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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

#231

Post by senderista »

zmgsabst wrote: post #229
senderista wrote: post #173

Earlier quoted context omitted.

The curvature scalar can be physically evaluated by measuring the volume of a small ball of freely falling test particles and comparing to its volume in flat spacetime. https://en.wikipedia.org/wiki/Scalar_curvature#Relation_betw...

Okay — why is it unphysical such a ball has unbounded curvature?

If this unbounded state were asymptotic then it wouldn't be unphysical, but a freely falling particle reaches the singularity in finite time. A freely falling observer crossing the event horizon could in principle perform this measurement as they approached the singularity, and general relativity could no longer describe their measurements.

Re: Black hole singularity is a surface not a point

#232

Post by gerdesj »

ambicapter wrote: post #146
gerdesj wrote: post #125

Earlier quoted context omitted.

No need for funky stuff, just do the classic (lol) Young's slits experiment which should be more than enough oddness for anyone. I was shown it at school, using microscope glass slides that we waved over a Bunsen burner running cold, to coat with soot. We then carefully etched parallel lines with a compass by hand. Our (~17 y/o) efforts were a bit random but we did get some smudgy banding results on the screen.

Using a compass is interesting, use two razor blades taped together is much easier and it’s close enough to perfect parallel lines.

It was the '80s so safety razor blades were available but in a posh public school (UK version) aged ~17, compasses were everywhere and large old style razor blades were not - the modern style razor with one or two thin blades was a thing. We could have probably got some needles from matron for a finer point, than a compass.

Your pair of razor blades is a great solution.

Re: Black hole singularity is a surface not a point

#233

Post by zmgsabst »

senderista wrote: post #231
zmgsabst wrote: post #229

Earlier quoted context omitted.

Okay — why is it unphysical such a ball has unbounded curvature?

If this unbounded state were asymptotic then it wouldn't be unphysical, but a freely falling particle reaches the singularity in finite time. A freely falling observer crossing the event horizon could in principle perform this measurement as they approached the singularity, and general relativity could no longer describe their measurements.

Okay — you’re still not explaining what actually breaks, just repeating they could measure over and over.

Please say what you specifically believe is unphysical about the situation — what trajectory reaches the singularity in finite time and why specifically is that unphysical?

My understanding is that you have a cusp singularity that is actually an infinite spike, ie, distance to the singularity is unbounded; that is, no matter how small a circle/sphere around the singularity, you have an infinite diameter. And so you will need to be much more explicit about where the problem lies.

Re: Black hole singularity is a surface not a point

#234

Post by sigmoid10 »

pdonis wrote: post #228
sigmoid10 wrote: post #225

Earlier quoted context omitted.

>You wrote that the singularity is a point in space Because it is. Remember: space, not spacetime. Hence the remark in brackets in the original comment and my reminder to read precisely in the other one. And in Eddington Finkelstein it is most obvious that it is a point in space (i.e. it has spatial coordinate r=0 where r has the metric signature of a spatial dimension) that you can hit at various points in (global)…

Sorry, you're just repeating the same wrong statement. I know you said "space", and I already explained that a spacelike line in spacetime cannot be a point in space. It can only be a moment of time. You are quite correct that, since the singularity is a line in spacetime, different incoming light rays (or free-falling observers, for that matter) can hit it at different points. Depending on how you choose your coordi…

Sorry, you are still arguing against things I never said or that you desperately want to misinterpret in a disingenuous way. If I was actually wrong about anything I said, all you had to do was write down the explicit metric and point out exactly where it disagrees with what I said. But if you did, you would immediately see that your argumentation falls apart.

Re: Black hole singularity is a surface not a point

#235

Post by XorNot »

Sharlin wrote: post #192
XorNot wrote: post #84

Earlier quoted context omitted.

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.

Which is exactly my point. Under normal circumstances you inexorably move towards the future. But we generally believe we have freedom of action about the spatial coordinates.

A black hole is interesting because you inexorably move towards the singularity - which is a defined location in spacetime, and also has a boundary - the event horizon.

So now your freedom of action is reduced: you must move towards the singularity, but you also can't actually move outside of the event horizon either.

Re: Black hole singularity is a surface not a point

#236

Post by pdonis »

sigmoid10 wrote: post #234
pdonis wrote: post #228

Earlier quoted context omitted.

Sorry, you're just repeating the same wrong statement. I know you said "space", and I already explained that a spacelike line in spacetime cannot be a point in space. It can only be a moment of time. You are quite correct that, since the singularity is a line in spacetime, different incoming light rays (or free-falling observers, for that matter) can hit it at different points. Depending on how you choose your coordi…

Sorry, you are still arguing against things I never said or that you desperately want to misinterpret in a disingenuous way. If I was actually wrong about anything I said, all you had to do was write down the explicit metric and point out exactly where it disagrees with what I said. But if you did, you would immediately see that your argumentation falls apart.

> you are still arguing against things I never said

You said the singularity is a point in space. That's what I'm arguing against.

The rest was an attempt to try to help you understand the correct physics. Evidently it was wasted effort. I won't do it again.

> all you had to do was write down the explicit metric and point out exactly where it disagrees with what I said.

Sure, it's the one in terms of t' and r in the Wikipedia article on Eddington Finkelstein coordinates. [1]

> if you did, you would immediately see that your argumentation falls apart.

No, I see that yours does.

The article is using the timelike signature convention, so positive ds^2 is timelike and negative ds^2 is spacelike. Vertical lines in a spacetime diagram (like the one just a little bit below the metric, on the right, that shows the light cones) are intervals where only dt' is nonzero. It is obvious from the metric that for any r cannot be a point in space. It can only be a moment of time. The fact that it is vertical on the diagram does not change that.

[1] https://en.wikipedia.org/wiki/Eddington%E2%80%93Finkelstein_...

Re: Black hole singularity is a surface not a point

#237

Post by ben_w »

eru wrote: post #169
cscheid wrote: post #127

Earlier quoted context omitted.

Pauli exclusion is a QM thing. Black holes are a GR thing. Famously, we don’t know how to mix the two.

We don't know how to mix the two in general. There are plenty of tame enough special cases where we know how a mixture has to look like. Finding a tame enough special case was how Hawking discovered his radiation.

> Finding a tame enough special case was how Hawking discovered his radiation.

It's how it was formulated, but the radiation is far too weak to measure to know it's a real thing on an actual black hole; from this calculator, a 1 solar mass black hole has a Hawking radiation power of 9e-29 W: https://www.vttoth.com/CMS/physics-notes/311-hawking-radiati...

We do see analogous effects in physical analogues of black holes, but we don't know for sure that Hawking radiation actually comes off of actual black holes.

Worse, when they're small enough(!) to be luminous enough to actually observe, they should be hot enough to be spewing out a whole load of exotic nonsense particles (not just photons) that we don't really know how to model correctly with regards to Hawking radiation even if it is part of whatever ends up unifying QM and GR.

Given even the event horizon of a BH doesn't play well with QM, it's probably best to wait for some physicists to work out how to combine QM and GR better.

Re: Black hole singularity is a surface not a point

#238

Post by ben_w »

inigyou wrote: post #155
XorNot wrote: post #78

Earlier quoted context omitted.

Common sense - the experience gained from your common everyday experience of reality - does not apply in black holes. Common sense would tell you they can't exist at all because you can't compress atoms - you have lived your entire life with atoms being entirely incompressible for the practical purpose of anything you do. Leaning on common sense to discuss fundamental physics has been wrong since round about the star…

Common sense actually says that atoms don't exist and if you squish cheese really hard you just get really hard and slightly smaller cheese, or maybe you invent a new type of dairy product.

Dunno, the ancient Greeks had roughly this idea (ἄτομον) in their philosophy, so while actual atomic theory is not common sense, merely having atoms is also not-*not* common sense.

Re: Black hole singularity is a surface not a point

#239

Post by ben_w »

GoblinSlayer wrote: post #204
XorNot wrote: post #78

Earlier quoted context omitted.

Common sense - the experience gained from your common everyday experience of reality - does not apply in black holes. Common sense would tell you they can't exist at all because you can't compress atoms - you have lived your entire life with atoms being entirely incompressible for the practical purpose of anything you do. Leaning on common sense to discuss fundamental physics has been wrong since round about the star…

There's indeed a problem with existence of black holes. Why would they exist? They are usually postulated into existence.

They're an automatic outcome of fairly benign starting conditions in GR: Simply having more than a certain amount of mass within a certain volume.

The required density for that stuff goes down as the volume goes up, so a solar-system sized object (~77 AU radius) at mere normal sea-level (Earth) air density (1.2 kg/m^3) would just be one automatically.

Given that the maths of GR requires spacetime to not have a singularity*, and yet it predicts a singularity from benign starting conditions, I take this as a sign that GR is not correct.

But black holes are nonetheless an outcome, not a presumption.

* it's more complicated than that

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