5 ms·

Regarding your last point: that depends on your reference frame. From the outside it does appear to take "forever" to get into a black hole, but if you were the

by podiki 9y ago

Regarding your last point: that depends on your reference frame. From the outside it does appear to take "forever" to get into a black hole, but if you were the one falling in...it happens much quicker, certainly not taking an infinite amount of time.

ars 9y ago
That's true. But it's things from the outside that are falling in, and that make a black hole. Since they take an infinite amount of time to fall in, the black hole never forms in the first place. (I'm less sure about this paragraph, but I believe that) An object falling in also never sees a black hole, since the other things falling in are dilated relative to him, so there's still not enough mass to make a black hole, even for the object falling in.
- saagarjha 9y ago
  > But it's things from the outside that are falling in, and that make a black hole. Since they take an infinite amount of time to fall in, the black hole never forms in the first place. See this excellent answer on Physics StackExchange: https://physics.stackexchange.com/questions/5031/can-black-holes-form-in-a-finite-amount-of-time#146852 https://physics.stackexchange.com/questions/5031/can-black-h.... In a nutshell (if I'm interpreting this correctly), yes, to an outside observer you never observe the black hole form since it would take an infinite amount of time for light signals from the newly formed black hole to reach you. However, to any observers that fell into the black hole (and so were in the same frame of reference), the black hole would form in finite time.
  - monocasa 9y ago
    Except Hawking radiation means that it doesn't exist for an infinite amount of time. So an observer falling in would just see it fizzle away as they get closer to it.
    - pdonis 9y ago
      Even in the presence of Hawking radiation, this statement is not correct. An observer falling in would fall right through the horizon and be destroyed in the singularity; then, much, much later (something like 10^70 years for a black hole of 10 solar masses), the hole would be fully evaporated away by Hawking radiation and some of the light rays in that outgoing Hawking radiation would carry information about the observer falling through the horizon 10^70 years before.
- short_sells_poo 9y ago
  You seem to misunderstand the way an event horizon works. From the infalling object's (and the hole's) reference frame, the object crosses the event horizon in finite time and falls into the singularity. It is only from an observer's perspective that the object never seems to cross the horizon.
  - ars 9y ago
    No, that's exactly what I understand. But we don't care about the infalling object. We care about the observer, because any infalling object initially is an observer, and because we (us humans) are observing the black holes. Since from an observers POV nothing can actually fall into the black hole, no black hole can form. The fact that an infalling object reaches the black hole makes no difference to us. Because of time dilation, we can observe no black holes. So our telescopes will never see a black hole.
    - teamonkey 9y ago
      > Since from an observers POV nothing can actually fall into the black hole, no black hole can form. The event horizon isn't really a physical boundary in that sense. It's the mathematical boundary at which, according to general relativity, a particle must have velocity equal to the speed of light in order to escape. A density change inside the star can change the size and shape of that boundary without things falling into it in the usual sense. In the same way, a density wave can 'travel' faster than the speed of sound (or light) in a medium because the wave is a mathematical construct that emerges from a physical situation.
      - grkvlt 9y ago
        A better way of thinking about the event horizon is that it is the boundary beyond which events happening cannot be assigned a 'when' in our universe - i.e. the object falling 'in' to the black hole is never seen to pass this horizon, but to the object it seems that they do, however those events happen to it after an infinite time in our universe, i.e. never, due to time dilation. The contents of a black hole are the events that occur outside/after our universe relative to that boundary. I found these explanations on an awesome PBS youtube video [0] about black holes and space-time. 0. https://youtu.be/vNaEBbFbvcY https://youtu.be/vNaEBbFbvcY
- dragonwriter 9y ago
  > But it's things from the outside that are falling in, and that make a black hole. Since they take an infinite amount of time to fall in, the black hole never forms in the first place. They only take an infinite time to fall in once a black hole has formed; OTOH, unless I misunderstand, the time would asymptotically approach infinity up to that point, which seems to have a similar effect.
  - pdonis 9y ago
    > They only take an infinite time to fall in once a black hole has formed No, they don't. See my other comments upthread.
- 4ad 9y ago
  You keep repeating this misconception even though you have been repeatedly been corrected over and over again. What you don't seem to realize is that the choice of coordinates in GR is arbitrary and local. There are types of coordinates that are singular at the event horizon, and there are others which are not. This has nothing to do with infalling vs. the exterior observer. You seem to think that time is somehow fixed for the exterior observer, but that couldn't be further from the truth. This is really, really trivial and really basic GR. I don't really know what to suggest apart from MTW[1], except maybe this basic course from http://theoreticalminimum.com/courses/general-relativity/2012/fall http://theoreticalminimum.com/courses/general-relativity/201.... If you understand what Penrose diagram are, and how to compute them, the answer is immediately obvious. [1] https://en.wikipedia.org/wiki/Gravitation_(book) https://en.wikipedia.org/wiki/Gravitation_(book)
  - r-bryan 9y ago
    I can recommend: Bernard F. Schultz, _A first course in general relativity_ It's accessible to undergrads! (who have studied SR, vector calculus, electrostatics).
- podiki 9y ago
  Also, did we forget of LIGO's recent observations of black hole mergers, in perfect agreement with the GR calculations? Nothing else (known, or proposed) can match that.

by podiki 9y ago

ars 9y ago

That's true. But it's things from the outside that are falling in, and that make a black hole. Since they take an infinite amount of time to fall in, the black hole never forms in the first place. (I'm less sure about this paragraph, but I believe that) An object falling in also never sees a black hole, since the other things falling in are dilated relative to him, so there's still not enough mass to make a black hole, even for the object falling in.

saagarjha 9y ago
> But it's things from the outside that are falling in, and that make a black hole. Since they take an infinite amount of time to fall in, the black hole never forms in the first place. See this excellent answer on Physics StackExchange: https://physics.stackexchange.com/questions/5031/can-black-holes-form-in-a-finite-amount-of-time#146852 https://physics.stackexchange.com/questions/5031/can-black-h.... In a nutshell (if I'm interpreting this correctly), yes, to an outside observer you never observe the black hole form since it would take an infinite amount of time for light signals from the newly formed black hole to reach you. However, to any observers that fell into the black hole (and so were in the same frame of reference), the black hole would form in finite time.
- monocasa 9y ago
  Except Hawking radiation means that it doesn't exist for an infinite amount of time. So an observer falling in would just see it fizzle away as they get closer to it.
  - pdonis 9y ago
    Even in the presence of Hawking radiation, this statement is not correct. An observer falling in would fall right through the horizon and be destroyed in the singularity; then, much, much later (something like 10^70 years for a black hole of 10 solar masses), the hole would be fully evaporated away by Hawking radiation and some of the light rays in that outgoing Hawking radiation would carry information about the observer falling through the horizon 10^70 years before.
short_sells_poo 9y ago
You seem to misunderstand the way an event horizon works. From the infalling object's (and the hole's) reference frame, the object crosses the event horizon in finite time and falls into the singularity. It is only from an observer's perspective that the object never seems to cross the horizon.
- ars 9y ago
  No, that's exactly what I understand. But we don't care about the infalling object. We care about the observer, because any infalling object initially is an observer, and because we (us humans) are observing the black holes. Since from an observers POV nothing can actually fall into the black hole, no black hole can form. The fact that an infalling object reaches the black hole makes no difference to us. Because of time dilation, we can observe no black holes. So our telescopes will never see a black hole.
  - teamonkey 9y ago
    > Since from an observers POV nothing can actually fall into the black hole, no black hole can form. The event horizon isn't really a physical boundary in that sense. It's the mathematical boundary at which, according to general relativity, a particle must have velocity equal to the speed of light in order to escape. A density change inside the star can change the size and shape of that boundary without things falling into it in the usual sense. In the same way, a density wave can 'travel' faster than the speed of sound (or light) in a medium because the wave is a mathematical construct that emerges from a physical situation.
    - grkvlt 9y ago
      A better way of thinking about the event horizon is that it is the boundary beyond which events happening cannot be assigned a 'when' in our universe - i.e. the object falling 'in' to the black hole is never seen to pass this horizon, but to the object it seems that they do, however those events happen to it after an infinite time in our universe, i.e. never, due to time dilation. The contents of a black hole are the events that occur outside/after our universe relative to that boundary. I found these explanations on an awesome PBS youtube video [0] about black holes and space-time. 0. https://youtu.be/vNaEBbFbvcY https://youtu.be/vNaEBbFbvcY
dragonwriter 9y ago
> But it's things from the outside that are falling in, and that make a black hole. Since they take an infinite amount of time to fall in, the black hole never forms in the first place. They only take an infinite time to fall in once a black hole has formed; OTOH, unless I misunderstand, the time would asymptotically approach infinity up to that point, which seems to have a similar effect.
- pdonis 9y ago
  > They only take an infinite time to fall in once a black hole has formed No, they don't. See my other comments upthread.
4ad 9y ago
You keep repeating this misconception even though you have been repeatedly been corrected over and over again. What you don't seem to realize is that the choice of coordinates in GR is arbitrary and local. There are types of coordinates that are singular at the event horizon, and there are others which are not. This has nothing to do with infalling vs. the exterior observer. You seem to think that time is somehow fixed for the exterior observer, but that couldn't be further from the truth. This is really, really trivial and really basic GR. I don't really know what to suggest apart from MTW[1], except maybe this basic course from http://theoreticalminimum.com/courses/general-relativity/2012/fall http://theoreticalminimum.com/courses/general-relativity/201.... If you understand what Penrose diagram are, and how to compute them, the answer is immediately obvious. [1] https://en.wikipedia.org/wiki/Gravitation_(book) https://en.wikipedia.org/wiki/Gravitation_(book)
- r-bryan 9y ago
  I can recommend: Bernard F. Schultz, _A first course in general relativity_ It's accessible to undergrads! (who have studied SR, vector calculus, electrostatics).
podiki 9y ago
Also, did we forget of LIGO's recent observations of black hole mergers, in perfect agreement with the GR calculations? Nothing else (known, or proposed) can match that.