# Naked singularities, timelike singularities

1. Sep 13, 2012

### bcrowell

Staff Emeritus
Is there a logical connection between the concept of a naked singularity and the concept of a timelike singularity?

On a Penrose diagram, black hole and big bang singularities are always spacelike.

Global hyperbolicity (Hawking and Ellis, p. 206) basically means two conditions: (1) no CTCs, and (2) $\forall p,q\ J^+(p) \cap J^-(q)$ is compact. (J+ and J- mean future and past timelike lightcones, and compactness essentially says the set doesn't contain any singularities or points at infinity.) The point of global hyperbolicity is that it guarantees existence and uniqueness of solutions to Cauchy problems.

If I try to imagine Penrose diagrams where condition 2 fails, it seems like I need to have a timelike singularity. It doesn't seem like it matters whether there's a horizon, and yet, it seems like the whole reason we care about naked singularities is that they violate uniqueness of solutions of Cauchy problems. (I.e., anything can pop out of a naked singularity, even "green slime and lost socks.") So I'm thinking there must be some link between timelike singularities and naked ones.

I got started on trying to understand these definitions because someone told me that the generalization of the Hopf-Rinow theorem to semi-Riemannian spaces requires hyperbolicity. Hopf-Rinow in a Riemannian space basically says that any two points can be connected by a geodesic, and that geodesic's length is extremal. If I try to see why this would fail if hyperbolicity fails in the semi-Riemannian case, it seems like it could clearly fail if I had a timelike singularity, because the singularity could lie between two spacelike-related points p and q, blocking what would have otherwise been a spacelike geodesic connecting them.

On a separate but somewhat related topic, can anyone help me understand the distinction between the weak and strong causality conditions (H&E pp. 190, 192)? I don't understand what their definition of the latter is trying to express, and I don't understand the example they give in a figure where only weak causality holds. Violating it would seem to mean that a nonspacelike curve would have to pass through a neighborhood twice, but I don't see that happening in the figure.

Last edited: Sep 13, 2012
2. Sep 13, 2012

### PAllen

I would think there needs to something more to the generalization, at least the part about length being extremal. Maybe: if two events are causally connected, there is at least one timelike geodesic connecting them, and any such geodesic is a local maximum in the variational sense; one of them will be a global maximum. Most any stronger statement I can conceive of is trivially false in perfectly normal spacetimes (including attempting to pose any extremal property for spacelike geodesics is false even in Minkowski space).

3. Sep 13, 2012

### bcrowell

Staff Emeritus
I'm sure I'm getting the details wrong. I don't have a source that gives the exact statement of the generalization. All I'm working from is a general, brief summary by someone else: http://math.stackexchange.com/questions/194353/geodesic-flows-and-curvature/194827#194827 (See user "student's" comment on the answer.) I would actually be grateful if anyone could point to a complete formulation.

4. Sep 13, 2012

### PAllen

He says:

" In any case, what is generally understood to be the "equivalent" of Hopf-Rinow in Lorentzian geometry is the theorem that states that in any globally hyperbolic spacetime, two causally related points can always be joined by a maximizing geodesic. Global hyperbolicity here would be the substitute for completeness."

Which is basically identical to what I guessed above, without ever having heard of Hopf-Rinow by that name.

5. Sep 13, 2012

### bcrowell

Staff Emeritus
Would "maximizing" mean maximizing the arc length in a -+++ signature, i.e., minimizing proper time? Or should this really be "extremizing?"

Would spacelike and timelike geodesics be treated differently? The definition of a hyperbolic spacetime doesn't treat them symmetrically, and, e.g., we expect to have closed spacelike curves even if we don't have CTCs.

6. Sep 13, 2012

### PAllen

As he(?) states the generalization, it doesn't apply to spacelike geodesics, as I guessed. The key phrase is:

"two causally related points" - his formulation
"if two events are causally connected" - my formulation.

Maximizing would be relative to a +--- signature, for causally connected events.

7. Sep 13, 2012

### Naty1

Roger Penrose says more than that in THE ROAD TO REALITY:
and he goes on to discuss his Weyle curvature hypothesis applicable to the physical universe.

gotta go be back later or tomorrow....

8. Sep 13, 2012

### PAllen

Except that Hawking himself, in conceding a bet, admitted that naked singularities are possible without exotic matter for implausibly symmetric initial conditions. His restated challenge is to require that initial conditions (in the ADM initial value formulation) leading to a naked singularity form an open set. This is his mathematical formulation of 'generic initial conditions'.

9. Sep 14, 2012

### Naty1

I checked and could find no further information in the Penrose book, but here is a contrary view to the Penrose quote above, from a paper previously referenced in these forums:

http://arxiv.org/pdf/gr-qc/0109051v2.pdf
Why do naked singularities form in gravitational collapse?
Pankaj S. Joshi∗, Naresh Dadhich† and Roy Maartens‡

Are those last few sentences taking a different view than "implausibly symmetric initial conditions" mentioned by PAllen. ?? I am surprised by that last sentence in particular.

But maybe what they mean is that given perfect symmetry, one still needs a density gradient, shear, for a naked singularity.
I wonder when this paper was published.

Last edited: Sep 14, 2012
10. Sep 14, 2012

### bcrowell

Staff Emeritus
Thanks, Naty -- the Penrose quote was exactly what I was looking for. That led me to some good info via googling; Penrose himself seems to have been the one who first formalized the definition:

Penrose, Gravitational radiation and gravitational collapse; Proceedings of the Symposium, Warsaw, 1973. Dordrecht, D. Reidel Publishing Co. pp. 82-91.

Basically you want to formalize a definition that describes the formation of a singularity by gravitational collapse from nonsingular initial conditions, such that signals can escape to infinity. He takes two shots at formalizing this, first with a global definition and then with a local one.

The global definition involves an absolute event horizon: in an asymptotically flat spacetime, the boundary of the set of events from which a timelike curve can get to i+. A naked singularity is one that is not on or outside the horizon. This definition has the problem that the big bang is considered a naked singularity, and the definition only applies to asymptotically flat spacetimes

The local definition is that a naked singularity is a timelike one, i.e., there exists an observer who can have it both in his past and in his future. ("Timelike" isn't automatically defined without reference to some observer, because the singularity isn't a point-set.) Basically the singularity is one that you can both anticipate and remember; metaphorically, it's OK if God creates you, and you may see God when you die, but you're not usually going to see God in person during your lifetime.

Another global definition I hit by googling: Rudnicki, 2006, http://arxiv.org/abs/gr-qc/0606007

There is no obvious way to prove a link between the global definition and the local one. They are connected, because they both cause Cauchy hypersurfaces not to exist. (Penrose references a theorem by Geroch, J Math Phys 11 (1970) 437.) Penrose remarks:

I think the part about "the normal picture of spherically symmetric collapse" is basically just the result of Birkhoff's theorem.

So AFAICT we have various definitions of naked singularities. It still seems to be an open problem what is the most useful definition and how to relate the various definitions.

11. Sep 20, 2012

### George Jones

Staff Emeritus
Just after you posted, I spent some time thinking about this, but I couldn't come with a non-spacelike curve that passed through the neighbourhood twice. I then forgot about this until yesterday, when, for some reason, it popped into my mind again. After thinking some more I finally found the curve, which I have put on the attached diagram. The curve can't be closed, so the weak causality condition is satisfied. This type of curve can intersect any arbitrarily small neighbourhood of p twice. Why didn't Hawking and Ellis put this curve on the diagram as, e.g., a dotted line?

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12. Sep 21, 2012

### Staff: Mentor

Shouldn't this be "not on or inside the horizon"?

But there is no spacetime with a big bang singularity that is asymptotically flat, is there? (Except for the obvious case of "big bang" solutions that are really Minkowski spacetime in disguise, like the Milne universe.)

13. Sep 21, 2012

### bcrowell

Staff Emeritus
Yep, thanks.

Hmm... What Penrose actually says is: "Consider, now, the set E consisting of all events from which a timelike curve (or a null curve -- it makes little difference) can be drawn into the future to infinity." He defines an absolute horizon as the boundary of E. I guess I'm confused about what this definition is really saying. He seemed to me to be referring to curves escaping to $i^+$ and $\mathscr{I}^+$. I don't see how one would apply this definition, for example, to a closed, matter-dominated cosmology with a Big Crunch. I guess a realistic cosmological model might have a Penrose diagram on which it would make sense to label an $i^+$. Anyway, my statement was clearly wrong.

Cool, thanks, George! My copy of H&E is at work -- I'll puzzle over this on Monday.

14. Sep 21, 2012

### Staff: Mentor

This matches the definitions I'm familiar with (for example, I believe Hawking & Ellis uses a basically identical definition).

That's my understanding from the definitions I'm familiar with.

I know I've seen Penrose diagrams of the various FRW spacetimes somewhere, but I can't dig them up right now. From what I remember, the closed cosmologies don't have any "infinity" at all: no $i^+$ or $\mathscr{I}^+$, no $i^-$ and $\mathscr{I}^-$, and no $i^0$. Their Penrose diagrams just look like a box, with the past singularity at the bottom, the future one at the top, and the left and right sides corresponding to "antipodal points" in the closed 3-sphere of each spacelike hypersurface in between the singularities.

The open cosmologies have $i^+$ and $\mathscr{I}^+$, but no $i^-$ or $\mathscr{I}^-$. Their Penrose diagrams look like a right triangle with the horizontal base being the big bang singularity, the vertical left side being the spatial origin, and the hypotenuse going up and to the left at 45 degrees, representing $\mathscr{I}^+$. I think the open ones have an $i^0$, which corresponds to the lower right vertex of the triangle (the top vertex, on the left, of course being $i^+$).

15. Sep 24, 2012

### bcrowell

Staff Emeritus
Aha, now I get the strong causality thing, thanks to George's post. Basically strong causality says that neighborhoods can always be made small enough so that you can't visit them twice. Geroch has a more readable presentation of this topic on p. 196, where he explains that the motivation is that we don't just want to forbid CTCs, we also want to forbid things that are infinitesimally close to being CTCs.