Is 'Local Flatness' the Right Term for Describing Spacetime?

[atyy]

OK, but hopefully everyone who objects to "local flatness" also objects to one of the traditional statements of the equivalence principle: gravity is locally equivalent to acceleration.

 

[Orodruin]

OK, but hopefully everyone who objects to "local flatness" also objects to one of the traditional statement of the equivalence principle: gravity is locally equivalent to acceleration.

There are clearly tensor properties of curved spacetime that are not equivalent to just having acceleration in Minkowski space (the tensor properties do not care about whether or not you use an inertial frame).

I think a better formulation would be "locally indistinguishable from" as measuring curvature requires parallel transport around small loops returning small^2 changes in the transported vectors. This makes reference to the measuring procedure rather than the mathematical formulation.

Of course, in the end this just underlines the difficulty in inventing a precise enough popular language to use when we engage in B- and I-level threads on GR. I mean, I am sure (or assume) that we all agree on the actual maths involved in GR, the issue is one of nomenclature alone.

 

[atyy]

Of course, in the end this just underlines the difficulty in inventing a precise enough popular language to use when we engage in B- and I-level threads on GR. I mean, I am sure (or assume) that we all agree on the actual maths involved in GR, the issue is one of nomenclature alone.

But if it weren't for all this terrible language, we wouldn't have the pleasure (?) of radiating charge and the equivalence principle threads from time to time :P

I think a better formulation would be "locally indistinguishable from" as measuring curvature requires parallel transport around small loops returning small^2 changes in the transported vectors. This makes reference to the measuring procedure rather than the mathematical formulation.

BTW, Ohanian even objected to this in this old paper of his: https://doi.org/10.1119/1.10744
"The strong principle of equivalence is usually formulated as an assertion that in a sufficiently small, freely falling laboratory the gravitational fields surrounding the laboratory cannot be detected. We show that this is false by presenting several simple examples of phenomena which may be used to detect the gravitational field through its tidal effects; we show that these effects are, in fact, local (observable in an arbitrarily small region)."

It's pretty much like the famous objection by Synge I mentioned earlier.

 

[PAllen]

But if it weren't for all this terrible language, we wouldn't have the pleasure (?) of radiating charge and the equivalence principle threads from time to time :P
BTW, Ohanian even objected to this in this old paper of his: https://doi.org/10.1119/1.10744
"The strong principle of equivalence is usually formulated as an assertion that in a sufficiently small, freely falling laboratory the gravitational fields surrounding the laboratory cannot be detected. We show that this is false by presenting several simple examples of phenomena which may be used to detect the gravitational field through its tidal effects; we show that these effects are, in fact, local (observable in an arbitrarily small region)."

It's pretty much like the famous objection by Synge I mentioned earlier.

The mathpages article you linked previously has a pretty effective refutation of Ohanian's examples, to whit, they all ignore (sometime subtly) the time aspect of local spacetime region.

Synge's objection is purely mathematical, and amounts to the same as Orodruin's - that curvature is defined at each point of the manifold.

The charged particle debates (classically) all boil down to the fact that radiation is not a strictly local phenomenon, and can be shown to follow from inability to construct a global inertial frame.

However, I do share the same question Peter raised about one part of this article, and I also choose not to accept Einstein's definition of gravity, as described therein (this being purely a choice of terminology).

 

[PAllen]

For Riemannian manifolds, I have seen the term “locally Euclidean” used. This avoids the flat vs curved conundrum, while also not having to discuss coordinates. Would the “locally Minkowski” make you @Orodruin happy?

 

[Jimster41]

Is there a microscopic description of these "tidal forces" - that (if I understand correctly) betray non-zero curvature even in an infinitesimal inertial frame?

https://en.wikipedia.org/wiki/Tidal_tensor

My cartoon is that they represent (result from) geometric phase or "Pancharatnam-Berry Phase" (non-zero holonomy)?

I get that there is a frequency shift (in light for example) as a function of a gravitational field (gravitational lensing). But my understanding of that is that it would not be detectable from within the inertial frame?

Are there any experiments that would detect a changing value of the field (curvature) from inside an inertial frame? Is there just some simple electrostatic gradient effect that can be measured? I was assuming the answer is no?

Would the Aharonov-Bohm effect reflect such change? Not sure how that effect is measured but I gather it's not just a simple magnetometer.

Would the spontaneous collapse of entanglement (somehow absent other causes) be indicative, or some change in the stability of entanglement as a function of alignment with the change (gradient) in the field?

 

[atyy]

The mathpages article you linked previously has a pretty effective refutation of Ohanian's examples, to whit, they all ignore (sometime subtly) the time aspect of local spacetime region.

Interesting! I only linked the mathpages article for Synge's remark and had not read the rest of it. I wonder whether Ohanian includes these in the latest edition of his textbook with Ruffini.

 

[PAllen]

Is there a microscopic description of these "tidal forces" - that (if I understand correctly) betray non-zero curvature even in an infinitesimal inertial frame?

https://en.wikipedia.org/wiki/Tidal_tensor

My cartoon is that they represent (result from) geometric phase or "Pancharatnam-Berry Phase" (non-zero holonomy)?

I get that there is a frequency shift (in light for example) as a function of a gravitational field (gravitational lensing). But my understanding of that is that it would not be detectable from within the inertial frame?

Are there any experiments that would detect a changing value of the field (curvature) from inside an inertial frame?

Would the spontaneous collapse of entanglement (somehow absent other causes) be indicative, or some change in the stability of entanglement as a function of alignment with the change in the field?

Would the Aharonov-Bohm effect reflect such change? Not sure how that effect is measured but I gather it's not just a simple magnetometer.

I would say that the SEP (strong equivalence principle) is a testable proposition, and that if any test could be devised that, when performed in an arbitrarily small spacetime region with any finite precision, could distinguished a local inertial frame in a region with curvature from one without, you would have a violation of SEP.

The SEP is making a claim that local physics is precisely as indistinguishable from SR as local geometry is from Euclidean for a Riemannian metric. If you look at the various equivalent definitions of geometric curvature, they all require infinite precision to execute:
- limit of change of vector transported around quadrilateral as its size goes to zero divided by the area. The actual change goes to zero, and still goes to zero if divided by e.g. a diagonal of the quadrilateral.
- limit of angular defect in a triangle as its size goes to zero, divided by the area of the triangle. Again, the angular defect itself goes to zero, and you need the division by area to measure the second order effect.
- limit of the difference between 1 and ratio of circumference or area to the euclidean formula, divided by area, as the size goes to zero. The ratios themselves go to 1, and the difference from 1 still goes to zero if divided by circle diameter.

 

[andrewkirk]

@Orodruin
I agree that it is misleading. I'd like to make two points.

First, comparable uses of the word 'locally' in topology agree with your principle that the property must apply in some neighbourhood of each point - which flatness does not. The examples I think of are locally connected, locally path connected and locally compact. 'Locally flat' does not adhere to this principle. There is no neighbourhood of a point in which the curvature is constant at zero. So use of the term 'locally flat' does not follow standard practice for the term 'locally' in topology.

Second, is it not the case that, if we exclude singularities from a spacetime manifold (which IIRC we can do without inhibiting our ability to calculate) then any achievable spacetime manifold is everywhere 'locally flat'? I am not completely sure of that, or whether 'local flatness' may not apply at the event horizon of a black hole. But if I guessed correctly, then saying a spacetime is locally flat is saying nothing, and we lose nothing by discarding the phrase.

I would have thought that saying the spacetime is differentiable (or $C^n$ for some $n$) tells us all that is needed. It would be better to simply say that we can approximate a spacetime to an arbitrarily high degree of accuracy near a point by taking a small enough neighbourhood of the point.

 

[PAllen]

@OrodruinSecond, is it not the case that, if we exclude singularities from a spacetime manifold (which IIRC we can do without inhibiting our ability to calculate) then any achievable spacetime manifold is everywhere 'locally flat'? I am not completely sure of that, or whether 'local flatness' may not apply at the event horizon of a black hole. But if I guessed correctly, then saying a spacetime is locally flat is saying nothing, and we lose nothing by discarding the phrase.

I don’t know what you mean here. @orodruin’s complaint is that most useful GR manifold’s (Schwarzschild, Kerr, FLRW, etc.) are nowhere flat, though the first two are asymptotically flat at spatial infinity.

On the other hand, the definition of equipping a manifold with a metric requires that it be locally Euclidean or Minkowski to second order. This is all mathematical definition. The physical question is then whether such models with a mapping to measurements correspond to reality.

The singularity can’t be part of the manifold - it isn’t a choice. And no part of the manifold, horizon or arbitrarily close to a singularity can avoid being locally Minkowski or Euclidean, because the definition Riemannian or pseudoriemannian forces this by design.

 

[andrewkirk]

@orodruin’s complaint is that most useful GR manifold’s (Schwarzschild, Kerr, FLRW, etc.) are nowhere flat, though the first two are asymptotically flat at spatial infinity.

Yes, I understand that that is part of Orodruin's point, and I agree with it. But I don't understand why you think what I wrote does not agree with that.

 

[Orodruin]

For Riemannian manifolds, I have seen the term “locally Euclidean” used. This avoids the flat vs curved conundrum, while also not having to discuss coordinates. Would the “locally Minkowski” make you @Orodruin happy?

It is better, although I am not completely sure how I feel about it yet. I have to sleep on it I think.

 

[PAllen]

Yes, I understand that that is part of Orodruin's point, and I agree with it. But I don't understand why you think what I wrote does not agree with that.

Perhaps I misunderstood you. Your first paragraph seemed to reject local flatness, while your second embraced it. But I think I missed the significance of your scare quotes.

In that case, whatever the best term is, I think it is crucial to know that equipping a manifold with a metric intentionally gives it some universal local properties. And your comment about the horizon is exactly why it is crucial - a horizon is locally indistinguishable from my living room per GR. We need a name for this property that tells you it is impossible to say things like time stops at the horizon in GR.

 

[JustTryingToLearn]

My understanding of "local flatness" is the following. Around any spacetime point (with local flatness), there exists a region of spacetime (a neighborhood) within which the results of any experiment cannot be distinguished from the results of an experiment performed in completely flat spacetime. In other words, there is some region around the point such that, should you perform an experiment there, you would not be able to take the results and prove that special relativity is not the "true" theory (more simply, that special relativity is not valid) in that region of spacetime. If you do perform such an experiment and can show that SR is not valid, then you have chosen too large a neighborhood.

If I am misinterpreting this, I'd welcome feedback as this is something I am trying to learn in more detail.

 

[Physics4Funn]

I see many posts by several different people referring to spacetime being "locally flat" with the intended meaning of being locally indistinguishable from Minkowski space, i.e., being able to rewrite the metric on orthonormal form and not being able to measure curvature on some local scale. I do not think this is an appropriate nomenclature and the more appropriate nomenclature would be to refer to a local inertial frame. I am aware that some textbook authors, such as Schutz, use the term in this way as well. These are (some of) my issues with the terminology:

• "Local flatness" is typically defined in a different manner in topology, where it is a property of a submanifold. The entire point of using differential geometry is that spacetime can be described without reference to it being a submanifold of some higher-dimensional space.
• Not withstanding the previous point, we otherwise use "local" to describe a property that is only true in a point or in a neighbourhood of that point. "Flat" refers to the curvature being zero. Putting those two together as "locally flat" would therefore typically mean that the curvature at the given event (or neighbourhood) would be zero. This is not generally true as curvature invariants can be computed to be non-zero even though there are local inertial frames at all events.
• There exists other alternative terminology to describe precisely the ideas that "locally flat" intends to convey. The existence of a "local inertial frame" or similar comes to mind.

Any thoughts? Am I just being picky?

Yes, I am bothered by this also.
I am concerned that most of the mathematics that physics uses only applies to flat spacetime.
I call this "flat spacetime prejudice."
I offered to teach an undergraduate class in general relativity to try to get more physicists fluent with curved space. I wonder if the trouble finding a unified field theory is hindered by the lack of workers.
For many reasons, it didn't happen.

To your question:
I don't feel qualified to answer your question.
I would really like to hear from a mathematician about this.

Maybe there is an answer in the many replies here.

 

[PAllen]

My understanding of "local flatness" is the following. Around any spacetime point (with local flatness), there exists a region of spacetime (a neighborhood) within which the results of any experiment cannot be distinguished from the results of an experiment performed in completely flat spacetime. In other words, there is some region around the point such that, should you perform an experiment there, you would not be able to take the results and prove that special relativity is not the "true" theory (more simply, that special relativity is not valid) in that region of spacetime. If you do perform such an experiment and can show that SR is not valid, then you have chosen too large a neighborhood.

If I am misinterpreting this, I'd welcome feedback as this is something I am trying to learn in more detail.

That is a correct statement of the Einstein Equivalence Principle as defined by e.g. Clifford Will. Its ability to be true in GR is, indeed, closely related to “local behavior of a pseudoRiemannian manifold”. The gist of this thread is what is the best compact verbal description of this local behavior that we all agree on the mathematics of. The equivalence principle names the physics. What we seek consensus on is a name for corresponding math of the manifold.

 

[atyy]

I think a better formulation would be "locally indistinguishable from" as measuring curvature requires parallel transport around small loops returning small^2 changes in the transported vectors. This makes reference to the measuring procedure rather than the mathematical formulation.

The mathpages article you linked previously has a pretty effective refutation of Ohanian's examples, to whit, they all ignore (sometime subtly) the time aspect of local spacetime region.

I would say that the SEP (strong equivalence principle) is a testable proposition, and that if any test could be devised that, when performed in an arbitrarily small spacetime region with any finite precision, could distinguished a local inertial frame in a region with curvature from one without, you would have a violation of SEP.

The SEP is making a claim that local physics is precisely as indistinguishable from SR as local geometry is from Euclidean for a Riemannian metric. If you look at the various equivalent definitions of geometric curvature, they all require infinite precision to execute:
- limit of change of vector transported around quadrilateral as its size goes to zero divided by the area. The actual change goes to zero, and still goes to zero if divided by e.g. a diagonal of the quadrilateral.
- limit of angular defect in a triangle as its size goes to zero, divided by the area of the triangle. Again, the angular defect itself goes to zero, and you need the division by area to measure the second order effect.
- limit of the difference between 1 and ratio of circumference or area to the euclidean formula, divided by area, as the size goes to zero. The ratios themselves go to 1, and the difference from 1 still goes to zero if divided by circle diameter.

So if we have infinite precision, are we able to detect deviations from flatness, even at a point? For example, could geodesic deviation be detected? In other words, is there a physical counterpart to the objection to the terminology of "local flatness"?

 

[jbriggs444]

So if we have infinite precision, are we able to detect deviations from flatness, even at a point?

Clearly not. If we have infinite precision we can detect deviations from flatness using measurements drawn from a neighborhood of arbitrarily small extent. But not from a neighborhood with no extent.

 

[atyy]

Clearly not. If we have infinite precision we can detect deviations from flatness using measurements drawn from a neighborhood of arbitrarily small extent. But not from a neighborhood with no extent.

But the definition of curvature (ie to mathematically say that the curvature is non-zero at a point) also requires a neighbourhood?

 

[jbriggs444]

But the definition of curvature (ie to mathematically say that the curvature is non-zero at a point) also requires a neighbourhood?

Same as a derivative, f'(x). It is defined for a point but the definition depends on behavior near the point.

Or, consider the definition of a limit of a function at a point. The definition is for a point but is independent of the function value at that point.

 

[PAllen]

So if we have infinite precision, are we able to detect deviations from flatness, even at a point? For example, could geodesic deviation be detected? In other words, is there a physical counterpart to the objection to the terminology of "local flatness"?

The value at a point is the result of a limit. Thus, you can’t measure it at a point. However, classically, you could measure geodesic deviation in a ball a billionth of a plank length with tiny instruments of arbitrarily great precision.

I don’t have any real objection to local flatness treated as a name for math that both @Orodruin and I agree on. But I can also sympathize with the objection. Thus I am open to agreeing to other terminology as preferred for this site. I am not enamored of having to say something involving coordinates, because the local behavior is coordinate independent. I have suggested “locally Minkowski” as a possibility.

Note, unlike some, I have no problem with practical definitions of coordinate independent features (e.g. asymptotic flatness or spherical symmetry) that involve the existence of coordinates in which the metric takes a certain form. However, I want a name to emphasize that the feature itself is coordinate independent.

 

[Orodruin]

I would be interested to hear exactly what meaning different people include in this usage of "local flatness". Exactly which properties does the spacetime (or manifold if we become a bit more general) need to satisfy for you to call it "locally flat"?

 

[PAllen]

I would be interested to hear exactly what meaning different people include in this usage of "local flatness". Exactly which properties does the spacetime (or manifold if we become a bit more general) need to satisfy for you to call it "locally flat"?

For me, it is a universal feature, by design, of any Riemannian or pseudoRiemannian manifold. It has no meaning if you don’t equip the manifold with a metric. Riemann‘s aim in his definitions was to allow geometry in the large and topology to be wildly different from Euclidean, while preserving local Euclidean behavior.

 

[atyy]

I would be interested to hear exactly what meaning different people include in this usage of "local flatness". Exactly which properties does the spacetime (or manifold if we become a bit more general) need to satisfy for you to call it "locally flat"?

Local flatness is a property of all (semi)-Riemannian manifolds. Thus a manifold that is nowhere flat is everywhere locally flat. Yes, I sympathize with your peeve, but I think it is tied up with the physics of the equivalence principle. I don't think one can totally avoid misleading terminology in the discussion, but I would prefer to handle it by keeping the traditional terminology, and just explaining the details of the physics.

Normal coordinates are one mathematical tool corresponding to the physics notion of local flatness, and quantifying deviations from it.

 

[Orodruin]

Thus a manifold that is nowhere flat is everywhere locally flat.

This certainly should not be the case. If it is the concept is meaningless.

Riemann‘s aim in his definitions was to allow geometry in the large and topology to be wildly different from Euclidean, while preserving local Euclidean behavior.

Did Riemann use ”local flatness”?

If it is supposed to be a property of all manifolds with a metric I do not see the point of introducing the term at all.

I also assume that you want to latch on the condition that the connection is Levi-Civita. To me it is flat (edit: pun not intended, but it is funny now that I reread it...) out misleading to talk about flatness at all without actually referencing the connection and a priori the connection need not be tied to the metric.

I have trouble seeing why you need to introduce this nomenclature at all if it is just supposed to refer to a smooth manifold with a metric as all you need to say is that it locally looks like Euclidean/Minkowski space in the sense that there is a smooth map from a neighbourhood to a set in E/M space. (Of course with varying amounts of technicality depending on who you are talking to.)

 

[atyy]

I also assume that you want to latch on the condition that the connection is Levi-Civita. To me it is flat (edit: pun not intended, but it is funny now that I reread it...) out misleading to talk about flatness at all without actually referencing the connection and a priori the connection need not be tied to the metric.

Yes, to be more careful, the metric compatible connection is needed.

 

[Orodruin]

Yes, to be more careful, the metric compatible connection is needed.

... and torsion free!

 

[Paul Colby]

Are there any experiments that would detect a changing value of the field (curvature) from inside an inertial frame?

I would argue (probably a losing battle) that LIGO qualifies. LIGO is much smaller than the GW wavelengths detected. As a matter of principle there are no real measurements "at a point" or "at an instant" only ever improving approximations of such.

 

[andrew s 1905]

An interesting discussion that I have only just seen. As one who might want a B level answer I think it worth considering how I would understand "locally flat"

To me I would assume that local ment something the size of a lab or the apparatus being used, not some mathematical idea. Flat would mean the analysis of the experiment could use special relativity rather than general or given the context Newtons/Galileo's.

I am sure this is all wrong given the above.

However, I feel one has to give up some precision to get concepts across to us less able but interested questioners. While it maybe ideal not to teach "wrong" physical concepts I don't see how this can be avoided unless you can somehow give me all the necessary mathematical tools up front. I have tried and keep trying to gain more of these but at 67 it's hard.

Regards Andrew s

 

[atyy]

... and torsion free!

Oops, indeed. Anything else I forgot ... ?