As is nicely visualised by it's Penrose Diagram, e.g. https://jila.colorado.edu/~ajsh/insidebh/penrose_schw.gif
I enjoyed this Veritasium video on the subject, which includes Penrose Diagrams.
Black hole singularity is a surface not a point
41–50 of 239 posts
Re: Black hole singularity is a surface not a point
#42Earlier quoted context omitted.
Or read Susskind's "The Theoretical Minimum: General Relativity". For a non-spinning blackhole at least, not only is the singularity not a point, it is a surface in time, not space (as the book explains, the space and time coordinates switch places as you cross the event horizon).
> the space and time coordinates switch places as you cross the event horizon I'm sorry but this is blowing my mind. What???
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Re: Black hole singularity is a surface not a point
#43Should be pointed out that this is a critique of common popsci journalism tropes and not a fancy new research result. Anyone who has taken a graduate level class in General Relativity would have been able to tell you the same.
Or read Susskind's "The Theoretical Minimum: General Relativity". For a non-spinning blackhole at least, not only is the singularity not a point, it is a surface in time, not space (as the book explains, the space and time coordinates switch places as you cross the event horizon).
> the space and time coordinates switch places as you cross the event horizon
If Susskind's book does in fact say that, it's extremely disappointing to me, because, as a number of other GR textbooks will tell you (e.g., Misner, Thorne & Wheeler and Wald, the two great classic GR textbooks), the "switch places" is an artifact of a particular choice of coordinates (Schwarzschild coordinates), and does not represent anything physical. So it's not something that should be relied on. (Not to mention the confusion it causes when pop science sources repeat the statement and then draw all manner of wrong conclusions from it.)
The part about being "a surface in time" might be all right, assuming that by that he means "a surface representing a moment in time, not a place in space"--in more technical language, a spacelike surface. That is correct, and it's an invariant that does not depend on any choice of coordinates. But that invariant fact can be described without having to talk about the "switch places" thing at all.
Re: Black hole singularity is a surface not a point
#44Earlier quoted context omitted.
Consider the magnetar. https://en.wikipedia.org/wiki/Magnetar "A magnetar's 10^10 tesla field, by contrast, has an energy density of 4.0×1025 J/m3, with an E/c2 mass density more than 10,000 times that of lead."
Yes, magnetars are considerably more rare than black holes and considerably more interesting to study in terms of raw horsepower. Imagine a type-2 civilization using them as engines or launchers for spacecraft to zip around the galaxy.
the radiation from a magnetar exceeds any other star, overcoming that would seem implausible
still trying to wrap my mind around kilonovas (colliding neutron stars)
ie. they can pop out earth-sized chunks of gold, in theory, and since they aren't black holes that would be VISIBLE, albeit also "in theory" lol
* https://www.nasa.gov/image-article/unfolding-story-of-kilono...
maybe Roman can spot one someday, that would be something
Re: Black hole singularity is a surface not a point
#45Should be pointed out that this is a critique of common popsci journalism tropes and not a fancy new research result. Anyone who has taken a graduate level class in General Relativity would have been able to tell you the same.
I think it's not even a valid critique of that and it's sort of playing games with what the definition of a singularity is to reach the claim that it's making. I think the topology of the singularity is not even a well defined question and certainly not well understood enough to bear the strong claims in the paper.
Unfortunately you are wrong. Everything the paper is saying about the singularity and its properties in GR, and more generally about the black hole solutions it describes, is well understood and has been for decades. The definition of "singularity" that the paper is using is perfectly fine, and its topology is perfectly well-defined. A good textbook treatment is that of Wald (1984).
Some of the things the paper points out are not emphasized in other sources, which is probably why the authors chose to write it. But there is nothing in the paper that is in the least questionable or ill-defined; it's all standard General Relativity as applied to the Schwarzschild and Kerr black hole solutions.
Re: Black hole singularity is a surface not a point
#46Off-topic, but it makes me think of "reasoning black holes": you get enough like-minded people together that they start reinforcing each other's logic and beliefs until not only those people get completely detached from reality, but anyone who interacts with them gets sucked in as well unless their own logic ("velocity") is adequate to skirt the edge and escape, forever altered by the experience.
Similar questions arise: how would you know if you were inside one? The laws of logic ("physics") seemingly don't apply, but there's no way to test them in that environment.
Re: Black hole singularity is a surface not a point
#47Earlier quoted context omitted.
I think it's not even a valid critique of that and it's sort of playing games with what the definition of a singularity is to reach the claim that it's making. I think the topology of the singularity is not even a well defined question and certainly not well understood enough to bear the strong claims in the paper.
Singularities suggest incomplete theories.
This is the opinion of most physicists, yes, but it does not in any way justify the GP's claims or cast doubt on anything that is said in the paper. Note that the paper talks explicitly about the limitations of GR as the singularity is approached and how a quantum gravity theory, if we ever find and confirm one, might fix those issues.
Re: Black hole singularity is a surface not a point
#48Earlier quoted context omitted.
> the space and time coordinates switch places as you cross the event horizon I'm sorry but this is blowing my mind. What???
Because it's very misleading. Time and space do not switch places past the event horizon. What happens is that the direction/path between an object and the singularity becomes a timelike dimension, and the direction that plays the role of time outside of the event horizon becomes a spacelike dimension. That is not the same as them swapping or that time becomes space and space becomes time not to mention that space ha…
> Really what it means is that past the event horizon you can use the direction in space between you and the singularity as a way to measure time
That's not correct. There is a relationship between the radial coordinate r you are at and the time it will take you, by your clock, to reach the singularity (at least assuming you are freely falling), but that relationship can't be described the way you are describing it.
To put the issue with what you say as starkly as possible: at any event inside the black hole's horizon, there are spacelike curves in every direction from you that will hit the singularity! So "the direction in space between you and the singularity" is not even well-defined.
To be clear, you are right that Susskind's statement is very misleading (I posted my own criticism of it upthread).
Re: Black hole singularity is a surface not a point
#49How does one descr blackhole to a non-physicist without losing much accuracy? I just it of a very-dense-object.
"A really interesting and cool thing for astronomers to talk about...but you might want to pray that not one of 'em ever comes within a million trillion miles of the Earth."
Unfortunately, there is a rather large one not one sixth of that distance away (a million trillion miles is actually rather large - 170 kly - approximately double the size of our galaxy)
Re: Black hole singularity is a surface not a point
#50Is there such a thing as a "point" in the universe?