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

[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!

 

[Herman Trivilino]

That would be the clever insult Dirac threw at a student

It was actually Wolfgang Pauli who is famously credited for saying "that is not even wrong". It's not clear if he said it as a reaction to a paper submitted to him by a young physicist asking for his comments, or as a comment at a seminar talk by another physicist. Or perhaps both. But it seems it was perhaps the former, and was not directed at or stated in the presence of the author.

But either way perhaps Pauli didn't mean it as an insult, but rather his comments were directed at the idea, not at the author of the idea.

Some people take these things personally even though they are not meant that way. Richard Feynman writes at length about his own tendency to simply speak his mind about an idea without giving any thought to how it might be perceived by the originator of the idea, because he is attacking the idea, not the person. He would even start with "You're crazy, that cannot be right because ..." never thinking at the time about how it would be perceived, because he was focused on the shortcomings of the idea, not at all on those of the author of the idea. Only later, often during subsequent interactions with the person, would he become aware of having delivered a perceived insult.

He even talks about Neils Bohr near the end of his career, approaching Feynman about an idea he had. Bohr said he knew Feynman wouldn't hesitate to tell him if his idea was wrong, whereas others would because they cared too much about offending the famous and senior Bohr.

P.A.M. Dirac was a actually a shy and soft spoken person. Kind of the opposite of Pauli's personality type.

 

[Foretranimal]

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?”

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.

That makes sense. Next time I'll close my OP with something like, "I have no expertise in this area, so there may well be something basic I am just missing." So everyone knows I am seeking information rather than proposing a resolution...

Oh, wait... well, that is exactly what I did say. Perhaps the mistake was placing it at the end rather than the beginning?

 

[PeterDonis]

Next time I'll close my OP with something like, "I have no expertise in this area, so there may well be something basic I am just missing."

You missed the part where @Dale said not to propose a resolution of your own. Pretty much all of your OP, as quoted below, was proposing a resolution of your own.

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

Does the equivalence principle say my perspective is valid? If it does, is it possible that that person really never crossed the event horizon?

The thing I have always read is that when the in-faller crosses, they feel nothing special. But this paradox seems to assume the crossing happens. If the crossing never happens, is there an information paradox at all?

If it takes forever to cross the event horizon, and a black hole's existence is less than forever, would the black hole evaporate before that unfortunate person crosses the horizon?

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?

 

[Foretranimal]

You missed the part where @Dale said not to propose a resolution of your own. Pretty much all of your OP, as quoted below, was proposing a resolution of your own.

The first point I made was what I think I would be observing, and is not proposing anything.

The remaining 4 points I posted end with question marks, so it would be clear I was asking, not proposing something.

How can questions be interpreted as proposing a resolution?

 

[Foretranimal]

Not if the hole evaporates. An evaporating hole is different from an "eternal" hole, which is where the "slow down, freeze, and redshift to nothing" description comes from.

The second sentence:

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

It was intended to make clear that the scope of my question pertained to evaporating black holes.

 

[Foretranimal]

If you mean that relativity of simultaneity is involved in the "barn and ladder" scenario, yes, that's true.

But relativity of simultaneity is not involved in what we're discussing in this thread.

It may not be what you are discussing, but it actually is at the core of what I'm trying to ask. Again, I may be committing the sin of not knowing how to articulate this. So.... shame on me?

 

[PeterDonis]

The first point I made was what I think I would be observing, and is not proposing anything.

The remaining 4 points I posted end with question marks, so it would be clear I was asking, not proposing something.

How can questions be interpreted as proposing a resolution?

The second sentence:

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

It was intended to make clear that the scope of my question pertained to evaporating black holes.

I think you're misunderstanding my point. We all realize that you didn't understand when you made your OP to this thread that you were proposing your own resolution instead of just asking as @Dale advised. (And apparently you still don't understand that.) That's why we kept trying to tell you that during the course of the thread, right up through my last post before this one. But instead of listening to us, and taking a step back, and thinking, wait, what if these people, who all know a lot more about this subject than I do, are trying to give me important information about how I should change my entire approach to this question, you kept arguing that you weren't doing what we kept telling you you were doing. Just as you're doing now, in what's quoted above.

Ultimately, of course, it's up to you what you say, what questions you ask, how you approach any subject you're trying to find out more about. I'm glad that you found a paper that seems to capture things better for you.

I'm just saying that, when multiple people who all know a lot more about a subject than you do keep telling you the same thing about your approach to that subject, it might pay to stop and listen, even if it's something you might not want to hear.

 

[PeterDonis]

It may not be what you are discussing, but it actually is at the core of what I'm trying to ask. Again, I may be committing the sin of not knowing how to articulate this. So.... shame on me?

If you're asking why relativity of simultaneity is not involved with what we're discussing in this thread, it's because whether or not the infalling person crosses the horizon is an invariant fact about the spacetime geometry and how that person's worldline (trajectory through spacetime) is embedded in it, which is the same regardless of what coordinates you choose or what simultaneity convention you adopt.

The idea that by changing your "perspective" you can change whether or not some actual, real event happens is simply wrong. And when you bring in relativity of simultaneity to this question and say it's significant, you're saying, whether or not you realize it, that by changing your "perspective" you can change whether or not the actual, real event of the infalling person crossing the horizon happens. Which you can't.

 

[Foretranimal]

No site is immune to this; it's common in all physics forums.

Agreed: it is not black and white, some sites do it better than others.

To avoid problems, you have to ask knowing if your question is correct and practically knowing the answer.

Another way to avoid problems is to consider what you are reading as an assertion is actualy a question.

I'm old-fashioned and like to use question marks for that purpose, but I'm totally open to using whatver works here if someone would let me know what that is.

That ensures you're never censored for asking.

It didn't ensure that in this thread.

But if you know the answer in advance and if an answer is correct, what are you doing asking in a physics forum?

At this point, I'm here trying to figure out how to make it clear to members in this forum that I am asking a question when flattly stating "I am making no assertions, but asking questions" is not clear, so not working for some members.

Do you have any suggestions?

 

[PeterDonis]

how to make it clear to members in this forum that I am asking a question

The issue is not how to tell an assertion from a question. The issue is what questions you are asking and whether they are answerable without you having to unlearn everything you think you know and start fresh by re-learning basic SR and GR--which is beyond the scope of a PF thread. (And yes, I know that "asking questions" is not supposed to be the same thing as "proposing your own resolution", which is what I talked about in post #67. But they're really just different manifestations of the same root problem.)

Again, I understand that when you posted the OP to this thread, you weren't even considering the possibility that what you were asking wasn't answerable without you having to unlearn everything you think you know and start fresh by re-learning basic SR and GR. That's why we kept trying to tell you that that might be the case over the course of the thread. I understand that's not a pleasant thing to hear. But it might be true.

One way to get an indication about whether it's true is for you to read my post #68 and see whether it makes sense to you.

 

[Foretranimal]

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

Good question. I suppose the answer depends on what studying relativity means in your questions.

If it means obtaining an advanced degree in the area, the answer is that that degree of time and financial resource commitment would need to be for something I intend to do professionally, and I know myself well enough that I know I don't have enough knack in the topic to be professionally successful. I'm not terrible at theory, but I've always been much better at practical engineering, so that is what I pursued.

If it means teaching myself as best as I can for the enjoyment of learning something I find interesting, accepting my understanding will have limitations, I've been doing this since mid-1990s, although not exclusively - I have broad interests I like to engage in parallel.

 

[javisot]

Agreed: it is not black and white, some sites do it better than others.



Another way to avoid problems is to consider what you are reading as an assertion is actualy a question.

I'm old-fashioned and like to use question marks for that purpose, but I'm totally open to using whatver works here if someone would let me know what that is.



It didn't ensure that in this thread.



At this point, I'm here trying to figure out how to make it clear to members in this forum that I am asking a question when flattly stating "I am making no assertions, but asking questions" is not clear, so not working for some members.

Do you have any suggestions?

Looking at the entire conversation, it's as simple as correcting you and explaining the standard solution. Personally, I usually use those "first personal approximations", but I don't think the moderators acted or spoke disrespectfully.

You have to familiarize yourself with the harsh language that's often used. The price of not seeking mathematical answers is receiving short, harsh answers instead. What's wrong is wrong, and what's right is right.

 

[Dale]

At this point I think this thread is unlikely to provide any further educational value. Thank you for everyone who contributed.

 

[Nugatory]

Good question. I suppose the answer depends on what studying relativity means in your questions.

If it means obtaining an advanced degree in the area….
…If it means teaching myself as best as I can for the enjoyment of learning something I find interesting, accepting my understanding will have limitations, I've been doing this since mid-1990s, although not exclusively - I have broad interests I like to engage in parallel.

This is a false dichotomy. For purposes of constructive discussion of special relativity, forty or so hours with Taylor and Wheeler’s “Spacetime Physics” will suffice, and that’s a few months of hobby/spare time not the years that you’re talking about. Taylor and Wheeler first edition is free online and understandable by a high school senior or college freshman (maybe needing some help over a hard spot if you get stuck - something this forum is pretty good at).

General relativity is a heavier lift, but https://preposterousuniverse.com/wp-content/uploads/2015/08/grtinypdf.pdf will get you started, provide enough background that you will know how to proceed from there.