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Black hole singularity is a surface not a point

arxiv.org

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Re: Black hole singularity is a surface not a point

#21

Post by pantulis »

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.

It's a region of space from where not even light can scape.

You can get a region like that by squashing a lot of mass in a small space, like happens when a star collapses under its own gravity. So here the intuition of "high density" makes sense.

But at the center of galaxies you have the so called "supermassive black holes" which are more or less comparable in size to the solar system and yes, they have a lot of mass but they are not very dense, a pop-sci trope is comparing it's density to cotton candy or even the air we're breathing right now.

So it's a matter of how you distribute mass/energy in a given diameter, not exactly of density.

Re: Black hole singularity is a surface not a point

#22

Post by Certhas »

moralestapia wrote: post #18
Certhas wrote: post #3

As is nicely visualised by it's Penrose Diagram, e.g. https://jila.colorado.edu/~ajsh/insidebh/penrose_schw.gif

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.

Re: Black hole singularity is a surface not a point

#23

Post by imzadi »

Would this apply also to the singularity at the beginning of the universe? I guess I thought that the singularity was where all matter is compressed so much that it occupies a zero dimension point. I'm not sure if that applies equally to black holes and the singularity at the beginning of the universe. I'm kind of dumb on this stuff even though it fascinates me.

Re: Black hole singularity is a surface not a point

#24

Post by ben_w »

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 because speed of light is the speed of causality: events that happen further in, are "over the horizon" for you, they cannot causally influence you.

Re: Black hole singularity is a surface not a point

#25

Post by bell-cot »

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.

"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."

Re: Black hole singularity is a surface not a point

#26

Post by My_Name »

ck2 wrote: post #6

what's really going to blow your mind is while you probably assumed or knew spinning black holes move space around them spinning black holes also move TIME around them * https://www.science.org/doi/10.1126/sciadv.ady9068 so in theory a spinning black hole that's been around for billions of years has a time drag around it in a path that is billions of years old (no we can't navigate it because yes that would be time t…

Another thing that may blow the minds of some is that M87* is less dense than air at 0.44kg/m³ so if you could bring it to sea level (in a large enough theoretical test area) it would float like a helium balloon (sea level air is 1.2kg/m³).

Of course if you did do that, the air itself would collapse into a black hole larger than M87*...

Re: Black hole singularity is a surface not a point

#27

Post by dash2 »

kwoff wrote: post #12
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.

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???

Re: Black hole singularity is a surface not a point

#28

Post by 1970-01-01 »

srean wrote: post #17
ck2 wrote: post #6

what's really going to blow your mind is while you probably assumed or knew spinning black holes move space around them spinning black holes also move TIME around them * https://www.science.org/doi/10.1126/sciadv.ady9068 so in theory a spinning black hole that's been around for billions of years has a time drag around it in a path that is billions of years old (no we can't navigate it because yes that would be time t…

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.

Re: Black hole singularity is a surface not a point

#29

Post by measurablefunc »

imzadi wrote: post #23

Would this apply also to the singularity at the beginning of the universe? I guess I thought that the singularity was where all matter is compressed so much that it occupies a zero dimension point. I'm not sure if that applies equally to black holes and the singularity at the beginning of the universe. I'm kind of dumb on this stuff even though it fascinates me.

It could also be an infinite dimensional ball which technically also has 0 surface area & volume even though it has a non-zero radius.
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