Urban legends in authoritative astronomy

[Ken G]

Every now and then we come across a deeply ingrained falsehood or half-truth in places that otherwise seem authoritative, and we wonder how that came to be. I think they spread in a manner very similar to "urban legends", where each authority takes a previous authority as its basis, without independently checking the conclusion. When a slight variation is included each time, this process can quickly lead to outright falsehoods.

My favorite is the explanation often given for why high-mass main-sequence stars are so much more luminous than low-mass main-sequence stars. The typical story is that nuclear fusion is highly sensitive to temperature, and massive stars have higher temperature cores so that the pressure can support the greater mass of the star, all leading to the high luminosity. The logic of that causation is completely false, the sensitivity of nuclear fusion to temperature only tells you an estimate of the core temperature, the luminosity then follows from the least sensitive dependences on core temperature-- the overall force balance, which sets the stellar radius, which sets the luminosity, which refines the core temperature. The usual logic is exactly backwards-- things that are sensitive to temperature are not set by temperature, they set the temperature.

I can only imagine that the many authors and websites that give the false argument are taking it from each other. What other examples of particularly unfortunate effects of this process can people cite?

 

[Wallace]

1) Almost every time someone uses the phrase 'expansion of space' to describe the expanding universe
2) That The Big Bang implies all the material in the Universe was initial at one point in space that 'exploded'

Actually there are plenty of examples to do with the Big Bang and expansion of the Universe, they are just a couple of biggies. They get perpetuated all the time, often by people who should know better!

 

[Laura1013]

1) Almost every time someone uses the phrase 'expansion of space' to describe the expanding universe
2) That The Big Bang implies all the material in the Universe was initial at one point in space that 'exploded'

I was just going to say those - The Big Bang "explosion" is my favorite.

The size of the Universe. There's crazy confusion here if you try to look it up using Internet sources, some of which really should know better.

 

[Ken G]

Yeah, I second those, especially "expansion of space". I even see authoritative sources say that "new space is constantly filling in between the galaxies", as if space was a fluid with physical attributes that quantify its "amount". Perhaps it someday will be, but I haven't seen it so far-- and indeed, a lot of relativity seems predicated on that not being the case.

 

[Wallace]

Here's a pedantic point of nomenclature that I find somewhat annoying; that it came to be that the current value of H is dubbed 'the Hubble constant', when a better term would be something like 'the current value of the Hubble parameter'. Not really an 'urban legend' as such, but a confusing use of terminology none the less.

 

[DaveC426913]

I see a difference between the OP and the subsequent posts.

My impression (and correct me if I'm wrong) is that the first example seems to be one of a bit of knowledge held as dogma by otherwise very-informed people within the science.

The others seem to be more of the garden-variety, armchair amateurs with a simplistic knowledge of the science.

It's one thing if Joe the blacksmith thinks the world is flat, it is entirely another if Galileo himself declares that it's flat.


Or am I reading too much into this?

 

[kdv]

1) Almost every time someone uses the phrase 'expansion of space' to describe the expanding universe
!

Howo would you describe the expansion of the universe?

 

[turbo]

One glaring example of an urban legend in cosmology/astronomy is the notion that structure formed in a hierarchical manner since the BB and that the metallicity of astronomical objects evolves over time as generations of stars go supernova.

Unfortunately for these concepts, the inverse-square law for absolute magnitudes requires that the black holes powering the most distant quasars (z~6.5) be extraordinarily luminous, and according to conventional models they would have to consist of black holes of several billion solar masses cannibalizing host galaxies of at least a trillion solar masses. These observations turn the hierarchical model of structure formation on its head. How could such monsters have formed in only a few hundred million years? Also, if structures continue to accrete gravitationally, where are the later, closer analogs of these monsters? Nobody knows.

This has been pointed out by the SDSS team, and here is a nice presentation to the Space Telescope Science Institute by Michael Strauss, the SDSS science spokesperson.

http://www.stsci.edu/institute/itsd...ScienceColloquiaFall2005/MichaelStrauss110205

Other anomalies include the observation that the SDSS quasars from z~5.7-6.5 all exhibit Solar or super-Solar metallicities. How can this be if they reside at a distance corresponding to a BB age of ~800 M years? Where did they get the time to accrete materials from generation upon generation of supernovae to achieve these metallicities? Also, against all theories, there is no evolution in either the absolute nor relative metallicities with redshift.

Finally, it is expected that the great column densities of the highest-redshift quasars would greatly increase the odds that they would be lensed. Not a single one of the z>5.7 quasars is lensed. Watch Strauss' presentation with an open mind. He is not some wild-eyed radical, but is the science spokesperson for the most eminent observational astronomy consortium in existence.

There is some revolutionary stuff on the table, boys and girls. Perhaps some of the quandries can be resolved if we consider that quasars might not be at the cosmological distances implied by their redshifts, but that concession does not resolve all the problems. There are other underlying assumptions that need to be re-examined if we are to make sense of the SDSS data.

 

[Ken G]

Here's a pedantic point of nomenclature that I find somewhat annoying; that it came to be that the current value of H is dubbed 'the Hubble constant', when a better term would be something like 'the current value of the Hubble parameter'. Not really an 'urban legend' as such, but a confusing use of terminology none the less.

Yes, that leads to a lot of confusion, as does "dark matter".

 

[Ken G]

My impression (and correct me if I'm wrong) is that the first example seems to be one of a bit of knowledge held as dogma by otherwise very-informed people within the science.

The others seem to be more of the garden-variety, armchair amateurs with a simplistic knowledge of the science.

The difference I see is that the OP is a real misconception held by the authorities, while the others have more to do with misconceptions promoted by the authorities in the interest of "dumbing down" an actual understanding. But in some cases the authorities themselves may hold the misconception to some degree, it isn't clear. I would count it a "half-truth" instead of a "falcity", there's ambiguity there.

 

[Ken G]

Howo would you describe the expansion of the universe?

Just like that.

 

[Ken G]

How could such monsters have formed in only a few hundred million years? Also, if structures continue to accrete gravitationally, where are the later, closer analogs of these monsters?

Is it possible that this is related to the reason quasars "turn off"-- they deplete their "food supply"? In other words, the hierarchy might describe the potential for mass sources, but the actual mass sources can be depleted by previous history. That would be contrasted to the "trickle down" hierarchy of a turbulent cascade, which is in equilibrium so cannot run dry.

Other anomalies include the observation that the SDSS quasars from z~5.7-6.5 all exhibit Solar or super-Solar metallicities. How can this be if they reside at a distance corresponding to a BB age of ~800 M years? Where did they get the time to accrete materials from generation upon generation of supernovae to achieve these metallicities? Also, against all theories, there is no evolution in either the absolute nor relative metallicities with redshift.

Lost in the noise perhaps? Metallicity varies greatly from place to place. Also, it is hard to infer metallicities at high Z except in bright objects that might not track the prevailing conditions-- maybe the problem is that the theory is overgeneralized but not completely wrong?

Finally, it is expected that the great column densities of the highest-redshift quasars would greatly increase the odds that they would be lensed. Not a single one of the z>5.7 quasars is lensed.

Small-number statistics? Or is there really something fundamentally amiss here?

 

[DaveC426913]

The difference I see is that the OP is a real misconception held by the authorities, while the others have more to do with misconceptions promoted by the authorities in the interest of "dumbing down" an actual understanding.

Yes, well said. That's what I was trying to say, badly.

 

[Ken G]

Yes, well said. That's what I was trying to say, badly.

I understood what you meant-- and it's a valid point.

 

[ray b]

turbo
I see the quasar as a huge black hole forming
from two or more smaller but still massive black holes
as they orbit each other perhaps very close
they would move thru each others matter disks
and or disrupt the orbits of the disk matter

this would allow more matter to interact with the hole faster
then a non moving BH with infalling matter as that does not look like it would allow the massive show nore the jets
once there is only one massive BH the show is over

 

[Chronos]

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.

 

[kdv]

Just like that.

So if you were teaching cosmology and a student would ask "what do you mean by the universe is expanding"? you would say "I mean...that it is expanding." and not say anything else?

Do you agree that the proper distance between points in a FLRW universe is increasing with time? This must mean that space is inflating.

 

[Hurkyl]

Do you agree that the proper distance between points in a FLRW universe is increasing with time?

Out of curiousity, how are you making sense of an absolute notion of position? And how are you making sense of proper distance without specifying a path?

 

[Ken G]

So if you were teaching cosmology and a student would ask "what do you mean by the universe is expanding"? you would say "I mean...that it is expanding." and not say anything else?

No, I would say "the distances between galaxy clusters are increasing as the universe ages. There are many ways to picture how that happens, but the most elegant one is to imagine that space itself is expanding and carrying those clusters "along with it". This picture emerges from the way we choose to coordinatize cosmological space, which brings me to the Cosmological Principle...". Now, isn't that much better than telling students that space is raisin bread? That tends to replace old misconceptions with new ones.

Do you agree that the proper distance between points in a FLRW universe is increasing with time?

I'm not sure what you mean by proper distance (I see it as a proper time), but the various ways we use to measure distance do yield increases with proper time.

This must mean that space is inflating.

How do you figure that? There isn't a theory for the action of space.

 

[kdv]

Out of curiousity, how are you making sense of an absolute notion of position? And how are you making sense of proper distance without specifying a path?

I am not using any notion of absolute position. I am not sure which of my statement refers to an absolute notion of position. I am talking about relative distance between galaxies.

I mean the following: if you install a mirror on a remoet aglaxy (far enough so that it is carried away by the Hubble flow) and you send a beam of light to that mirror and wait until it comes back, don'tyou agree that it takes a longer time for the beam of light of come back to us as time passes (on our clock)? If I send two signals one day apart, they won't come back one day apart, no?

 

[kdv]

No, I would say "the distances between galaxy clusters are increasing as the universe ages. There are many ways to picture how that happens, but the most elegant one is to imagine that space itself is expanding and carrying those clusters "along with it". This picture emerges from the way we choose to coordinatize cosmological space, which brings me to the Cosmological Principle...".

fair enough. If I was a student, the sentence "...emerges from the way we choose to coordinatize space" would leave me with the impression that maybe the whole thing is not "real" and just an artifact due to a choice of coordinates. But that's probably just me.

Now, isn't that much better than telling students that space is raisin bread? That tends to replace old misconceptions with new ones.

I'm not sure what you mean by proper distance (I see it as a proper time), but the various ways we use to measure distance do yield increases with proper time..

At least we agree that there is an increase of distance with proper time.

(What I meant was that if you install a mirror on a remote galaxy (carried away by the Hubble flow) and you send two light signal to it one day apart, they will come back with a time delay superior to one day because the second light signal will have needed to travel a longer distance)).

we agree that the distance increases with proper time. And I think we agree that this is not because the galaxies are moving away in already existing space. So I don't see why we can't conclude that there is more space to travel through as time increases (unless you say that if something has to travel more distance it does not apply that it traveled through more space. Then it becomes an issue of terminology. I guess I take this as a definition: that if there is more distance between two points, there is more space between them.)

 

[my_wan]

At least we agree that there is an increase of distance with proper time.

I certainly wouldn't be very tolerate to anybody that demanded this wasn't so. Yet fundamentally how do we know this is the case? Is there any empirical differences in this and the Hubble gauge in Weyl geometry approach?

(What I meant was that if you install a mirror on a remote galaxy (carried away by the Hubble flow) and you send two light signal to it one day apart, they will come back with a time delay superior to one day because the second light signal will have needed to travel a longer distance)).

This would be a nice experiment but in no way can it be said that this has ever even in principle been done. The redshift alone cannot be construed as a round trip light signal in proper time. I give a simple thought experiment here to demonstrate the effect;
https://www.physicsforums.com/showthread.php?p=1647178#post1647178

I would agree that your interpretation is very likely. However, I will object to calling it factual so long as empirical determinate exist that we have yet to measure. If you think that some empirical measure has already determined it factually I would love to hear about it. I expect that empirical data will be available within a few years.

 

[Ken G]

I mean the following: if you install a mirror on a remoet aglaxy (far enough so that it is carried away by the Hubble flow) and you send a beam of light to that mirror and wait until it comes back, don'tyou agree that it takes a longer time for the beam of light of come back to us as time passes (on our clock)? If I send two signals one day apart, they won't come back one day apart, no?

So yes, you are talking about proper time, applied to a light signal. And yes, they will not come back one day apart. But the "reason" for that is coordinate dependent-- and one cannot state that "space is expanding" as if that was a physical principle. One can only say "in one particularly useful way of coordinatizing space, this is consistent with imagining that space itself is expanding." That is what I'd say, even to a nonscience person, because I don't believe that creating an illusion of understanding is a good replacement for not fully understanding.

 

[Ken G]

fair enough. If I was a student, the sentence "...emerges from the way we choose to coordinatize space" would leave me with the impression that maybe the whole thing is not "real" and just an artifact due to a choice of coordinates.

That is why I would use it as a springboard to address the Cosmological Principle, which is the only thing that makes it not purely an artifact of the coordinatization. If that still left you wondering, it would hopefully prompt you to ask, "does that mean it's all just an artifact of the choice of coordinates", which would then usher in the next key lesson about what are invariant observables and what are arbitrary concoctions in our minds. In short, it spurs the process of learning relativity and cosmology at the same time, rather than shutting off that process, the way saying "space is expanding and carrying everything with it" does.

And I think we agree that this is not because the galaxies are moving away in already existing space.

I'd have to stop you there-- what is "already existing space"? How do you construct a physical meaning for space? There isn't even a unique means to establish the distance between two events.

So I don't see why we can't conclude that there is more space to travel through as time increases (unless you say that if something has to travel more distance it does not apply that it traveled through more space. Then it becomes an issue of terminology. I guess I take this as a definition: that if there is more distance between two points, there is more space between them.)

But if you apply your definition to our galaxy and the LMC, you have a problem. You would clearly say that our galaxy and the LMC are "actually moving", but which one is the one that's moving? We can reference it to the CMB, but that's just a chosen coordinatization, that's what I'm talking about-- most people use "comoving frame coordinates", but relativity tells us that is not an absolute, it's just an arbitrary choice. The concept of "space" acts differently in different chosen coordinatizations (and I use that word instead of "reference frame" because I think of a reference frame as a local property of an observer, whereas a coordinatization is a global mathematical set of instructions for labelling events).

The invariants are things like the time on your clock it takes your light signal to get back-- but many different "pictures" may be used to correctly calculate that number. This is the core lesson of relativity, I would say: the best any observer can do is identify an equivalence class of descriptions for what is observed. Why would we teach that in relativity, and contradict it in our first breath of cosmology?

 

[my_wan]

That is why I would use it as a springboard to address the Cosmological Principle, which is the only thing that makes it not purely an artifact of the coordinatization.

Even if we accept Cosmological Principle at face value it is nonetheless an arbitrary inertial frame we simply chose for convenience of parameterization. Defining our solar system as having a sun in the middle is also a convenience of parameterization. To then call that more than a convenience, whether solar system or Universe, "not purely an artifact of the coordinatization" flies in the face of everything we know about relativity.

If that still left you wondering, it would hopefully prompt you to ask, "does that mean it's all just an artifact of the choice of coordinates", which would then usher in the next key lesson about what are invariant observables and what are arbitrary concoctions in our minds. In short, it spurs the process of learning relativity and cosmology at the same time, rather than shutting off that process, the way saying "space is expanding and carrying everything with it" does.

Yes ask yourself if you have subverted your own question, "does that mean it's all just an artifact of the choice of coordinates", by some assumption about the Cosmological Principle. Convenient yes, absolute or more than an artifact of the parameterization no.

I'd have to stop you there-- what is "already existing space"? How do you construct a physical meaning for space? There isn't even a unique means to establish the distance between two events.

In general no, with respect to a particular observer yes. In infamous words it is what we measure. This is essentially the operational definition kdv defined soon after. I therefore fail to see your objection to kdv using "already existing space" in reference to something he denies can be characterized that way.

But if you apply your definition to our galaxy and the LMC, you have a problem. You would clearly say that our galaxy and the LMC are "actually moving", but which one is the one that's moving? We can reference it to the CMB, but that's just a chosen coordination, that's what I'm talking about-- most people use "comoving frame coordinates", but relativity tells us that is not an absolute, it's just an arbitrary choice. The concept of "space" acts differently in different chosen coordinatizations (and I use that word instead of "reference frame" because I think of a reference frame as a local property of an observer, whereas a coordinatization is a global mathematical set of instructions for labelling events).

Now you are taking issue with "actually moving", yet the very definition of "actually moving" is to move wrt something, in this case one of the galaxies which he clearly stated. Would not every observer in the Universe agree that the galaxies are moving wrt each other?

The invariants are things like the time on your clock it takes your light signal to get back-- but many different "pictures" may be used to correctly calculate that number. This is the core lesson of relativity, I would say: the best any observer can do is identify an equivalence class of descriptions for what is observed. Why would we teach that in relativity, and contradict it in our first breath of cosmology?

So the invariant you speak of here assumes the very operational definition that you denied kdv. Except he used space whereas you used its inverse, time.

ETA: Removed a modifier "not".

 

[Ken G]

The redshift alone cannot be construed as a round trip light signal in proper time. I give a simple thought experiment here to demonstrate the effect;
https://www.physicsforums.com/showthread.php?p=1647178#post1647178

I have a question about this thought experiment. Is it not true that if the mass is "added to the sphere" by drawing it in in a spherically symmetric way, that it will only affect the time dilation to outside observers who the drawn in matter passes? As for the crucial two observers inside the flat spacetime, would the potential in there not already be affected by that mass, so drawing it into the sphere would have no effect? It seems to me you need the mass of the sphere to increase sort of magically out of nowhere, and that violates other principles. To me it basically sounds like putting a region where the laws of physics don't apply outside the region where they do. That's not necessarily wrong but it's contrived, which is the basic problem with "tired light" in the first place. Or another way to say this, GR is a differential theory that requires boundary assumptions does it not? So if one is modifying the boundary assumptions in some arbitrary way, new results become possible, but what evidence do we need that these results are physical?

 

[Ken G]

Even if we accept Cosmological Principle at face value it is nonetheless an arbitrary inertial frame we simply chose for convenience of parameterization.

That is true, but the point is, the Cosmological Principle is what makes it convenient. I did not say the Cosmological Principle made it an absolute frame of reference, I said it rendered it the elegance of the "expanding space" coordinatization. We are not disagreeing.

Defining our solar system as having a sun in the middle is also a convenience of parameterization. To then call that more than a convenience, whether solar system or Universe, "not purely an artifact of the coordinatization" flies in the face of everything we know about relativity.

Now we have a slight disagreement. I agree that we may use any coordinatization of the solar system we like, and that will control the language we choose ("the sun rose" versus "the Earth turned", etc.). But that does not mean that every result we obtain from a given choice is "purely an artifact". That the Sun is the source of gravity that rules the solar system is known, so placing the source of the gravity at the center of the effects of that gravity is not "pure artifact", it has a logical basis. Hence certain simplifications that subsequently arise stem directly from that logical basis, they are not coincidences. Similarly with the comoving frame choice, it is not sheer coincidence that this achieves the simplifications it does-- it stems from the Cosmological Principle, a real principle not an artifact of our choices. It is that sense that I say the "expanding space" picture is not purely an artifact of our choices, it is a result of our choices coupled with a physically real aspect of the universe (apparently). Still, I think we do agree on the crucial issue-- the words we use to describe "what is happening" are deeply affected by choices we make, and should not be confused with statements of some mind-independent reality.

Yes ask yourself if you have subverted your own question, "does that mean it's all just an artifact of the choice of coordinates", by some assumption about the Cosmological Principle. Convenient yes, absolute or more than an artifact of the parameterization no.

Again I argue that the convenience itself is real, as it stems from an observed property our universe did not need to have-- so is not purely an "artifact."

In general no, with respect to a particular observer yes. In infamous words it is what we measure.

But even distance measurements come in many forms, and they are not all proportional to each other. Which defines the "real" space, even just for that one observer?

This is essentially the operational definition kdv defined soon after. I therefore fail to see your objection to kdv using "already existing space" in reference to something he denies can be characterized that way.

The objection stems from the arbitrariness in the concept of globally integrated distance. Surely if there was "already existing space" for some observer, it would admit to some special means for measuring its distances, a means that reflected this meaning of "space" in some non-arbitrary way.

Now you are taking issue with "actually moving", yet the very definition of "actually moving" is to move wrt something, in this case one of the galaxies which he clearly stated. Would not every observer in the Universe agree that the galaxies are moving wrt each other?

Not when those galaxies are cosmologically separated, no. Indeed, the normal explanation of cosmological redshifts is that they are from galaxies that are not moving relative to each other (unless you refer to peculiar motions, but we agreed those are not absolute and they can use the CMB-- they don't need any other galaxies to get a relative motion).

So the invariant you speak of here assumes the very operational definition that you denied kdv. Except he used space whereas you used its inverse, time.

You seem to be missing what it was that I "denied" kdv. I denied no invariants, that would be foolish. I denied that the invariants prescribe a unique meaning to the concept of quantity of "existing space".

 

[Hurkyl]

One can only say "in one particularly useful way of coordinatizing space, this is consistent with imagining that space itself is expanding." That is what I'd say, even to a nonscience person, because I don't believe that creating an illusion of understanding is a good replacement for not fully understanding.

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.

 

[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.

That is an extremely important point, but can you not attribute that to tidal gravitational effects leading to a proper-time dependence in the metric? I don't see why I have to attribute physical attributes to space to understand why that volume changes. In other words, the volume is an observable, but I still need a coordinatization to distinguish what is happening to my coordinates versus motion through those coordinates.

Futhermore, it seems to me a more natural way to attribute a change in the metric is a change in the measuring standards, not something happening to "space" (this reminds me of how we set our clocks ahead in daylight savings time instead of simply doing everything an hour earlier-- we are forever forcing the universe to conform to our standards, instead of just allowing our standards to be dynamical. One might argue that it is the purpose of a standard to be fixed, and that is certainly a valid perspective, but I think it lends to an ethnocentrist view that perpetuates at ever more subtle levels the errors of the ancient geocentrists.)

Personally, I view space as a completely make-believe concept, with no observable attributes other than as a placekeeper for time-of-flight. If so, it would help constrain the possible ways that dark energy could act.

 

[Ken G]

Perhaps we should branch the discussion on expanding space to the cosmology section. Is there anyone who thinks that the role of core temperature in making massive stars more luminous is not an "urban legend", or who has other examples of "urban legends" that have become engrained in otherwise authoritative astronomy sources?

 

[my_wan]

I have a question about this thought experiment. Is it not true that if the mass is "added to the sphere" by drawing it in in a spherically symmetric way, that it will only affect the time dilation to outside observers who the drawn in matter passes? As for the crucial two observers inside the flat spacetime, would the potential in there not already be affected by that mass, so drawing it into the sphere would have no effect? It seems to me you need the mass of the sphere to increase sort of magically out of nowhere, and that violates other principles. To me it basically sounds like putting a region where the laws of physics don't apply outside the region where they do. That's not necessarily wrong but it's contrived, which is the basic problem with "tired light" in the first place. Or another way to say this, GR is a differential theory that requires boundary assumptions does it not? So if one is modifying the boundary assumptions in some arbitrary way, new results become possible, but what evidence do we need that these results are physical?

The thought experiment was rather simplistic. However, I don't see how the particular objection here is more than a contrivance. I didn't imagine the mass source as symmetrically waiting in another mass sphere outside the first but so what if it was, the effect is the same. The depth of field inside a symmetric hollow sphere is always the same as the surface of the sphere. If you compress a sphere into a smaller radius the surface gravity increases, increasing depth of field at the surface. You could do the same thought experiment without adding mass, just shrink the radius of the sphere. So here I need no extra source of mass, much less a magical one, even though the objection lacked validity in the original form from redistribution of distant asymmetrical masses. You speak as though X amount of mass dictates an X depth of field regardless of the proximity of mass.

The "tired light" hypothesis in most forms has empirical difficulties that go well beyond a simple complaint of contrived. If the complaint of contrived was legitimate grounds for falsification no science would be safe. I did not modify the boundary "assumptions" I simply defined the boundary "conditions".

You asked, "GR is a differential theory that requires boundary assumptions does it not?" That question is just plain weird. GR is a differential theory that predicts certain boundary values given a set of boundary conditions, i.e. constraints. In this case the distribution of masses. Why can't I distribute mass in any arbitrary way I choose. Is there something unphysical about about a large beach ball? I didn't even need anything unusual, like negative mass for warp drives etc., just plain old everyday baryonic mass.

 

[Hurkyl]

Ken G: The point is, to the best of my knowledge, that is (one expression of) the technical definition of the technical phrase "space is expanding". It is coordinate-free, and what physicists mean when they use the term.

 

[Ken G]

The thought experiment was rather simplistic. However, I don't see how the particular objection here is more than a contrivance. I didn't imagine the mass source as symmetrically waiting in another mass sphere outside the first but so what if it was, the effect is the same.

That is precisely what I am not convinced of, but lack the GR ability to find the appropriate fully consistent dynamical solution.

Note The depth of field inside a symmetric hollow sphere is always the same as the surface of the sphere. If you compress a sphere into a smaller radius the surface gravity increases, increasing depth of field at the surface. You could do the same thought experiment without adding mass, just shrink the radius of the sphere.

Both of these versions appear to be applying Newtonian concepts to a GR problem. You might be able to get away with that in a time steady situation, but your situation is only meaningful if there is explicit time dependence, so I think you need to do a time-dependent GR solution to prove this point. Subtle things may happen when you include a fully self-consistent treatment of the motion and energy implicit in this model. For example, the signal of the mass distribution changes could not propagate faster than c, so you could not be affected by anything happening to mass too far away to see. Thus would we not be able to see the effects of such a change in universal mass distribution? There might be even more fundamental problems, leading to theorems that the thought experiment violates. It is an interesting experiment, but I don't find it meaningful for doing GR. I really only know one thing about GR, which is that you don't know anything until you've actually found the solution to the equations.

You speak as though X amount of mass dictates an X depth of field regardless of the proximity of mass.

I am well aware of how potentials are calculated, and I also know that they are of limited use in time dependent GR problems.

If the complaint of contrived was legitimate grounds for falsification no science would be safe.

But then there's always Occam, which is used not for falsification but for prioritization. After all, we could all be "hooked up to the Matrix" for all we know-- why is that not an example of your "complaint of contrived"?

I did not modify the boundary "assumptions" I simply defined the boundary "conditions".

What you did not do is general relativity.

Why can't I distribute mass in any arbitrary way I choose.

You did more than distribute it-- you put it in motion, but failed to include the laws of dynamics. I can only assume you are modifying the boundary conditions in an aphysical and ad hoc way to achieve a preconceived result, when in fact a "natural" theory is desired.

 

[Ken G]

Ken G: The point is, to the best of my knowledge, that is (one expression of) the technical definition of the technical phrase "space is expanding". It is coordinate-free, and what physicists mean when they use the term.

But maybe they shouldn't, especially when communicating with non-technicians. I agree one should understand what is meant when a phrase is invoked, but a separate discussion may ensue on the appropriateness of the terms. If physicists go around saying "space is expanding" meaning that test particles diverge, will the general public not get the idea that "space really is expanding", meaning that the redshifts are caused by expanding space? Whereas I would argue that whatever it is that causes those masses to diverge is what causes the redshifts, and "expanding space" is nothing but a particular picture that can be used as a stand-in label for whatever that cause really is. Mistaking stand-in labels for real physical phenomenon is what I am decrying, because it happens quite a lot and leads to misconceptions and many forum questions. That is in turn because, all too often, people apply the stand-in labels as if they had literal (i.e., not technical) meaning.

I also think that explaining the difference makes an excellent entry point for striving for a more complete understanding, rather than a literal recipe for replacing understanding. I know that all educators must choose how far down the path they wish to tread, versus when do they want to stop and content themselves with conveying an illusory understanding that can get similarly-worded questions right on the final exam. But don't we feel a bit uncomfortable using words that clearly suggest that space is a ponderable medium that serves as a causal agent for conveying material along with its own motion? Isn't there some inverted logic in that description?

 

[my_wan]

Now we have a slight disagreement. I agree that we may use any coordinatization of the solar system we like, and that will control the language we choose ("the sun rose" versus "the Earth turned", etc.). But that does not mean that every result we obtain from a given choice is "purely an artifact". That the Sun is the source of gravity that rules the solar system is known, so placing the source of the gravity at the center of the effects of that gravity is not "pure artifact", it has a logical basis. Hence certain simplifications that subsequently arise stem directly from that logical basis, they are not coincidences. Similarly with the comoving frame choice, it is not sheer coincidence that this achieves the simplifications it does-- it stems from the Cosmological Principle, a real principle not an artifact of our choices. It is that sense that I say the "expanding space" picture is not purely an artifact of our choices, it is a result of our choices coupled with a physically real aspect of the universe (apparently). Still, I think we do agree on the crucial issue-- the words we use to describe "what is happening" are deeply affected by choices we make, and should not be confused with statements of some mind-independent reality.

You say, "But that does not mean that every result we obtain from a given choice is "purely an artifact"". 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 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.

Again I argue that the convenience itself is real, as it stems from an observed property our universe did not need to have-- so is not purely an "artifact."

The convenience is all that is real about it. It could actually be argued that the Cosmological principle is needed under most reasonable assumptions about the Universe. In fact the hardest thing to explain theoretically would be a failure of the Cosmological principle. 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. I did a quick and dirty thought experiment here;
https://www.physicsforums.com/showthread.php?t=221068
I don't take it serious but I will investigate.

But even distance measurements come in many forms, and they are not all proportional to each other. Which defines the "real" space, even just for that one observer?

Yes including an infinite number of purely arbitrary ones so long as transforms were defined to correspond to what we measure. They are not proportional in the sense of quantity but neither do they conflict with each other. They are proportional in that a general transform can be defined to translate between them. If by "real" you mean "absolute" we both know better. 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.

The objection stems from the arbitrariness in the concept of globally integrated distance. Surely if there was "already existing space" for some observer, it would admit to some special means for measuring its distances, a means that reflected this meaning of "space" in some non-arbitrary way.

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.

Not when those galaxies are cosmologically separated, no. Indeed, the normal explanation of cosmological redshifts is that they are from galaxies that are not moving relative to each other (unless you refer to peculiar motions, but we agreed those are not absolute and they can use the CMB-- they don't need any other galaxies to get a relative motion).

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

Here you seem to be referring to comoving coordinates to say "not moving relative to each other". Again this is an arbitrary coordinate system of convenience and presumably disagrees but does not conflict with the notion of proper distance. 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. You would have a case on pure technicality if kdv hadn't provided an operation definition that happen to be consistent with proper distance.

You seem to be missing what it was that I "denied" kdv. I denied no invariants, that would be foolish. I denied that the invariants prescribe a unique meaning to the concept of quantity of "existing space".

Yet he did provide an operational definition. His operational definition exactly matched your operational definition of the "time on your clock it takes your light signal to get back". In fact your definition was simply the inverse, by definition. His definition of "existing space" is simply the inverse of your definition of time.

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

 

[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.