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This is a repost of a question I saw here:

Could the spacetime manifold itself end at the event horizon?

which was closed because it apparently didn't seem clear as to what the poster there was asking. However when I saw it I think I had a fairly immediate idea of what the asker is supposed to be asking about and I suspect there is a valid question here - however if I'm wrong, you can close this one too.

Namely, what I'm asking (and believe the OP of that Q was asking as well and I'm more asking it to resurrect the question in a better form) is the series of these questions, each of which builds on top of each other and it is asking in particular about the topological restrictions on spacetime manifolds in general relativity:

  1. is it permissible for a space-time manifold in general relativity to have an edge in the same sense that a piece of paper (a 2D manifold) has an edge? (That is, the manifold has boundary points in the sense in topology, with the set of boundary points being of dimension one less than its own dimension i.e. if $M$ is the manifold then $\partial M \ne \emptyset$ and $\mathrm{dim}\ \partial M = (\mathrm{dim}\ M) - 1$)

  2. if so, is it permissible for it to contain a "hole" in the same sense as if you punched a hole in said sheet using a hole punch (this is another 2D boundary, but we can enclose it with a loop and perhaps higher-dimensional enclosures in its higher dimensional analogue - another way to say this may be that the higher-dimensional boundary results in the manifold being not simply connected)?

  3. if that is so, could the event horizon of a black hole be just such a boundary (or connected set of boundary points) of the spacetime manifold?

I note that it appears the regular singularity of an ordinary black hole is (at least from what I gather in readings) a boundary of dimension 1 - so the question basically is if you can have a boundary of dimension 3, and thus the manifold simply stops - as the sheet of paper does at its edge or better yet a hole punched in it - at the black hole's event horizon, so that the black hole is literally a hole in spacetime. One simplistic model of what is being asked would be just take the Schwarzschild spacetime and delete the interior part. No observer outside could tell the difference, right, so this would be consistent empirically, no? And anything falling in simply ceases to exist when it hits the horizon as that represents the termination of its worldline, same as with the singularity but here the "singularity fills the entire volume of the horizon"?

The context seems to be quantum-gravitational theories involving a black hole "firewall" and the idea is that at least on a cursory reading of some of the papers put forward suggested the firewall was just this kind of abrupt termination (mathematical boundary) of the manifold.

I also note however this question is fairly old - from 2013 - so I'd also be curious in knowing if that progress on the firewall problem since then has definitively been able to rule out the idea of such a space-time hole and if so, how exactly it did so (provided that was a proper characterization in the first place). Even modulo any connection to the firewall theories, what exactly would prevent a black hole from being such a "literal hole in space" as outlined in points (1)-(3) above?

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is it permissible for a space-time manifold in general relativity to have an edge in the same sense that a piece of paper (a 2D manifold) has an edge?

Not really. The Einstein field equations only make sense at a point that has an open neighborhood of spacetime surrounding it, so we can only apply them on a manifold, not on a manifold-with-boundary. We do sometimes talk about a manifold-with-boundary in GR, but usually the context is that we're describing idealized points and surfaces that have been added to the spacetime, such as $\mathscr{I}^+$ or $i^0$. These are like vanishing points in perspective art. They are not actually part of the spacetime.

The fundamental reason that we do relativity on a manifold, not a manifold-with-boundary, is the equivalence principle. One way of stating the e.p. is that every region of spacetime is locally describable by special relativity. That is baked into the structure of GR and the Einstein field equations, and it would be violated at a boundary.

if so, is it permissible for it to contain a "hole" in the same sense as if you punched a hole in said sheet using a hole punch

GR doesn't impose any constraints on the topology of spacetime, so you can have holes. However, a hole does not imply a boundary in topology. If you take the Cartesian plane and remove the closed unit circle $r\le1$, you get a manifold, not a manifold-with-boundary.

if that is so, could the event horizon of a black hole be just such a boundary (or connected set of boundary points) of the spacetime manifold?

There is no physical motivation for doing this in classical GR. Nothing special happens, locally, at the event horizon. The event horizon is a set of points defined only in relation to distant points.

Historically, the misbehavior of the Schwarzschild metric, expressed in Schwarzschild coordinates, was not clearly understood at first. Later people realized that it was only a coordinate singularity. In GR, we aren't normally interested in spacetimes that are not maximally extended. When a spacetime has a proper extension, that is usually interpreted as meaning that something has just been artificially deleted from it. For example, you can take Minkowski space and delete a point, or delete everything at $t\ge 0$, but this is considered the kind of silly, artificial example that we want to rule out. We only want to talk about incomplete geodesics if the geodesics end at a singularity (a real singularity, not a coordinate singularity).

The reason that proposals such as firewalls are so radical is that they violate the equivalence principle. When, for example, people attempt to do semiclassical gravity and wind up with a prediction that something diverges at the event horizon of a black hole, it's a sign that their technique for doing semiclassical gravity isn't working properly. They can try to do things like renormalizations in order to get rid of this unphysical behavior. The basic problem is that semiclassical gravity lacks any clearly defined foundational principles. We have no reason to think that the techniques people use are valid approximation schemes.

I note that it appears the regular singularity of an ordinary black hole is (at least from what I gather in readings) a boundary of dimension 1

Not true. There is no standard way to define its dimensionality. See Is a black hole singularity a single point?

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  • $\begingroup$ Wow. Thanks - and this is the exactly kind of answer I would have thought the original question should've gotten (which is why I decided to rephrase to make it better.). I am wondering though - if semiclassical gravity is as suspicious as you say in that one paragraph, does this mean even results like Hawking Radiation which are relatively "uncontroversial" can't totally be trusted? Or is there some guideline that generally says despite these issues it is "good" and still trustworthy in at least some cases but not others? $\endgroup$ – The_Sympathizer Mar 18 '18 at 20:47
  • $\begingroup$ (Although granted, of course, its absolute trustworthiness or lack thereof cannot be established with anything short of hard experimental/observational data - but that is obviously quite unfeasible to get in anywhere near the short-term, medium-term, or foreseeable long-term future for Hawking Radiation given that to go to a black hole is essentially interstellar travel now and we have no way to create a miniature black hole (and could be dangerous if it's wrong!). Yet it's taken as being "very likely" to be so, e.g. it was even used as one possible argument to (cont'd) $\endgroup$ – The_Sympathizer Mar 18 '18 at 20:50
  • $\begingroup$ (cont'd) dismiss concerns that the Large Hadron Collider could cause damage if it had produced black holes (and it doesn't seem to have so far, but that's a separate question from whether HR would have "saved the day" if it had.).) $\endgroup$ – The_Sympathizer Mar 18 '18 at 20:51
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    $\begingroup$ @The_Sympathizer: We're unlikely ever to be able to detect Hawking radiation directly, so we may never know. There seem to be results in semiclassical gravity that are widely accepted (Hawking radiation) and others that few people really believe (divergences of fields at an event horizon). I'm not aware of any definite criterion for telling which results should be believed, or which methods should be trusted. $\endgroup$ – Ben Crowell Mar 18 '18 at 22:45
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Is it permissible for a space-time manifold in general relativity to have an edge in the same sense that a piece of paper (a 2D manifold) has an edge?

Such models are being considered. In the recent 20 years there has been a great interest in such models in connection with Hořava–Witten theory:

  • Hořava, P., & Witten, E. (1996). Eleven-dimensional supergravity on a manifold with boundary. Nuclear Physics B, 475(1-2), 94-114, doi, arXiv.

There the strongly coupled limit of the $E_8 \times E_8$ heterotic superstring is identified with M−theory compactified on a $S_1/\mathbb{Z}_2$ orbifold (a line segment with two endpoints) with $E_8$ gauge fields on each orbifold fixed plane. Those fixed planes are two parallel boundaries of the 11 dimensional space-time: Hořava–Witten domain walls. Of course, the original scenario provides a new compactification method, rather than a way to include boundaries in the usual 4D spacetime. So in a sense, this model is somewhat similar to a braneworld cosmologies.

if so, is it permissible for it to contain a "hole" in the same sense as if you punched a hole in said sheet using a hole punch

One of the insights from Hořava–Witten models (and quite a lot of earlier works) is that there must be dynamical degrees of freedom living on such boundaries. So the geodesic incompleteness that would generally be a feature of manifold with boundary does not lead to indeterminism: if something 'flies' into the boundary there must be equations telling us which boundary degrees of freedom would be excited, how the boundary is deformed etc. This is usually done by specifying both bulk and boundary actions for the theory. For example, we could construct the pair using supersymmetry:

  • Belyaev, D. V. (2006). Boundary conditions in supergravity on a manifold with boundary. Journal of High Energy Physics, 2006(01), 047, doi, arXiv.

The takeaway is that we cannot just "punch holes" in spacetime manifold, we must do it consistently so that both bulk and boundary are solutions for corresponding equations of motion.

could the event horizon of a black hole be just such a boundary

In black hole physics there is membrane paradigm, a model that considers a black hole as a thin membrane microscopically close to the black hole's event horizon.

  • Thorne, K. S., Price, R. H., & MacDonald, D. A. (Eds.). (1986). Black holes: the membrane paradigm. Yale university press.

Such membrane would have physical properties, mass, temperature, electrical resistivity etc. From the point of view of this model, there is nothing 'inside' of this membrane, everything that falls into the black hole is incorporated into the membrane. Of course, this is usually considered a simplified visual and computational aid.

However, there are models of black holes (or to be more precise 'exotic compact objects', ECO, since many of those models do not have a horizon) that assume the existence of new physics microscopically close to the (would be) horizon. Since the ringdown modes from mergers of black holes/ECOs would contain information about this physics, there is hope that LIGO (or next generation of gravitational wave detectors) could offer some observational evidence of this new physics.

One interesting analysis:

  • Abedi, J., Dykaar, H., & Afshordi, N. (2017). Echoes from the Abyss: Tentative evidence for Planck-scale structure at black hole horizons. Physical Review D, 96(8), 082004, doi, arXiv.

finds 'tentative evidence for Planck-scale structure near black hole horizons'.

As a sample of models for the ECO that are being considered have a look at

  • Maggio, E., Pani, P., & Ferrari, V. (2017). Exotic compact objects and how to quench their ergoregion instability. Physical Review D, 96(10), 104047, doi, arXiv.

where authors consider ECO model which is Kerr metric for $r>r_0$ and a reflective membrane at some $r=r_0$. This model most closely resembles 'horizon as a boundary' suggestion.

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