3 ms·

> the singularity is still a point (or rather surface) in the future instead of a point in space. It's a spacelike line on the Kruskal diagram, yes. > The iss

by pdonis 2mo ago

> the singularity is still a point (or rather surface) in the future instead of a point in space. It's a spacelike line on the Kruskal diagram, yes. > The issue is that these diagrams are for eternal, static black holes The full Kruskal diagram is, yes. But the essential features of the black hole portion of that diagram are still there in a black hole that forms by gravitational collapse of a massive body. That includes the singularity being a spacelike line, and there being spacelike curves inside the horizon that are infinitely long. I agree that Eddington-Finkelstein coordinates can help with intuitions about this spacetime geometry as well.

sigmoid10 2mo ago
>It's a spacelike line on the Kruskal diagram It also is in Schwarzschild coords, so you've gained nothing with respect to the original issue from switching coordinates. Kruskal-Szeres really is only useful for demonstrating that the event horizon is not such a weird place, but it does nothing for the singularity at the center. >the essential features of the black hole portion of that diagram are still there in a black hole that forms by gravitational collapse The issue is even the limited diagram doesn't really show you that and the full one goes crazy with white holes. So not a good place if you don't want to confuse laypeople.
- pdonis 2mo ago
  > It also is in Schwarzschild coords True--indeed, the statement that it's a spacelike line is an invariant, independent of any choice of coordinates. But it's a lot harder to see that in Schwarzschild coordinates. > Kruskal-Szeres really is only useful for demonstrating that the event horizon is not such a weird place, but it does nothing for the singularity at the center. I disagree, and I think many GR physicists would as well. Indeed, both classic GR textbooks, Misner, Thorne & Wheeler (1973) and Wald (1984) discuss Kruskal coordinates and their associated spacetime diagram (MTW in Box 31.2 and sections 31.5 and 31.5, Wald in section 6.4), and those discussions contain a good deal more than just showing that the horizon is "not such a weird place".
  - sigmoid10 2mo ago
    Of course you can show stuff with them too. You can show tons of things with all of them. I was talking in relation to Schwarzschild coords.

by pdonis 2mo ago

sigmoid10 2mo ago

>It's a spacelike line on the Kruskal diagram It also is in Schwarzschild coords, so you've gained nothing with respect to the original issue from switching coordinates. Kruskal-Szeres really is only useful for demonstrating that the event horizon is not such a weird place, but it does nothing for the singularity at the center. >the essential features of the black hole portion of that diagram are still there in a black hole that forms by gravitational collapse The issue is even the limited diagram doesn't really show you that and the full one goes crazy with white holes. So not a good place if you don't want to confuse laypeople.

pdonis 2mo ago
> It also is in Schwarzschild coords True--indeed, the statement that it's a spacelike line is an invariant, independent of any choice of coordinates. But it's a lot harder to see that in Schwarzschild coordinates. > Kruskal-Szeres really is only useful for demonstrating that the event horizon is not such a weird place, but it does nothing for the singularity at the center. I disagree, and I think many GR physicists would as well. Indeed, both classic GR textbooks, Misner, Thorne & Wheeler (1973) and Wald (1984) discuss Kruskal coordinates and their associated spacetime diagram (MTW in Box 31.2 and sections 31.5 and 31.5, Wald in section 6.4), and those discussions contain a good deal more than just showing that the horizon is "not such a weird place".
- sigmoid10 2mo ago
  Of course you can show stuff with them too. You can show tons of things with all of them. I was talking in relation to Schwarzschild coords.