Urban legends in authoritative astronomy

[Ken G]

In fact the every result we obtain with any choice of coordinates is the same "artifact". We may even derive transforms to prove the equivalence.

The "equivalence" of what? You have already said there is a convenience issue that is not equivalent. I've already agreed there is a formal equivalence.

The convenience is the only pure artifact. To say that defining one of an infinite number of physically equivalent choices has a logical basis borders on giving (x, y, z, t)=0 absolute meaning.

I am saying that if there is a logical basis for taking one origin (the source of gravity, roughly), then one should expect a certain non-coincidental convenience to attach to that choice, making it non-arbitrary. I should think that would be noncontroversial enough, given how often we do it. If one has a logically based convenience that attaches to a certain choice, I do not call that an "artifact", I call it a logicaly based convenience-- as for comoving coordinates and the Cosmological Principle. What is controversial in that statement?

The convenience is all that is real about it.

The way I would say that same thing is, the Cosmological Principle is all that is real, the convenience of comoving coordinates stem from that.

It could actually be argued that the Cosmological principle is needed under most reasonable assumptions about the Universe.

It would require entering into undemonstrable philosophy to claim prior knowledge of "what is reasonable". I wait for experiment to tell me that.

In fact the hardest thing to explain theoretically would be a failure of the Cosmological principle.

I can agree there-- we are fortunate not to have to.

Even so how sure can you be that the Hubble flow wouldn't appear isotropic regardless of our peculiar motion such that any apparent anisotropy is simply a doppler shift.

Personally, not sure at all. One would only look for higher-order than dipole variations though, a la Occam's razor.

If by "real" you mean what we measure then the most straightforward definition is proper distance, which is consistent with the operational definition that kdv gave. This could even be communicated to a distant civilization using Planks constant as a standard.

What I'm saying is that there are two ways to think of space. One is that space is really there, and the other is that we made it up in our minds to explain various observations. I realize that at some level everything we talk about is made up in our minds, but what I mean by "made up in our minds" is that we have no direct measurements that correspond to what we have made up-- it always requires some construction. Proper time, for example, is directly measurable between two causally connected events. Proper distance is not. So when dealing with distance, we need to invent indirect measures, like timing light. I admit that we need a concept of "infinitesmal distance" to calibrate a clock, but we don't have any way to measure cosmological distances that are not indirect, so require a model to interpret the measurement, and that is why there are so many different and essentially arbitrary distance measurements. I take that as a major clue that space is not "real" the way proper time is. It simply is not directly measurable, you measure something else and use it to infer a distance, so the latter concept is just invented.

The presents of an observers alone dictates the means of measurement. They may not all generally agree on distances but they can agree on the symmetries of their disagreements and all will agree that spatial separation exist. These symmetries are non-arbitrary.

Then we may talk about the symmetries as real-- not the "space". The usual concept of space is what I am talking about, not the replacement of that concept with more precise groups of symmetries. The latter I am fine with calling real-- as I said, we measure something else and invent a concept of space to make sense of it. Space is a theory.

Question: Are you claiming here that the Hubble flow does not constitute a change of "proper" distance with time?

I am saying that "proper distance" is a made up concept, as it is by definition not directly measurable. It is also a theory. A useful one, like many theories are, but we made it up because we can't measure it. We can measure time.

Here you seem to be referring to comoving coordinates to say "not moving relative to each other".

I am repeating the standard descriptions that I am criticizing as being only pictures-- how many times have you heard it stated that cosmological redshifts are "not due to relative motion"? Many, over here.

Again this is an arbitrary coordinate system of convenience and presumably disagrees but does not conflict with the notion of proper distance.

That is the argument I am presenting, yes. A coordinate system where there is no relative motion is as arbitrary as one where there is, even if some real effect makes it more convenient.

If a two way light signal sent tomorrow takes longer to return than one sent today its proper distance has increased even though its comoving distance might not have.

That isn't proper distance, it's proper time in distance units. Is that all you mean by proper distance? Again, note it is not directly measurable, you measure a time and decide to multiply by c, for whatever reason. Even if you choose units where c=1, you are still measuring a time and calling it a distance.

You would have a case on pure technicality if kdv hadn't provided an operation definition that happen to be consistent with proper distance.

It sounds like you are claiming that if successive light pulses take longer to return, that means space is expanding. That doesn't sound very reasonable at all, given that pulses to the Voyager spacecraft also do that. My objection to kdv was never anything other than the claim that mean cosmological redshifts are categorically caused by the expansion of space. I said that this was a coordinate choice, and Hurkyl pointed out that when "expanding space" is used technically it actually means something different than what kdv seemed to mean (it really meant "gravitational redshift", in essence). Hence the only way to refute my objection is for kdv to say that Hurkyl's limited meaning was all he really intended, in which case I would say, that could have been made more clear.

His definition of "existing space" is simply the inverse of your definition of time.

You have claimed two things, it seems, one is that distance can be defined to be a light travel time (that sounds like a time to me, but you did call it an operational definition so I must accept it), and the other is that distance is the way to determine "how much space there is". Combining those claims suggests that space is expanding between us and the Voyager spacecraft -- after all, there's more space between us, the space must have expanded. Those are all the aspects of kdv's position that you have cited, are they not? What did I leave out?

I agree this definition of "existing space" is a purely an "artifact" of the chosen definition. Yet it is a particularly useful one if we consider sending long range probes.

I agree that we certainly like to imagine that the one-way speed of light is a constant, so we do indeed make up a concept of distance that is inferred from light travel times. I do not think that tells us anything at all about "what is happening to space" in our solar system, other than the very useful picture that it is being compressed laterally and stretched radially as it is sucked into the Sun.

You argue that this operational definition is invalid yet you argue that placing the sun at the center of our solar system is not "pure artifact". The sun centered solar system is itself a purely operational definition.

No, it isn't any kind of definition at all. It is a convenience stemming logically from a fact.

It is no more nor any less valid than kdv's operational definition of "existing space".

The parallel you draw actually makes my argument. Had kdv said that the coordinate-generated concept of "existing space" was nothing but a convenience stemming logically from a fact (the Cosmological Principle), I would have agreed completely. Indeed, that is just what I said.

All observers may not agree on how much but all observers will agree that there is some space between you and I, inversely identical to time. Sounds like a reasonable operational definition of "existing space" to me.

Except that it is only true if we adopt the axiom that the one-way speed of light is constant. But more to the point, you seem to be using the term "operational definition" to mean "coordinate choice". If the operational definition requires a certain (albeit elegant) coordinate choice, how are you distinguishing the coordinates from the definition? All I've said is that it is a coordinate choice, so if your argument will be that this is the same thing as an operational definition, you are simply creating a semantic identity.

 

[Ken G]

You can say, for any point in space, whether or not space is expanding at that point, in an entirely coordinate-independent way. John Baez gives a vivid explanation -- if you place at that point a small cloud of non-interacting dust whose particles are initially comoving, you can observe if its volume starts increasing, decreasing, or stays constant, corresponding to expansion, contraction, and neither.

Let's return to this comment, I have a question about it. I presume by "initially comoving", you mean they all see an isotropic CMB. Is there any other way to tell if a "small cloud" of test particles are initially comoving? If not, then are we saying that we need a smooth CMB to have a concept of expanding space? To me, that shows it is a coordinate-correlated effect, in the sense that it only occurs in a universe where a certain coordinate is special, if a dynamically similar but non-homogeneous universe has no such concept. I suppose what I am really saying is that the concept of expanding space stems from the Cosmological Principle alone, so it is not a "property of space", as the latter would presumably need to exist in a universe without a Cosmological Principle. That sounds like a more subtle type of coordinate-connected effect, if not a coordinate effect per se.

 

[turbo]

Metallicity in the early universe is not a difficult pill to swallow. Massive pop III stars were going nova all over the place in the first few hundred million years. Black holes [quasars] were also doing their part in churning out metallicity during that epoch. I do not view that as a serious argument agains the redshift - distance relationship.

There is no observed evolution in either absolute or relative metalliticities in SDSS quasars with redshift (or in any other observable property), which cannot be explained by any conventional cosmology. You should watch the presentation before you comment.

 

[Chronos]

An unsupported assumption, turbo. I have offered numerous papers discordant with your perceptions. Offering a rare paper favoring your unorthodox views is not compelling. Recent papers to consider:

Gamma-Ray Bursts, new cosmological beacons
Authors: V. Avila-Reese (1), C. Firmani (1,2), G. Ghisellini (2), J. I. Cabrera (1) ((1)IA-UNAM, Mexico; (2) INAF-OAB, Italy)
http://arxiv.org/abs/0802.2578

Metal-Poor Stars
Anna Frebel
http://arxiv.org/abs/0802.1924

This merely a slice of virtually daily submissions that do not support your assertions.

 

[Hurkyl]

Let's return to this comment, I have a question about it. I presume by "initially comoving", you mean they all see an isotropic CMB.

No, I meant approximately parallel worldlines. (Which, of course, only makes intrinsic sense when the cloud is sufficiently small)

 

[my_wan]

The "equivalence" of what? You have already said there is a convenience issue that is not equivalent. I've already agreed there is a formal equivalence.

Quote me where I claimed a convenience issue is not equivalent to a less convenient coordinate designation. My complaint was that you defined one of an infinite set of equivalent coordinate choices and called one of them a non-arbitrary artifact simply on the grounds of convenience.

I am saying that if there is a logical basis for taking one origin (the source of gravity, roughly), then one should expect a certain non-coincidental convenience to attach to that choice, making it non-arbitrary. I should think that would be noncontroversial enough, given how often we do it. If one has a logically based convenience that attaches to a certain choice, I do not call that an "artifact", I call it a logicaly based convenience-- as for comoving coordinates and the Cosmological Principle. What is controversial in that statement?

It is fundamentally no more or less logical than any other coordinate system. Harder does not make it less logical.

The way I would say that same thing is, the Cosmological Principle is all that is real, the convenience of comoving coordinates stem from that.

You designation of real and not real seems to be based on what taxes your imagination the least.

It would require entering into undemonstrable philosophy to claim prior knowledge of "what is reasonable". I wait for experiment to tell me that.

Which is why I said "it could be argued". Yet it was postulated long before there was any empirical basis for it.

Personally, not sure at all. One would only look for higher-order than dipole variations though, a la Occam's razor.

But if such is the case it is at least in principle empirically verifiable.

What I'm saying is that there are two ways to think of space. One is that space is really there, and the other is that we made it up in our minds to explain various observations. I realize that at some level everything we talk about is made up in our minds, but what I mean by "made up in our minds" is that we have no direct measurements that correspond to what we have made up-- it always requires some construction. Proper time, for example, is directly measurable between two causally connected events. Proper distance is not. So when dealing with distance, we need to invent indirect measures, like timing light. I admit that we need a concept of "infinitesmal distance" to calibrate a clock, but we don't have any way to measure cosmological distances that are not indirect, so require a model to interpret the measurement, and that is why there are so many different and essentially arbitrary distance measurements. I take that as a major clue that space is not "real" the way proper time is. It simply is not directly measurable, you measure something else and use it to infer a distance, so the latter concept is just invented.

So here you are designating proper time as real but proper distance is not? Why is it not just as valid to say space it real and time is an illusion of space? After all you can't measure time without counting up the motions of something through space. This appears to be at the heart of your objection to kdv's definition. In fact neither has any meaning without a definition of the other and choosing time as real/space imaginary is nothing more than a convenience that few people apparently need.

Admittedly we can't stretch a tape measure to the nearest star but the method does in fact exist to measure "proper" distance. Radar ranging achieves the same result, comoving coordinates have a much larger question mark as to how it relates to proper distance. In fact when tethered galaxy problems are considered radar ranging is often how proper distance is defined, as opposed to comoving distance. Proper and comoving distance has traditionally been considered equivalent but that is heavily debated today.

Am. J. Phys., 2003
http://arxiv.org/abs/astro-ph/0104349
accepted for publication in MNRAS
http://arxiv.org/abs/astro-ph/0610590
Physical Review D
http://arxiv.org/abs/gr-qc/0203074

Then we may talk about the symmetries as real-- not the "space". The usual concept of space is what I am talking about, not the replacement of that concept with more precise groups of symmetries. The latter I am fine with calling real-- as I said, we measure something else and invent a concept of space to make sense of it. Space is a theory.

Without space there is no symmetry to call real. The symmetry is that space and time vary inversely to each other. Saying time varies does not define a symmetry! Defining one real, one not, is no different from defining the motion of A real and the motion of B imaginary. Electric fields real and magnetic fields imaginary.

I am saying that "proper distance" is a made up concept, as it is by definition not directly measurable. It is also a theory. A useful one, like many theories are, but we made it up because we can't measure it. We can measure time.

Yes we can and do measure it. It is what a carpenters tape measures. To call "proper distance" a made up concept is denying the symmetry between space and time.

I am repeating the standard descriptions that I am criticizing as being only pictures-- how many times have you heard it stated that cosmological redshifts are "not due to relative motion"? Many, over here.

Yes but it has traditionally been considered an increase of proper distance that lacked inertial forces. How else would you describe the Universe as more dense in the past? Yet many of these assumptions are being challenged with increasingly sophisticated arguments.

That is the argument I am presenting, yes. A coordinate system where there is no relative motion is as arbitrary as one where there is, even if some real effect makes it more convenient.

So are you now conceding that convenience does not constitute a non-arbitrary choice?

That isn't proper distance, it's proper time in distance units. Is that all you mean by proper distance? Again, note it is not directly measurable, you measure a time and decide to multiply by c, for whatever reason. Even if you choose units where c=1, you are still measuring a time and calling it a distance.

There is no difference between proper distance and proper time in distance units! That is the very definition of the symmetry!

It sounds like you are claiming that if successive light pulses take longer to return, that means space is expanding. That doesn't sound very reasonable at all, given that pulses to the Voyager spacecraft also do that. My objection to kdv was never anything other than the claim that mean cosmological redshifts are categorically caused by the expansion of space. I said that this was a coordinate choice, and Hurkyl pointed out that when "expanding space" is used technically it actually means something different than what kdv seemed to mean (it really meant "gravitational redshift", in essence). Hence the only way to refute my objection is for kdv to say that Hurkyl's limited meaning was all he really intended, in which case I would say, that could have been made more clear.

No, if successive light pulses take longer to return then proper distance is increasing for whatever reason, expansion or no. kdv's definition was specifically stated to be defined as a change of proper distance. He made the traditional assumption that the Hubble flow increased the proper distance between masses comoving with it. The same assumption presumably needed for Big Bang cosmology.

You have claimed two things, it seems, one is that distance can be defined to be a light travel time (that sounds like a time to me, but you did call it an operational definition so I must accept it), and the other is that distance is the way to determine "how much space there is". Combining those claims suggests that space is expanding between us and the Voyager spacecraft -- after all, there's more space between us, the space must have expanded. Those are all the aspects of kdv's position that you have cited, are they not? What did I leave out?

I must cut you off now. In place of my quote defining "existing space" you have quoted me to say "how much space there is", even though I included, "All observers may not agree on how much but all observers will agree that there is some space between you and I, inversely identical to time", in my definition of "existing space". That's just plain ugly.

Yes I'm saying proper distance can be defined by measuring time, exactly the same way time is defined by measuring motion in space.

I agree that we certainly like to imagine that the one-way speed of light is a constant, so we do indeed make up a concept of distance that is inferred from light travel times. I do not think that tells us anything at all about "what is happening to space" in our solar system, other than the very useful picture that it is being compressed laterally and stretched radially as it is sucked into the Sun.

Must I really do the old one way argument? You still haven't satisfied yourself on the empirical legitimacy of the one way speed of light?

No, it isn't any kind of definition at all. It is a convenience stemming logically from a fact.

So now you say defining (x, y, z, t)=0 at the center of mass is not a definition.

The parallel you draw actually makes my argument. Had kdv said that the coordinate-generated concept of "existing space" was nothing but a convenience stemming logically from a fact (the Cosmological Principle), I would have agreed completely. Indeed, that is just what I said.

If he had said that then it would have been nothing more than cerebral diarrhea. Proper distance is a well defined concept that goes back to 1905 and plays a fundamental role in the derivation of Relativity. It even has its own mathematical notation. Proper distance can be defined for any observer by any observer simply by determining the space-time interval (an invariant). Why then do you need a similar term defined identically explicitly defined as an imaginary construct representing a fact that has something to do with the Cosmological Principle? Not once did the Cosmological Principle play any part in defining proper distance in relativity.

Except that it is only true if we adopt the axiom that the one-way speed of light is constant. But more to the point, you seem to be using the term "operational definition" to mean "coordinate choice". If the operational definition requires a certain (albeit elegant) coordinate choice, how are you distinguishing the coordinates from the definition? All I've said is that it is a coordinate choice, so if your argument will be that this is the same thing as an operational definition, you are simply creating a semantic identity.

Here's the one way speed of light again. I am forced to make presumptions about this debate I do not wish to. I'm done...

 

[Ken G]

No, I meant approximately parallel worldlines. (Which, of course, only makes intrinsic sense when the cloud is sufficiently small)

But how do you tell that the wordlines are parallel? Remember, the precision required to be meaningful in this context is the width of the cloud times H/c, is it not? That's the width of the cloud measured in units of 2 kiloparsecs.

 

[Hurkyl]

But how do you tell that the wordlines are parallel? Remember, the precision required to be meaningful in this context is the width of the cloud times H/c, is it not? That's the width of the cloud measured in units of 2 kiloparsecs.

I never claimed the experiment was a practical one!

 

[Ken G]

Quote me where I claimed a convenience issue is not equivalent to a less convenient coordinate designation.

I never claimed you even said that.

My complaint was that you defined one of an infinite set of equivalent coordinate choices and called one of them a non-arbitrary artifact simply on the grounds of convenience.

And I responded that grounds of convenience are indeed not arbitrary, they spring logically from some truth about the situation.

It is fundamentally no more or less logical than any other coordinate system. Harder does not make it less logical.

It certainly makes it less logical to use, which is the only sense to which I was applying the term.

You designation of real and not real seems to be based on what taxes your imagination the least.

On the contrary, my definition invokes my imagination not in the least. It is based on what can be directly measured-- I do not think science is fundamentally an exercise in imagination, we have other names for the latter.

Which is why I said "it could be argued". Yet it was postulated long before there was any empirical basis for it.

Many things were "postulated" before there was empirical evidence for it. Some of them later acquired that evidence, others were refuted. What conclusions does that lend to? That winners write the history?

But if such is the case it is at least in principle empirically verifiable.

And it has been looked for, very carefully, but has not been seen. Certainly we all bear reminding that the universe might be more complicated than we think, but I don't see any other particular value in the hypothesis.

So here you are designating proper time as real but proper distance is not? Why is it not just as valid to say space it real and time is an illusion of space? After all you can't measure time without counting up the motions of something through space.

I thought you'd ask that, because it is a good question. But it is not necessary to count the movement of something through space to use a clock-- it is not necessary to do anything but measure the frequency of it. It makes no difference at all if you conceptualize that as something "moving through space", that's the whole point-- you only have to be able to identify quartz when you find it. You have to be able to "name the atom" you are using for your clock, but you do not have to be able to measure distances, or even conceptualize them, except for a concept of "infinitesmal distance" that we may well need to be able to apply to our immediate environment to function. But in cosmology, distance is a construct that is not directly measurable. That's why there are so many versions-- so many choices of indirect measures.

This appears to be at the heart of your objection to kdv's definition. In fact neither has any meaning without a definition of the other and choosing time as real/space imaginary is nothing more than a convenience that few people apparently need.

I have established the difference, and why it is untrue that either needs the other. For a second difference, note that causality distinguishes the two, as does relativity. Causality distinguishes spacelike separations (constructs) from timelike separations (measurables). Relativity distinguishes them in that each observer perceives everything as happening in the same spatial point but at different temporal points. Things that happen at different spatial points at the same time are indirect constructs, which is why relativity requires a simultaneity postulate but not a "same location" postulate.

Admittedly we can't stretch a tape measure to the nearest star but the method does in fact exist to measure "proper" distance.

No, this is precisely the point. We cannot stretch that tape measure even in principle-- the events it connects would need to be causally connected so we can only measure proper time intervals between them.

Radar ranging achieves the same result, comoving coordinates have a much larger question mark as to how it relates to proper distance.

Radar ranging is a proper time measurement, once again.

In fact when tethered galaxy problems are considered radar ranging is often how proper distance is defined, as opposed to comoving distance.

I can call my foot a hand if I like, but a proper time measurement is a proper time measurement. Will you use radar ranging without having a clock? If not, you are doing a time measurement.

Proper and comoving distance has traditionally been considered equivalent but that is heavily debated today.

Am. J. Phys., 2003
http://arxiv.org/abs/astro-ph/0104349
accepted for publication in MNRAS
http://arxiv.org/abs/astro-ph/0610590
Physical Review D
http://arxiv.org/abs/gr-qc/0203074

Those should be interesting papers to consult, thank you. Of course, we must allow that a professional parlance often uses words that are literally imprecise but are given precision by their usage. I already mentioned that if by "distance" kdv really means "time" of flight, and by "space" he means "the construct we produce by timing light signals and imagining a model of space that includes a constant one-way speed of light", then we have no disagreement.

Without space there is no symmetry to call real. The symmetry is that space and time vary inversely to each other.

I would say the symmetry involves the relative proper times of different observers. Space is not a necessary construct to establish that symmetry.[/quote] Defining one real, one not, is no different from defining the motion of A real and the motion of B imaginary.[/quote]It isn't like that at all.

Electric fields real and magnetic fields imaginary.

That analogy also fails because, among other things, E dot B is an invariant, making it impossible to allow that only E or B is real. There is no analogous invariant with space and time (is there?). Also, E and B are part of a tensor, not a 4 vector. Note also that I did not say that the structure of spacetime is that of a 1-vector not a 4-vector, I said that spacetime is a construct.

Yes we can and do measure it. It is what a carpenters tape measures. To call "proper distance" a made up concept is denying the symmetry between space and time.

The carpenter's tape cannot be used to measure the distance between objects that are in relative motion, without also having a simultaneity convention. In short, the tape measures only its own length, and that only by definition-- attributing that to distances between events is part of the construct I am talking about, and leads to the "pole in barn" paradox. This is what I mean by pointing out that you cannot measure proper distance, only proper time.

Yes but it has traditionally been considered an increase of proper distance that lacked inertial forces. How else would you describe the Universe as more dense in the past?

It sounds like you are defining "motion" as an "increase in time of flight between objects that is traceable to inertial forces". This requires we know the history of a system to determine what is and what is not going to count as motion. But of course then the Earth is not moving, because an orbit does not involve "inertial forces". Also, the peculiar motions of astronomy, all generally due to gravity, are also not "motion". So the Voyager spacecraft is moving through space, but a distant star that is receding from us is not. I submit that will never become the standard meaning of the term "motion", because some nearby galaxies have blueshifts that cannot be traced to inertial forces.

Yet many of these assumptions are being challenged with increasingly sophisticated arguments.

It will be interesting to see what comes of that. We always need people looking at all possibilities.

So are you now conceding that convenience does not constitute a non-arbitrary choice?

I have maintained that all along. All I have claimed is that convenience does not determine causation, that reverses the proper logic. In other words, convenience does not establish reality, reality establishes what is convenient. We always choose the latter, but not the former.

There is no difference between proper distance and proper time in distance units! That is the very definition of the symmetry!

One does not define a symmetry, one measures something and uses it to unpack the symmetry. As I said, the symmetry is in the proper times measured by separate observers. Constructing a concept of distance-that-is-really-time-of-flight adds nothing to the symmetry, it is nothing but a convenient language we construct around the observations. That's what I have been saying, space is a model.

No, if successive light pulses take longer to return then proper distance is increasing for whatever reason, expansion or no. kdv's definition was specifically stated to be defined as a change of proper distance.

But what we was defining was the expansion of space, was he not? Was not our discussion all about the meaning of "expansion of space"? I can't recall there ever being a dispute about the meaning of "increasing distance", once a choice has been made of how we will construct our distance concept.

Yes I'm saying proper distance can be defined by measuring time, exactly the same way time is defined by measuring motion in space.

But time isn't measured that way, that is your construct. It is just measured by observing the action of a clock, without preconception of why the clock acts that way.

Must I really do the old one way argument? You still haven't satisfied yourself on the empirical legitimacy of the one way speed of light?

There is no empirical legitimacy of the one way speed of light, other than, of course, convenience. The discussion would simply parallel our current discussion of the meaning of "expanding space".

So now you say defining (x, y, z, t)=0 at the center of mass is not a definition.

Yes, it is a coordinatization.

Proper distance is a well defined concept that goes back to 1905 and plays a fundamental role in the derivation of Relativity.

It was always proper time. But the distinction was a small matter compared to a new theory.

Proper distance can be defined for any observer by any observer simply by determining the space-time interval (an invariant).

Using... a clock!

Why then do you need a similar term defined identically explicitly defined as an imaginary construct representing a fact that has something to do with the Cosmological Principle? Not once did the Cosmological Principle play any part in defining proper distance in relativity.

I realize that. As I said, it's a proper time.

 

[turbo]

An unsupported assumption, turbo. I have offered numerous papers discordant with your perceptions. Offering a rare paper favoring your unorthodox views is not compelling. <snip>

This merely a slice of virtually daily submissions that do not support your assertions.

A paper? I linked to a streaming video presentation given by Michael Strauss at the Space Telescope Science Institute. It would be a good idea for you to view the "paper" before dismissing it out-of-hand.

When theory cannot predict or explain observations the two most likely explanations are 1) the observations are improperly interpreted and 2) the theory is wrong. The hint that option 2) may be the answer brings out the knee-jerk reactions of outright rejection from those who wish to maintain status-quo in the face of new observations, and that is detrimental to science.

Before you trot out some more papers on theoretical models that try to allow current theory wriggle-room to avoid confrontation with observation, PLEASE watch the presentation and give it some thought. Strauss is the science spokesperson of the Sloan Digital Sky Survey and is one of our generation's pre-eminent observational astronomers. Along with Xiaohui Fan and other members of the SDSS team he has published dozens of papers about astronomical bodies at high redshift. These guys are not theorists - they are observational astronomers who are skilled at data-reduction and interpretation. Their results are compelling.

 

[my_wan]

That analogy also fails because, among other things, E dot B is an invariant, making it impossible to allow that only E or B is real. There is no analogous invariant with space and time (is there?).

Bolding mine. Yes there is, which is the point. It's called the space-time interval $$s^2 = c^2\Delta t^2 - \Delta r^2$$, which is an invariant. Note the time component $$\Delta t$$, and the spatial component $$\Delta r$$? These must be define in proper distance and proper time.

 

[Ken G]

I never claimed the experiment was a practical one!

I don't mean practical that it can be carried out-- it can be pure thought experiment. But it does have to have that precision to have meaning, that's all I mean. It's not a hypothetical question, I need an experimental description for understanding what a neighborhood of comoving test particles actually is.