# Could black holes have no singularity?

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1. Dec 25, 2014

### zerospin

I have a problem with the concept of a singularity, defined as something that has a property which is infinite. Infinities do not belong in our reality, and in my opinion are just hints that our understanding of the phenomenon is incomplete or wrong.

From my understanding, during the collapse of a star, there are certain stages that can prevent the formation of a singularity, if the star is not too massive. At first the pressure, then the repelling force between particles, and then we are told, that there is nothing left that could prevent the collapse into a single point.

But would it not be more likely, rather than having a point of infinite density, that there actually is something that prevents the formation of a singularity? Like for example the pressure of the quantum foam or dark energy, or some other force or phenomenon that we haven't discovered yet.

What is the common answer to this idea? How sure are we, that singularities exist?

I have this idea, that even if we compress the entire Universe into a single point, it will not collapse into a singularity, but rather into something really tiny, maybe as small as a Planck length or smaller, but that some force will still prevent an infinite value.

Is there actually something that proves that a singularity exist, other than equations that produce infinities? Or do we take it "on faith"?

2. Dec 25, 2014

### Staff: Mentor

Neither.
You're right that there's nothing that proves that the singularity exists, as it's on the wrong side of the event horizon (which forms whether there's a singularity or not) so we can never observe it. However, that doesn't mean that anyone is taking its existence for granted. On the contrary, the most common view is probably that the singularity does not form and that the infinities that appear in our equations are a strong hint from nature that out current theories break down at very small distances from the center - as you say "that there actually is something that prevents the formation of a singularity".

You'll find analogous situations in classical mechanics. The equations for both Newton's law of gravity and Coulomb's electrostatic law give infinities when you set $r=0$, but we neither see nor expect to see gravitational point sources and infinities near charged particles.

Last edited: Dec 25, 2014
3. Dec 25, 2014

### phinds

Then you would be well served to use the common description of a black hole singularity not as a place where things are infinite but a place where, as Nugatory points out, our model just breaks down and we don't know WHAT is going on.

4. Dec 25, 2014

### DaveC426913

Concur with others.

You're looking at it backwards.

It is those infinities that tell us our models are incomplete at describing reality.

5. Dec 25, 2014

### Matterwave

To add on to the answers already provided. The reason people don't think singularities are legitimate is that for things that are very very small we need to use quantum mechanics. For things that are very massive, we need to use general relativity. But for things that are both very small and both very massive, we don't yet have a theory to describe them. In other words, we don't have a quantum theory of gravity. That singularities exists is a general feature of General Relativity, see the singularity theorems: http://en.wikipedia.org/wiki/Penrose–Hawking_singularity_theorems . But the cosmic censorship conjecture says that we can't ever observe these singularities anyways since they are always hidden behind event horizons, so we can't observationally verify that a singularity does exist. See: http://en.wikipedia.org/wiki/Cosmic_censorship_hypothesis (note however, that these are, at this point, simply conjectures - well educated guesses).

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6. Dec 26, 2014

### bcrowell

Staff Emeritus
Cosmic censorship doesn't apply to the big bang singularity, which isn't hidden behind a horizon.

But even putting aside the horizon issue --

Let P be the proposition that the singularities predicted by GR really exist, and let Q be the proposition that they are somehow eliminated by quantum-gravity effects. I don't think there's any plausible possibility that any observation can distinguish P from Q empirically. We don't have any way of probing the Planck scale. Probing the Planck scale would require godlike mastery over vast amounts of matter and energy, which means that humans will never be able to do it. Long before we became able to do it, we would have become something beyond human. See, e.g.,
Rothman and Boughn, "Can Gravitons Be Detected?," http://arxiv.org/abs/gr-qc/0601043 .

7. Dec 26, 2014

### Matterwave

But isn't the big bang singularity hidden behind every "decoupling epoch" from all types of potential observation, so even if it's a naked singularity, it's essentially hidden behind a different kind of horizon? We can't see light back past the CMB because that's the light-decoupling epoch, we can't even see neutrinos past the weak decoupling epoch...the only thing I can possibly think of to detecting the BB singularity might be gravity waves...?

8. Dec 26, 2014

### bcrowell

Staff Emeritus
I agree that we have no way of directly accessing the big bang singularity through observation, and I agree that we are not ever likely to be able to do such a thing. I don't think the specific argument you gave is 100% conclusive, though, since, e.g., we can probe the early universe through models of big bang nucleosynthesis and maybe baryogenesis.

Maybe we should distinguish the notion of accessing a real singularity observationally from the notion of examining an almost-singularity at the Planck scale. If a real singularity existed, then there would be infinitely many orders of magnitude separating its energy scale from any fixed energy scale. I don't see how any process of observation can allow us to infer what's going on across a span of infinitely many powers of 10. Observations need to be interpreted, and we can't interpret an observation of conditions that are infinitely different from anything else we have experience with.

Last edited: Dec 26, 2014