn’t your hypothetical orbit behave just like any satellite’s orbit?

That is, if you push something outside of a satellite towards the “earth” then the center of moments stays the same while the bulk of the satellite shifts slightly away from earth.

Or does the gravity of a black hole do things that are not similar to the gravity of a planet?

If not, then expand the thought experiment to consider throwing a ball towards the hole while orbiting just outside the event horizon. Would it behave in the same manner as was discussed in thought experiments about this action with an earth orbiter?

Yes, what most people are forgetting is that this is a thought experiment. They’re right that it would be impossible to orbit in this way but that’s not really the point.

The rest of your post is wrong, I think. Nothing can leave a black hole. You are essentially putting your hand into a black box. At the cutoff point nothing can return. That includes nerve signals, blood, or visual contact. And the flesh itself. It cannot be pulled back. Your hand is gone. It doesn’t matter that the rest of you is right there. The part of you that is inside the horizon would have to move faster than light to return. It can’t. There is no force in the universe that can pull it out.

I don’t understand how this makes the model of an event horizon wrong. It seems more that your understanding of it is wrong.

Then tell me how it is wrong.

I am in a jet pack…trying to move at near the speed of light, hanging within arms length of the even horizon. Say the black hole is so huge there is no spagettification (like the OP says) I reach out my hand inside the event horizon…now what exactly is stopping me from pulling it back out again? It can’t be gravity because I am not really feeling it. I have a hard time believing that I will be sitting there yanking on my hand but can’t get it out because it is ‘stuck’.

No…I think something else is going on that the visualization doesn’t cover…

If you treat the black hole like space is no longer there…that anything you put in ceases to exist…maybe…but this seems ‘wrong’ somehow…something entering a black hole is still there…

IANOC(osmologist)…so I am speaking from ignorance here…but that doesn’t mean my questions are necessarily crap :slight_smile:

ZenBeam has it right. Even in a thought experiment, we must obey the laws of physics, and one consequence of the laws of physics is that you can’t orbit stably just above the event horizon. You could in principle maintain position just a little above the event horizon by firing stupendously powerful rockets, or hanging from an unobtainium rope, or the like. In that case, if you stick your hand over the edge and across the horizon, your hand will be ripped off, but you’d interpret that from your reference frame as an effect of the incredible acceleration your rocket is providing, not as an effect of the black hole itself. Alternately, you could just fall freely into the hole, in which case your hand would not be ripped off, and you’d still feel it normally for the entire time, but any nerve signal which originated inside the horizon would not reach your arm until your arm was also inside the horizon, and so on.

In any event, two things are true: You won’t ever get any information from inside the hole to outside the hole, and you won’t ever notice anything particularly special right at the location of the horizon.

I think that’s where things aren’t going to work out: as ZenBeam already mentioned, there are no stable orbits closer to the event horizon than the photon sphere at 1.5 Schwarzschild radii – that’s the point where you’d have to be going at c to orbit. To get any closer, you’d have to continually accelerate, with the acceleration necessary to not fall in going to infinity as you approach the actual event horizon, which means that instead of spaghettified, you’ll be pancaked long before.

ETA: Eh, forgot to refresh. Chronos said it better.

Of course you’re feeling gravity. You’re feeling immense amounts of gravity. Your jet pack is fighting gravity all the way, as you would find out in a instant if you turned it off.

The gravity is so immense that it pulls back inside anything traveling at the speed of light or slower. That includes your arm. It may not cease to exist in an existential sense but no information about it can reach you. All sight and feeling have vanished because those are physical signals not moving at greater than the speed of light.

Didn’t Einstein famously imagine traveling at the speed of light to understand the consequences?

I don’t see anything physically impossible in this thought experiment. Unlikely to impossible with any real materials, true, but don’t many thought experiments posit impossibly perfect materials?

And you will notice something special at the horizon. It must be black because no photons are escaping it. If you could see into a horizon then you’d have information about it, wouldn’t you?

Yes, and the consequences he came up with were contradictory, so he concluded that traveling at the speed of light is impossible, and then set out to understand why that was so.

And I was freely granting the existence of an unobtainium rope that can suspend you just barely above the horizon. But while I’ll grant the existence of a material with an arbitrarily large but finite strength, I won’t grant the existence of an infinitely strong material, which is what you’d need to pull your hand back out.

As observed by many wise pop culture folks over the decades.

"Once you go black, you’ll never come back "

There’s another way to think about it. Photons that are emitted radially right at the horizon stay at the horizon. Since photons must locally travel at c you’d see the horizon moving towards you at almost c.

In order not to be engulfed, your rocket would have to undergo unbelievable acceleration, which would crush most of you against the hull, but since your arm is unsupported it would just be ripped off.

(Lots of only partially correct stuff in this post)

You can’t just hover with a rocket in a gravity field where you need 99.99999999% of the speed of light just to stay in place. I had forgotten that you can’t orbit at the event horizon, because the lowest orbit allowed by the laws of physics is an orbit at c, which means that orbit must be well above the event horizon.

You can’t just hover right outside the event horizon of a black hole with any real world rocketship, because you’d exhaust your propellant long before you came anywhere close to the event horizon. Even if your rocket is powered by total conversion you still need reaction mass, and rockets have a finite amount of reaction mass.

Note that if you’ve got Star Trek style warp drives that can propel you faster than light, you don’t need to worry about being trapped inside the event horizon of a black hole, since you can travel faster than light, as well as travel in time and violate causality and suchlike. Event horizons don’t mean doodly-squat to you.

The real answer to the question is that you’re imagining sitting in space, you stick your arm out, and can’t figure out why you shouldn’t be able to pull it back in. But if you’re that close and traveling at substantial fractions of c, your intuitions about how things work are often going to be wrong. If you’re really close to the event horizon, so close that part of you can be inside and part can be outside, the amount of energy needed to accelerate the rest of your body so that it escapes the black hole will be very very very large. Like, really large. So large that without some magical space warp you’re not going to have enough energy to accelerate yourself to 99.999999999999999999% of c. And even if you did, the acceleration would tear your body apart. Or smooosh you to a nanofilm.


Can you fall into / orbit a really large black hole (with low tidal forces) so that you pass through the event horizon relatively slowly ? With slowly being on the order of a fraction of a second to many hundreds of seconds ?

Cause, if so, I can see that having at least a couple very unusual, if not bad effects on the human body even with benign tidal forces.

No, not from your point of view. If part of you, say your foot, is inside the event horizon (and stays attached to you), in the time it takes for any information to get to your head, your head will be inside the horizon.