Hacker News Board Hacker News, browsed like a classic message board
Live data from Hacker News

Black hole singularity is a surface not a point

arxiv.org

Post reply on HN
Print view View original

31–40 of 239 posts

Re: Black hole singularity is a surface not a point

#31

Post by empath75 »

sigmoid10 wrote: post #2

Should 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.

Re: Black hole singularity is a surface not a point

#32

Post by lstodd »

dash2 wrote: post #27
kwoff wrote: post #12

Earlier 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???

Yup. It's that weird.

Also read Nick Gorkavyi: The Oscillating Universe: Einsteinian Cosmology of Black Holes and Gravitational Waves

Re: Black hole singularity is a surface not a point

#33

Post by 0x20cowboy »

ben_w wrote: post #24
dekdrop wrote: post #13

How does one descr blackhole to a non-physicist without losing much accuracy? I just it of a very-dense-object.

Normally people think of gravity as pulling on objects. You can instead think of it as pulling on the space those objects are in. A black hole happens when there is enough gravity that space gets pulled inwards somewhere, at at least the speed of light. Gravity falls off with distance, and the distance where space is being pulled inwards at exactly the speed of light is called the "event horizon". It has this name be…

> Normally people think of gravity as pulling on objects. You can instead think of it as pulling on the space those objects are in.

(Very uneducated person here) I’ve always wondered if large objects caused gravity, or if maybe large objects form in the places where there is a lot of gravity. This is probably elementary, but I’ve never looked in to it. Maybe today is the day!

Re: Black hole singularity is a surface not a point

#34

Post by MathMonkeyMan »

dash2 wrote: post #27
kwoff wrote: post #12

Earlier 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???

[This video][1] and the one before it on the playlist are a good no nonsense explanation of the topic.

[1]: https://www.youtube.com/watch?v=O_2vnb_eVGE

Re: Black hole singularity is a surface not a point

#35

Post by ben_w »

0x20cowboy wrote: post #33
ben_w wrote: post #24

Earlier quoted context omitted.

Normally people think of gravity as pulling on objects. You can instead think of it as pulling on the space those objects are in. A black hole happens when there is enough gravity that space gets pulled inwards somewhere, at at least the speed of light. Gravity falls off with distance, and the distance where space is being pulled inwards at exactly the speed of light is called the "event horizon". It has this name be…

> Normally people think of gravity as pulling on objects. You can instead think of it as pulling on the space those objects are in. (Very uneducated person here) I’ve always wondered if large objects caused gravity, or if maybe large objects form in the places where there is a lot of gravity. This is probably elementary, but I’ve never looked in to it. Maybe today is the day!

Dark matter is mysterious enough to be compatible with both at the same time, I think.

(Is a collisionless gas really even an "object"?)

Re: Black hole singularity is a surface not a point

#36

Post by ImHereToVote »

empath75 wrote: post #31
sigmoid10 wrote: post #2

Should 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.

Singularities suggest incomplete theories.

Re: Black hole singularity is a surface not a point

#37

Post by metalliqaz »

dash2 wrote: post #27
kwoff wrote: post #12

Earlier 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???

it might help to think of the singularity as not a point in space but rather a future that cannot be avoided. All possible paths through space and time, no matter what happens, will go towards the singularity.

Re: Black hole singularity is a surface not a point

#38

Post by DeluluDon »

Certhas wrote: post #22
moralestapia wrote: post #18

Earlier quoted context omitted.

Sorry man, that's not what this is about.

It's not? Schwarzschild has a space-like singularity. That's the wiggly horizontal line at the top left of the diagram. If you are in the black hole you can't avoid hitting it. Seems to be exactly what the paper is remarking on.

[flagged]

Re: Black hole singularity is a surface not a point

#39

Post by Kranar »

dash2 wrote: post #27
kwoff wrote: post #12

Earlier 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???

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 has 3 dimensions and time has only 1 dimension so how could they even swap places.

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, specifically the amount of time left before you reach the singularity. It's not so mind blowing when you interpret it that way now is it? You can imagine many things in ordinary life that you use to measure time without claiming that time has literally swapped places with it. On a road trip, the number of kilometres to your exit tells you how long you have left, that's using space as a proxy for time... big deal. The notable difference between a road trip and a black hole is that on a road trip you could stop for a break, you could maybe take a detour, you could decide to go back home... and these would all break your use of space as a proxy for measuring time. Well with a blackhole you can't do any of those things, there is no going back, there is no detour, the relationship between the spatial direction towards the singularity and time is fixed and causal and there's nothing you can do about it.

The phrasing used is used almost certainly to evoke some kind of voodoo mind-blowing mystery that completely disappears when you get down to the more strict formalism.

Re: Black hole singularity is a surface not a point

#40

Post by altairprime »

dekdrop wrote: post #13

How does one descr blackhole to a non-physicist without losing much accuracy? I just it of a very-dense-object.

Consider a balloon. I don’t imagine in visuals but if you do, either a solid color or a patterned balloon works. Let’s say it’s a cow print design.

Deflate it, then stretch the balloon over a vacuum cleaner tube and put on a rubber band to keep it in place.

If you pour sand on it, you can only get a small bump of sand and then it’ll run off the sides. Reasonable, logical, normal behavior. Clearly it’s a surface — it’s holding sand, it’s pouring sand in different directions over the edge, the sand is not all compacted into a single grain.

Turn on the vacuum cleaner. Assume a balloon stretchier than the strongest vacuum cleaner in the universe. What happens? Several things, each of which are perfectly reasonable:

1) The end of the tube is still a circle, and the balloon is still attached and covering the tube, so it’s still a two-dimensional circle.

2) A single grain of sand can’t block the vacuum tube, so it clearly hasn’t collapsed to a point.

3) The covered end of the vacuum cleaner tube is still the same circle, with the same diameter, as it was before you turned on the vacuum.

4) You can pour buckets more of sand onto that stretched circle of balloon than the handful you could before.

5) If you pour enough sand onto the circle, it’ll behave just like it did before: the sand will form a small mound and then newly-poured sand will run off whichever side the sand was poured on.

6) The rubber band is going to catch some of the overflowing grains of sand and hold onto them (‘accretion’), near but just outside the circle.

Next: Consider a more powerful vacuum cleaner. How much more? Lots. The most. An atomic Dyson powered by nuclear fusion. (This is a bit unrealistic, but that’s astrophysics for you.)

How much sand can you pour onto that two-dimensional, circular, balloon surface?

Lots. The most. Some of it will spill around the edges and get caught in the accretion band, but somehow that circle, that’s still the same size and clearly still blocking the vacuum tube, can hold an entire universe of sand.

That’s how black holes work :)

ps. For those who dislike the crudity of my teaching analogy and want to pop the spherical cow balloon: Topologically, the surface covering the vacuum tube is always a circle, even if you have an infinitely-powerful vacuum cleaner. At no point — pun intended — can a vacuum cleaner apply a transformation applied that reduces the dimensionality of the surface, thus it must remain, topologically, a circle.

pps. So clearly I must choose the circle in front of me! Hahaha! Aaaahahahah!

ppps. dies

Post reply on HN