Do Black Holes Really Exist? - Comments

[rootone]

A neutron consists of three quarks bound by gluons, so in principle that is what a disintegrated neutron should turn into.
Whether or not that condition of free quarks and gluons can remain stable is (I think) unknown.
Matter in this state has been hypothesised though for two cases.
Firstly there *may be* a stable condition of this sort when a neutron star collapses instead of collapse directly to a black hole, (hypothetical quark star)
Quark-gluon plasma is also hypothesised as being a possible state of the Universe very shortly after the big bang.

 

[Jonathan Scott]

What happens to the neutrons which collapse? Do they disappear? Just saying its loosely described as neutron collapse says little. What happens to the mass-energy of the neutrons which collapse?

As far as I know, there is very little evidence as to what might happen in this case, but I think the general opinion is that neutrons might collapse into their components, i.e. quarks and gluons, in which case the total mass-energy would remain unchanged, but the density could be somewhat higher. If the pressure was removed, the quarks and gluons would form the original number of neutrons (or protons and electrons) again. If the density is able to increase significantly relative to a neutron star, this would directly trigger collapse to a black hole anyway, otherwise it may be possible to add a bit more mass before that happens.

 

[rjbeery]

Excellent article PeterDonis! Personally, my objection doesn't lay in the math of GR, but in our definition of "existence". In the past when I've pressed on this point the typical response is that the discussion has turned semantic and philosophical. I disagree. I believe the entire subject rests on what we mean, precisely, when we say that something exists*. The definition of this word is where the discussion should be taking place if we want to discuss the subject at all.

*such as a definition of "exists" which applies to a region of spacetime that is not in the causal past of future null infinity

 

[Bernie G]

A neutron consists of three quarks bound by gluons, so in principle that is what a disintegrated neutron should turn into.

From the Wiki article on protons: "it is now known to be composed of three valence quarks: two up quarks and one down quark. The rest masses of the quarks contribute only about 1% of the proton's mass, however.[2] The remainder of the proton mass is due to the kinetic energy of the quarks and to the energy of the gluon fields that bind the quarks together."

It says the quark mass contribution is only 1%(! ) and the rest is energy. Sure sounds like ultra-relativistic stuff to me. Question: Suppose you disintegrated 10 neutrons, resulting in quark matter and energy, then half the energy was lost, and you recombined what was left. Would you have about 5 neutrons?

 

[Martin0001]

From the Wiki article on protons: "it is now known to be composed of three valence quarks: two up quarks and one down quark. The rest masses of the quarks contribute only about 1% of the proton's mass, however.[2] The remainder of the proton mass is due to the kinetic energy of the quarks and to the energy of the gluon fields that bind the quarks together."

It says the quark mass contribution is only 1%(! ) and the rest is energy. Sure sounds like ultra-relativistic stuff to me. Question: Suppose you disintegrated 10 neutrons, resulting in quark matter and energy, then half the energy was lost, and you recombined what was left. Would you have about 5 neutrons?

Quarks formed this way will also form a sort of *degenerate matter*, very much like initial neutrons did but a denser one
Gross of energy will be retained in the system and pressure of degeneracy of degenerate quark matter will take care of it.

 

[Jonathan Scott]

Question: Suppose you disintegrated 10 neutrons, resulting in quark matter and energy, then half the energy was lost, and you recombined what was left. Would you have about 5 neutrons?

The energy is what is keeping the quarks apart (and they cannot be completely isolated). There could also be extra quark / anti-quark pairs produced from interactions involving the excess energy. If you removed enough energy for the quarks to recombine, they would combine back into 10 neutrons or the equivalent in protons (plus leptons). They could in theory initially combine back into heavier particles, for example including strange or charm quarks, but the final result could not be less than 10 protons in rest mass because of baryon number conservation.

(From Special Relativity, if you have a system containing lots of internal kinetic energy but the overall momentum is small, the kinetic energy effectively counts towards the rest mass of the overall system).

 

[rjbeery]

When discussing whether or not black holes exist there are three options that I see:

1) Define "exists" and "black holes" to exclude them from existence
2) Reject the word "exists" completely as an unscientific term
3) Define "exists" and "black holes" to include them in existence

The definition for black holes in the Insight article is given as a region of spacetime that is not in the causal past of future null infinity, and that's the one I'll use here.

Regarding #1, we could try to define "exists" in terms of simultaneity. There are obvious ambiguities in doing this, but it's a start. Events A and B could be said to co-exist if observer C claimed they were simultaneous; we could then say that A exists with B for C...but that's all. Co-existence is relative to an observer; however, this does give us a chance to put bounds on co-existence. If two events A and B are lightlike or timelike separated then there is no observer C who could make the claim that they are in co-existence. Using these definitions, no events A at or within a hypothetical event horizon co-exist with events B outside of the event horizon because there is no observer C who can make the claim that they are simultaneous.

Regarding #2, ambiguity suggests that perhaps "exists" should not enter a scientific discussion. I disagree with this. By the same logic we should not discuss velocity in a scientific context due to its relative nature. In any event, if we choose to reject "existence" completely then we certainly can't also make the claim that black holes exist.

That leaves us with #3. As external observers to any theoretical black hole, we could make the claim that the black hole exists due to the thought experiment of simply imagining observer B traveling to the region of a suspected black hole A and "falling in". The black hole exists for B because he can cross the event horizon A in finite proper time, right? The math is clear on this, but there is baggage with the view that this constitutes existence; namely, it requires a block universe in which timelike separated events are considered to co-exist. There is simply no external observer C who can make the claim that A and B co-exist until B reaches A...and that never happens for C, D, E or any other external observers. If we simply declare that A and B co-exist by fiat, or by definition, then we must also accept that Julius Caesar and Christmas Day of 2100 co-exist. This is a consequence that I doubt most people would accept.

 

[Jonathan Scott]

I think that philosophical discussion about the meaning of "exists" is outside the scope of these forums.

The scientific position as I understand it is that standard GR accurately predicts observations relating to weak gravitational fields and also predicts that extremely dense objects will collapse gravitationally into what we describe as a black hole, which has some rather bewildering and awkward properties. There are modified versions of GR and other gravitational theories which do not predict such collapse, although they are typically far more complex and contrived than GR and hence are less satisfactory, by Occam's razor. We therefore eagerly await experimental confirmation or refutation of whether objects predicted by GR to be black holes actually have the predicted properties.

(Although I accept GR as the best theory we have so far, I personally have a strong suspicion that Einstein's Field Equations might effectively be only a weak field approximation. It would certain stir things up a bit if someone could spot an object soon that GR says should be a black hole but which clearly isn't, such as a pulsar around 30 solar masses!).

 

[Martin0001]

When discussing whether or not black holes exist there are three options that I see:

1) Define "exists" and "black holes" to exclude them from existence
2) Reject the word "exists" completely as an unscientific term
3) Define "exists" and "black holes" to include them in existence

The definition for black holes in the Insight article is given as a region of spacetime that is not in the causal past of future null infinity, and that's the one I'll use here.

Regarding #1, we could try to define "exists" in terms of simultaneity. There are obvious ambiguities in doing this, but it's a start. Events A and B could be said to co-exist if observer C claimed they were simultaneous; we could then say that A exists with B for C...but that's all. Co-existence is relative to an observer; however, this does give us a chance to put bounds on co-existence. If two events A and B are lightlike or timelike separated then there is no observer C who could make the claim that they are in co-existence. Using these definitions, no events A at or within a hypothetical event horizon co-exist with events B outside of the event horizon because there is no observer C who can make the claim that they are simultaneous.

Regarding #2, ambiguity suggests that perhaps "exists" should not enter a scientific discussion. I disagree with this. By the same logic we should not discuss velocity in a scientific context due to its relative nature. In any event, if we choose to reject "existence" completely then we certainly can't also make the claim that black holes exist.

That leaves us with #3. As external observers to any theoretical black hole, we could make the claim that the black hole exists due to the thought experiment of simply imagining observer B traveling to the region of a suspected black hole A and "falling in". The black hole exists for B because he can cross the event horizon A in finite proper time, right? The math is clear on this, but there is baggage with the view that this constitutes existence; namely, it requires a block universe in which timelike separated events are considered to co-exist. There is simply no external observer C who can make the claim that A and B co-exist until B reaches A...and that never happens for C, D, E or any other external observers. If we simply declare that A and B co-exist by fiat, or by definition, then we must also accept that Julius Caesar and Christmas Day of 2100 co-exist. This is a consequence that I doubt most people would accept.

All fine, but why to make issue complicated, if one can keep it simple?
Let's define a BH as a compact object, detectable only by its immense gravity which does not have a hard surface up to the area where EH should be present.
Such object should only give away to our investigation few details like mass, angular momentum and electric charge but only information about mass is easy to access.
Forget what is inside, so for example object with mathematical singularity as well as one with hard surface of compact star just below EH are both BH.

Now let's get more observations and find out if existing candidates meet these objections.
So for example if we are finding that candidates are showing magnetic fields which cannot be explained by accretion discs we must consider them not to be BH.
There are several classes of such objects, which might pretend to be BH but they are not.
So let's exclude some objects like compact stars made of degenerate matter of any sort, then exclude MECO, then exclude "fuzzballs", gravastars etc.
Once these are excluded let's assume that we have "proven" BH to exist, but *if* we have proven that object is MECO, fuzzball, gravastar or something alike, let's assume that BH do not exist.

Let's hope that current efforts to produce images of Sag. A by arrays of radiotelescopes will deliver some more information in this respect.

 

[rjbeery]

I think that philosophical discussion about the meaning of "exists" is outside the scope of these forums.

Outside the scope of a thread entitled "Do Black Holes Really Exist"?

 

[maline]

Events A and B could be said to co-exist if observer C claimed they were simultaneous

The idea that an observer "claims" that two events are simultaneous- meaning that there is a well-defined inertial frame centered on that observer- is true in SR & locally in GR but not in situations involving large distances & gravity. Any coordinate map you define on spacetime has some objects with coordinate acceleration that does not come from any force. So to ask whether a spacelike separated object "exists now" is really and truly meaningless- if you like, and work at it long enough, you can draw up a map that will label your point of interest with the same time coordinate as your present. It will be just as valid as any other map, so long as you work out all the Christoffel symbols etcetera whenever you want to predict anything. As far as I know you can do this for the interior of a BH as well, as long as it's not in your future light cone.

 

[jambaugh]

Unquestioned belief in existence of BH displayed by many here is breath taking.

What we can say is that some ultradense objects which do not appear to possesses a surface are detected.
Of course there might be a very red shifted surface indeed and such an object would not be a BH.
There are few credible alternatives indeed...

Your remaining post not withstanding, here's my position. There are 2 questions:
i. Is GR right or mostly right? (e.g. to the extent that its prediction of BH's requires they exist in galactic centers given the amount of mass there.)
and
ii. Do Black Holes Exist? (as a more general question than above.)

The first question we can consider by doing other experiments and so in so far as GR has been confirmed by evidence, there you go.
Since the original concept of a BH was as a prediction of GR to answer the second question which I take to be the OP's question, I would argue that "some ultradense objects which do not appear to possesses a surface" comes mighty close to an "if it quacks like a duck" definition of a BH. However as this doesn't quite address this begged question I've pointed out then we really should answer that one, first...

If we wish to generalize the idea to theories distinct from Einstein's GR, what do we mean by a "Black Hole"?

Keep in mind also that when we speak of curved space-time this is a MODEL for GR and other such theories of gravity, not the theory itself. So do not try to describe a BH in terms of the geometry of invisible space-time but rather in terms of what is or can be observed. (e.g. one cannot communicate observations across an event horizon.)

But Martin, my belief in BH's is not "unquestioned" any more than is my belief in quarks or the Higgs particle or even the photon itself. All my beliefs are tentative subject to change with new data. Now if you want to speak of unquestioned beliefs we could open a discussion on Anthropogenic Global Warming! ;)

 

[rjbeery]

The idea that an observer "claims" that two events are simultaneous- meaning that there is a well-defined inertial frame centered on that observer- is true in SR & locally in GR but not in situations involving large distances & gravity. Any coordinate map you define on spacetime has some objects with coordinate acceleration that does not come from any force. So to ask whether a spacelike separated object "exists now" is really and truly meaningless- if you like, and work at it long enough, you can draw up a map that will label your point of interest with the same time coordinate as your present. It will be just as valid as any other map, so long as you work out all the Christoffel symbols etcetera whenever you want to predict anything. As far as I know you can do this for the interior of a BH as well, as long as it's not in your future light cone.

C's choice of coordinates and simultaneity calculation is irrelevant because the only thing that matters in my argument is causality. No valid choice of coordinates can change causal ordering.

 

[rjbeery]

Your remaining post not withstanding, here's my position. There are 2 questions:
i. Is GR right or mostly right? (e.g. to the extent that its prediction of BH's requires they exist in galactic centers given the amount of mass there.)
and
ii. Do Black Holes Exist? (as a more general question than above.)

My position is that GR does not predict that BH's exist in galactic centers given the amount of mass there. I'm claiming that GR could be perfectly correct but that we are misinterpreting (or simply not clarifying precisely) what we mean by "exists".

 

[jambaugh]

My position is that GR does not predict that BH's exist in galactic centers given the amount of mass there. I'm claiming that GR could be perfectly correct but that we are misinterpreting (or simply not clarifying precisely) what we mean by "exists".

At the classical level (where this discussion currently resides) a (categorical) objects existence is quite meaningful. An object of category X exists if a physical object has been observed with properties similar to one of category X so that current orthodox theory forbids it being other than of category X. By orthodox theory I mean theory which has as yet not been overturned by virtue of empirical evidence and which has already supplanted a prior theory the same way.

Note the word "exists" here is rather more like "is actualizable" i.e. "can exist". For example it is not that the Tooth Fairy doesn't exist because she has been killed last week, but rather her existence is not possible.

(Now if you want to get into whether the singularity of a BH exists we got issues since we are getting into the quantum domain where ontology breaks down and we have to get phenomenological.)

Now I am not above playing semantics games and we should be clear that we agree to the meaning of the words we use, but... Stars exist. Roc's do not exist. Black holes? Well that depends on the precision of the latest observations and your degree of skepticism. The M87 observations: http://arxiv.org/abs/1503.03873 point to a mass sufficiently large and compressed that infalling matter is not impacting a surface outside the calculated S. Rad.

Quack Quack Quack!

 

[russ_watters]

Unquestioned belief in existence of BH displayed by many here is breath taking.

Be careful that you aren't projecting a preconception here, because what you think you see does not exist: no one has expressed anything anywhere close to "unquestioned belief".

By their nature, black holes cut us off from observation of many of their features/structure, leaving the list of observable features relatively short. So we'll never have anywhere close to the certainty I have in, for example, the existence of my car. And even at that, no scientist would ever claim 100% proof of a theory.

One small caveat: a black hole is a theoretically predicted object and the term is just a name. Jupiter was Jupiter long before humans knew it wasn't a God's flaming chariott. For black holes, the name and prediction came before the detection, but it is possible that the name will remain even if the theory is found to be largely wrong. Why? Because the objects are real/exist (to close to the maximum level of scientific certainty) and even if their description changes, it is hard to unstick a name. Jupiter is still Jupiter even though we now know it isn't a God.

 

[maline]

C's choice of coordinates and simultaneity calculation is irrelevant because the only thing that matters in my argument is causality. No valid choice of coordinates can change causal ordering.

Huh? If I understand you at all, your issue is with the word "exists" being in present tense. That's why my response is that the "present" is arbitrary. Why would causality be relevant at all? An object does not need to have any effect on us in order to exist...
Anyhow, in the previous thread someone pointed out that the event of a non-negligible mass falling into a BH actually is in the causal past of future null infinity. I don't know much about this topic, but why are you ignoring that point?

 

[rjbeery]

Huh? If I understand you at all, your issue is with the word "exists" being in present tense. That's why my response is that the "present" is arbitrary. Why would causality be relevant at all? An object does not need to have any effect on us in order to exist...
Anyhow, in the previous thread someone pointed out that the event of a non-negligible mass falling into a BH actually is in the causal past of future null infinity. I don't know much about this topic, but why are you ignoring that point?

That's because that point is false. An infalling event at finite time would allow that event to enter an observer's past light cone. This cannot happen, and that's my point in referencing causality. Causal ordering means that you can't swap timelike separated events by a mere coordinate change.

There may be ambiguity about what exists "now" but causal ordering allows an observer to unambiguously define what does not exist now -- namely anything in his future light cone. Objects in his past light cone could be said to have had existed, and presumed persistence of an object could plausibly grant that object existence now, but that simply does not happen with black holes.

 

[maline]

An infalling event at finite time would allow that event to enter an observer's past light cone. This cannot happen, and that's my point in referencing causality.

Here is a what Nugatory posted in your thread:
However, if you are considering processes in which infalling objects increase the mass and radius of the black hole, then you can no longer use the approximation that the infalling mass is near-as-no-never-mind zero, and the Schwarzschild spacetime is not a solution of the Einstein Field Equations under those conditions. Instead, you have to use something like the Oppenheimer-Snyder spacetime, in which the mass of the black hole changes over time and "growth events" can and do appear in the past light cone of outside observers.
But I don't think it's even necessary to discuss that. Black holes can form because the equations work in their own spacetime. Why shouldn't we say they exist? What does our own past light cone have to do with it?

Objects in his past light cone could be said to have had existed, and presumed persistence of an object could plausibly grant that object existence now, but that simply does not happen with black holes.

Do you consider that a necessary condition? As in, the regions of the universe outside of our cosmic horizon do not exist? If so, then yes, the interior of a Black hole "doesn't exist". But this isn't a very intuitive nor useful terminology, nor something worth arguing about in a physics context.

 

[rjbeery]

But I don't think it's even necessary to discuss that. Black holes can form because the equations work in their own spacetime. Why shouldn't we say they exist? What does our own past light cone have to do with it?

By that logic the singularities exist as well, and all of the problems associated with them. You won't find many physicists who will accept that.

Do you consider that a necessary condition? As in, the regions of the universe outside of our cosmic horizon do not exist? If so, then yes, the interior of a Black hole "doesn't exist". But this isn't a very intuitive nor useful terminology, nor something worth arguing about in a physics context.

Well, for one thing, if black holes don't exist as we believe then the information paradox doesn't need solving.

 

[maline]

By that logic the singularities exist as well, and all of the problems associated with them. You won't find many physicists who will accept that.

According to GR, the singularities do exist. We find that problematic, so work is in progress to modify GR for those density scales. But we're in the context of GR here, so I don't see your point. Anyway the question of "when" the singularity would exist is irrelevant- if your theory predicts a singularity somewhere, that's a problem.

Well, for one thing, if black holes don't exist as we believe then the information paradox doesn't need solving.

If spacetime as we understand it has information loss somewhere, why is that not a problem?
Of couse, if we're discussing conservation of something, we have to integrate it over spacelike slices, which can get complicated around black holes. But that's real physics, and it's been done.

 

[Nugatory]

By that logic the singularities exist as well, and all of the problems associated with them. You won't find many physicists who will accept that.

I don't see how that follows. That logic allows me to make predictions about the structure of spacetime inside the event horizon (and it is possible, in principle, for me to validate these predictions by crossing the horizon myself - in practice I choose not to sacrifice the rest of my life just so that I can satisfy my curiosity). These predictions don't have to include the existence of a singularity at $r=0$; I expect that most physicists would say that the Schwarzschild solution is a describes the vacuum region both inside and outside event horizon, and that the vacuum region doesn't extend all the way to $r=0$ because something stops the classical singularity of GR from forming.

 

[Bernie G]

Reading the tit-tat in the comments section after this brief article has broadened my horizons. One believes in "GR black holes" and the other is a skeptic. Neither is enthusiastic about string theory:
http://news.sciencemag.org/physics/2014/11/what-powers-black-holes-mighty-jets

 

[jines]

I need some clarity regarding black holes.

First: A collection of dense gases. Due to the force of gravity, the particles fuse (nuclear fusion), and a star is born.

Then: When the star "uses up" all its matter, (I guess nothing more to fuse?), it starts to die, and matter is squeezed into a tiny space, and this results in very strong gravity that not even light can escape (blackhole).
If the black hole has used up all its matter, and it is now dying, what type of matter is squeezed into the tiny space? Are these types of matter similar to the particles that were present when the star was born? How could this matter squeezed into a tiny space create such a strong gravitational force?

 

[rootone]

I need some clarity regarding black holes ...

Only the very largest stars (most massive that is, not their radius) can collapse due to gravity at the end of their life to become a stellar mass black hole.
Large stars but not the largest of all will collapse to become a neutron star, in which case what used to be atoms becomes reduced to densely packed free neutrons with some ionized atom nucleii and free electrons mixed in.
We don't really know what happens to matter which collapses further still inside black holes since it's impossible to observe inside a black hole's event horizon..
Some speculate about objects called 'Quark stars', but there is no evidence at all that such things can actually exist.

Most stars are the red dwarf type which at the end of their fusion days just fizzle out and slowly cool down.
Somewhat bigger stars similar to our Sun will shed their outer layers eventually leaving the core behind as a white dwarf consisting of densely packed atoms of mainly oxygen and carbon, which again slowly cools down.

Whatever kind of state a star ends up in, it does not increase in mass, so its gravitational field does not become greater.
If hypothetically the Sun was compressed to be a black hole, the planets would continue orbiting as if nothing had changed.

 

[D2Bwrong]

Consider that a BH is not a solid object - a spiraling stream of particles collapsing to a gravitational center. In essence a vortex.

 

[Drakkith]

Consider that a BH is not a solid object - a spiraling stream of particles collapsing to a gravitational center. In essence a vortex.

Not true. A black hole exists whether there's an accretion disk of infalling material or not.

 

[Solon]

"..densely packed free neutrons"
I thought free neutrons decayed in 10.3 minutes?

 

[Drakkith]

"..densely packed free neutrons"
I thought free neutrons decayed in 10.3 minutes?

Not under the pressures encountered inside a neutron star.

 

[Nugatory]

If the black hole has used up all its matter, and it is now dying, what type of matter is squeezed into the tiny space? Are these types of matter similar to the particles that were present when the star was born? How could this matter squeezed into a tiny space create such a strong gravitational force?

At first nearly all the matter in the star is hydrogen gas, because hydrogen is by far the most common element. At the temperatures and pressures found at the center of a star, the hydrogen fuses to form helium; this reaction releases a tremendous amount of energy that resists further collapse and keeps the star burning for most of its lifetime. When the star runs out of hydrogen at the center, collapse resumes until the pressure at the center is enough to start helium fusing into yet heavier elements, releasing more energy and stopping the collapse again. However, this has to stop at some point because the heavier the element the less energy is released by fusing it; and fusing iron and anything heavier actually consumes energy instead of releasing it. Eventually the star runs out of elements whose fusion will release enough energy to resist collapse - and then the star collapses catastrophically.
Thus, at the time of collapse the star still has plenty of matter, as a mix of elements up to and including iron.

As for what happens to this matter when it collapses to down to the center of a black hole? We don't know. We have a good theory for very strong gravitational fields (general relativity) and a good theory for very small things (quantum mechanics) but no good theory for very small things in very strong gravitational fields.

 

[PeterDonis]

We pass through event horizons constantly. space-like hyper-surface is an event horizon, the future, and past light cones of any space-time event are examples of an event horizon, i.e. a boundary across which causal signals and matter can only travel one way.

None of these are "event horizons" in the sense that the event horizon of a black hole is; they are not boundaries of regions that cannot send light signals to future null infinity.

 

[jambaugh]

What the heck is "future null infinity"?
[edit] I mean by that, operationally. You must make some rather strong assumptions about cosmology to infer there is such a direction. [end edit]
Here is Ridler's original definition:

"We shall define a horizon as a frontier between things observable and things unobservable."
"An event-horizon, for a given fundamental observer A, is a (hyper-) surface in space-time which divides all events into two non-empty classes: those that have been or will be observable by A, and those that are forever outside A's possible powers of observation."
["Visual Horizons in World Models" Monthly Notices of the Royal Astr. Society. 116 (6): 662–677]

The only problem with this is that the observer must know his entire future world line to define an event horizon for him in this context. Choosing an observer at an instant (imagine the observer is very short lived as such) and you have my definition. Specifically the event horizon for a momentary observer is the past light cone of her final event-point of existence.

The point I was trying to get across is that if you were say falling into a sufficiently massive black hole crossing it's stationary event horizon, you wouldn't notice a thing. Later in your finite future you'd begin feeling the tidal effects until after a short time (circum-radius/c) your spatial universe in two directions squeezes to radius 0, you are stretched infinitely in the third and time stops at the singularity. (One may presume a theory of quantum gravity might refine that singularity in time better.)

 

[PeterDonis]

What the heck is "future null infinity"?

For the quick definition, see here:

https://en.wikipedia.org/wiki/Absolute_horizon#Definition

For the gory details, the definitive reference is Hawking & Ellis.

You must make some rather strong assumptions about cosmology to infer there is such a direction.

Yes, strictly speaking, only asymptoticallly flat spacetimes have a future null infinity, and the class of spacetimes used to describe the universe as a whole in cosmology, the FRW spacetimes, are not asymptotically flat. However, there is an analogous concept that works in Schwarzschild-de Sitter spacetime, which, since we believe our universe has a positive cosmological constant, is an appropriate model for our universe for this discussion. Roughly speaking, the black hole in Schwarzschild-de Sitter spacetime is the region of spacetime that cannot send light signals to the cosmological horizon.

Hm. I might need to add an addendum to the Insights article that mentions this.

Here is Ridler's original definition

As you will see if you consult Hawking & Ellis, or indeed any GR textbook since the early 1970s, including MTW and Wald, this definition is outdated. The term "event horizon" in the usage of any of the references I have just described is defined as I defined it in post #66.

The only problem with this is that the observer must know his entire future world line to define an event horizon for him in this context

The event horizon is not a property of an observer. It is a property of the spacetime geometry. What is true is that we must know the entire future of the spacetime geometry to know if there are any true event horizons, and if so, where they are. I make this point in the article.

The point I was trying to get across is that if you were say falling into a sufficiently massive black hole crossing it's stationary event horizon, you wouldn't notice a thing.

This is true, but it has nothing whatever to do with the definition of an event horizon.

 

[jambaugh]

The event horizon is not a property of an observer. It is a property of the spacetime geometry. What is true is that we must know the entire future of the spacetime geometry to know if there are any true event horizons, and if so, where they are. I make this point in the article.

Agreed with the first two sentences. But the last qualifier makes for a bad definition imnsho but that's a quibble about semantics and who's value system one chooses. Let me then define a causal horizon to be what I earlier defined as an event horizon rather than what your Wikipedia reference refers to as an absolute horizon.

This is true, but it has nothing whatever to do with the definition of an event horizon.

It has something to do with understanding the causal/event horizon of a black hole which was the OP issue. Consider...

A contracting spherical cloud of dust of radius just beyond its Schwarzschild radius for its mass. At its center, a strobe flashing out pulses of light which trace over the future light-cones of its pulsing events. In the space-time of this progressing collapse, these light cones are bent more and more forward (in the time direction) so that become closer and closer to a 3-cylinder (in distant time x angular x circumradial coordinates). They "flare" back out as they leave the cloud and move farther and farther away from it.

Then the future event horizon of that flash that reaches the outer surface of the cloud just as it shrinks to its Schwarzschild radius will remain forever on the resulting black hole's event horizon. This light cone has been bent into a light cylinder which is the event horizon for the history of the black hole (unless of course its mass changes.)

Any future infalling observer will see this sequence of strobe pulses including that horizon pulse as he crosses the BH's event horizon. Assuming the BH is massive enough that the tidal effects there are negligible the observer is, locally, just passing through another light cone.

So locally, as I see it, a light cone is simply an event horizon that hasn't been bent by a central mass into this cylindrical shape. I will concede the semantic debate and call it a causal horizon if you like. The event horizon of an idealized black hole is the future null 3-surface of a specific space-time event and without those asymptotic assumptions, it is qualitatively no different. Certainly not in terms of local space-time geometry.

 

[PeterDonis]

the last qualifier makes for a bad definition imnsho

Which qualifier? The one about needing to know the entire future? GR is a deterministic theory, so in any GR model you do know the entire future, so knowing where the event horizons, if any, are in the model is straightforward.

The issue, if there is one, comes when we talk about how we test whether the model matches reality. It is true that we can never know for sure that there is an event horizon in reality--we can never know for sure that the model matches reality in that respect. But we still need a term for the feature of the model in question, and "event horizon" is the term that physicists have settled on for that.

If one wants to make it absolutely clear that we are talking about the phenomenon we observe in reality, I would use the term "apparent horizon", since that basically describes what we observe in reality: a boundary around a region of spacetime into which things fall, but from which nothing is ever observed to come out. Then the issue described above can be stated as: we can never know for sure that an apparent horizon that we observe actually is an event horizon. We can construct a model in which it is, but we can never know for sure that that aspect of the model matches reality.

the future event horizon light cone of that flash that reaches the outer surface of the cloud just as it shrinks to its Schwarzschild radius will remain forever on the resulting black hole's event horizon

See my edit in the quote above. "Future light cone" is the general term for, well, the future light cone of an event: the null surface formed by the maximal future extensions of all null geodesics passing through the event. Calling it a "future event horizon" just co-opts a term which already has a different, well-defined meaning, for no good reason, since we already have the term "future light cone" for what you are talking about here.

You could say that there is a particular event at $r = 0$ in this spacetime whose future light cone is the event horizon: but that just concedes the point that not all future light cones are event horizons, only some of them.

locally, as I see it, a light cone is simply an event horizon that hasn't been bent by a central mass into this cylindrical shape. I will concede the semantic debate and call it a causal horizon if you like.

Or you could just call it a future light cone, as above. That's the standard term. I think trying to gerrymander the term "event horizon" to cover all future light cones, which is basically what you are suggesting, just obfuscates things.

As for the term "causal horizon", that is even more general, since any null surface is a causal boundary; you don't even need to consider whether that surface is part of any light cone of interest. But that also means the term is so general that it is not very useful. Usually we are not interested in all causal boundaries, but only in particular ones that have particular properties.