Are singularities part of the manifold?

[TrickyDicky]

Mod note: Posts split off from https://www.physicsforums.com/showthread.php?p=4468795

Dun dun duuuun :)

Hi, WN, might the OP be referring to GR instead of SR, more specifically to the expanding FRW universe in which it is impossible to even consider the notion of exansion without agreeing about an "everywhere at once" notion?, after all the current opinion in physics is that our universe is globally described by GR (which implies locally by SR of course) rather than by Minkowski flat spacetime where relativity of simultaneity truly belongs.

 

[Nugatory]

Hi, WN, might the OP be referring to GR instead of SR

He might be, but so what? The answer is the same either way: "at the same time" is observer-dependent.

 

[TrickyDicky]

He might be, but so what? The answer is the same either way: "at the same time" is observer-dependent.

You mean expansion is observer-dependent?
I mean if you apply the relativity of simultaneity strictly to extended objects like the universe there is no possible BB model.

 

[Nugatory]

If you examine Einstein's Equivalence Principle, it shows what Einstein was talking about... something physically, absolutely moved in Einstein's "external world.

Einstein's equivalence principle is about acceleration not motion; you'd have to replace the word "moved" with "accelerated" to make what you said above correct.

 

[WannabeNewton]

For the FLRW universes, everything is described in terms of the observers comoving with the Hubble flow. The global simultaneity slices are with respect to the congruence defined by these observers so again the global simultaneity is only for said family of observers. A different family of observers in the same space-time that define a congruence with non-vanishing twist won't even have global simultaneity slices.

 

[TrickyDicky]

For the FLRW universes, everything is described in terms of the observers comoving with the Hubble flow. The global simultaneity slices are with respect to the congruence defined by these observers so again the global simultaneity is only for said family of observers. A different family of observers in the same space-time that define a congruence with non-vanishing twist won't even have global simultaneity slices.

Certainly, but the relativity of simultaneity argument you guys are using to answer the OP forbids the existence of such family of observers when strictly applied (and even more something physical like a last scattering surface and the CMBR everyone in the universe, any family of observers must agree about). This was my point when making the distinction SR/GR.

 

[TrickyDicky]

A different family of observers in the same space-time that define a congruence with non-vanishing twist won't even have global simultaneity slices.

That family of observers is free to define congruences as they see fit, but the mainstream theory says they should still observe the CMB and be able to relate the doppler shift they observe it with to the family of observers that define global simultaneity slices (global "now instants") and that allow the notion of a homogeneous universe at each instant and a density change with respect to those instants (spatial expansion).

 

[Dale]

There is no absolute simultaneity implied by GR either. You can take the FLRW spacetime, do a coordinate transform with a different global simultaneity convention, and still be able to correctly describe all physics. The GR vs SR distinction doesn't make a difference here. Simultaneity is relative either way.

 

[king vitamin]

DaleSpam: If you're referring to my post, I agree with you completely on the relativity of simultaneity. My qualifier was on my statement that there always exists a global synchronization procedure. I don't feel comfortable generalizing this to GR (does it simply depend on whether the manifold admits a global coordinate system?).

 

[TrickyDicky]

There is no absolute simultaneity implied by GR either. You can take the FLRW spacetime, do a coordinate transform with a different global simultaneity convention, and still be able to correctly describe all physics. The GR vs SR distinction doesn't make a difference here. Simultaneity is relative either way.

You woul need to define what you mean by "absolute" here, I don't think it is a scientific term.
Also I'm not sure what you mean by a coordinate transformation with global simultaneity convention, what global convention? you surely know there is no global coordinates in GR.
And exactly relative to what, is the simultaneity of the CMB?

 

[PeterDonis]

the expanding FRW universe in which it is impossible to even consider the notion of expansion without agreeing about an "everywhere at once" notion?

This is not correct; the expansion of the congruence of "comoving" observers in FRW spacetime is independent of coordinates and independent of any choice of simultaneity convention.

You mean expansion is observer-dependent?

No. See above.

I mean if you apply the relativity of simultaneity strictly to extended objects like the universe there is no possible BB model.

Sure there is; the spacetime geometry described by the BB model is independent of coordinates and independent of any choice of simultaneity convention, just like any spacetime geometry in GR.

the relativity of simultaneity argument you guys are using to answer the OP forbids the existence of such family of observers when strictly applied (and even more something physical like a last scattering surface and the CMBR everyone in the universe, any family of observers must agree about).

No, it doesn't. The "last scattering" surface is independent of coordinates and independent of any choice of simultaneity convention. So is the CMBR.

What you should be saying is that there is only one set of coordinates and one simultaneity convention in which the last scattering surface is a surface of constant coordinate time, and in which the isotropy of the CMBR is manifest in the coordinates. But that statement doesn't justify the other claims you're making.

That family of observers is free to define congruences as they see fit, but the mainstream theory says they should still observe the CMB and be able to relate the doppler shift they observe it with to the family of observers that define global simultaneity slices (global "now instants") and that allow the notion of a homogeneous universe at each instant and a density change with respect to those instants (spatial expansion).

And all of this is indeed what such a family of observers will be able to do. Why do you think it wouldn't be?

 

[Dale]

You woul need to define what you mean by "absolute" here, I don't think it is a scientific term.

You are correct. By "absolute simultaneity" I simply mean the converse of "relative simultaneity" where simultaneity is a matter of convention and different conventions lead to different notions of simultaneity.

Also I'm not sure what you mean by a coordinate transformation with global simultaneity convention, what global convention? you surely know there is no global coordinates in GR.

That isn't true, in general. Many space times do admit global coordinates. However, to make my statement applicable to general space times you can weaken it to "non-local" rather than "global".

And exactly relative to what, is the simultaneity of the CMB?

The CMB doesn't have an intrinsic simultaneity. You will have to explain what you mean by simultaneity of the CMB.

 

[TrickyDicky]

Peter, you are making Exactly the same point I am, I don't know how you manage to make it look like you are arguing with me. ;)

 

[TrickyDicky]

That isn't true, in general. Many space times do admit global coordinates. However, to make my statement applicable to general space times you can weaken it to "non-local" rather than "global".

No, not in general, I said in GR so I was referring to curved spacetimes.

The CMB doesn't have an intrinsic simultaneity. You will have to explain what you mean by simultaneity of the CMB.

Sure I mean the relativity of simultaneity of observers very far apart but at rest with the CMB.

 

[PeterDonis]

Peter, you are making Exactly the same point I am, I don't know how you manage to make it look like you are arguing with me. ;)

Because it seems like you're saying the opposite. This has happened before.

 

[Dale]

Peter, you are making Exactly the same point I am, I don't know how you manage to make it look like you are arguing with me. ;)

It seems to me that you are making opposing points.

 

[PeterDonis]

I mean the relativity of simultaneity of observers very far apart but at rest with the CMB.

Relativity of simultaneity according to what simultaneity convention? Remember that the global simultaneity convention of the "comoving" FRW chart (in which each surface of constant coordinate time is a surface of simultaneity for each "comoving" observer) "lines up" with the local simultaneity convention of each "comoving" observer--i.e., within the local inertial frame of each "comoving" observer, events which are simultaneous with respect to the global FRW coordinates are also simultaneous with respect to the LIF.

 

[Dale]

No, not in general, I said in GR so I was referring to curved spacetimes.

Me too, specifically I was referring to the FLRW spacetime.

The FLRW spacetime admits a traditional coordinate chart which defines the simultaneity convention you are focused on, but it also admits many other coordinate charts. The notion of simultaneity defined by those alternate coordinate charts is every bit as "global" as the notion of simultaneity on the traditional chart, although they disagree. All of the physics of the FLRW chart is the same regardless of the simultaneity convention adopted. Therefore, simultaneity is relative in the FLRW spacetime also, the CMB notwithstanding.

 

[TrickyDicky]

My only point has been that relativity of simultaneity is a local notion, how is this making opposing points?

 

[TrickyDicky]

Me too, specifically I was referring to the FLRW spacetime.

I guess we are referring to different things when talking about global coordinates, I mean the standard sense in which global means covering the whole manifold. I'm sure now you will agree with me. ;-)

 

[TrickyDicky]

Relativity of simultaneity according to what simultaneity convention? Remember that the global simultaneity convention of the "comoving" FRW chart (in which each surface of constant coordinate time is a surface of simultaneity for each "comoving" observer) "lines up" with the local simultaneity convention of each "comoving" observer--i.e., within the local inertial frame of each "comoving" observer, events which are simultaneous with respect to the global FRW coordinates are also simultaneous with respect to the LIF.

Again there seems to be some confusión here about what are global coordinates, FRW coordinates are not global in the same sense Minkowski coordinates are in Minkowski spacetime, and this is key to understand the difference between a local and a global simultaneity convention.

 

[Dale]

I guess we are referring to different things when talking about global coordinates, I mean the standard sense in which global means covering the whole manifold. I'm sure now you will agree with me. ;-)

In any spacetime which admits a global chart (standard sense) you can do a coordinate transform to obtain a different global chart with a different notion of simultaneity. So no, I don't agree with you. Simultaneity is relative in GR also including curved spacetimes. Relativity of simultaneity is not merely a local concept.

 

[TrickyDicky]

In any spacetime which admits a global chart (standard sense) you can do a coordinate transform to obtain a different global chart with a different notion of simultaneity. So no, I don't agree with you. Simultaneity is relative in GR also including curved spacetimes. Relativity of simultaneity is not merely a local concept.

To be sure, you are saying you consider FRW coordinates global in the same sense cartesian coordinates are in Euclidean space?

 

[PeterDonis]

FRW coordinates are not global in the same sense Minkowski coordinates are in Minkowski spacetime

I don't understand what you're saying here. FRW coordinates cover the entire spacetime, just as Minkowski coordinates do, and that's the sense of "global" you said you were using.

 

[Dale]

To be sure, you are saying you consider FRW coordinates global in the same sense cartesian coordinates are in Euclidean space?

I didn't say that. I think that the answer to that depends on the topology of the universe.

What I am saying is that simultaneity is relative, regardless of whether you are talking about SR or GR.

 

[TrickyDicky]

I didn't say that.

Good, then we are agreeing on that.

What I am saying is that simultaneity is relative, regardless of whether you are talking about SR or GR.

And you'd have to quote me saying that simultaneity is not relative. Locally in GR and globally in SR.

 

[WannabeNewton]

In what sense is global simultaneity not relative? Two different time-like congruences in the same space-time don't necessarily pick out the same one-parameter family of orthogonal hypersurfaces foliating the space-time (assuming that such families actually exist for the two congruences). In other words, the global simultaneity slices of two different families of observers in a given space-time (assuming such slices can actually be defined) need not agree.

 

[TrickyDicky]

In what sense is global simultaneity not relative?

Mate, I just invited anyone to quote me saying simultaneity is not relative, I never wrote such thing, I merely made a distinction between the concept in GR and SR, it is local in the former and global in the latter but in both simultaneity is relative. Is my english that bad or what?

 

[Dale]

Mate, I just invited anyone to quote me saying simultaneity is not relative, I never wrote such thing, I merely made a distinction between the concept in GR and SR, it is local in the former and global in the latter but in both simultaneity is relative. Is my english that bad or what?

Yes. At least three of us got a different impression from your posts. Specifically, this quote makes it seem like you believe relativity of simultaneity does not belong in GR:

our universe is globally described by GR ... rather than by Minkowski flat spacetime where relativity of simultaneity truly belongs.

In any case, the relativity of simultaneity is not merely local in GR.

 

[TrickyDicky]

Yes.

Yes to my bad english?!

At least three of us got a different impression from your posts.

Fine, then, we all know this things are democratic so yes I wrote somewhere in the thread "simultaneity is not relative".

Specifically, this quote makes it seem like you believe relativity of simultaneity does not belong in GR:

Make it seem is opinable, Simultaneity of relativity is an SR concpt historically and from any point of view you might want to choose. It so happens that GR is locally Minkowskian and that makes it part of GR.

In any case, the relativity of simultaneity is not merely local in GR.

This sounds as if it made you mad that SR is merely local in GR. But if it were global it would be SR not GR don't you think?