21
$\begingroup$

Let's assume there is an astronaut with a very long rope trailing behind him. As he approaches a very large black hole, he can look back and see the rope behind him trailing off into the distance.

What would he see after he crosses the event horizon and looks back along the rope while a portion of the rope is still outside the event horizon?

$\endgroup$
  • 34
    $\begingroup$ As always in these type of questions you must beware of the extreme warping of space-time near a black hole. When you say "what would he see" you are going to be understood to mean "what do his eyes make of the photons they see coming in to them" when what you might really mean is "what is going on" - and this brings up tricky issues of simultaneity which is ill-defined in this region of space-time. $\endgroup$ – Paul Young Aug 29 at 22:50
  • $\begingroup$ More on ropes & event-horizons. $\endgroup$ – Qmechanic Aug 30 at 15:10
  • $\begingroup$ Does this answer your question? Does someone falling into a black hole see the end of the universe? $\endgroup$ – Deschele Schilder Aug 30 at 16:48
  • $\begingroup$ @descheleschilder This doesn't really answer my question. I understand someone falling into a black hole would not see the end of the universe. I'm asking what someone would see once inside the event horizon. $\endgroup$ – Andrew Aug 30 at 20:51
  • 3
    $\begingroup$ I have deleted a number of comments -- many were attempting to answer the question and others were starting to become inappropriate. If somebody has an answer to the question, please post it as an answer. Thanks! $\endgroup$ – tpg2114 Aug 31 at 20:49
43
$\begingroup$

Dale's answer is correct, but I want to further emphasize that nothing special happens in the vicinity of an event horizon. It's just like any other region of spacetime.

Here's an analogy. Suppose you're in a building that's rigged to explode at a certain time. If you're in the building and too far from an exit at a late enough time, you won't be able to escape before the explosion even at your top speed. If it's a single-story, square building and you can exit at any point on the edge, then the region from which you won't be able to escape at a given time is square. It starts in the center of the building and expands outward at your maximum running speed. The boundary of that region is the "escape horizon".

If you don't escape and die in the explosion, then the escape horizon will necessarily sweep over you at some point before your death. When it passes you, nothing special happens. You don't notice it passing. You can't detect it in any way. It isn't really there. It's just an abstract concept that we defined based on our foreknowledge of the future.

The event horizon of a black hole is defined in the same way, with a singularity in place of the explosion and the speed of light in place of your running speed. If your worldline ends at the singularity, then the event horizon will sweep over you, at the speed of light, at some earlier time. But you won't notice. You can't detect it in any way. It isn't really there.

People get confused about this because there's phenomenology associated with black hole horizons: the closer you get to them, the faster you have to accelerate to avoid falling through, the slower your clock runs, the hotter you get from the Hawking radiation, and so on. They also behave like electrical conductors for some purposes, though it's not mentioned as often.

The thing is, if you mispredict where the singularity is going to be, and try to escape from what you think is the horizon but actually isn't, all of those same things happen. Any event horizon defined by any future spacetime points, whether singular or not, has these properties, even in special relativity. (See Rindler coordinates and Unruh effect for more about the special-relativistic case.)

So the answer to any question about what you'd see while falling through an event horizon is always the same as if the event horizon was somewhere else.

| cite | improve this answer | |
$\endgroup$
  • 1
    $\begingroup$ This answer deserves better than the first! $\endgroup$ – Deschele Schilder Aug 31 at 6:44
  • 9
    $\begingroup$ ... horizons are unphysical but they also conduct electricity? What? If they conduct electricity then can't my ohm-meter tell me where they are? $\endgroup$ – user253751 Aug 31 at 10:58
  • 1
    $\begingroup$ @user253751 I toned down the claim about conductivity and added a Wikipedia link. $\endgroup$ – benrg Aug 31 at 15:55
  • 1
    $\begingroup$ What is confusing to me is that if the falling observer does not notice anything "special" they should still think that they can climb out again, for example using that rope? (Assuming it's not falling with them but is attached somewhere "outside" the event horizon). Maybe they are not even falling but very carefully crossing that threshold, holding on to the rope... $\endgroup$ – Peter - Reinstate Monica Sep 1 at 11:18
  • 1
    $\begingroup$ One thing though. What if one body part enters the zone of no return? It is a special kind of piece of spacetime. $\endgroup$ – Deschele Schilder Sep 1 at 20:48
30
$\begingroup$

He would still see the rope trailing behind him. There is nothing that prevents light from the rope to fall from the rope inward through the event horizon and to his eyes.

| cite | improve this answer | |
$\endgroup$
  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ – tpg2114 Aug 31 at 12:42
  • $\begingroup$ In essence, this is indeed the answer. Do you mind me making a minor edit to your answer? You'll go up to 30... $\endgroup$ – Deschele Schilder Sep 6 at 1:36
  • $\begingroup$ Sure, go ahead. If I object then I can revert it, but if I agree then it is good to have additional input $\endgroup$ – Dale Sep 6 at 2:15
14
$\begingroup$

It depends on your point of view. For the astronaut, nothing, in particular, happens.
For us, as distant observers, the astronaut will get frozen in time. Someone once wrote (in this question I read on this site) that upon entering the BH's event horizon, the astronaut's hand will be pulled off his body, due to the fact that time stands still on the horizon. This is obvious nonsense. For the astronaut, nothing seems to be going on. He's just falling freely through the horizon. Looking back he just sees the rope falling with him. And because the BH's mass is that big he won't get spaghettified (this may seem strange because of the enormous mass of a galaxy-like BH, but the radius of the event horizon is huge too).
If he/she is hovering above the event horizon ( which takes an enormous amount of energy) put's his/her into the hole, he/she won't see, feel, or is able to pull his/her hand back. When they start moving away from their stationary hovering position, they...I'll let you think about this.

His/her information content is completely lost and we won't be able to tell from the Hawking radiation what he/she looked like. Contrary to the ADS/CFT correspondence approach which is based on string theory and states that information isn't lost. I don't accept string theory to correspond to reality, so neither do I accept that information isn't lost.
We will never know what will happen if she falls on and on. Maybe she'll end up in another Universe, maybe she'll get spaghettified, but for sure she's lost forever.

In short: Because someone (in a special suit) is in free fall and as long the someone is not torn apart by tidal forces, that someone will observe nothing strange. He/she would see everything as here on earth. It's only for us as distant observers that time- and one space-coordinate take each other's place. In free fall, the time, t-, and spatial (radius, in the case of polar or spherical coordinates), r-coordinates don't interchange.

| cite | improve this answer | |
$\endgroup$
  • $\begingroup$ Possibly the knowledge that your trajectory will end in a singularity in short, finite own time, might be a little bit disturbing, even if going below the event horizon itself is not traumatic. $\endgroup$ – peterh - Reinstate Monica Aug 30 at 17:24
  • 4
    $\begingroup$ @SolomonSlow As you have a transporter, I have a suit in which I can survive anywhere! Seriously! $\endgroup$ – Deschele Schilder Aug 30 at 21:58
  • 1
    $\begingroup$ "His/her information content is completely lost and we won't be able to tell from the Hawking radiation what he/she looked like." That is false. The evolution of quantum states is time-reversible. $\endgroup$ – Acccumulation Aug 31 at 3:59
  • 2
    $\begingroup$ I just posted a link saying that Hawking thought that information is destroyed. I don't know why you're posting what you're "pretty sure" of without any citations. $\endgroup$ – Acccumulation Aug 31 at 4:26
  • 2
    $\begingroup$ @descheleschilder, I don't remember. But with regards to "hovering...." While it's true that the gravity gradient at the horizon of a super-massive black is not great, the amount of force that your engine must produce in order to hover there approaches the infinite as your ship approaches the horizon. If you were hovering close enough to put your arm across, then you would be experiencing G forces from your ship's engine on an astronomical scale. Even if you were hovering much farther away, the sheer weight of your arm dangling over the side would have torn it right off of your body. $\endgroup$ – Solomon Slow Sep 1 at 12:17

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.