# Event Horizons in General Relativity

Discussion in 'Astronomy, Exobiology, & Cosmology' started by RJBeery, Apr 17, 2019.

1. ### NotEinsteinValued Senior Member

Messages:
1,986
It's called being open-minded, and not jumping to conclusions until you've seen all the evidence.

If you are not interested in discussing your ideas, then why did you post them in the first place?

3. ### RJBeeryNatural PhilosopherValued Senior Member

Messages:
4,215
Umm, we're on post #42. You tell me to give you references, so I do. You tell me to explain the logical contradiction, and I try. Not sure what's left -- at some point the words I type have to have meaning to you or we're wasting each other's time.

5. ### NotEinsteinValued Senior Member

Messages:
1,986
Let's recap:
Post #1: You link an LHC report.
Post #2: I comment to a single line of the OP, asking for sources.
Post #3: You give the reference to a picture you used, and said: "I'd be happy to deluge you with sources."
Post #4: I point out your reference doesn't seem to back up what you are claiming it says, so I ask for those additional sources.
Post #11: I again ask for more sources.
Post #12: You post a link to a definition, and you say you don't want to post any more sources.
Post #13: I call out your contradiction with respect to the "deluge of sources" you claimed you would be happy to provide me with.
Post #23: You mention Gravitation by Misner, Thorne and Wheeler, a book which you consider to be flawed, so I'm not sure if I should count that as a source/reference or not. (Using the first edition, check out chapter 34, and especially figure 34.6. Interesting...)
Post #42: You claimed that you gave me references to what I asked for.

So in conclusion: you did not provide me with the sources I asked for, even though you said you would. And now you are claiming you did. Please stop lying.

But what you did is not how you explain a logical contradiction. You start with premises, do deductions, and eventually show a contradiction. All you have are: some specific parts of GR you like, some assumptions, and a jump to a conclusion. It's that middle-part (the deduction of the conclusion) that I'm interested in. You haven't explained that at all.

Funny, I'm getting the same feeling from my side. I've been responding to you, but you seen unwilling or unable to respond to a lot of it. You keep misusing words ("straight lines", "past light cones"), you keep skipping parts of my posts, you keep making claims you can't back up (the way the line cones were drawn in your picture), you keep misrepresenting my position, and now you are misrepresenting the events of this thread.

Look, if you can't be bothered to have an honest discussion, that's fine. Next time, just post in the appropriate section of the forum, and save yourself that trouble.

7. ### RJBeeryNatural PhilosopherValued Senior Member

Messages:
4,215
The contradiction lies in claiming that the micro black holes "existed" but then subsequently evaporated due to Hawking radiation. For something to have "existed" they necessarily reside in our past light cones. Do you disagree with this innocuous statement?

8. ### NotEinsteinValued Senior Member

Messages:
1,986
If that's part of the logical contradiction, then please show how you came to that conclusion from the premises. (Note: I think you've gone circular: didn't you use the contradiction to conclude the black holes couldn't have existed?)

What definition of "existed" are you using? Because if I take the standard GR approach, that's not true. The black hole can be said to meaningfully exist, even though the inside of the event horizon is not in our causal past.

As you can see, it all depends on your precise definition of "existed".

9. ### RJBeeryNatural PhilosopherValued Senior Member

Messages:
4,215
Citation needed. What "standard GR approach" are you referring to? What does "meaningfully exist" mean, specifically?

The reason I'm asking is that you're using typical arguments for massive black holes, such as those which "may exist" in the center of galaxies, whereas I'm crucially referring to micro black holes which, barring any sort of evaporative process, would exist and have existed in our laboratories, able to be detected and studied, according to the mathematics of GR. That's the contradiction. We would not normally get to reside in an area of spacetime where a massive black hole "used to exist" but no longer does. Micro black holes are different -- they don't exist currently in our labs (which we agree on, correct?), and GR forbids that they reside in our past light cones...therefore they didn't, don't and cannot exist at all.

10. ### NotEinsteinValued Senior Member

Messages:
1,986
Oh, so I have to provide references for my claims, but you don't? Well, that seems fair.

Here's the 'citation': Gravitation by Misner, Thorne and Wheeler, first edition. The book is a standard textbook, so it contains the "standaard GR approach". And specifically entire part VII is about deriving the existence of black holes, their properties, and their effects in that context.

Wait... Are you claiming that massive black holes are able to exist in out past light cones? Why are you all of a sudden making that distinction? It's extremely intellectually dishonest to change the goalpost this late in a discussion.

No, you now doing a 180 on black holes existing is a contradiction with many of your earlier statements. Why would black holes in the center of galaxies exist, but in labs not? Both are embedded in our past light cone in exactly the same way.

For example, take a look at the picture you posted (the one in which you later drew the red lines). Notice how that picture scales just fine from a micro black hole to a massive black hole. Both situations are equivalent, only the scale is different.

What do you mean by "normally"? (There's at least one exception that I can see: if the black hole evaporated through Hawking radiation.)

I wouldn't make a hard non-existence claim, but there's currently no evidence for them that I'm aware of, correct.

Except that you've just said that's not true; apparently black hole CAN exist in our past light cones, according to you. So please provide evidence massive black holes can, but micro black holes can't.

Again, what definition of "exist" are you using, because what you are saying here looks tautological. I've pointed out before that an object doesn't need to be in our past light cone in order for it to exist: in the case of a black hole, because the event horizon is on the border of our past light cone, we do see its effect: the curvature of spacetime it causes, etc. It obviously exists for any meaningful definition of that word, because it has a measurable effect.

11. ### RJBeeryNatural PhilosopherValued Senior Member

Messages:
4,215
The difference between massive and micro black holes is that we are capable of traveling to the location of the "supposedly existing" micro black holes. The logic for massive black holes falls apart for the same reason (i.e. black holes of any size cannot exist in our past light cones), but we obviously cannot investigate them in the same manner.

12. ### NotEinsteinValued Senior Member

Messages:
1,986
In other words, our sun can't be orbiting the center of our galaxy, because there's a massive black hole there? That's certainly a novel position of you to take...

So your comment in post #44 and post #46 distinguishing explicitly between micro and massive black holes was misleading? OK, glad we cleared that up.

Except that we can (in principle). That we currently don't have the technology to do so doesn't mean the sun hasn't been doing exactly that for millions of years.

13. ### RJBeeryNatural PhilosopherValued Senior Member

Messages:
4,215
I don't follow anything you just wrote. If and when we are able to travel to the center of a galaxy, after the black hole-like structure has evaporated, we would find nothing...just as we do in our labs. And our conclusion will be the same -- a black hole cannot have existed in that region because that would place it in our past light cones.

If you have a problem with this logic please just say what that problem is, because I'm getting tired wrestling with pigs.

14. ### RJBeeryNatural PhilosopherValued Senior Member

Messages:
4,215
Event horizons and any form of finite-time evaporation process of black hole-like regions are fundamentally incompatible concepts. You cannot have both.

15. ### NotEinsteinValued Senior Member

Messages:
1,986
But the black hole at the center of our galaxy isn't evaporating any time soon. In fact, it's probably been there for a long, long time, plenty of time for our sun's light to reach it (and vice versa).

Right, so a black hole exists until it evaporates, and then it gets magically erased from history.

You claimed we can't interact with black holes in a lab. You say there's no distinction between micro and massive black holes. So it automatically follows we can't interact with the black hole at the center of our galaxy. Thing is, the sun has been doing that for many millennia: exchanging particles and such. Therefore, at least one of your two premises must be wrong.

I'm merely responding to what you write. Look at all the contradictions I pointed out. Many of them you corrected by re-formulating your statements because the original statements weren't correct as written. There are two parties in a discussion, you know?

Assuming that's true, and we definitely seem to have event horizons (see the NASA picture posted earlier), I have to conclude that means you are arguing against Hawking radiation. Which is not a subject I'm intimately familiar with, so I'll leave you to that.

16. ### RJBeeryNatural PhilosopherValued Senior Member

Messages:
4,215
First of all, I'm sorry about the wrestling with pigs comment, I get frustrated. Anyway, the fact that we can't interact with black holes in the lab, assuming GR is correct, is proof that they evaporate by some process. There is nothing special about micro black holes except for the fact that they apparently evaporate before we can study them. I'm not saying that we can't interact with the black hole-like region in the center of our galaxy, I'm saying it cannot contain an event horizon. If it did, then we would just have to wait a few trillion millennia for it to evaporate (by the same process that affects our laboratory micro black holes) then travel to that region (now that it's survivable) and draw the logical conclusion that a black hole never fully formed there.

17. ### NotEinsteinValued Senior Member

Messages:
1,986
No problem, although I am getting a bit tired of all the insults you throw at me.

Except, that as I pointed out, we are. Well, the sun is.

Or, as your own reference in the OP states, they don't form in our current labs in the first place.

Or, as your own reference in the OP states, they don't form in our current labs in the first place.

Except that it does: it's the only explanation that fits the NASA picture. In fact, the NASA picture's appearance was pretty much perfectly predicted by GR's black hole model.

Except that, as per your own set-up of the situation, it obviously did. And the fact we can measure the evaporation, send probes to it, and (as I previously pointed out) the curvature, it's obvious that there is an event horizon there. The event horizon doesn't magically get erased from history after it's gone: that's madness (in the context of GR).

18. ### RJBeeryNatural PhilosopherValued Senior Member

Messages:
4,215
It is madness, because the logical contradiction does not go away, regardless. Pictures of the black hole-like region do not prove there is an event horizon there, they only prove that there is extreme spacetime curvature there (as would be expected). The external properties of a "frozen star" and a proper black hole containing an event horizon would be virtually indistinguishable. A frozen star would have asymptotically red-shifted light coming from its center exactly as a proper black hole would from its event horizon.

Also if you ever feel that I'm insulting you I apologize. After 20 years of posting in physics forums, I have developed a subtle barb in my comments. Genuine intellectual exchanges are extremely rare on the internet.

19. ### NotEinsteinValued Senior Member

Messages:
1,986
And the only explanation in GR is an event horizon; the frozen star concept was superseded by the black hole concept when people started exploring these objects in more detail. If I remember correctly, the crucial way to see this is to calculate what happens to a free-falling observer falling into the black hole. This is a standard GR exercise. Just go through section 31.2 of Gravitation (first edition) and see for yourself that a frozen star concept doesn't hold up under scrutiny.

No worries, it takes much, much worse abuse to get me riled up.

The first step to improving yourself is admitting you have a problem, so good for you!

They actually are not, but since there are many, many pseudoscience peddlers walking around, they have retreated into more private forums and chatrooms.

20. ### RJBeeryNatural PhilosopherValued Senior Member

Messages:
4,215
NotEinstein...do you believe that the standard treatment of black holes in GR is completely free from logical contradictions? Do you believe that an event horizon can exist currently (according to some frame of reference), subsequently evaporate due to some process, disappear completely, and not reside in the past light cones of some other frame of reference? It's easy to dismiss cranky comments like "black holes don't exist!!!!" but I sincerely want to know how you are reconciling this in your mind. As I said, even if God Almighty told me otherwise I would still side with logic. If you can provide me with a logical explanation I would love to hear it.

21. ### NotEinsteinValued Senior Member

Messages:
1,986
No, not with any kind of certainty. However, nobody that I know of has ever been able to produce one, even after 100+ years of some of the most extreme skepticism. It's quite worthy of the title "theory".

Isn't the event horizon dependent on the observer? I'm going to assume you mean something along the lines of: "the event horizon as seen/measured by the observer, associated with the same black hole as the other observer saw/measured", then the second part is true per definition: the event horizon and its contents are per definition not part of the causal past of an observer, so assuming a black hole exists (first part), then yes, I believe this to be the case. But it's rather tautological, so I don't know if this is what you meant?

I reconcile this by not assuming things that aren't part of my causal past don't exist. We can argue about the future-part, but obviously the things in the elsewhere-section of the light cone exist as much as anything in the past-section.

I've sided with logic in this entire thread, so that's good to hear.

No, it's you making the claims. The burden of proof is on you.

22. ### RJBeeryNatural PhilosopherValued Senior Member

Messages:
4,215
Point of clarification here but, no, it's a defined area agreed upon by all observers, determined by the mass (and spin, charge, etc) of the black hole.
The contradiction isn't in the fact that "some item" isn't in your causal past. Have you ever heard of Rindler horizons? There's a sphere at the very edges of our universe which, due to cosmic inflation, is falling out of our future light cones. In other words, there are stars that will be forever lost to us even if we could travel at the speed of light for infinity. You and I would both agree that those stars still exist. Not where I'm headed with this.

I think you're missing the part about traveling to the area where a black hole was "said to have existed". That area is necessarily in our past light cone. Your computer is in your past light cone. Your high school is in your past light cone. The laboratory where micro black holes are speculated to have been created is within your past light cone. Black holes cannot have existed in that laboratory without also existing in your past light cone. There is a mental leap here that you aren't making, or I'm doing a terrible job at describing the problem.

23. ### NotEinsteinValued Senior Member

Messages:
1,986
Except that it's not; see Gravitation (first edition) page 361, the paragraph below equation 31.6. And http://ned.ipac.caltech.edu/level5/March01/Carroll3/Carroll7.html "The surface r = 2GM, while being locally perfectly regular, globally functions as a point of no return"...

The free-falling observer doesn't see anything like an event horizon at $r=2GM$, as I already pointed out earlier. That's why the Schwarzschild radius calculation always starts with a variant of the phrase "for an observer infinitely far away ...".

Edit: Let me clarify this a bit, because I see I've been sloppy with my language: the event horizon may be seen to be in the same place by all (outside) observers, but an observer approaching it will pass it without trouble, and in finite (proper) time. It's clearly not a physical barrier (as I already pointed out earlier). It's clearly not a "frozen star".

I am aware of it, yes.

Good, because I wasn't headed there either.

See, that's the point I disagree with. You keep jumping to a conclusion without showing how you got there. Why must an object be in ones causal past to be said to have existed? I already pointed out that the curvature sure didn't magically disappear when the event horizon formed. You continue claiming there's a contradiction here without backing it up. Please deduce this from GR principles.

It's actually both: you are making a mental leap that I can't follow, because you are doing a terrible job at describing the problem.