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There is an idea that the geometry of physical space is not observable(i.e. it can't be fixed by mere observation). It was introduced by H. Poincare. In brief it says that we can formulate our physical theories with the assumption of a flat or curved space by changing some assumptions and these two formulations are empirically indistinguishable. Here is a topic related to it: Can general relativity be completely described as a field in a flat space? . This idea is also accepted by many contemporary physicists including Kip Thorne, see:

Thorne, K. S. 1996. Black Holes and Time Warps, New York: W.W. Norton, pp.400-2,

But here is my question:

Is the topology of the universe observable? By observable, I mean can we find it by observations, or is the topology also subject to indeterminacy by experiment (similar to geometry)? What do physicists claim about the topology of the universe?

This question is of importance because when we move from a formulation in a curved and closed geometry to a flat and infinite geometry and we claim both formulations are equivalent, then it seems that, at least ,global topological properties like boundedness are not observable. In the post mentioned it has been stated that only differential topology is observable, in agreement to what I said.

I want to know what physicists say about observability of topology, and why? (I am not asking what is the topology of space, I know that question was asked before. I am asking about a deeper issue: whether it can be observed in principle or not?)

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In short: it could be observed, but we don't so far. At least not spacetime topology. However, we detect topological effects on gauge theories (regarding the topology of the gauge group). –  Dox Jul 22 at 3:08
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There is a real risk of applying a double standard here with regard to the ontological definitions you choose. There is at least one natural definition of "geometry" for which it most certainly can be observed, and there is another definition for which it cannot. The same dichotomy holds for "topology." Would constructing a non-contractible worldsheet count as proving a topological statement about the universe? It depends if you allow one to write the laws of physics in an equivalent (but seldom used) way that makes no reference to standard topological notions. –  Chris White Jul 22 at 3:54
    
Chris, here is the phrase from Thorne:"The flat spacetime paradigm’s laws of physics can be derived, mathematically, from the curved spacetime paradigm’s laws, and conversely. This means that the two sets of laws are different mathematical representations of the same physical phenomena, in somewhat the same sense as 0.001 and 1/1000 are different mathematical representations of the same number". so, what do you mean when you say " it most certainly can be observed"?, if you mean after accepting some conventions , geometry becomes observable, then i agree with you, –  Amirpouyan Jul 22 at 4:01
    
Possible duplicates: physics.stackexchange.com/q/1787/2451 and links therein. –  Qmechanic Jul 22 at 4:22
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See this paper. The consequences of the topology (Circle searching...) have not been observed, so the model is not correct, but it is interesting. –  Trimok Jul 22 at 9:15

3 Answers 3

up vote 15 down vote accepted
+50

I'm not going to provide a full answer here, because I don't know the answer, but I want to give some statements that illustrate quite nicely the kind of problems one would face when determining topology of anything:

We know spacetime is a manifold. That means, locally, it looks just like $\mathbb{R}^4$. That's already a bummer. We can't do jack at one place to find out anything about topology. But, as soon as we move, we get into all the complications of reference frames and whatnot. So, experimentally, whether or not we can principally detect topology, it's going to be one hell of a challenge.

But it gets worse. You know how we always suppose that fields fall off at infinity? That's one of the natural reasons principal bundles arise in gauge theories. If we want to make precise the notion of a field $A$ falling off at infinity, we say it has to be a smooth function and have a well-defined value $A(\infty)$. And what's $\mathbb{R}^n$ together with $\infty$? The one-point compactification, also known as the sphere $S^n$. But it is not quite feasible to find global solutions to the equations of motion of a gauge theory on $S^n$, thanks to the hairy ball theorem and others. So we say: Alright, let's solve the e.o.m. locally on some open sets $U_\alpha,U_\beta$ homeomorphic to the disk (think of the hemispheres overlapping a bit at the equator), and patch the solutions $A_\alpha,A_\beta$ together on the overlap by a gauge transformation on $U_\alpha \cap U_\beta$. Now, we've got our field living naturally on the sphere $S^n$ if we want a global solution. Does this mean that we actually live on an $S^n$, or just that we are inept to find a coherent description of the physics on $\mathbb{R}^n$? What would that even mean?

I can hear the people saying "We can always examine what the curvature is - $S^n$ has non-vanishing one, $\mathbb{R}^n$ has vanishing one.". That's alright, but the above gauge argument forces us the either accept that there is no globally well-defined gauge potential $A$ on $\mathbb{R}^n$ or to think of some $S^n$ on which a patched-together solution lives. What's more real? What would it even mean to say one of these views is more meaningful than the other?

So, you might be inclined to say: "Screw these weird gauge potentials, we're living on a spacetime, and not some bundle!" But there are topological effects of these bundles such as instantons or the Aharonov-Bohm effect. Spacetime alone is not enough. And what would be a meaningful distinction between "These bundles are not where we live, they're 'above' spacetime" and "We live on the bundles, and most often only experience the projection on spacetime"?

What I am trying to say is that it is not even clear what we should regard as the universe we live in. The ordinary, 4D spacetime is not enough to account for all the strange things that might happen.

And as I said in the beginning, don't take this as an answer. I am biased from being immersed in gauge theories, and only having superficial knowledge of the intricacies of GR. But from what I see, all "non-trivial topology" can also be seen as arising from patching together local solutions to the physical laws that otherwise don't match well.

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Note that the $\infty$ conditions are not statements about fields on cosmic scales but about the "isolation" of our situation. Another way to put is to speak about "physics in a box" where we just take a "large box limit" hoping to get a box-independent theory aproximately applicable in appropriate situations. Drawing cosmic-topology conclusions is not entirely appropriate without further discussion. –  Void Jul 22 at 11:59
    
Or, to put it differently, the limiting process makes it obvious that the singularity the no-hair theorems refer to can be at $\infty$ and we are in fact always talking about a topological disc. –  Void Jul 22 at 12:08

As has been discussed in many questions around here (e.g. here), relativity tells us only about local properties and behavior of a space-time. There are some exceptions when we make global assumptions - if we have a space of globally and strictly constant positive curvature, non-trivial topology is imminent because the space has to be the 3-sphere $\mathbb{S^3}$.

But we can also add nontrivial topology without much constraints. The full richness can be explored e.g. through quotienting the "canonical" space-slices $\mathbb{E^3},\mathbb{S^3},\mathbb{H}^3$ (flat euclidean, 3-sphere, 3-hyperbolic) by a discrete symmetry group $\Gamma$. I.e. for $\Gamma$ a group of discrete translations in all directions "cutting up" $\mathbb{E}^3$, we get a topological 3-torus $\mathbb{E^3}/\Gamma = \mathbb{T^3}$.

The intuitive picture is that the space looks locally exactly as our good olde' flat euclidean space $\mathbb{E^3}$, but after a certain distance (the translation), we get to the same place. Naturally, as this is the same place, we should find the same things at these places up to their movement and evolution during the time we weren't there.

As for cosmological observation, if we are to detect nontrivial topology with current methods, the space or the non-trivialities must be "sufficiently small". Imagine we are on a sphere and we are restricted by observation to see a very very small patch of it - there will be no way we can conclude it is a sphere.

If we however see beyond say one of the discrete translations of $\Gamma$, we should be in principle able to detect multiple images of the same object. The problem is, since light took longer to travel from the image farther away, we will see the more distant object to be "younger" than the closer one and most probably under a different angle. For a decently large universe with other effects like redshift and obscuring, this is probably a deal-breaker.

Nevertheless, the endurance of scientists is endless. We may detect a repetition in the images when collecting large amounts of data for all visible objects and using certain correlation methods to evaluate them. Certain types of topological non-triviality would then be visible as peaks or "spikes" in the correlation indicators.

The deepest image of the universe is the CMB, of which we have a very detailed dataset. CMB can be viewed as a snapshot of a large sphere at luminous distance $\chi_{CMB}$. If this sphere intersects a topological non-triviality, we should see "circles" or certain pattern repetitions in the CMB. So long, the few tests of the data however did not reveal any of this. If anything, an $\mathbb{R^2}\times \mathbb{S^1}$ (a "3-tube") topology is conjectured in association with the slightly preferred direction of the CMB.


There are possible indirect tests suggested by the discussion of ACuriousMind - a non-trivial topology imposes different boundary conditions on fundamental fields and other possible objects. Note however that the theory we develop means "very far" by $\infty$. That is e.g. in particle experiments, "very far" may be a distance of few meters, not cosmological scales. Effects due to topologically different boundary conditions would most probably play an important role in the very early universe and might provide indirect tests of the cosmic topology.


My main source for this answer are this and this review article.

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Every single comment and answer was very useful. I think i have found an answer in an old paper by Clark Glymour (Minnesota studies in philosophy of science, volume III, pp.50-60):

"It has recently been noted (Ellis, 1971; Dautcourt, 1971; Ellis and Sciama, 1972; Glymour, 1972; Trautman, 1965) that in some general relativistic cosmologies various global features of space-time may necessarily escape determination. In contrast to classical space-time theories, the fundamental group of space-time may itself be such a feature in a relativistic space-time. A precise account of what it means for two space-times to be "indistinguishable" will permit us to prove some elementary propositions concerning the classification of indistinguishable space-times which have distinct global topologies".

It says that it is possible for two space times to be empirically equivalent and yet have different topological properties, so it seems that the topology is also subject to indeterminacy by experiment at least in some of its aspects. The question of interest could be which aspects of topology are observable and which one unobservable?,

I wasn't able to get access to Ellis's paper but here is its address: Ellis, G. F. R. (1971). "Topology and Cosmology," General Relativity and Gravitation, vol. 2, p. 7., if someone has it ,then i would be grateful if he explains it here for us,

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I am slightly annoyed, because the paper of Ellis(the linked is public access but bad quality) says basically the same things as stated in my post. –  Void Aug 21 at 19:40
    
You are right. I voted for your answer just now. But i think the other answer was better fitted with the aim of question: whether topology is conventional or not?. However if you think otherwise, i am open to discussion. –  Amirpouyan Aug 21 at 21:20
    
"...can we find it by observations, or is the topology also subject to indeterminacy by experiment (similar to geometry)? What do physicists claim about the topology of the universe?" the title of your question is "Is topology of universe observable?" And you have "general-relativity" and "cosmology" in your tags. On the other hand, I don't care so much, so whatevs. –  Void Aug 21 at 21:46
    
But i didn't mean the experimental methods of detecting a non-trivial topology. Suppose someone declares that he has detected a non-trivial topology for universe, what i mean is that is it possible for us to tell him there are infinite class of topologies compatible with your data? I supposed everybody knows it when i said "like geometry", because geometrical conventionalism works in this way, if i would have asked "how we can observe topology?" then your answer was the right one, but i have asked "is topology observable?" which means "can we fix it after observing it?!, –  Amirpouyan Aug 22 at 13:01
    
Then the answer to your question of observability in physics would be no. About anything, every time. There is always more theoretical entities you can postulate just beyond the horizon of experimental possibilities, there is never any "fixing". A requirement of "fixing" is a misunderstanding of science. But a physicist (and a scientist in general) says something is observable if there exist distinct hypotheses about it that are falsifiable. Nothing more and nothing less. And in this sense the topology of the universe is observable. –  Void Aug 22 at 14:15

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