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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

#191

Post by Sharlin »

codesnik wrote: post #172
PaulHoule wrote: post #91

Earlier quoted context omitted.

The singularity in a non-rotating, non-charged black hole is as you say. It’s like in a finite amount of time you “run out of time”, like there isn’t any more time on that trajectory. The singularity in a rotating black hole is entirely different but the interior of classical Kerr (rotating) black holes is one of the most controversial if inconsequential topics in theoretical physics because there are reasons to beli…

and some rotation probably inevitable anyway, and non-rotating black holes could be just a mathematical construct, no?

Yes.

Re: Black hole singularity is a surface not a point

#192

Post by Sharlin »

XorNot wrote: post #84
SoftTalker wrote: post #67

Earlier quoted context omitted.

The way Brian Cox puts it, a singularity is a point in time: the end of time. I have trouble really conceptualizing black hole physics, I just think of it as a mass so great that nothing, including light, can escape its gravity. Works for me.

The more interesting component is that black hole physics is almost an anti-free will zone. Everywhere else in the universe with mass and energy you can do what you want (sort of). An event horizon throws a hard shroud over that and drastically reduces opportunities: your free will to use mass and energy is significantly curtailed (you must head towards the singularity).

Your free will outside a black hole is curtailed in exactly the same manner: you cannot do anything to avoid, or even delay, tomorrow.

Re: Black hole singularity is a surface not a point

#193

Post by sigmoid10 »

pdonis wrote: post #126
sigmoid10 wrote: post #99

Earlier quoted context omitted.

>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 bla…

> 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…

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.

Re: Black hole singularity is a surface not a point

#194

Post by sigmoid10 »

pdonis wrote: post #128
sigmoid10 wrote: post #105

Earlier quoted context omitted.

>the notion that the singularity is always in your future makes sense But it is just a mathematical artefact of weirdly chosen coordinates. In reality, the singularity is still just a point in space (or a line in spacetime), except that inside the event horizon all paths you are allowed to travel lead to it. There's no need for this whole "space turns into time" notion apart from the fact that you are guaranteed to h…

> it is just a mathematical artefact of weirdly chosen coordinates. No, that's not correct. The fact that the singularity is always in your future inside the horizon is an invariant, independent of any choice of coordinates. > the singularity is still just a point in space No, it's not. A point in space would be a timelike line in spacetime. But the singularity is a spacelike line in spacetime. That's a moment of tim…

Just draw the geometry in Eddington Finkelstein coordinates and you will see everything I wrote above is true at the technical level if you read precisely.

Re: Black hole singularity is a surface not a point

#195

Post by sfn42 »

Dylan16807 wrote: post #134
bell-cot wrote: post #74

Earlier quoted context omitted.

Yes - I took the grandparent comment's "descr to a non-physicist" as "describe to someone who really isn't into math or hard sciences". Those folks will hear "million trillion" as "a really Really REALLY REALLY big number". Not as "1 x 10^18".

I guess, but "light year" is easy to say, easy to understand, and much less likely to mislead. And most black holes would probably be safe at that distance. Though the issue of small black holes is worth bringing up. If you scrunch up one cubic kilometer of stone into a black hole, that's heavy to be stable for trillions of years, but light enough to pass between the Earth and the moon without causing serious problem…

You'd be surprised how may people don't even understand that a light year is a measure of distance.

Re: Black hole singularity is a surface not a point

#196

Post by phrotoma »

sfink wrote: post #46

Off-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 ar…

Perhaps you're thinking of something like this?

https://en.wikipedia.org/wiki/Group_polarization

Re: Black hole singularity is a surface not a point

#197

Post by irjustin »

DrJokepu wrote: post #165

I have a question for any physicists here. Due to my engineering background, I know just enough physics and mathematics to completely misunderstand general relativity and quantum mechanics. However, one pattern I have noticed is that one favorite past time of physicists is looking at the mathematical models, trying to find insane ass edge cases and then trying to interpret them. With that in mind, do these equations…

Is this a variation of a naked black hole/singularity? If so, PBS space time has one on it[0].

Upshot is if you spin it fast enough... acgoiawef.awef?

[0] https://www.youtube.com/watch?v=1Z5fnwUmTSY

Re: Black hole singularity is a surface not a point

#198

Post by raattgift »

amelius wrote: post #180

> Counterintuitively, in general relativity two points can be spatially close yet causally distant. Can the converse also be true in general relativity?

Yes, during cosmic inflation for example: two test objects initially close can end up many light-years apart. If we make these test objects null (i.e., lightlike) then we can always contrive an inflation that stretches them apart in such a way that they still meet again.

Some exotic spacetimes involving pp-wave sandwiches can focus initially non-converging and spatially distant light pencils onto each other at a caustic shortly after the passing of the stack of plane-parallel gravitational waves. One can hide some such processes in the early cosmos.

Re: Black hole singularity is a surface not a point

#199

Post by GoblinSlayer »

bmitc wrote: post #145
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.

> Anyone who has taken a graduate level class in General Relativity would have been able to tell you the same. You say that and yet this thread is full of people arguing about it, and there's an entire Wikipedia article on this: https://en.wikipedia.org/wiki/Gravitational_singularity . In fact, that article says: > No complete and precise definition of singularities exist in the theory of general relativity, So which…

> And things like naked singularities aren't proven to not exist either.

What do you mean by not exist? If you postulate the right black hole with a naked singularity, it would have a naked singularity.

> It seems obvious that better understanding what a black hole's singularity is would require quantum mechanics

If you postulate a classical black hole, it won't require quantum mechanics to understand.

Re: Black hole singularity is a surface not a point

#200

Post by evanb »

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!

Both / they're related. More mass gravitates more.

Suppose you had an infinite universe that was filled with a cool gas of low uniform density. Then the gravitational field at any particular point would be 0, by Gauss' law.

But, if you wait a brief moment the gas will not stay uniform, because each atom of the gas will have some velocity. You'll observe fluctuations: places with small over-density and small under-density (compared to the average). The places with over-density will gravitate more than the average and places with under-density will gravitate less, and gravity will cause the gas to clump.

Wait a few billion years and some places will have amalgamated whole galaxies' worth of matter around them and other places will be empty.

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