Why Black Hole Interiors Grow Almost Forever | Hacker News Reader

Why Black Hole Interiors Grow Almost Forever

quantamagazine.org

99 pointspavel_lishin69 comments

I wonder if the space region in the interior of a black hole expands the same way as regular space around it.

If so, it would seem to be possible for a super massive black hole to be so big that it would be impossible to actually reach the singularity. If there is enough space the singularity would "move" faster than light. Essentially another event horizon within the black hole protecting travelers from the singularity.

This would lead to a stable universe like ours. Though about this a few days ago and it seemed like an interesting idea.

I remember from a youtube video that the spacetime inside the black hole flipped around, where space becomes temporal dimension and time becomes spatial dimension (i.e. time is traversable in either direction while the singularity becomes an inevitable place in the "future").

> spacetime inside the black hole flipped around, where space becomes temporal dimension and time becomes spatial dimension

No, this is not correct. The "flip" is in a particular choice of coordinates, not in spacetime itself.

> time is traversable in either direction while the singularity becomes an inevitable place in the "future"

It is true that the singularity is inevitably in the future--that's because inside the horizon, the future "time" direction points towards the singularity. But time is still only traversable into the future; you can't "reverse" your travel in time inside the horizon.

This is from PBS Spacetime series [1]. A fantastic channel, and they did a number of videos on black holes.

[1] https://youtu.be/KePNhUJ2reI

I wonder if inside the black hole the singularity behaves more like past than it behaves like future.

No, it doesn't. The singularity inside a black hole is a future singularity, not a past singularity.

Just to clarify, you mean it's a future singularity in the entropy sense, or in some other sense?

Iff I understand correctly, what pdonis is saying is that it is in your future. It’s where (when?) you run out of future to go into, in a way which may or may not be analogous to the North Pole being the coordinate singularity where you run out of north to move in.

So it's the end of time in a sense?

That’s my understanding, but I’m just an enthusiastic amateur and that doesn’t take you very far with relativity.

That is true, the time and space axis gets flipped at the event horizon. This also happens to a fast moving objects due to relativistic effects, although the roatation is usually not very high unless you go really fast. Seehttps://en.m.wikipedia.org/wiki/Minkowski_diagram

However that doesn't mean you can't move when you cross the event horizon. You can do that like usual (except for gravity pulling you inside. You can. It only means outside observer won't be able to see what you're doing, because of the time dilation (and the event horizon of course).

This kinda implies time and space are the same thing, it just depends on your point of view. Still have to see about that :D

> the time and space axis gets flipped at the event horizon. This also happens to a fast moving objects due to relativistic effects

Neither of these things are true, and I have no idea how you are getting either of them from the Wikipedia article you linked to. Penrose-Terrell rotation, which affects how objects appear to observers when they are moving at relativistic speeds relative to those observers, does not "flip" the time or space axes; it's an optical effect due to the finite speed of light.

As I noted in another post upthread, the "flip" of time and space axes inside a black hole's horizon is a property of a particular choice of coordinates, not of spacetime itself.

> This kinda implies time and space are the same thing

No, they aren't, because there is still a fundamental difference between timelike and spacelike directions in spacetime. That is true regardless of how you choose coordinate axes.

People often mistakenly try to map space time onto an ether-like flat background, treating spacetime like a drawing where the paper is the background. It's not an uncommon kind of fumbling as someone tries to grasp these things.

To be pedantic, it's not that they are the same thing, but that they are intrinsically tied together: Time begins at the moment of first inflation.

If black holes are universes, then what happens when 2 black holes merge? What happens when universes merge? The sudden appearance of more stuff? More space? Existential cataclysm?

And what happens when they finally evaporate due to Hawking Radiation?

With our current theories such questions are nigh intractable.

Given that "uni" means one, what could "universes" even mean?

It's simply the plural of "universe". If there actually is more than one universe, it would imply that "universe" was a misnomer, much like "atom" turned out to be.

People might keep using the word with a new definition. The new definition could be something like, "a region of the multiverse that is causally disconnected from other regions of the multiverse under such-and-such conditions." If those conditions change, two universes could become causally connected and merge into one universe. I imagine inhabitants of those universes would (if they look in the right place) witness events with no recognizable cause until the merger was complete.

Or what we currently call the universe might be renamed a sub-universe within the actual "one" universe, which was much bigger than we thought. Then the question is what would happen when sub-universes merge. Same question, different glyphs.

The same implication as unicycles.

Check out Lee Smolin's Fecund Universes Theory. http://evodevouniverse.com/wiki/Cosmological_natural_selecti...

Whoa that sure sounds plausible.

OK, so what experimental evidence supports the idea that the interior volume of a black hole is growing? That is, what do we observe about black holes that'd be different if interior volume wasn't growing? Or is this just a consequence of current black hole theory, with no explicit experimental support?

Nothing. The event horizon is an absolute limit on observability and literally means “the threshold beyond which nothing ever happens”.

Yes. But TFA is all about what's happening there. So (and I apologize, if this seems rude) how is this different from talk about angels dancing on pins? Is it just that, somehow, this became a discussion of mathematics?

I am, obviously, a hard-core experimentalist ;)

I don't know if this is what is going on here, but I get the impression that these sort of discussions are ways to probe theories for inconsistencies. That seemed more clearly the case in this recent commentary by Sabine Hossenfelder on a paper invoking negative mass[1].

You don't have to experimentally observe a posited contradiction before thinking about whether it really is one, and if so, how it might be resolved. For one thing, you might find a way to investigate the issue experimentally.

[1] http://backreaction.blogspot.com/2018/12/no-negative-masses-...

Thanks. I do agree that generating testable hypotheses is a good thing, and that exploring contradictions, implications, etc re existing theory plays an important role. Still, it bugs me when these ideas get popularized, and people talk about them as if they were known. But I'm pretty sure that experts in the field keep it straight.

Observing/measuring a quantum state affects it, I always wondered if thinking about possibilities similarly could affect quantum mechanics.

So what? Observing a black hole could affect it? But because of the event horizon, we couldn't observe any such effect, right? I suppose that someone could drop in to check. And maybe then they'd know.

But what about the rest of us? I gather that Hawking eventually agreed that black holes preserve information, and could leak it (somehow, eventually). Rajaniemi exploited the idea that information could be encoded in Hawking radiation. But that's just SF, I think.

As I understand it, the ever-increasing interior volume of a black hole is deduced from the equations that support general relativity theory, which itself is supported by experimental evidence, so I think that's how it differs.

The scientific process requires experimental observation and is the gold-standard for determining truth. But the scientific process has limits and does not work on every problem. It is important to understand that not all research uses the scientific method--a lot of research does not. I think that's okay provided the authors do not try to dress up their research to make it seem like it is science when it is not.

Do you think, that scientific process cannot work with black hole interior? If it so, I believe it is a premature belief.

For experimental method was able to prove or disprove something, you need first some theoretic statement, based on which we can devise an experimental setup. Theoretic statements, which you can test experimentally, are not easily available in a countless quantities, theorists are searching for them, finding a lot of statements that can be drawn from paradigm, and some of this statments, happen to be reachable by an experiment.

This statement maybe is not can be proved by observation, but who knows maybe if one think a little more, he would be able to draw some conclusions from this statement, that would be possible to check experimentally? How can we know if it possible to reach such a conclusions or not? The only way is to discuss angles dancing on pins. The only way is to find a lot of statments and see if some of them can be proved or disproved experimentally. And you would never know, did you had found all of such statements, or just small part of them.

See? Scientific method is not just experimenting, scientific method is more about speculating and discussing dancing angles. Scientific method fully inherited such discussions from scholastics, but it differs from them because it added an experiment as a required tool. Scientific method is not negation of scholastic method, scientific method is an improved scholastic method. So, nothing wrong with discussing dancing angles, while we do not dismiss any opportunity to get an atomic miscroscope to count angels on a pin. If we do not see any opportunity, then we continue to discuss angels, until we see one.

I don't know if it will be possible in a future to measure a volume of a black hole. But who knows... Scientists are very inventive when it comes to an experiment, and sometimes they can find some clues, that can be counted as observational evidence. Maybe they'll create experimental setup where all the conditions would be right to volume grow endlessly, but without creating a black hole? Or, maybe they create a gravitational-wave laser, point it at a black hole, and draw some evidence from observing results? I don't know. Generally we cannot know future before it happened. If we could then future was already here.

I disagree with this characterization. A crucial part of the scientific method is hypothesis formation. Nearly all of theoretical physics (or theoretical any field) can he slotted into that part of the method. Unguided experimentation can sometimes produce unexpected results, but many of our best scientific results come as a result of theoreticians proposing complete theories in advance, and then using these theories to generate new experiments. Since you can’t exactly pick up a black hole at the corner store, this type of theory development is even more critical — since it might produce ideas for experiments that we could actually conduct. [\rant]

Could this be the source of dark energy and lead to the realization that the universe is a black hole interior?

that would seem to explain the "big bang" :) an explosion from an infinitely dense point that is ever-growing, which began in a higher-order universe as a black hole that we now inhabit.

(of course I know nothing of science, this is all just pop-sci in my head, but boy is it fun to think about)

One very important difference between the big bang and the interior of the black hole: Within the event horizon of a black hole, there is always a singularity in your future. In our universe, that singularity is in our past.

One interpretation: the big bang theory describes a white hole -- the time-reversed version of a black hole.

Einstein-Cartan Theory (https://en.m.wikipedia.org/wiki/Einstein%E2%80%93Cartan_theo...) introduces a torsion term to General Relativity’s field equations that is not incompatible with observations and provides both an avenue for explaining the cosmological constant (https://arxiv.org/abs/1607.01128) and “big bounce” motivation for the beginning of our current universal epoch (https://arxiv.org/abs/1512.06074).

Imagine our universe is contained within a blackhole.

If time and space invert at the event horizon, could it be that the Big Bang was the event horizon of our black hole, that we are forced forward through time by the pull of our black hole interior, and that the expansion of the universe is actually just metatime passing, and as we move through space we are actually moving forward and back in metatime, with the expansion of our universe being an expression of increasing constraints due to metatime’s metaspace as we get closer to the center of our blackhole?

I could imagine that the “Big Rip” is what we experience when we asymptotically approach the singularity. The strangeness of black holes could mean that falling into a single point is actually experienced as space flying apart faster than light.

Yes.

Be wary of saying things like "from an infinitely dense point" because you might mistakenly think the universe is expanding from some distinguished point in our space. Instead, the infinitely dense position was in time, and the universe has always been (we think) infinitely big.

Yeah it's like all points exploded at the same time, but we can only trace our light cone back to one of them.

Thanks for pointing this out. I've always thought about the initial singularity as a point in both time and space. Is it still correct to say all the matter that makes up our observable universe started out at a single point in space? Since the singularity was infinitely dense, this at least seems possible.

I'm about to do something terrible, but it helped me immensely to understand the Big Bang. Let's pretend we have a magic box that tells us how space expands over time:

    V(t)

So, t starts at '0', which we're gonna call 't0', and goes until 'now'. So V could be small, it could be big. We don't know, and it kinda doesn't matter. The reason is that the way the Big Bang works is like this:

    (t - t0) * V(t)

So, when t == t0 (the beginning of time), it doesn't matter what shape space (V) has, the size of the universe (spacetime) is '0'.

Turtles all the way down?

Can anyone perhaps explain a little better what is meant by the interior volume of a black hole grows forever (while the surface area of the event horizon stays relatively constant)? Felt like the article tried to touch on that but I certainly didn't understand it.

Imagine a cone with a fixed diameter for the base. That represents the black hole’s external event horizon. Now stretch the point of the cone away from the base... forever, as the singularity warps space time more and more. The volume of the cone (inside the event horizon) will grow as the point moves further away from the base.

Thank you! I think that's about the best possible layman's explanation that I could (in the weird world of trying to think in higher dimensions) understand.

Can you give a reference to a paper that describes the math behind this?

Neglecting evaporation, it's a lot closer to Yen Chin Ong's results (and those he cites, especially Christodoulou and Rovelli [2]) than most of the comments throughout this discussion, in that with a careful choice of time coordinate a spacelike spherical sector of a Schwarzschild black hole could evolve roughly similarly.

Of course, one has enormous freedom to choose spacelike hypersurfaces through a Schwarzschild black hole, so such a result is not especially interesting without some sort of uniqueness proof for the idea, and I don't think that's achievable. (cf [2]).

I'm also not sure this is what GP has in mind.

- --

[1] https://arxiv.org/abs/1503.08245 (§ 2) (simpler overview: https://www.kth.se/profile/ycong/page/the-interior-volumes-o... )

[2] https://arxiv.org/abs/1411.2854 (§ IV)

Thanks for the references. The Rovelli paper seems to be the key one deriving the claimed result. I have to say the result looks contrived to me: they are constructing spacelike surfaces with increasing volume only by what I would call severe gerrymandering. But that's probably off topic for this discussion.

It makes me think of Gabriel’s horn. Like, out here space is finite while time is infinite, but in there, space becomes infinite and time infinite. Very amateur opinion, mind you.

> Can anyone perhaps explain a little better what is meant by the interior volume of a black hole grows forever

There is no reference to an actual paper in the article (which is typical for these pop science sensationalist websites), so it's difficult to tell what this is supposed to refer to. As far as I know, it's actually an effect of a particular choice of coordinates inside the horizon, so it's debatable whether it actually has physical meaning.

> There is no reference to an actual paper in the article

Hover over the underlined "proposed a solution" in the first paragraph and see https://arxiv.org/abs/1810.11563 -- although the article certainly doesn't go so far as to point to e.g. §23 and the Conclusion, so I at least partly agree with you.

I don't have much time for AdS arguments, so haven't done more than quickly skim for relevance to the Quanta article (which I also just skimmed for much the same reason).

> these pop science sensationalist websites

I can't really agree with characterizing Quanta as either "pop science" or "sensationalist", and presumably the Simons Foundation, it's backer, would disagree even more strongly.

FWIW, Preskill liked the article. https://twitter.com/preskill/status/1070865055732916225

Even though I just skimmed, I certainly agree with the first sentence of Susskind's lecture's Conclusion, even without reading much of the rest of it. :-) (I also agree with the Conclusion's final sentence on p. 83. In fact, the whole last two paragraphs!) For my part, the Quanta article doesn't seem to have done Susskind or the lecture any great disservice. For that alone, I think the OP author should be commended.

> As far as I know, it's actually an effect of a particular choice of coordinates inside the horizon, so it's debatable whether it actually has physical meaning.

What's the point of science with no "physical meaning"?

I sense that a lot of advanced maths is like that. It may turn out to have a "physical meaning" at some point - or not.

Sure, but math <> science.

I've wondered if the increasing acceleration towards the center slows time down for a particle. As speed tends towards that of light, time slows down and space expands.

Is that vaguely in alignment with the actual models?

Armchair theoretical physics philosophizing— if complexity (~spacetime) inside the black hole is growing at the fastest rate possible, where does all this stuff go? It doesn’t stay within the black hole, instead it fuels dark energy expansion throughout the universe, through some unknown process akin to how oceanic subduction of tectonic plates recycles continental land through mountain-building and volcanism. There is some mysterious “mantle” of the universe where all this space gets tumbled and transformed into dark energy expansion.

Possible? Or crackpot theory?

More likely that there’s just A LOT of room at Planck scales, once you override the strong and weak nuclear forces, and the classic understanding of subatomic equilibrium that permits electrons to grant atomic nuclei their usual elbow room.

Consider our traditional model for the atom, according to quantum electro/chromo-dynamics.

Now take all that extra space provided, and fill it to the brim with squished, crushed star detritus.

It’s on the order of pinheads to football stadiums. What if you filled up a cubic football field with pinheads snipped off the top of a pin?

Now, not just the field, but the whole stadium, including the cheap seats. Fill all the seats with pure atomic nuclei the size of a head of a pin, and nothing else.

Take a single helium atom, and use that normal volume as a pre-defined knapsack to stuff more matter into.

Now take the mass of as many helium atoms as you can get your hands on, and fill up the volume of that first helium atom with the mass of its peers, until every last planck length is retaining its full quantity of potential mass.

Furthermore, I’m thinking it’s likely that black holes are so extreme that you get a runaway crush that overrides the normal volume of all the usual protons, electrons and neutrons, to get degenerate quark soup.

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

I’m not talking about the matter within the black hole, but the spacetime itself that is constantly “flowing” into the hole and getting stretched infinitely towards the singularity.

What if, before reaching the singularity, which is perhaps not possible, the spacetime is “shredded” into Planck-scale entities, which populate an invisible “backplane” that coexists with the visible universe (outside of black holes). the energy of this backplane somehow “pushes” dark energy to expand space everywhere in the universe.

  invisible backplane

Sounds like romanticized wish fulfillment. You want it to be true, so maybe it is?

On the one hand, we know there’s lots of spare empty space within an ordinary atom. We can take a known fact and build on that fact, to inform ourselves of possible details for observed phenomenon. The volume and behavior of degenerate matter represents hypothetical concepts that could be tested.

On the other hand, we have unknown unknowns and maybe we can hijack these blindspots and stuff ideas we like inside these cubby holes, even though there’s no way to test for such possibilities, but let’s formulate an untestable hypothesis because we think multiverses and wormholes and reality simulations are cool?

i said i was speculating for fun. if you don’t want to play, that’s fine.

Are we inside one of these things as well?

Yep, but the outer thing is a simulation running in a kubernetes cluster

Let's find a bug and get out this thing!

We must be living inside a black hole.

Help I'm trapped inside a universe factory!

I wonder if the authors here are just making an obscure Dr. Who "bigger on the inside" reference. Did they claim the exterior of the black hole, were it visible, would resembles a blue box?

That’s pretty funny!

I think black holes convert matter to dark matter that leaks across the universe in a different dimension. But my proof was a dream...

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