# What is meant by globally hyperbolic spacetime? [duplicate]

Can somebody explain what is meant by globally hyperbolic spacetime? What other kinds can there be?

• Did you google this? May 21, 2019 at 14:25
• Possible duplicate of Intuitive meaning of Globally Hyperbolic
– JMac
May 21, 2019 at 14:46
• @JMac I disagree that this is a duplicate because (A) this question does not ask for the intuitive explanation, so answers should reasonably include intuitive and formal explanations; and (B) this question additionally asks for what other types of spacetime there could be. Just my two cents.
– Mike
May 21, 2019 at 17:32

Wald has a nice chapter on "Causal Structure" that lays it all out very well. He points out that there are three distinct definitions of globally hyperbolic spacetimes, but that they are precisely equivalent. And I find Wald's to be the simplest, so that's what I'll explain here. The basic idea is that there is a single surface that we might think of like an "inital-data slice" for which, if you know everything about all physical fields on that surface (and maybe an infinitesimal neighborhood, so that you can define derivatives), then you can (in principle) predict everything about the future of the universe and retrodict everything about its past.

Start out with the notion of causality. Basically, a point $$p$$ in a spacetime is in the "causal future" of another point $$q$$ if $$p$$ can "be influenced" by a signal sent from $$q$$. In GR (and mainstream physics more generally), this is basically taken to mean that $$p$$ can be connected to $$q$$ by a timelike or null curve (representing possible paths of massive or massless particles, respectively).

Next, we think of an achronal set $$S$$. This is basically a slice through some part of the spacetime (we'd usually think of it as being a "spacelike" slice, though it could be null), for which no two points can be connected to each other by a timelike curve. That is, if we have two points $$p\in S$$ and $$q \in S$$, then — at best — only a null signal from one can influence the other. (Of course, there are usually other points that are not on $$S$$ that could be timelike connected to both, but such points would be in the past or future of $$S$$.) The achronal set doesn't have to extend very far, but it could extend "across the entire spacetime" in some sense.

Now, we can get to the idea of a domain of dependence for an achronal set $$S$$. The future domain of dependence $$D^+(S)$$ is the collection of points in the spacetime for which every single causal curve that goes through those points will go through $$S$$ (assuming the causal curves are extended far enough back in time). Roughly speaking, this means that the only information that can affect anything in $$D^+(S)$$ is information about $$S$$ itself. So $$D^+(S)$$ represents all the points that we can predict given knowledge of $$S$$. Similarly, we can talk about the past domain of dependence $$D^-(S)$$, representing all the points that we can retrodict given knowledge of $$S$$. And the full domain of dependence of $$S$$ is just the union of all those points: $$$$D(S) = D^+(S) \cup D^-(S).$$$$ This is the whole set of points that can be either predicted or retrodicted from knowledge of $$S$$.

Finally, we can define a globally hyperbolic spacetime as a spacetime for which there exists (at least one) achronal set $$\Sigma$$ for which $$D(\Sigma)$$ is the entire spacetime. That is, the entire past and future of the whole spacetime can be either predicted or retrodicted just from knowledge of $$\Sigma$$. Such a surface $$\Sigma$$ is called a "Cauchy surface".

Now, the question is what good is this definition? What other type of spacetime could there be? Wald says

There are some good reasons for believing that all physically realistic spacetimes must be globally hyperbolic.

So, I don't think there's any realistic suggestion that some relevant physical model involves non-globally hyperbolic spacetime. But its certainly possible to imagine them. One prime source of this imagined spacetimes is where there are points that are simply "missing". This is a weird idea that you might expect to see in Star Trek or something, and it shouldn't feel intuitive. But just imagine that some region of spacetime ceased to exist suddenly, and then came back into existence after a moment. When it comes back into existence, the physical fields will have some new values inside that region that may or may not have anything to do with how they were before the region blinked out of existence. That means that anything in the future domain of dependence of this region will depend on the values of the physical fields when they came back into existence. There's no reason to think you could have predicted those values from some much earlier achronal set, for example. Similarly, you couldn't retrodict the values from a later achronal set. Therefore, such a spacetime could not have a $$\Sigma$$ whose domain of dependence is the entire spacetime, and it's not globally hyperbolic.

Again, this isn't something that we expect to happen on physical grounds, but you can construct a mathematical model that has these features. And if we're trying to model physics with math, then we need to specify which mathematical models we find relevant to physics. That's all.

• Nice answer mike. From your answer though, it seems like global hyperbolicity =determinism. Which kind of makes me think that in order to account for quantum indeterminism one would have to consider non globally hyperbolic spacetime's. I don't believe GR itself requires spacetime to be globally hyperbolic Oct 22, 2021 at 2:08
• @R.Rankin What you say is essentially correct. Global hyperbolicity (GH) is basically a type of determinism. And GR itself doesn't require GH; it's just a reasonable suggestion that you're allowed to ignore. But remember that we don't claim that GR is a perfect and complete theory. In particular, GR itself doesn't incorporate quantum effects at all. You have to basically take GR as is and slap quantum theory on top of it, or move on to other theories that can't really be called GR. But to the extent that GR is a good approximation to physics, GH is a helpful guideline.
– Mike
Oct 22, 2021 at 5:12
• Thank you. Due to it's nonlinearities, GR may still have some surprises for us. Using techniques of geometric and differential topology, complicated spacetimes representing particles (in the spirit of Einstein-Rosen and later wheeler) are now analyzable (see Asselmyers papers for example) then the mathematical nature of EH action takes on forms similar to wittens topological quantum field theories. Even if not right, it's a fascinating area Oct 29, 2021 at 17:31
• "One prime source of this imagined spacetimes is where there are points that are simply "missing".......... just imagine that some region of spacetime ceased to exist suddenly, and then came back into existence after a moment." This isn't so wierd if you consider that it describes the opening and closing of a wormhole on a manifold (a change in topology). Wheeler showed that in this situation, nothing can actually traverse the wormhole due to the expansion and consequent contraction of spacetime in the vicinity osti.gov/biblio/4780637 Jan 28, 2022 at 19:35
• Geroch "Domain of dependence" (1970) states that "A set is said to be achronal if no two of its points may be joined by a timelike curve." What about null curves? Can a null curve be considered a Cauchy surface? I am posing this question, because in A. Wall "Maximin Surfaces, and the Strong Subadditivity of the Covariant Holographic Entanglement Entropy " (2016) ,page 9 in the footnote9 it is stated that "an AdS-Cauchy surface, using the convention in which a Cauchy surface is permitted to be null". Sep 19, 2022 at 0:52