Tl;dr: Someone please explain to me why some physicists think something could ever cross the event horizon of a black hole.
There is a conflict between my understanding of what the event horizon of a black hole is vs the way that many theoretical physicists talk about them.
I understand that a result in general relativity is that time progresses more slowly in the presence of energy, and this is why light bends around massive objects.
The way I understand the dynamics around a black hole is that the surface of an event horizon is the region of space where the energy is so great that time literally grinds totowards a halt (edit/clarification: from the perspective of an observer farther out). Light moves at the speed of causality, and when causality slows down, so does light. Light is bent and redshifted due to time dilation, and only when time stops does the wavelength of light go to zero. That’s the event horizon as I understand it.
If an object falls towards a black hole, it shouldn’t matter if we are that object or if we’re just observing it from farther away, everyone should agree that it never crosses the boundary of the event horizon.
From a spectating observer’s perspective, the object is redshifted until it fades entirely as it gradually stops moving through time (so light stops being emitted from it). But it will only ever approach the boundary asymptotically; it will never cross it.
From the perspective of the object itself, the universe around it will progressively speed up and the entirety of the history of the universe will play out behind it (Edit: that only happens if the object accelerates to remain stationary). An infinite amount of time would pass everywhere else before it crosses the horizon. Now, that will never happen if black holes evaporate in finite time (and we have good reason to think they do). The black hole will evaporate long before any eternity passes anywhere. The more slowly you move through time, the faster this process will appear to you. When you are more or less frozen in time, the black hole will be evaporating at a rate that approaches ‘instantaneously’ - so the closer you get to it, the hotter it will appear and the faster it evaporates. You and everything else would literally radiate away from this noticeably shrinking event horizon before ever crossing it.
So, in this view, I feel utterly confused by physicists talking about “what it’s like to cross the event horizon” or “what the interior of a black hole is like”. Either my understanding is incorrect, or these physicists are just indulging themselves with hypotheticals rather than thinking about physics (or working on alternative models where black holes are fundamentally nothing like what I describe).
It’s most likely me not understanding this properly, so… what am I missing?
Update
As I mentioned in this comment, it has been shown that an event horizon may never form at all, and that all one ever sees is a shell of fading signatures followed by radiation from all the matter that falls into it.
I have more to learn about the particular dynamics around the area surrounding a black hole, but I believe I’ve managed to reduce my antecedents to the assumption that quantum information is conserved and the following counterfactuals, which appear promisingly independent of whichever dynamical model one might prefer:
- A sufficiently long-lived asymptotic (sufficiently distant) observer would be able to identify a particular point in time at which a black hole will have fully evaporated.
- A sufficiently long-lived asymptotic observer would be able to track the signature of something falling towards a black hole until it is radiated out.
Counterfactual (1) is supported by the prediction of Hawking radiation and means that the black hole has a finite life span. Counterfactual (2) is supported by the common claim that, to an observer far away, the wavelength of emitted light from an infalling object will go towards infinity as they get closer to the event horizon.
This means that the observer just has to wait long enough to detect each subsequent photon until the source of the emission has been radiated out, and so the observer is a witness of the fact that the infalling object was never inside the event horizon. For information to be conserved, there can never be disagreement between the objective experience of the witness and the information encoded in the radiation, and so if the infalling observer were to be reconstructed after being spat back out by the black hole, it would agree that it was never inside the horizon.
Feedback on my reasoning would be very welcome.
You don’t get frozen on the event horizon. You appear that way to someone not there, but that’s not your experience. You don’t even notice there being a horizon. Well, besides the immense tidal forces. They may cause you some concern. But yeah, you’ll cross that horizon and hit that singularity in a finite time, and likely one that you think is pretty short. I think the problem in this case lies with you thinking time stops in a black hole. Which likely hasn’t been helped by many beginner physics articles saying that space time stops in black holes. It doesn’t, what does happen is that space time becomes infinitely curved and that fucks with things in a complicated way that can be easily ignored for most things (not astrophysics, obviously).
As for the hawking radiation and the black hole evaporating before you get there, yeah black holes will be like all things in the universe and eventually cease to be, that’s not a problem in this case cos you will be hitting it way quicker than what you’ve hypothesised above. Well, I say that, but as mentioned above the tidal forces are gonna spaghettifi you well before that. Although, depending on the type of black hole you’re in, you can make it past the horizon before this happens. But, again, you won’t notice this. Either way, you’re getting stretched waaaaaaaaay before that black hole is dying. If you do fall in a super massive black hole (one which will allow you cross the horizon before destroying you), you will see some really cool shit when you look back out. There will be blueshifting of the stars, they’ll change shape, some will move at different rates. Wild.
Edit: here’s a fun little answer that answers the first part way better than me. Also covers the space time curvature more accurately than I have: https://physics.stackexchange.com/questions/48026/in-general-relativity-gr-does-time-stop-at-the-event-horizon-or-in-the-centra
Excellent, this is precisely the perspective I want to understand! Let me try to pick this apart so that I may figure out why we disagree.
This is an interesting statement, because in my understanding, you most certainly would. Maybe not in practice, for the same reasons you might not notice if your brain undergoes rapid deterioration for any other reason. But in theory, if we assume that the object in question is a composite of multiple non-overlapping particles (fermions) interacting with each other, eventually particles closer to the event horizon would get close enough to it that they become unable to interact with particles further away in finite time.
What that means in practice for, say, a human brain, is that it would become subject to an increasingly extreme time dilation gradient, preventing it from functioning properly. The part of the brain that (for the sake of argument) passes the event horizon would simply become inaccessible to the part of it that’s still outside. The constituent particles of the part that’s inside would, again for the sake of argument, be subject to some absurd conditions: while particle interactions in our everyday spacetime are local and propagated at the speed of light in all spatial directions, inside some hypothetical black hole interior, all trajectories would converge to the same point and never point (much less intersect) in opposite directions to each other. Thus, no mutual causal interaction between any two things would be possible.
That’s not quite it. I think time does not progress at the event horizon. In order to enter the black hole, you would need to first go through the event horizon. But if time does not pass at the event horizon, you couldn’t possibly keep going any further if you were to reach it. If you start counting – “one, two, three, …” – out loud, you would exclaim “…, infinity!” before observing the first particle of your body passing through the horizon, and that’s just not going to happen. In effect, you just wouldn’t ever experience getting closer to the horizon after a certain point; you’d see it evaporating away from you for the rest of your life, however brief that is.
If what I said above is correct, the black hole would cease to be long before I reach the horizon. As a thought experiment, consider an eternal observer that doesn’t fall into the black hole. That observer would attest to the fact that the black hole evaporates long before I ever enter it, because it would evaporate in finite time whereas it would take an infinite amount of time for me to reach the event horizon.
This is what I thought too until just a few moments ago, but then I realised that this would only be true if (and only for as long as) you were accelerating away from the black hole. If you merely “free fall” towards it, I believe you would actually observe a redshifted universe behind you, since your relative velocity would approach the speed of light the closer you get to the horizon.