Realism about singularity

[windy miller]

I understand that most physicists reject the idea the idea that Big Bang is a singularity. I see, for example, in this survey, most reject associating the Big Bang with a singularity. https://arxiv.org/pdf/2503.15776. However , if we hypothetically took a realist view of singularities. I(n other words , they are not as most believe places where our physics breaks down but real physical descriptions of the universe at the Big Bang) ; in that scenario, is the spatial volume of the universe zero at the Big Bang?

 

[Ibix]

I think you'd have to find somebody who believes that and ask them.

As far as I understand it the question doesn't make sense because the concept of volume is supplied by the metric attached to the manifold and the singularity lies outside the manifold.

 

[PeterDonis]

if we hypothetically took a realist view of singularities. I(n other words , they are not as most believe places where our physics breaks down but real physical descriptions of the universe at the Big Bang);

You don't have to "hypothetically" take a "realist view". You can just ask what the model of the universe with an initial singularity says. The model itself is perfectly well-defined, independent of the question whether it represents what's actually physically real in our universe.

in that scenario, is the spatial volume of the universe zero at the Big Bang?

No, because, as @Ibix says, the singularity is not part of the manifold in the model, and so the question as you state it has no meaning.

What does have meaning is to ask what the limit is of the spatial volume of the universe as the initial singularity is approached by moving backwards in time along any comoving worldline. That limit is zero. But the limit is just a mathematical limit. It does not describe any actual property of the spacetime itself, since, as above, the singularity is not part of the spacetime.

 

[elias001]

I have a very naive question? How do we know with 100% certainty that the inside of a blackhole is s singularity? I mean nobody has been able to yest it ot physically confirm directly or indirectly? For all we know, the inside of a blackhole could consists of an infinite amount of volume of space.

 

[Ibix]

How do we know with 100% certainty that the inside of a blackhole is s singularity?

We don't. Prevailing opinion is that GR stops being an accurate model somewhere inside (or possibly even outside, although I think that's a minority view) a black hole and the singularity is only a feature of GR not reality, and that we hope a theory of quantum gravity will produce a singularity-free model. But we don't actually know either way.

For all we know, the inside of a blackhole could consists of an infinite amount of volume of space.

This is the case for classical GR black holes anyway, at least for the some definitions of "space".

 

[elias001]

@lbix who will we volunteer to go inside a blackhole to find out and tell us all about it? Maybe all the elderly folks in a nursing home.

 

[Ibix]

@lbix who will we volunteer to go inside a blackhole to find out and tell us all about it?

The point about an event horizon is that you cannot communicate from inside it to outside it, so dropping someone inside is pointless. Quantum gravity might, however, predict something that looks like a black hole but only has an apparent horizon rather than a true event horizon and in that case escape is possible in principle.

That said, I rather suspect that direct probing of black hole interiors will remain beyond our capabilities for a long time.

 

[martinbn]

I have a very naive question? How do we know with 100% certainty that the inside of a blackhole is s singularity? I mean nobody has been able to yest it ot physically confirm directly or indirectly? For all we know, the inside of a blackhole could consists of an infinite amount of volume of space.

You can say that about many things. How do we know with 100% certainty what happens inside the Sun?

 

[elias001]

@martinbn I think we have a greater chance of studying the inside of the sun via some sort of future remote sensing technology. Also escaping the sun's gravity is not impossible in the sense of having to travel faster than light speed.

 

[martinbn]

@martinbn I think we have a greater chance of studying the inside of the sun via some sort of future remote sensing technology. Also escaping the sun's gravity is not impossible in the sense of having to travel faster than light speed.

My point is that noone or nothing human made has been inside a star, but that doesn't stop us from deducing what happens there. Why is it different for black holes!?

 

[phinds]

My point is that noone or nothing human made has been inside a star, but that doesn't stop us from deducing what happens there. Why is it different for black holes!?

Do the math

 

[fresh_42]

Why is it different for black holes!?

Because we can reproduce the processes in the sun in a lab. That's a big difference.

 

[martinbn]

Do the math

Penrose did it sixty years ago.

Because we can reproduce the processes in the sun in a lab. That's a big difference.

You cannot reproduce even the gravity in the sun's core in a lab.

 

[fresh_42]

You cannot reproduce even the gravity in the sun's core in a lab.

You can extrapolate gravity from small to large scales, but you cannot calculate what's going on in a black hole. All calculations remain untestable hypotheses. Penrose also calculated a cyclic universe, but that doesn't make it one.

 

[PeterDonis]

You can extrapolate gravity from small to large scales, but you cannot calculate what's going on in a black hole.

Yes, you can. The literature is full of calculations of what goes on inside black holes.

All calculations remain untestable hypotheses.

In the sense that, if what we call "black holes" truly are black holes, with true event horizons, we, remaining outside their horizons, can never get direct evidence of what happens inside the horizons, yes, that's true.

However, that doesn't mean quite what you appear to think it means. We can calculate what conditions would be like inside black holes of various masses, and we can find other scenarios, not inside black hole horizons, with similar physical parameters (the most important being spacetime curvature), and we can test what happens in those other scenarios. That gives us indirect evidence of what we would expect to happen if we fell inside a black hole. That is not nothing, even if we'd never be able to communicate what we experienced after falling into the hole to those left behind outside. So it's simply not true that we know nothing about the inside of black holes.

 

[martinbn]

You can extrapolate gravity from small to large scales, but you cannot calculate what's going on in a black hole.

Why not?

All calculations remain untestable hypotheses.

May be untestable, but they are not hypotheses, they are conclusions. And if they are based on a well established theory, why dismiss them?

Penrose also calculated a cyclic universe, but that doesn't make it one.

This seems inconsistent. You don't think there are singularities, but you also don't like a proposal that goes beyond the initial singularity of the standard cosmological model.

 

[fresh_42]

However, that doesn't mean quite what you appear to think it means. We can calculate what conditions would be like inside black holes of various masses, and we can find other scenarios, not inside black hole horizons, with similar physical parameters (the most important being spacetime curvature), and we can test what happens in those other scenarios.

The sun tests our calculations. They are observable. Which experiments could ever test the calculations regarding the inside of an event horizon? E.g., I do not think that reality allows singularities, but I admit that there might be models without it. But how could we ever know? I still think that there is a basic difference between a star and a black hole. One is observable, the other one is not.

Admittedly, this question might turn into a philosophical one, as any observation is finally the observation of its impact on the objects around it. In this sense, there is no difference. Nevertheless, we can measure the neutrinos that escape the sun, but we cannot measure whatever the calculations beneath the event horizon tell us. Or can we? In that case, I retract my statement and claim the opposite.

 

[fresh_42]

This seems inconsistent. You don't think there are singularities, but you also don't like a proposal that goes beyond the initial singularity of the standard cosmological model.

This is not inconsistent. I believe that there are physical constraints on what we can know and what we cannot know. I don't think that models about the Big Bang can ever be falsified since we have no access to the time before there was radiation. It remains a mathematical result.

 

[PeterDonis]

Which experiments could ever test the calculations regarding the inside of an event horizon?

As I said, experiments in other scenarios where the spacetime curvature and other physical parameters are similar to those that our models predict for inside an event horizon. Similar physical parameters means similar predictions. We do that in science all the time.

I still think that there is a basic difference between a star and a black hole. One is observable, the other one is not.

I've already explained why I don't think this is a "basic difference". It's something we need to be aware of, sure. But I simply don't agree that it has the huge implications you seem to think it has. We rely on indirect evidence in science all the time.

And, as has already been said, we don't even know for sure that the objects we currently call "black holes", such as Sagittarius A at the center of our galaxy, are true black holes with true event horizons. There are other possibilities for what they could be that don't have true event horizons, meaning that what's inside could be observable even in the strict sense you are using that term.

we cannot measure whatever the calculations beneath the event horizon tell us. Or can we?

As I've said, we can make measurements in other scenarios with similar physical parameters. Whether that counts as "measuring whatever the calculations beneath the event horizon tell us" to you is your call. But if I come to you with measurements made in a similar scenario, and you just flat out refuse to believe those measurements have any relevance to what might be inside a black hole, then you simply don't believe in doing physics the way we actually do physics.

 

[fresh_42]

But if I come to you with measurements made in a similar scenario, and you just flat out refuse to believe those measurements have any relevance to what might be inside a black hole, then you simply don't believe in doing physics the way we actually do physics.

One essential point in science is falsifiability. That's what makes the difference to me, whether an observation is physics or a mathematical result. As I mentioned, I believe that there are hard boundaries beyond which we cannot falsify results anymore, mainly because we need a form of radiation for an observation. This makes, in my opinion, a difference between a star that radiates and a star that does not. We can also have an experimental setup of the former (particle accelerators, fusion reactors, H-bomb), but not for the latter. I only answered the question of whether there is a difference between the sun and Sagittarius A, and I think there is, simply because one emits radiation that can be examined, and the other one does not.

However, I also admitted that there is no difference on a philosophical or metaphysical level as far as indirect observations are concerned. In this sense, we can or can't even be sure about $F\sim\ddot x$ since we only observe the consequences of this formula.

Hence, I do not doubt "the way we actually do physics"; I only make distinctions on the experimental level, which vanish on the metaphysical level. In this sense, I expressed my opinion that the sun and Sagittarius A are different in what we can know and what we cannot know.

 

[berkeman]

I don't think that models about the Big Bang can ever be falsified since we have no access to the time before there was radiation. It remains a mathematical result.

Have you read "The First Three Minutes" by Steven Weinberg? It's an easy read, but has lots of interesting information about our current observations of the makeup of matter in the Universe and how that plays back to the early Universe and the Big Bang scenarios. There is a lot of experimental evidence that goes into our current theories of the early Universe.

 

[berkeman]

And, as has already been said, we don't even know for sure that the objects we currently call "black holes", such as Sagittarius A at the center of our galaxy, are true black holes with true event horizons. There are other possibilities for what they could be that don't have true event horizons, meaning that what's inside could be observable even in the strict sense you are using that term.

Interesting. Could you link to a couple papers about this? Thanks.

 

[fresh_42]

Have you read "The First Three Minutes" by Steven Weinberg? It's an easy read, but has lots of interesting information about our current observations of the makeup of matter in the Universe and how that plays back to the early Universe and the Big Bang scenarios. There is a lot of experimental evidence that goes into our current theories of the early Universe.

I know, but those results are mathematics up to ten to the power of negative something seconds. Observations are restricted to a time after 380,000 years, when the universe became transparent, as far as I know.

I may be wrong, so correct me if so. I think what we have are computer models that either lead to what we observe or do not observe. That doesn't rule out the existence of other models, even if we found one that fits.

 

[berkeman]

I know, but those results are mathematics up to ten to the power of negative something seconds. Observations are restricted to a time after 380,000 years, when the universe became transparent, as far as I know.

But we are able to do experiments on particles now that reach back to how they most likely behaved very soon after the Big Bang, no? It seems unlikely that their behavior and interactions have changed since then, so our extrapolations are limited only by our current accelerator energies and those experiments. That takes us back very close to those first 3 minutes, IMO.

 

[fresh_42]

But we are able to do experiments on particles now that reach back to how they most likely behaved very soon after the Big Bang, no? It seems unlikely that their behavior and interactions have changed since then, so our extrapolations are limited only by our current accelerator energies and those experiments. That takes us back very close to those first 3 minutes, IMO.

The same has been said about the ether. It is a consideration of the likelihoods of models, not measurements.

 

[elias001]

@berkeman there is a big difference between the first three minutes and at time $t=0$. As $t\to 0$, the more speculative our information becomes about the physical beginning process of the universe. Also, experimental confirmation of probably many things done in a lab. I am certain there are still a lot of unknown variables that have not been accounted for. But then that is the exciting thing about science. The endless "to be continued" part.

 

[berkeman]

@berkeman there is a big difference between the first three minutes and at time $t=0$. As $t\to 0$, the more speculative our information becomes about the physical beginning process of the universe. Also, experimental confirmation of probably many things done in a lab. I am certain there are still a lot of unknown variables that have not been accounted for. But then that is the exciting thing about science. The endless "to be continued" part.

Have you read the book that I mentioned? It goes back a lot farther than the 3 minute mark...

 

[elias001]

@berkeman in this youtube video , at the 9:53 mark where it says there are parts of the universe that have moved past the boundary of the observable universe due to cosmic expansion. So if you apply the same logic in the other direction, how do we know there are a nunch of minutes in the past we have nit able to observe or possibly might never be able to observe. Ok, this is probably a simple kinematics problem in three dimensions. I am not going to even try to answer the questions because I have a feeling it would involve general relativity since we are talking about how light moves across both space, time which I am assuming have to account for gravity. I am not doubting or casting shade on the book you are referencing. To answer your question, I have not read the book. I probably will have more appreciation for it after properly learning about GR. I just think if there are things beyond the observable universe we cannot see. Shouldn't there are things from the other direction coming toward us or rather that have not caught up to where we can see it with our telescope regardless of how powerful we make them to be.

 

[PeterDonis]

One essential point in science is falsifiability.

According to one view of science, yes. But that is not the only possible view.

 

[PeterDonis]

one emits radiation that can be examined, and the other one does not

I think this is a very simplistic view.

Even if the event horizon of Sag A (assuming that Sag A is in fact a true black hole with a true event horizon) does not itself emit radiation, objects outside the horizon can and do. The reason astronomers believe Sag A is a black hole is that they have observed radiation emitted by objects orbiting it, and have calculated from those observations both the mass of Sag A and the approximate volume of the region of space that it occupies, and the only objects allowed by our best current physical theories that can fit that much mass into that small a volume are a black hole, and the various more exotic solutions that look from the outside like black holes (but don't have true event horizons). In other words, Sag A can't be a normal star, or a cluster of stars, or a cluster of white dwarfs or neutron stars, etc., etc., etc. None of those things can cram that much mass into that small a space.

Of course we have no way of getting direct evidence that Sag A has a true horizon, if it does. And even if it doesn't, if it turns out to be one of those other more exotic things, we won't be able to tell the difference for a time roughly equal to the Hawking evaporation time for a black hole of the mass of Sag A, which is something like $10^{90}$ years. So even in the case where it would be possible in principle to get direct evidence from inside Sag A, it won't happen for a looooooooooooooooooooooooooooooooooong time. Which means that we won't know for sure about the thing that you say makes such a big difference, for that same amount of time.

To you, that appears to mean that we have to just throw up our hands and wait. To me, it means the thing you say makes such a big difference, doesn't. It doesn't make any practical difference to our ability to do physics.