I I don't see how a black hole's event horizon can be crossed

[PeroK]

What you see as a "path to learning" looks like a confusion of ideas to me. That said, you have to choose your own path. Where that path leads is another matter.

 

[Ibix]

This thread has moved quite fast and I've not fully caught up, so apologies if this repeats things already said.

There are different models of black holes, and those models have different properties. The comment about watching an infalling object slow down, freeze, and redshift to "nothing," but never cross the event horizon in the OP is a statement about an eternal black hole - a model that does not include Hawking radiation. It isn't true about a model that does include black hole evaporation. It's also a statement of what an observer outside the hole directly sees - not a statement about what that observer can infer from what they see. That last bit often gets confused, because with an eternal black hole it's easy to construct coordinate systems that don't assign time coordinates at or below the horizon and then take them too literally.

With evaporating black holes you do see the horizon crossing at the same time as you see the hole explode (at least in principle - actually picking it out of the mess might be difficult). So there's no "it didn't form but now it's exploded" paradox. The mistake is mixing true statements about eternal black holes with wondering about non-eternal holes.

To be fair, physicists will often make statements about "black holes" without specifying which models they're thinking of. It's a continuing problem with science communication that general statements are always false, but precise ones are often too nitpicky (about nits the public doesn't even know need to be picked) and turn off the public.

 

[Foretranimal]

What you see as a "path to learning" looks like a confusion of ideas to me. That said, you have to choose your own path. Where that path leads is another matter.

I'm here to be wrong and corrected, and offer my thanks in advance.

Yes, I do have a confusion of ideas: when I think about this topic, there are ideas I have that don't make sense to me. I love to be wrong, and I want to have some degree of understanding where and how I am wrong. I'm sorry if I presented anything as if I knew anything - I don't. I want to understand where my mistakes are and learn.

Usually, when I hit one of these perceived paradoxes, I want to understand to some degree what I missed. My background IS going to limit what I am able to understand to the degree of an expert - I am not an expert on this topic, I am just really curious.

That said, I do not do too badly with partially getting an answer when someone explains something. I may not get it at first. I may never get it in part. I may never get it at all.

But my warm thanks to everyone responding to me, and for taking the time to engage with this question.

I registered on this site earlier today - because this has been driving me nuts, and there is just NO substitute for back-and-forth communication when you have a question.

 

[Dale]

I may have made conflicting claims. But honestly, I did not mean to make a claim.

By conflicting claims I am talking about your OP.

If black holes "evaporate" via Hawking radiation, then they cannot exist forever.

So, from my external perspective, watching the person fall in, they slow down, freeze, and redshift to "nothing," but never cross the event horizon.

These are indeed genuinely conflicting claims. But the thing is that they are not merely different “perspectives” (coordinate charts). They are different spacetimes (manifolds). In a spacetime where one statement holds, the other does not.

But this paradox seems to assume the crossing happens. If the crossing never happens, is there an information paradox at all?
…
It seems more like the event horizon has no "there" or "inside" the event horizon - the horizon is an asymptotic limit of what exists. There is no inside, like there is no "before" the Big Bang. If you remove the assumption, there was a crossing, is there still a paradox?

Here is the problem. What you have found are genuinely conflicting statements, but not a paradox. And no discarding of assumptions is warranted by the conflict.

What you have found is like this: Suppose a theory produces the equation $$(x-3)(x+4)=0$$ One scientist decides to write a book to make as much money as possible, so he decides to talk about this equation, but only using words and not math. He talks about the positive root and says “it is odd”.

Another scientist sees all the money he makes and decides that she wants some too. She wants her book to be different so she writes about the negative root and says “it is even”.

While “it is odd” and “it is even” are genuinely conflicting claims, there is no paradox because each statement is referring to a different “it”.

The resolution is not to suppose that maybe we shouldn’t assume there are any even numbers. The resolution is to recognize that two different solutions are being discussed.

 

[Grinkle]

I respectfully dissagree

One "B" level poster to another ....

What I read from @PeroK is not that using the word 'perspective' is objectively wrong, rather that 'perspective' not a concept that physicists would directly wrap math around to map 'experience' (another fuzzy word I'm using here, I acknowledge) from one reference frame to another; if you invest some time into what are meant by co-ordinates, events and measurements and consider how different combinations of these more fundamental elements combine to form what you are calling a perspective, that will help you mature your intuition along the lines of your questions in this thread.

 

[PeroK]

One "B" level poster to another ....

What I read from @PeroK is not that using the word 'perspective' is objectively wrong, rather that 'perspective' not a concept that physicists would directly wrap math around to map 'experience' (another fuzzy word I'm using here, I acknowledge) from one reference frame to another; if you invest some time into what are meant by co-ordinates, events and measurements and consider how different combinations of these more fundamental elements combine to form what you are calling a perspective, that will help you mature your intuition along the lines of your questions in this thread.

Yes, that's an excellent summary.

There is a further point that an observer is not obliged to anayse things using any particular reference frame. An observer on the ground, trying to calculate what happens on a moving train or aeroplane can choose to use a reference frame in which the train or plane is at rest. The observer does not actually have to board the train or aircraft in order to use that reference frame!

The observer is restricted in the direct measurements they can make; but, not they are not restricted in terms of the reference frame or coordinate system they can choose to use to analyse the problem.

And, in fact, in many problems (whether relativistic or non-relativistic), switching to and from different reference frames is an important technique. Another example is using the zero-momentum reference frame to analyse a collision (whether relativistic or not).

 

[PeterDonis]

There is intrinsic value in looking at a flawed analogy for the purpose of figuring out why the analogy is flawed

Not if the answer is "it's not even wrong". Your analogy is too far off to even be corrected. You're better off just throwing it away and starting from scratch.

Please tell me how my premises are flawed

I can't; as above, they're too far off to even be corrected. You're simply using the wrong tools for the problem. If you're trying to drive screws with a hammer, and someone tells you that's wrong, it's no good asking them how you can correct your use of the hammer so it'll work. You need to drop the hammer and get a screwdriver.

What are my specific background limitations you are referring to?

Um, the fact that you admit you have no expertise in this area?

I'm asking so I might know where I can improve them.

By taking the time to learn what GR actually says about black holes and evaporating black holes, from a textbook. That will mean taking some time to learn GR in general. From what I can see, you need quite a bit of background knowledge about GR before you can properly approach this problem, or even understand the proper approach. That's getting beyond what we are likely able to do for you in the context of a PF thread.

I should not ask questions I don't already have a specific background to ponder and ask?

Not questions based on anything you think you know about this problem now, no. As far as I can tell, everything you think you know about this problem is wrong, so it will do you no good to use that knowledge to formulate questions.

Are you saying I should just stop thinking about this because I am not an expert?

No, I'm saying that I think you are drastically underestimating the amount you need to learn--and unlearn--to properly understand the scenario you're asking about. I don't think it's a matter of tweaking one or two premises in your understanding. I think it's a matter of throwing away everything you think you know and starting fresh.

I am not predicting anything

Yes, you are:

from my external perspective, watching the person fall in, they slow down, freeze, and redshift to "nothing," but never cross the event horizon.

That's a prediction--a wrong one. It's something you think you know, that's simply wrong, that you need to unlearn.

I am only referring to observations made in different referance frames.

There's no such thing. Observations aren't made "in" a particular reference frame. Observations are concrete, invariant facts that every reference frame has to agree on. For example, in your "barn and ladder" scenario, every reference frame has to agree on the four observations I listed in post #25.

This is another example of where I don't think it's a matter of just tweaking a few of your premises. As far as I can tell, your entire understanding of how special relativity work (never mind general relativity) is wrong and you should throw it away and start fresh.

 

[Dale]

I am not so pessimistic about @Foretranimal because of their openness here. There is quite a bit to do, but they do not seem resistant to learning. I think that willingness to learn is probably more important than having mostly correct concepts to begin with.

 

[PeterDonis]

But there is no way to demonstrate any existance outside your personal light cone experience. How can we know for sure there is anyting real there?

This is a question of philosophy, not physics. In physics, we build models and use them to make predictions. Sometimes the predictions are of things that we can't directly observe even in principle, such as the predictions about falling into a black hole. But the models and the predictions they make are still perfectly well-defined, and when physicists talk about what happens to someone who falls into a black hole, that's what they're talking about. The underlying theory on which the black hole models are based, General Relativity, is extremely well confirmed everywhere we've tested it, so physicists feel pretty confident about trusting its predictions in regimes like black holes.

(There is a caveat to this, that physicists do consider alternate models, of things that look like black holes from the outside for a very long time, but don't have any actual event horizons or singularities. If you search PF for "Bardeen black hole", you'll find some threads about one such alternate model. Nobody knows for sure which, if any, of the models we currently have, in a regime like this where we have no direct tests, will end up being the right one, the one that actually occurs in our actual universe. But the key point is that, in order to discuss a scenario at all in terms of "what happens", you have to pick one specific model to use in order to predict what happens. You can't mix them.)

In the OP of this thread, as has already been pointed out, you mixed up two different models: the model of a black hole that evaporates, and the model of an idealized "eternal" black hole that lasts for an infinite time. These are two different models, and it makes no sense to mix up predictions from one with predictions from the other.

Is that unobsevable place there for you and me when we can never affect it?

You can affect the black hole; you can drop things into it. Things at or inside the horizon can't affect you (because they can't send light signals to you).

 

[PeterDonis]

I think that willingness to learn is probably more important than having mostly correct concepts to begin with.

I agree with this, but I think one also has to be realistic about just how much one has to learn--and unlearn.

 

[PeterDonis]

I am asking for exactly that.

The correct premises are the black hole models that have been built using GR. Understanding them requires taking some time to learn them, which, as I've already pointed out, means learning the underlying framework of GR that they're built from. (And, as I've also pointed out, you might need to learn SR properly first.)

For a good introduction to SR and GR, you could try Carroll's online lecture notes:

https://arxiv.org/abs/gr-qc/9712019

They discuss black holes (but not, IIRC, evaporating ones, only "eternal" ones) in one of the later chapters.

The key differences between the "eternal" black hole model and the evaporating black hole model (or at least the basic evaporating black hole model that Hawking originally used--there's been a lot of more advanced work in this area since) have been described earlier in this thread.

 

[DaveC426913]

This is worth repeating, I think, so as not to discourage the OP:

That's getting beyond what we are likely able to do for you in the context of a PF thread.

He may not be getting he answers he asks for, but it's not because people aren't bothering, it's because a forum thread is not the best place to teach the required physics and maths.

 

[sbrothy]

This is worth repeating, I think, so as not to discourage the OP:

He may not be getting he answers he asks for, but it's not because people aren't bothering, it's because a forum thread is not the best place to teach the required physics and maths.

In effect, he has to use a different set of coordinates for his choice of media.

 

[Foretranimal]

"it's not even wrong".

That would be the clever insult Dirac threw at a student who made a wild guess about the wavelength of visible light when he didn't know. The student guessed something quite large (a few feet if I recall), and Dirac cut back into him,

"Am I blurry?"

The crux of the insult is that the wavelength would have to be smaller than a human pupil for eyes to work.

The correct premises are the black hole models that have been built using GR. Understanding them requires taking some time to learn them, which, as I've already pointed out, means learning the underlying framework of GR that they're built from. (And, as I've also pointed out, you might need to learn SR properly first.)

I was looking for a place a bit more welcoming to people like me, and I read in the FAQ:

Is Physics Forums only for professionals or academics?
No, Physicsforums.com is open to everyone, regardless of their level of expertise. Whether you’re a student, educator, enthusiast, or professional, you’re welcome to join and participate in discussions.
Source: https://www.physicsforums.com/insights/about-physics-forums/

I'm an enthusiast, and was initially pretty enthusiastic, and it seemed like there were no prerequisites required to ask questions, even ones that "aren't even wrong." But several people are stating with varying degrees of politeness that I don't have the right background to engage in discussions or ask questions, and have no business here until I... something... I don't even really know - maybe spend several years reading in meek silence before I dare talk?

If anyone knows a better place for people like me, my thanks in advance if you could steer me in a better direction.

 

[Foretranimal]

I am not so pessimistic about @Foretranimal because of their openness here. There is quite a bit to do, but they do not seem resistant to learning. I think that willingness to learn is probably more important than having mostly correct concepts to begin with.

It is hard, if not impossible, to maintain an openness to learn anything when you sincerely ask someone where they are making a mistake, and the answer is essentially, "everywhere." I really tried not to take it personally and learn something, but the tone was so persistently insulting, it sort of became impossible.

Some of the responses I got seemed to be honest answers to something I want to understand better, and I do appreciate that.

 

[PeroK]

I was looking for a place a bit more welcoming to people like me, and I read in the FAQ:

Is Physics Forums only for professionals or academics?
No, Physicsforums.com is open to everyone, regardless of their level of expertise. Whether you’re a student, educator, enthusiast, or professional, you’re welcome to join and participate in discussions.
Source: https://www.physicsforums.com/insights/about-physics-forums/

I'm an enthusiast, and was initially pretty enthusiastic, and it seemed like there were no prerequisites required to ask questions, even ones that "aren't even wrong." But several people are stating with varying degrees of politeness that I don't have the right background to engage in discussions or ask questions, and have no business here until I... something... I don't even really know - maybe spend several years reading in meek silence before I dare talk?

If anyone knows a better place for people like me, my thanks in advance if you could steer me in a better direction.

I admit haven't done a scientific analysis, but I think it's almost impossible to understand relativity without studying it from an undergraduate textbook. Note that does not mean "three years in meek silence". It does mean formal study - like a physics undergraduate is expected to do.

Sadly, enthusiasm is not enough. You have to be prepared to put in the hard work. I would try to help, but I have no magic powers to convey the theory of GR. All I can suggest is lots of hard work.

Note that GR is an advanced undergraduate subject, so you have (like many) jumped in at the deep end.

 

[Ibix]

'm an enthusiast, and was initially pretty enthusiastic, and it seemed like there were no prerequisites required to ask questions, even ones that "aren't even wrong." But several people are stating with varying degrees of politeness that I don't have the right background to engage in discussions or ask questions, and have no business here until I... something... I don't even really know - maybe spend several years reading in meek silence before I dare talk?

The question you need to answer is what you want out of the discussion. If you want facts about black holes then almost any source is good. The answer to your question in the OP has been given: things don't take forever to fall in to black holes (and with evaporating holes you even get to see them fall in, at least in principle), so there is no paradox (Edit: or rather, yes there is a paradox, the information paradox, but no contradiction earlier than that in the reasoning). I'd add that the chain of reasoning that leads people to think you can't fall in contains an error analogous to saying you can't walk though the north pole because you'd teleport across the top edge of a Mercator map.

The problem is that you look to be trying to fit the things you've read about black holes into a coherent framework. That is, you want to be able to understand and reason about them. If that's what you want, that's great. But the fact is that for questions about things like evaporating black holes we have no such framework to give you except the full mathematical treatment (Hawking himself said that the "negative energy particles fall in" explanation he gave for Hawking radiation was essentially a lie, but he couldn't think of any way to express what his maths actually says in English). Everything else fails because we don't have a common pool of experience of black holes to ground our conversation - just the mathematics. And there looks to be quite a gap between where you are now and that, I'm afraid.

You can learn this in your spare time (I did it, mostly on my commute - although I do have a physics background I haven't done physics for money in decades, and never did anything relativity related) but it's not a small undertaking. Carroll's lecture notes, linked above, are an excellent source, although if you don't have a physics background you almost certainly need to learn special relativity first.

I'd actually suggest reading the three relativity texts that @bcrowell, a former mentor here, produced (see https://lightandmatter.com/books.html). Relativity for Poets is a fairly solid non-mathematical introduction, and his GR text is a significantly less brutally mathematical introduction than Carroll.

 

[timmdeeg]

Isn't the core of all these posts that these two spacetimes, the one where something crosses the event horizon in its proper time and the other one, where it doesn't even reach in coordinate time the event horizon in finite time the fact that the principle of simultaneity just can't be applied to the two "perspectives"?
If not, what else has to be clarified?

 

[Ibix]

Isn't the core of all these posts that these two spacetimes, the one where something crosses the event horizon in its proper time and the other one, where it doesn't even reach in coordinate time the event horizon in finite time the fact that the principle of simultaneity just can't be applied to the two "perspectives"?

I don't think that makes sense either.

There are two distinct spacetimes being considered. In both of them the infaller crosses the horizon. In the evaporating black hole case the distant observer will eventually see this (in principle), but not in the eternal case. The fact that you don't see something doesn't mean it doesn't happen, though. In both cases you can pick coordinates where the object falling in reaches the horizon at some finite time coordinate. The difference is that in the evaporating case you can't pick coordinates where it doesn't (and even that isn't true, depending on how picky you want to be).

 

[javisot]

If anyone knows a better place for people like me, my thanks in advance if you could steer me in a better direction.

No site is immune to this; it's common in all physics forums. To avoid problems, you have to ask knowing if your question is correct and practically knowing the answer. That ensures you're never censored for asking. But if you know the answer in advance and if an answer is correct, what are you doing asking in a physics forum?

 

[Dale]

the tone was so persistently insulting

It was. We are supposed to be an educational site. That is our stated goal and mission.

I hope my colleagues here can look at your experience and think “would I appreciate a teacher that talked to me or my child like this?”

maybe spend several years reading in meek silence before I dare talk?

Just FYI, it took me 7 years of occasional study to finally “get” special relativity. This stuff is difficult.

You don’t have to be silent during that time. But it is better to just ask “what is the resolution to this apparent conflict” than to propose a resolution of your own.

 

[timmdeeg]

I don't think that makes sense either.

There are two distinct spacetimes being considered. In both of them the infaller crosses the horizon. In the evaporating black hole case the distant observer will eventually see this (in principle), but not in the eternal case. The fact that you don't see something doesn't mean it doesn't happen, though. In both cases you can pick coordinates where the object falling in reaches the horizon at some finite time coordinate. The difference is that in the evaporating case you can't pick coordinates where it doesn't (and even that isn't true, depending on how picky you want to be).

Agreed. My comment is related to the title of the Thread, not to Black holes evaporation, which makes things more complicated.

 

[PeterDonis]

several people are stating with varying degrees of politeness that I don't have the right background to engage in discussions or ask questions, and have no business here

No, nobody has said that. What we have said is that what questions you can usefully ask will depend on what background you have, so you need to take that into account. Unfortunately, that does mean that many questions you are tempted to ask before you have the requisite background won't be answerable. And getting the requisite background takes time and effort. There's no way around that.

@Ibix in post #47 is saying a more detailed version of the above.

 

[PeterDonis]

these two spacetimes, the one where something crosses the event horizon in its proper time and the other one, where it doesn't even reach in coordinate time the event horizon

No, that's not the two spacetimes. Both of the things you describe are true in the same spacetime--the "eternal" black hole spacetime (with the "coordinate time" being Schwarzschild coordinates in the exterior region).

The two spacetimes are that "eternal" black hole spacetime, and an evaporating black hole spacetime. In an evaporating black hole spacetime, the first of the two things you mention is still true, but the second is not.

 

[Foretranimal]

Well, good news: I was able to find someone who helped me find a fascinating paper on exactly what I was asking.

I'm not an academic or professional, so I don't know the proper vernacular to articulate clearly what I am asking. Sorry if that offended anyone. I didn't think to talk about Penrose diagrams, and make it crystal clear I was asking specifically about "non-eternal evaporating black holes," becasue I didn't have those words.

That said, a few of you were pretty condescending, and I don't think I did anything to deserve that. It felt lousy.

Anyway, this paper better articulates what I had in my head. I think if I read it a few hundred times over the next few years, I might begin to understand it poorly, which I look forward to.

https://arxiv.org/pdf/1907.04879

This thread has moved quite fast and I've not fully caught up, so apologies if this repeats things already said.

There are different models of black holes, and those models have different properties. The comment about watching an infalling object slow down, freeze, and redshift to "nothing," but never cross the event horizon in the OP is a statement about an eternal black hole - a model that does not include Hawking radiation. It isn't true about a model that does include black hole evaporation. It's also a statement of what an observer outside the hole directly sees - not a statement about what that observer can infer from what they see. That last bit often gets confused, because with an eternal black hole it's easy to construct coordinate systems that don't assign time coordinates at or below the horizon and then take them too literally.

With evaporating black holes you do see the horizon crossing at the same time as you see the hole explode (at least in principle - actually picking it out of the mess might be difficult). So there's no "it didn't form but now it's exploded" paradox. The mistake is mixing true statements about eternal black holes with wondering about non-eternal holes.

To be fair, physicists will often make statements about "black holes" without specifying which models they're thinking of. It's a continuing problem with science communication that general statements are always false, but precise ones are often too nitpicky (about nits the public doesn't even know need to be picked) and turn off the public.

I really appreciate your taking the time to figure out my badly phrased question. Thank you.

 

[PeterDonis]

I didn't think to talk about Penrose diagrams

In fairness, those are not a tool that would normally occur to someone not familiar with the subject. But they are a very useful tool, yes.

 

[PeterDonis]

this paper better articulates what I had in my head

That's good. Note that the paper answers your original question: see the answer to the first bullet at the top left of p. 2. (The reason why the answer to that question is yes is the same reason why the answer to your question of whether someone can fall through the horizon of an evaporating black hole is yes.)

 

[Ibix]

Anyway, this paper better articulates what I had in my head. I think if I read it a few hundred times over the next few years, I might begin to understand it poorly, which I look forward to.

Spacetime diagrams are a very powerful tool. I would suggest not reading that paper hundreds of times, but rather reading Carroll's lecture notes. They do discuss Penrose diagrams towards the end of chapter 7. Then read the paper again.

Like I say, you may well need to read an SR text first.

 

[martinbn]

Anyway, this paper better articulates what I had in my head. I think if I read it a few hundred times over the next few years, I might begin to understand it poorly, which I look forward to.

https://arxiv.org/pdf/1907.04879

Why not study relativity first and then read the paper once or twice!

 

[PeroK]

Anyway, this paper better articulates what I had in my head. I think if I read it a few hundred times over the next few years, I might begin to understand it poorly, which I look forward to.

That's a unique approach to learning!