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Nereid
2010-Jul-06, 04:01 PM
I'm starting this thread in response to OT comments in another thread, in a different part of BAUT.

It's not that I don't think the answer to the question is anything other than a resounding YES!

Nor is it that I cannot provide an answer.

Rather that there seems to be a lot of confusion and misunderstanding on this topic, and that I hope a thread like this will go a long way to clearing that confusion up, and reducing the misunderstanding.

To be clear: I'm referring to astronomical observations, or 'things in the sky'.

Ken G
2010-Jul-06, 04:11 PM
There is no direct diagnostic that distinguishes them in a given observation, so it is only in the context of a larger model that distinctions can be indirectly inferred. Nevertheless, that model might have a great deal of independent observational support, so you might feel on a firm footing while disentangling local gravitational redshifts from global cosmological redshifts. (The reason I put in "local" and "global" is that to me, a cosmological redshift is a form of gravitational redshift, but the common parlance is leave implicit the local/global distinction and reserve gravitational redshift for local gravitational deviations from the cosmological principle, rather than the gravitational consequences of the expansion obeying the cosmological principle, which gets called cosmological redshifts.)

Having said that, I will still always be a wee bit uncomfortable making sweeping distinctions between gravitational and cosmological redshifts, as long as we have the significant unknowns of dark matter and dark energy in the mix. I can agree that the modern standard model is our best model to date, and may continue to be supported, but I also allow the possibility that we are doing something wrong at the moment. If significant flaws in the cosmological principle are found, or if our understanding of gravitational redshifts requires significant modification, we could be led down the primrose path.

speedfreek
2010-Jul-06, 05:36 PM
The simplistic way I understand it, cosmological redshift can be considered as a form of gravitational redshift, if we consider the universe to have been denser in the past.

astromark
2010-Jul-06, 09:43 PM
No, its not a simple mater of observation... When I see a bluish tinted star I can not say with any assurance that it is moving closer to us.

Some stars are emitting a blueish hue because they are emitting a blueish hue... Being super hot.
Just as the opposite applies. Red stars are not always red shifted... some are red.
Only by observation can a case for red shifted be built. Direct observation of changing position can be shown to be proof of movement.

Ken G
2010-Jul-06, 09:49 PM
No, its not a simple mater of observation... When I see a bluish tinted star I can not say with any assurance that it is moving closer to us.

Some stars are emitting a blueish hue because they are emitting a blueish hue... Being super hot.
That's not really the same thing-- blue-hot stars have a bluer continuum, but all the lines are in the places they always are. Cosmological and gravitational redshifts refer to redshifting of the lines, not the continuum, as the latter is ambiguous but the former is more clear.

Nereid
2010-Jul-06, 10:03 PM
You're right, Ken G and speedfreak, the two are one and the same, from many perspectives. I should have spelled out what I meant more clearly, in the OP.

By gravitational redshift I mean what the Pound-Rebka experiment (http://en.wikipedia.org/wiki/Pound%E2%80%93Rebka_experiment) measured, what recent Hubble Space Telescope observations (http://hubblesite.org/newscenter/archive/releases/2005/36/) estimated for Sirius B, and what x-ray observations of AGNs (http://www.astro.umd.edu/~chris/Research/X-ray_studies_of_AGN/x-ray_studies_of_agn.html) have inferred (and so on).

By cosmological redshift I mean the redshift in the Hubble redshift-distance relationship (http://astro.berkeley.edu/~mwhite/darkmatter/hubble.html).

publius
2010-Jul-06, 11:38 PM
Such distinctions are problematic. Consider a two co-accelerating observers in a Rindler Frame, and a third, inertial observer, free-falling from the POV of the Rindler observers.

The two Rindler observers, seperated by a constant distance in their own frame, start sending light signals back and forth. The higher observer sees a redshift, and the lower observer sees a blueshift. They can call this gravitational, as the metric in their coordinates has such terms in it, conceptually the same as Schwarzschild or any other.

But from the POV of the inertial, "free faller" there is no shift at all. That observer sees the signals getting progressively redshifted as the other two accelerate away from him, of course, but once a light pulse is emitted, it suffers no red or blue shift as it travels through space. It is emitted at the frequency it is emitted at and stays that way. The reason for the perceived red and blue shifting is because of what happens to the clocks and rulers of the accelerating observers as they accelerate faster in the light travel meantime.

So what is happening there? What kind of shift do you call that, if any at all? Space-time there is completely flat, and if you adopt the view that "gravity" is only invariant space-time curvature (tidal gravity), then you wouldn't call it gravitational at all. But the Rindler observers (and Einstein) would, in their coordinates, be liable to call it gravitational.

And via the Equivalence Principle, we know that will be same situation for a similiar trio of observers in the Earth's gravitational field, so long as the invariant curvature is not significant over the distances and times of the experiment. A Pound-Rebka experiment would be very nearly the same for Rindler observers as those on earth.

Everybody agrees that one observer measures a different frequency than the one who emitted it, but the reason why is not so invariant, is it?


-Richard

astromark
2010-Jul-06, 11:50 PM
I'm starting this thread in response to OT comments in another thread, in a different part of BAUT.

It's not that I don't think the answer to the question is anything other than a resounding YES!

Nor is it that I cannot provide an answer.

Rather that there seems to be a lot of confusion and misunderstanding on this topic, and that I hope a thread like this will go a long way to clearing that confusion up, and reducing the misunderstanding.

To be clear: I'm referring to astronomical observations, or 'things in the sky'.

As you must see... some confusion prevails; Why ? ...
because that is what looks like a question or at the least ' A point of interesting contention' which is why I quite deliberately answered the question as it was written... Yet am now told thats not what this is about...and you wonder why we get confused ? Please explain ?

Nereid
2010-Jul-07, 12:02 AM
Such distinctions are problematic. Consider a two co-accelerating observers in a Rindler Frame, and a third, inertial observer, free-falling from the POV of the Rindler observers.

The two Rindler observers, seperated by a constant distance in their own frame, start sending light signals back and forth. The higher observer sees a redshift, and the lower observer sees a blueshift. They can call this gravitational, as the metric in their coordinates has such terms in it, conceptually the same as Schwarzschild or any other.

But from the POV of the inertial, "free faller" there is no shift at all. That observer sees the signals getting progressively redshifted as the other two accelerate away from him, of course, but once a light pulse is emitted, it suffers no red or blue shift as it travels through space. It is emitted at the frequency it is emitted at and stays that way. The reason for the perceived red and blue shifting is because of what happens to the clocks and rulers of the accelerating observers as they accelerate faster in the light travel meantime.

So what is happening there? What kind of shift do you call that, if any at all? Space-time there is completely flat, and if you adopt the view that "gravity" is only invariant space-time curvature (tidal gravity), then you wouldn't call it gravitational at all. But the Rindler observers (and Einstein) would, in their coordinates, be liable to call it gravitational.

And via the Equivalence Principle, we know that will be same situation for a similiar trio of observers in the Earth's gravitational field, so long as the invariant curvature is not significant over the distances and times of the experiment. A Pound-Rebka experiment would be very nearly the same for Rindler observers as those on earth.

Everybody agrees that one observer measures a different frequency than the one who emitted it, but the reason why is not so invariant, is it?


-Richard
Thanks publius.

I'm not sure how this is relevant to my question, which is about redshifts observed (or inferred) by astronomers on the surface of the Earth (or aboard the Hubble, XMM-Newton, etc; virtually, of course); or, in the case of Pound-Rebka, either at the top of a clock tower, on the surface of the Earth, or at its bottom.

Can you clarify please?

Nereid
2010-Jul-07, 12:07 AM
As you must see... some confusion prevails; Why ? ...
because that is what looks like a question or at the least ' A point of interesting contention' which is why I quite deliberately answered the question as it was written... Yet am now told thats not what this is about...and you wonder why we get confused ? Please explain ?
I don't follow this at all astromark.

Your earlier post seemed, to me, to have nothing to do with redshifts, which I assumed (perhaps incorrectly) is something well understood by the BAUTians I expected to respond.

How did you interpret "redshift", may I ask?

And FWIW, this thread is going very nicely indeed ... it is bringing out all manner of assumptions/interpretations/etc, all of which is well worth discussing!

Ken G
2010-Jul-07, 12:07 AM
Everybody agrees that one observer measures a different frequency than the one who emitted it, but the reason why is not so invariant, is it?
This is the key point, and it really gets to the heart of what physics is for. Physics is really about two things, it's about making predictions that work at some desired level of accuracy, and it's about having a framework for understanding why those predictions "make sense" in terms of unifying principles and symmetries. The predictions are pretty objective (though the accuracy target is subjective), but the "understanding" part is much more subjective, and nonunique. We have many ways to "skin the cat", conceptually, and oftentimes the different ways appear more naturally when viewed in different coordinatizations.

A coordinatization in physics is an approach for generating numbers on the way to the final prediction, and those intermediaries are not unique to that prediction, but they tend to create a different language for saying "what happened." A classic example is if a witness to a bank robbery says the robber entered from the left and crossed to the right, it is clear this is not a statement of "what happened", it is only an interpretation of what happened based on a particular coordinatization (and a particular point of view, which is related). Yet on the surface it can sound like a completely objective description of the events-- it takes a moment of thought to recognize the arbitrariness. That is even more true in relativity, when we talk about "how far things are" or "how long ago they happened", or even whether we are seeing "gravitational redshift." As such, the distinction is closely associated with the cosmological principle, which is why the language here is inherently cosmological, rather than being related to the local laws of physics themselves.

I would be tempted to classify any redshift that requires the equations of general relativity to correctly interpret to be gravitational redshifts, regardless of whether they are cosmological or local to the environment of the source, but I realize that the common lexicon (and what Nereid means) is that gravitational redshifts are due to the local peculiarities of either the source or the observer, and cosmological redshifts are those that appear even when the sources and observers have no peculiarities in their local environment (be it a blazing rocket engine, or a nearby black hole). Hence the distinctions are closely related to the cosmological principle, and for this reason, the relevant language is the language of cosmology, rather than anything that appears in the local laws of physics themselves.

tommac
2010-Jul-07, 12:07 AM
The simplistic way I understand it, cosmological redshift can be considered as a form of gravitational redshift, if we consider the universe to have been denser in the past.

Umm ... does this connote that the universe may not be expanding but just getting less massive?
OR
In other words ... how much of the redshift is from us being in a deeper gravitational well in the past(GR) vs galaxies moving away from us ( SR )

Nereid
2010-Jul-07, 12:08 AM
Umm ... does this connote that the universe may not be expanding but just getting less massive?
OR
In other words ... how much of the redshift is from us being in a deeper gravitational well in the past(GR) vs galaxies moving away from us ( SR )
Not at all relevant to the question in the OP, tommac; perhaps you'd like to start a new Q&A thread on it?

Nereid
2010-Jul-07, 12:11 AM
This is the key point, and it really gets to the heart of what physics is for. Physics is really about two things, it's about making predictions that work at some desired level of accuracy, and it's about having a framework for understanding why those predictions "make sense" in terms of unifying principles and symmetries. The predictions are pretty objective (though the accuracy target is subjective), but the "understanding" part is much more subjective, and nonunique. We have many ways to "skin the cat", conceptually, and oftentimes the different ways appear more naturally when viewed in different coordinatizations.

A coordinatization in physics is an approach for generating numbers on the way to the final prediction, and those intermediaries are not unique to that prediction, but they tend to create a different language for saying "what happened." A classic example is if a witness to a bank robbery says the robber entered from the left and crossed to the right, it is clear this is not a statement of "what happened", it is only an interpretation of what happened based on a particular coordinatization (and a particular point of view, which is related). Yet on the surface it can sound like a completely objective description of the events-- it takes a moment of thought to recognize the arbitrariness. That is even more true in relativity, when we talk about "how far things are" or "how long ago they happened", or even whether we are seeing "gravitational redshift."

I would be tempted to classify any redshift that requires the equations of general relativity to correctly interpret to be gravitational redshifts, regardless of whether they are cosmological or local to the environment of the source, but I realize that the common lexicon (and what Nereid means) is that gravitational redshifts are due to the local peculiarities of either the source or the observer, and cosmological redshifts are those that appear even when the sources and observers have no peculiarities in their local environment (be it a blazing rocket engine, or a nearby black hole).
All good points, but also irrelevant to the question I'm actually asking (which is, as you state, about the terms as actually used, in observational astrophysics).

Is my clarification, in post #6, unclear in any way?

Ken G
2010-Jul-07, 12:14 AM
All good points, but also irrelevant to the question I'm actually asking (which is, as you state, about the terms as actually used, in observational astrophysics).

Is my clarification, in post #6, unclear in any way?
No, your clarification makes it perfectly clear what you are using those terms to mean, but it is the artificiality of the distinction that is what is related to the OP question. Understanding the artificial nature is the first step in understanding why the answer is so model-dependent, rather than being something you can constrain just from the observations alone. You need a consistent logical framework (which in turn suggests, or connects with, a particular coordinate choice) to distinguish those sources of redshift, and if you later discover your conclusions were wrong, there need not be anything wrong with the observations themselves-- you may simply decide a different coordinate choice serves you better. That could happen, for example, if the cosmological principle is found wanting.

Luckmeister
2010-Jul-07, 12:40 AM
As you must see... some confusion prevails; Why ? ...
because that is what looks like a question or at the least ' A point of interesting contention' which is why I quite deliberately answered the question as it was written... Yet am now told thats not what this is about...and you wonder why we get confused ? Please explain ?

What it seems is not clear to you from the OP is that "observation" pertains to spectroscopic observation, not visual (just through a telescope) observation.

Mike

George
2010-Jul-07, 12:42 AM
You're right, Ken G and speedfreak, the two are one and the same, from many perspectives. I should have spelled out what I meant more clearly, in the OP.

By gravitational redshift I mean what the Pound-Rebka experiment (http://en.wikipedia.org/wiki/Pound%E2%80%93Rebka_experiment) measured, what recent Hubble Space Telescope observations (http://hubblesite.org/newscenter/archive/releases/2005/36/) estimated for Sirius B, and what x-ray observations of AGNs (http://www.astro.umd.edu/~chris/Research/X-ray_studies_of_AGN/x-ray_studies_of_agn.html) have inferred (and so on).

By cosmological redshift I mean the redshift in the Hubble redshift-distance relationship (http://astro.berkeley.edu/~mwhite/darkmatter/hubble.html).
I don't get all the physics in these examples but I think I get enough to better understand your question, though the other's answers will probably not change.

If you had a dynamic object moving around in a gravitational field, you might observe an occasional oddness to the spectral lines due to the special moments when the combination of Doppler shift and Gravitational shift dance together as demonstrated in the Pound-Rebka experiment. I would assume this would not be noticeable in the Sirius binary case, however, though you have two dynamic objects in a gravity well, and also dancing. :)

[I would give me about a 30% chance I'm "spot on".]

publius
2010-Jul-07, 12:59 AM
Thanks publius.

I'm not sure how this is relevant to my question, which is about redshifts observed (or inferred) by astronomers on the surface of the Earth (or aboard the Hubble, XMM-Newton, etc; virtually, of course); or, in the case of Pound-Rebka, either at the top of a clock tower, on the surface of the Earth, or at its bottom.

Can you clarify please?

Ken pretty much answered for me in his later posts. The point I was making is when we make distinctions between gravitational and cosmological redshift, that language is wedded to a particular coordinate system. In this universe we can say light is emitted at event A and received at event B with a perceived redshift. That much is invariant. Adopting a certain coordinate system, the standard comoving coordinates, we can say that x part of that redshift is "cosmological" and y part is "gravitational" and perhaps z part is "doppler", etc, etc. But in different coordinate systems, the balance of x, y and z will be different, and maybe not there at all (my Rindler example was one of a "not there at all" being a pretty plausible one).

My view is terms like cosmological and gravitational redshifts are not invariants and it's important to appreciate this because it will certainly modify how we ask the questions to begin with, before we even worry about the answers.

And for the record for any lurkers, we have a good theory that predicts that what the observations of A and B events will be to high accuracy -- that is the invariant, and on that the theory will live or die. But let's not get confused on what the theory is really about and the sideshow of how it resolves through the various coordinate lenses we observe through.


-Richard

Ken G
2010-Jul-07, 01:07 AM
And for the record for any lurkers, we have a good theory that predicts that what the observations of A and B events will be to high accuracy -- that is the invariant, and on that the theory will live or die. But let's not get confused on what the theory is really about and the sideshow of how it resolves through the various coordinate lenses we observe through. Though I don't claim to understand GR better than Richard, I'd say that's well put. Perhaps if we understood better the larger context that stimulated the OP question, we'd see why it's important to make that distinction.

speedfreek
2010-Jul-07, 05:13 PM
Umm ... does this connote that the universe may not be expanding but just getting less massive?
OR
In other words ... how much of the redshift is from us being in a deeper gravitational well in the past(GR) vs galaxies moving away from us ( SR )

In the context of this topic - i.e. cosmological redshift - the galaxies "moving" away from us are not (SR). It is all GR.

We can have a decreasing density as a result of expansion, but there are other ways where the density might decrease. For instance, what if the universe were static, but the galaxies were shrinking? What if it were the size of the matter that was changing, rather than the size of the background metric? The gravitational influence over cosmological distances would decrease in the same way, and the cosmological redshift would then obviously be due to the difference in the gravitational density of the universe when a galaxy emitted the light we see, when compared with the gravitational density of the universe as we see that light.

I say "what if", but as far as I know, the two "coordinatizations" are essentially equivalent and it is simply a choice we make when we decide to express the situation as an expansion of the distance between things, rather than the other way round. The same seems to be true of the examples that Nereid gave in post #6, if I have grokked the situation properly.

Jeff Root
2010-Jul-08, 01:45 AM
Nereid, Luckmeister,

Regarding Mark's post #4, I believe that he misunderstood what Nereid
meant by "distinguish", and interpreted it as simply "observe". He was
saying that it is not sufficient to just observe the color of a light to be
able to tell whether it is redshifted.

:-)

Mark,

As you undoubtedly know by now, from the additional posts by Ken and
others, Nereid was asking whether it is easy to distinguish gravitational
redshift from cosmological redshift, when light that reaches us from a
source at cosmological distance is observed to be redshifted.

-- Jeff, in Minneapolis

Nereid
2010-Jul-08, 06:37 PM
Start with the region of the electromagnetic spectrum between ~ the Lyman limit and ~mid-IR.

Observe the sky, and exclude solar system objects*.

The sky is populated by point sources and extended objects; there are also large regions whose boundaries are ill-defined, so we can't really call them objects, let's say 'diffuse emission' (or absorption).

With increasing angular resolution, many extended objects turn out to be great swarms of point sources.

The extended sources can be put into classes, based on the distribution of light within them: galaxies (elliptical, spiral, irregular), planetary nebulae, etc.

Take spectra of tens of millions of these; nearly all have lines, or bands (or both) in their spectra, emission, absorption, or both. Based on the spectra, point sources are either stars or quasars (disjoint classes); galaxies are either early-type or late-type (a cline, not a bimodal distribution); nebulae other than planetary nebulae are supernova remnants, HII emission nebulae, molecular clouds, etc.

Do a very great deal of work, over a century or more, and estimate the distance to all the (classes of) objects.

Develop a model to explain the distribution of these objects: stars and the various kinds of nebulae and clouds are found in galaxies or globular clusters (some exceptions); quasars are galaxies.

From their spectra, estimate the redshift; all objects for which redshifts can be reliably determined have lines (or bands) which are consistent with just a single redshift, or a small range in redshift - except (some) quasars.

Plot the redshifts of objects beyond our Local Group - galaxies, quasars, globular clusters, planetary nebulae (outside a galaxy), supernovae - against distance; an astonishingly clear trend emerges. Where galaxies etc seem to be organised into clusters, take the mean value of the redshifts of all cluster members and plot that as a single point; leave lone galaxies and small groups alone. The trend is even more clear. Give it a name, the Hubble redshift-distance relationship.

From high resolution spectra, estimate the surface gravity of all stars; estimate their masses (using various techniques); estimate their radii (using various techniques). For the few stars for which it is possible, estimate the difference between the predicted redshift due to motion towards (or away) from us and the observed redshift. Plot this difference against an appropriate mass-density-volume variable. Does a clear trend emerge?

Extend observations to the x-ray and gamma wavebands, and to the far-IR, microwave, and radio ones (we can also look at the EUV, but it's a desert, almost nothing to see except a bright, pretty uniform, haze).

Any new classes of objects? Yes, a few, radio lobes for example, and IR 'cirrus', and filamentary loops, and pulsars, and GRBs (more?). Lots of new stars, galaxies, quasars, and various nebulae and clouds, but they're not new classes of object. There's also an amazing all-sky uniform glow, which has an astonishing spectrum; easiest seen in the microwave region.

This is the observational background that is available for us to use when we try to decide whether a particular redshift is gravitational or cosmological.

Any questions, before my next post? The next post will examines how to decide - gravitational? or cosmological? (redshift, that is).

FYI, SDSS has some nice template spectra, of various common object classes, here (http://www.sdss.org/dr5/algorithms/spectemplates/index.html).

* easy to do, even for the zodiacal light; anyone not know how?

speedfreek
2010-Jul-08, 08:03 PM
From high resolution spectra, estimate the surface gravity of all stars; estimate their masses (using various techniques); estimate their radii (using various techniques). For the few stars for which it is possible, estimate the difference between the predicted redshift due to motion towards (or away) from us and the observed redshift. Plot this difference against an appropriate mass-density-volume variable. Does a clear trend emerge?
The greater the mass to radius ratio of a star, the larger the difference between the measured redshift (or blueshift) of that star and the predicted shift due to any peculiar motion of that star?

astromark
2010-Jul-08, 08:48 PM
Yes, Jeff in Minneapolis. That is now well clear.. I was making a point. Real red may not be actually red at all...

Mark in the South Pacific.

Strange
2010-Jul-08, 08:50 PM
Not at all relevant to the question in the OP, tommac; perhaps you'd like to start a new Q&A thread on it?

I think there is one already: http://www.bautforum.com/showthread.php/105658-Gravitational-redshifting-proposed-as-an-explanation-of-cosmological-redshifts

Ken G
2010-Jul-08, 09:53 PM
This is the observational background that is available for us to use when we try to decide whether a particular redshift is gravitational or cosmological.And that's also where the key distinction comes in, when it is asserted that one can distinguish observationally between two types of redshift-- the distinction between direct and indirect evidence. This is of course an artificial distinction, as all evidence is to some degree indirect, yet some data requires placing into a wider inferential milieu to reach conclusions about it, and other data "speaks for itself" to a much larger degree. In the former case, it's hard to say whether we are really "observationally" distinguishing what we think we are distinguishing-- it's more an issue of observations fed into a self-consistent framework that seems to be working for us, but may yet surprise us.

The reason I feel that distinction is worth making is it seems to me there are two kinds of conclusions one can reach from observations:

1) direct conclusions: these are conclusions that seem to emerge inescapably from the data, and for these to be wrong, there would have to be something wrong about the observation itself, or what information that observation was thought to contain. It's an error that is specific to that type of observation, like the error Hubble first made when using Cepeids to get the distance to other galaxies, since he didn't realize there was more than one type of Cepheid.

2) indirect conclusions: these are conclusions that we reach only because we favor some wider theoretical framework, and for these to be wrong, there might be nothing at all wrong with the observation, it might be the seemingly self-consistent inferential milieu that it was fed into. A classic example of this is the smallness of stellar parallax as the Earth orbits the Sun over several months. That is a true observation, even the ancient Greeks could put limits on how small that parallax is, and when they fed that observation into the larger inferential milieu that was the geocentric model, it made perfect sense. So when humanity later found that Kepler's planetary orbits worked much better, what was found to be wrong with the parallax observations? Nothing, those observations were fine-- the problem was with the inferential milieu that had rejected the possibility that stars might just be so staggeringly far away that their parallax could indeed be that tiny.

So the question I would pose is, to anyone who says that a set of redshift observations allows us to observationally distinguish between cosmological and gravitational redshift, by feeding those same observations into a larger inferential framework (like the cosmological principle), what if it later turns out that the partition they established is not supported by future work? What would need to be wrong about that set of redshift observations? If the answer is nothing would need to be wrong with the observations, it could all be the inferential milieu (like relaxing the cosmological principle), then we cannot actually claim we observationally distinguished those things in the first place, regardless of whether or not we are privy to potential flaws in our inferential milieu-- we must already admit that the distinguishing is being done theoretically, and can be undone theoretically just as easily. Such theoretical undoing, when it happens, can happen virtually overnight, whereas what we can legitimately claim is observationally determined is much harder to undo.

Whether or not this objection is really relevant to the overall point in the OP probably depends on what general context that point is intended for, which is as yet unclear, so I don't know how important what I'm saying here is-- I just do feel it is always useful to keep track of what we could easily change, and what would be very hard to change. A keener sense of that would have saved the Catholic church a big headache come Galileo's day, and we certainly don't want to put ourselves in that position.

tommac
2010-Jul-09, 03:32 AM
So the question I would pose is, to anyone who says that a set of redshift observations allows us to observationally distinguish between cosmological and gravitational redshift, by feeding those same observations into a larger inferential framework (like the cosmological principle), what if it later turns out that the partition they established is not supported by future work? What would need to be wrong about that set of redshift observations? If the answer is nothing would need to be wrong with the observations, it could all be the inferential milieu (like relaxing the cosmological principle), then we cannot actually claim we observationally distinguished those things in the first place, regardless of whether or not we are privy to potential flaws in our inferential milieu-- we must already admit that the distinguishing is being done theoretically, and can be undone theoretically just as easily. Such theoretical undoing, when it happens, can happen virtually overnight, whereas what we can legitimately claim is observationally determined is much harder to undo.



Wow ... this is a great post!

Staticman
2010-Jul-09, 09:03 AM
Nereid, your question is really odd, unless you tell us the source of your confusion here, or as Ken G says, clarify the context of your question, it is really difficult to answer. Without a model or theoretical frame, observations may be easy or difficult to distinguish depending on practical matters or technical considerations about the specific observation one is making, but this is really trivial so I don't think your question is about that, as I guess you coudl answer that yourself. As Ken and Publius are saying, observations have to be put in a theoretical context, in a specific coordinates, etc, to get to some useful interpretation. Hope by now you understand this.

Nereid
2010-Jul-09, 01:56 PM
From high resolution spectra, estimate the surface gravity of all stars; estimate their masses (using various techniques); estimate their radii (using various techniques). For the few stars for which it is possible, estimate the difference between the predicted redshift due to motion towards (or away) from us and the observed redshift. Plot this difference against an appropriate mass-density-volume variable. Does a clear trend emerge?The greater the mass to radius ratio of a star, the larger the difference between the measured redshift (or blueshift) of that star and the predicted shift due to any peculiar motion of that star?
This part of my post is rather vague - partly by design - but it's also rather sloppy.

I'll write an update/clarification later.

Nereid
2010-Jul-09, 02:02 PM
Nereid, your question is really odd, unless you tell us the source of your confusion here, or as Ken G says, clarify the context of your question, it is really difficult to answer. Without a model or theoretical frame, observations may be easy or difficult to distinguish depending on practical matters or technical considerations about the specific observation one is making, but this is really trivial so I don't think your question is about that, as I guess you coudl answer that yourself. As Ken and Publius are saying, observations have to be put in a theoretical context, in a specific coordinates, etc, to get to some useful interpretation. Hope by now you understand this.
Part of the clarification was in post #6, where I explained what I was referring to re 'cosmological redshift' and 'gravitational redshift'. I also explained that the only observer I am considering is one on the Earth, or in some semi-stable orbit around it or the Sun (think the HST, or XMM-Newton, or Planck).

As I fully expected, Ken G has a great post, in response to my previous post.

This thread is developing very nicely, from my POV! :)