4 ms·

OP's claim is that black holes infinite amount of time to form, because matter would take an infinite amount of time to fall into it. Personally, as a layman o

OP's claim is that black holes infinite amount of time to form, because matter would take an infinite amount of time to fall into it. Personally, as a layman on the subject of black holes, what I understand is that mass does take infinite amount of time to fall into an event horizon. But I don't know about the formation process of how/when that event horizon is formed. Perhaps you can help me, and him, understand why his claim is false--that it would happen in finite time?

JoeAltmaier 9y ago
Maybe they never finish forming (keep collapsing forever). But once you have an event horizon, you have a black hole, right?
Santosh83 9y ago
Signals take an infinte amount of time to reach a distant observer from just outside the event horizon. Which means we cannot see the event horizon or what is beyond, and we cannot also 'see' the event horizon being formed, or any matter crossing the event horizon. When the star collapses, light takes progressively longer to reach an outside observer, and eventually becomes too dim to detect. The same thing happens with an object falling into a black hole. But since the gravitational effect can still be felt, we cannot also say that the black hole never formed. At all events, a region of spacetime that emits no discernable signals has formed. You can call it a black hole or speculate further about what it is, but the phenomenon is still right there.
- wallace_f 9y ago
  >we cannot also 'see' the event horizon being formed I think what you're saying here is the confusing part. If no observer in the universe can ever witness a black hole being formed, it stands to reason they cannot exist (within the reference frame of observers outside it). Now I'm not asserting this as true or not, because I wouldn't assert such contrarian conclusions on a subject I have not properly studied; rather I am just highlighting what appears to be the confusing bit here.
  - Santosh83 9y ago
    What is meant when we say we expect to see a black hole, or event horizon, forming? If we mean that we can no longer detect any signal from particular region, then to that extent, we can assert that what we call as a black hole exists there. We're defining it by its negatives, but I can't see how one can say that even those negatives don't exist. Also its gravitational effects can be detected, so that is another reason why merely claiming a black hole doesn't exist, or never forms, seems to rather miss the mark.
podiki 9y ago
Put simply, matter certainly falls into a black hole in finite time. Black holes merge, as LIGO recently observed gravitational waves from, if that helps give some "direct" evidence. Yes, studying the exact formation for a black hole is a subject under investigation. From what little I've heard in the past, there is nothing surprising happening, but I guess people are hoping that could lead to some understanding. Not sure what they do, numerical GR I suppose, maybe compare to alternative gravity theories to see if any differences are predicted.

JoeAltmaier 9y ago

Maybe they never finish forming (keep collapsing forever). But once you have an event horizon, you have a black hole, right?

Santosh83 9y ago

Signals take an infinte amount of time to reach a distant observer from just outside the event horizon. Which means we cannot see the event horizon or what is beyond, and we cannot also 'see' the event horizon being formed, or any matter crossing the event horizon. When the star collapses, light takes progressively longer to reach an outside observer, and eventually becomes too dim to detect. The same thing happens with an object falling into a black hole. But since the gravitational effect can still be felt, we cannot also say that the black hole never formed. At all events, a region of spacetime that emits no discernable signals has formed. You can call it a black hole or speculate further about what it is, but the phenomenon is still right there.

wallace_f 9y ago
>we cannot also 'see' the event horizon being formed I think what you're saying here is the confusing part. If no observer in the universe can ever witness a black hole being formed, it stands to reason they cannot exist (within the reference frame of observers outside it). Now I'm not asserting this as true or not, because I wouldn't assert such contrarian conclusions on a subject I have not properly studied; rather I am just highlighting what appears to be the confusing bit here.
- Santosh83 9y ago
  What is meant when we say we expect to see a black hole, or event horizon, forming? If we mean that we can no longer detect any signal from particular region, then to that extent, we can assert that what we call as a black hole exists there. We're defining it by its negatives, but I can't see how one can say that even those negatives don't exist. Also its gravitational effects can be detected, so that is another reason why merely claiming a black hole doesn't exist, or never forms, seems to rather miss the mark.

podiki 9y ago

Put simply, matter certainly falls into a black hole in finite time. Black holes merge, as LIGO recently observed gravitational waves from, if that helps give some "direct" evidence. Yes, studying the exact formation for a black hole is a subject under investigation. From what little I've heard in the past, there is nothing surprising happening, but I guess people are hoping that could lead to some understanding. Not sure what they do, numerical GR I suppose, maybe compare to alternative gravity theories to see if any differences are predicted.