B Does the CMB give us absolute space and time in practice?

[S Holtom]

I believe you've asked a good question and made some valid observations, but rather than digest the answers you have been given you are now pushing a dubious idea too far.

What? I'm asking the question as a means of trying to digest the answers given so far.
Let's be clear: I have no agenda on this. I'm quite happy with the idea that the CMB cannot be used as a universal clock. What I am trying to get is an idea of why.

I wouldn't have started this thread if I just wanted to believe some position forevermore, right or wrong.

 

[Ibix]

What I am trying to get is an idea of why.

You can use it to define a universal definition of "now". What you cannot do is oblige anyone to use it, or show anything in the laws of physics to make anyone prefer to use it. Also, if I gave you a series of photos of the CMB taken over millions of years you could use CMB temperature to order them, but you could not use it to put a time stamp on any of them, not without other measurements that would depend on standard issue clocks.

 

[PeroK]

I'm quite happy with the idea that the CMB cannot be used as a universal clock. What I am trying to get is an idea of why.

Take the point I made earlier. Why would you use CMB time for particle physics experiments at CERN? Why not use local atomic clocks on Earth?

 

[S Holtom]

Take the point I made earlier. Why would you use CMB time for particle physics experiments at CERN? Why not use local atomic clocks on Earth?

I don't know what the relevance of these questions is.
My answer to both would be: "Use whatever clock you like". I'm not a lobbyist for the CMB consortium.

The OP is just about how, while in theory time intervals are relative, in practice there is a universal (lower case u) reference. One that doesn't require prior synchronization. That's all.

 

[Ibix]

The OP is just about how, while in theory time intervals are relative, in practice there is a universal (lower case u) reference. One that doesn't require prior synchronization. That's all.

But that's a false dichotomy. There is no universal reference built into the laws of physics, and this is true both in theory and in practice to the best of our knowledge. In your wording, all intervals are relative both in theory and in practice.

However, there can be ways to pick a simultaneity standard that depend on an accident of history. The CMB frame is one, correct. As V50 points out, however, we expect a Cosmic Neutrino Background which would probably define a slightly different frame (in any non-perfectly-ideal model). Which one is the "right" one?

 

[S Holtom]

There is no universal reference built into the laws of physics, and this is true both in theory and in practice to the best of our knowledge.

This objection doesn't make a lot of sense.
If something was explicitly laid out in the laws of physics then it would necessarily be true in theory.

So when I suggest that X might be true only in practice, not in theory, it seems strange to point out that X is not in the laws of physics as a kind of refutation. Because my statement already implies that.

 

[PeroK]

The OP is just about how, while in theory time intervals are relative, in practice there is a universal (lower case u) reference. One that doesn't require prior synchronization. That's all.

That's the $t$ in the conventional FLRW metric. See post #50.

 

[Ibix]

So when I suggest that X might be true only in practice, not in theory,

...then you are suggesting that theory is inconsistent with reality. Is that what you intended to suggest?

The theory says that there is no privileged frame, and that is also what we see in reality to the best of our knowledge. You are confusing a privileged frame with a frame that is well chosen to describe the universe. This is exactly the same mistake as confusing the Earth center-of-mass rest frame with a privileged frame. This frame is not privileged. It is simply extremely convenient for describing a range of physical phenomena near Earth, and two people on opposite sides of the Earth could arrive at this conclusion independently. It is a useful frame because there is a lot of mass on average at rest in the frame, and that has a lot of effects that are easier to describe in this frame. But there is no experiment that can detect this frame other than to measure the speed of the Earth - so it is not privileged.

All of that is also true of the CMB frame. Two disconnected observers can certainly arrive at the conclusion that it's a useful frame, and they will do so because there's a lot of mass at rest in that frame. But there is no experiment they can do to detect the frame other than measure the CMB dipole, and there is no consequence to choosing to use another frame.

 

[PeterDonis]

The OP is just about how, while in theory time intervals are relative, in practice there is a universal (lower case u) reference.

And this claim is false. The CMB "clock" you describe is not a time reference in the required sense. It does not reflect proper time along timelike worldlines, and you cannot infer proper time from it alone; you have to already have a different clock that does reflect proper time along timelike worldlines in order to know how to transform CMB "clock" readings into proper time readings. I have already explained this.

I'm quite happy with the idea that the CMB cannot be used as a universal clock. What I am trying to get is an idea of why.

I have explained that. See above.

Perhaps this will help: the CMB can be used as a universal simultaneity convention. You can adopt a convention that says "events at which the observed CMB temperature is the same happen at the same time, no matter where in space they are located". There is nothing wrong with that. Simultaneity conventions are conventions, and no such convention is preferred by the laws of physics, but particular conventions can be picked out by physical properties of particular physical systems. The CMB is an example of that.

But that is not the same as a "universal clock". The latter also requires a "tick rate" that matches proper time or can be used, without any other information, to infer proper time. The CMB "clock" doesn't meet that requirement, for the reasons I have already explained.

 

[Ibix]

Simultaneity conventions are conventions, and no such convention is preferred by the laws of physics, but particular conventions can be picked out by physical properties of particular physical systems.

@S Holtom - this is a shorter way of saying what my last two paragraphs say in my last post above.

 

[S Holtom]

...then you are suggesting that theory is inconsistent with reality. Is that what you intended to suggest?

Of course not. We frequently make a distinction in discussion between theory and practice, and rarely does doing so imply that the latter is against the law of physics. That's not what we mean by these expressions.
And note that I didn't bring up the laws of physics. I was addressing the point that you are trying to put to me that the fact that the CMB is not mentioned in the laws of physics as a privileged frame of reference means that it is not one in theory nor in practice.

Which is a flawed argument. You can say my OP is stupid, you can say I'm stupid. But the logic of claiming something is neither true in theory nor practice because it is not specifically spelled out in the laws of physics shows a misunderstanding of what is meant by "in practice".

 

[PeterDonis]

I didn't bring up the laws of physics.

Not explicity, but any time you ask about "absolute space and time", you are implicitly bringing up the laws of physics. If you didn't realize that before, you know it now.

the CMB is not mentioned in the laws of physics as a privileged frame of reference means that it is not one in theory nor in practice.

Which is a flawed argument.

No, it isn't. The laws of physics say there are no privileged frames of reference in practice as well as in theory. So if you are saying the CMB "frame" is one in practice (which it actually isn't, because it only gives you a simultaneity convention, not an actual clock, as I have explained, but leave that aside for the moment), you are making a claim that violates the laws of physics.

a misunderstanding of what is meant by "in practice".

The misunderstanding appears to be on your part, since you appear to think that saying "in practice" allows you to make claims that violate the laws of physics. It doesn't.

What the laws of physics do allow you to say, as I have already explained, is that the CMB is a physical system with particular physical properties, and that those properties allow you to pick out a particular simultaneity convention. But that is not the same as saying the "CMB frame" is a privileged frame of reference or defines an absolute space and time "in practice". It isn't and it doesn't.

 

[S Holtom]

The misunderstanding appears to be on your part, since you appear to think that saying "in practice" allows you to make claims that violate the laws of physics.

What have I said that violates the laws of physics? Everyone agrees that any arbitrary reference frame can be chosen, including the CMB.
Why are you trying so hard to straw man what I've said?

Not only have I not claimed that the CMB has a special status in the laws of physics, I have said precisely the opposite in the OP.

 

[PeterDonis]

Why are you trying so hard to straw man what I've said?

How is it a straw man to explicitly quote what you said?

Not only have I not claimed that the CMB has a special status in the laws of physics, I have said precisely the opposite in the OP.

That just means that you are contradicting yourself when you keep saying that the CMB defines a universal or absolute frame "in practice" even though it doesn't "in theory".

What have I said that violates the laws of physics?

I already explicitly responded to you in post #62 about this: see the second quote from you in that post and my response to it. Here, I'll even quote my response for you:

The laws of physics say there are no privileged frames of reference in practice as well as in theory. So if you are saying the CMB "frame" is one in practice (which it actually isn't, because it only gives you a simultaneity convention, not an actual clock, as I have explained, but leave that aside for the moment), you are making a claim that violates the laws of physics.

 

[Dale]

if there was a literal, visible grid overlaid over the whole universe we could call that "just another" frame of reference too. We would still likely consider it "special" though. And that's all I'm saying.

And that’s not enough to qualify the CMB as an “absolute” frame in the usual meaning of the term.

The CMB is indeed “special” only in the sense that it is the biggest and oldest thing that we can see. But to qualify as an “absolute” frame means that the laws of physics themselves uniquely identify that frame. And they don’t.

 

[vanhees71]

Hm, but in a sense it's a preferred frame to describe the large-scale structure of spacetime, because in any local rest frame of the CMBR you get a manifestly isotropic form of the metric coefficients, i.e., the usual coordinates for the FLRW metric. It's of course not a preferred local reference frame somehow derived from the fundamental laws, defining GR, because indeed there is no preferred frame due to general covariance, which is a gauge symmetry, i.e., the local physics is described by an equivalence class of coordinates modulo general diffeomorphisms.

 

[PeterDonis]

in a sense it's a preferred frame

It's a frame that corresponds to a set of symmetries (Killing vector fields) of the spacetime, and therefore makes the metric look simpler, yes. But that's not what "preferred frame" means in the physics literature; that term refers to a frame that is picked out by the laws of physics, not by particular symmetries of a particular solution to those laws. And of course examples of frames that are picked out by particular symmetries of a particular solution are ubiquitous in GR; the "CMB frame" is by no means the only one. That is why multiple posters have objected to using terms like "preferred frame" or "absolute space and time" to refer to FRW coordinates (which is what the "CMB frame" amounts to).

 

[Sagittarius A-Star]

Does the CMB give us absolute space and time in practice?

No. Terence and Stella from the twin "paradox" need to use their respective own watch to measure their own aging since their first meeting-event.

The twin "paradox" was verified in experiments, or in your terms: It was shown "in practice", that time is relative.

Bailey et al. (1977) measured the lifetime of positive and negative muons sent around a loop in the CERN Muon storage ring. This experiment confirmed both time dilation and the twin paradox, i.e. the hypothesis that clocks sent away and coming back to their initial position are slowed with respect to a resting clock.[28][29] Other measurements of the twin paradox involve gravitational time dilation as well.

Source:
https://en.wikipedia.org/wiki/Experimental_testing_of_time_dilation#Twin_paradox_and_moving_clocks

 

[S Holtom]

Thanks to everyone (except one) for the informative responses.

My understanding so far:

Space -- The CMB doesn't work so well as a reference frame over large distances, as two observers at rest relative to the CMB will be moving apart.
Time -- TBH I'm still not 100% clear on this one. Sorry, please be patient with me...
There seems to be a simple relationship between time and CMB temperature in the formula
T = T(t₀) (1+z)
(where T is temperature, t is time and z is redshift).
So I don't quite follow why knowing one doesn't imply the other. Is it because calculating z involves knowing things like dark matter density (and properties), a lot more precisely than we do now?

 

[PeterDonis]

Space -- The CMB doesn't work so well as a reference frame over large distances, as two observers at rest relative to the CMB will be moving apart.

This depends on what you want to use the reference frame for. This "frame" is the same as standard FRW coordinates, which cosmologists find useful for many purposes, even though they have to keep in mind that objects "at rest" in the coordinates (i.e., whose spatial coordinates are constant) are moving apart.

There seems to be a simple relationship between time and CMB temperature in the formula
T = T(t₀) (1+z)

This isn't a relationship between time and CMB temperature, it's a relationship between redshift and CMB temperature. It is not sufficient to know how much time has passed. Redshift is not the same thing as time. The relationship between redshift and time depends on the specific expansion history of the universe, which cannot be inferred from the information given.

 

[vanhees71]

It's a frame that corresponds to a set of symmetries (Killing vector fields) of the spacetime, and therefore makes the metric look simpler, yes. But that's not what "preferred frame" means in the physics literature; that term refers to a frame that is picked out by the laws of physics, not by particular symmetries of a particular solution to those laws. And of course examples of frames that are picked out by particular symmetries of a particular solution are ubiquitous in GR; the "CMB frame" is by no means the only one. That is why multiple posters have objected to using terms like "preferred frame" or "absolute space and time" to refer to FRW coordinates (which is what the "CMB frame" amounts to).

I said that repeatedly it's not preferred by the general laws of nature but by the physical situation considered (large-scale coarse-grained model for the Universe). There's of course no absolute space or time in the foundations, but the physical situation distinguishes the local rest frame of the CMBR as particularly "natural" to describe this situation. That's also the reason, why by definition (or if you wish convention) intrinsic properties of a medium (starting from elementary particles with their mass and spin; then temperature, chemical potential, particle density, various thermodynamic potentials to characterize thermal equilibrium of a medium) are defined in the (local) restframe of the medium. It's just a matter of convenience to define such quantities in a "preferred" (local) frame of reference and then formulate them in a manifestly covariant way to make them easily treatable in general frames.