2 ms·

> Black holes (...) take infinite time to form This is wrong, and based on not taking into account that in GR, simultaneity is not only a relative thing, but a

> Black holes (...) take infinite time to form This is wrong, and based on not taking into account that in GR, simultaneity is not only a relative thing, but also a local thing. There is much more detail in several great answers to this Stack Exchange question: http://physics.stackexchange.com/questions/5031/can-black-holes-form-in-a-finite-amount-of-time http://physics.stackexchange.com/questions/5031/can-black-ho...

ars 11y ago
Did you actually read the answers you linked to? They say exactly the same thing: A distant observer never sees a blackhole form, but infalling matter does. Because GR has no such concept of simultaneity the two observers can never agree on when the black hole formed. But since we are here on Earth we care about how it looks to a distant observer, and from our point of view black holes never form. And what the infalling matter sees is irrelevant to us. NOTE: You do not need black holes to make gravity waves, neutron stars will also make them. But time is still enormously dilated by neutron stars, and I'm wondering how that affects the gravity waves they make.
- semi-extrinsic 11y ago
  In the original comment I replied to, the point was "black holes cannot form because we cannot observe it, so black holes don't exist". This is a misunderstanding, and is what I was pointing out. Saying that something objectively "doesn't happen" because an observer infinitely far away can't see it is ridiculous.
  - ars 11y ago
    > This is a misunderstanding, and is what I was pointing out. No. You are misunderstanding things. > because an observer infinitely far away can't see it No, it's not that the observer can't see it (too hard to see or something like that). It never occurs in the timeline of the observer. From the point of view of the observer it never actually exists. That it exists from the point of view of someone else is irrelevant. It's not a semantic game about I can see it you can't. It literally and actually simply does not exist. (Not to mention the context is gravity waves, so if it never exists from the point of view of the observer it also can't make gravity waves.)
    - semi-extrinsic 11y ago
      Pardon me if I am being obtuse, but the point of the original comment was "black holes do not exist in our universe, since time dilation at the event horizon is infinite", yes? This is what I am arguing against. The event horizon is within the future null infinity of your observer; i.e. the observer may choose to travel towards to black hole, and will asymptotically fall into it and thus undeniably observe it. Hence it does actually exist even for this observer. Moreover, the observer can observe the shadow, effect on nearby masses and the gravitational lensing produced by the black hole, thus indirectly observing the black hole, even though he cannot see the actual event horizon.

ars 11y ago

Did you actually read the answers you linked to? They say exactly the same thing: A distant observer never sees a blackhole form, but infalling matter does. Because GR has no such concept of simultaneity the two observers can never agree on when the black hole formed. But since we are here on Earth we care about how it looks to a distant observer, and from our point of view black holes never form. And what the infalling matter sees is irrelevant to us. NOTE: You do not need black holes to make gravity waves, neutron stars will also make them. But time is still enormously dilated by neutron stars, and I'm wondering how that affects the gravity waves they make.

semi-extrinsic 11y ago
In the original comment I replied to, the point was "black holes cannot form because we cannot observe it, so black holes don't exist". This is a misunderstanding, and is what I was pointing out. Saying that something objectively "doesn't happen" because an observer infinitely far away can't see it is ridiculous.
- ars 11y ago
  > This is a misunderstanding, and is what I was pointing out. No. You are misunderstanding things. > because an observer infinitely far away can't see it No, it's not that the observer can't see it (too hard to see or something like that). It never occurs in the timeline of the observer. From the point of view of the observer it never actually exists. That it exists from the point of view of someone else is irrelevant. It's not a semantic game about I can see it you can't. It literally and actually simply does not exist. (Not to mention the context is gravity waves, so if it never exists from the point of view of the observer it also can't make gravity waves.)
  - semi-extrinsic 11y ago
    Pardon me if I am being obtuse, but the point of the original comment was "black holes do not exist in our universe, since time dilation at the event horizon is infinite", yes? This is what I am arguing against. The event horizon is within the future null infinity of your observer; i.e. the observer may choose to travel towards to black hole, and will asymptotically fall into it and thus undeniably observe it. Hence it does actually exist even for this observer. Moreover, the observer can observe the shadow, effect on nearby masses and the gravitational lensing produced by the black hole, thus indirectly observing the black hole, even though he cannot see the actual event horizon.