Do Black Holes Really Exist? - Comments

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  • #51
russ_watters
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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.
 
  • #52
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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?
 
  • #53
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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.
 
  • #54
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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.
 
  • #55
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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.
maline said:
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.
 
  • #56
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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.
 
  • #57
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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.
 
  • #59
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I need some clarity regarding blackholes.

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 blackhole 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?
 
  • #60
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I need some clarity regarding blackholes ...
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.
 
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  • #61
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Consider that a BH is not a solid object - a spiraling stream of particles collapsing to a gravitational center. In essence a vortex.
 
  • #62
Drakkith
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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.
 
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"..densely packed free neutrons"
I thought free neutrons decayed in 10.3 minutes?
 
  • #64
Drakkith
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"..densely packed free neutrons"
I thought free neutrons decayed in 10.3 minutes?
Not under the pressures encountered inside a neutron star.
 
  • #65
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If the blackhole 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.
 
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