Talk:Redshift/Archive 9

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[edit] History of Redshift

The idea of redshift was first recorded in 1783 by John Michell:

"Let us now suppose the particles of light to be attracted in the same manner as all other bodies with which we are acquainted; that is, by forces bearing the same proportion to their vis inertia, of which there can be no reasonable doubt, gravitation being, as far as we know, or have any reason to believe, a universal law of nature. Upon this supposition then, if any one of the fixed stars, whose density was known by the above-mentioned means, should be large enough sensibly to affect the velocity of light issuing from it, we should have the means of knowing its real magnitude [emphasis added]."

See Philosophical Transactions of the Royal Society of London, November 27, 1783.

--Thangalin 14:44, 25 November 2006 (PST)

[edit] frequency: ν or f

f for frequency was changed in ν. That may be more common in physics books, but f is also in use. IMO, f is much clearer for non-specialists, while ν is easily confused with v for velocity. Harald88 01:16, 5 August 2006 (UTC)

Agreed, as long as we mention that f = frequency. --Iantresman 10:03, 6 August 2006 (UTC)

[edit] Edits to the headings

I've changed History to a level 1 heading (I think this was left as a level 2 heading from a previous edit) and I've also removed the word redshift from some of the headings where it's obvious that we're talking about redshift (eg. History of redshift -> History). I'm leaving a note here since there is one change I'm not so sure about: Quantifying and interpreting redshift: z -> Quantification and interpretation: z. If I've done wrong, please feel free to revert. Cheers, darkliight[πalk] 14:17, 18 August 2006 (UTC)

[edit] Cosmological redshift

The cosmological redshift is not a Doppler effect, but is caused by the stretching of space. It is analagous to the Dopler effect. The redshift caused by motion through space (eg of stars) is a Doppler effect (not just analogous). See Gravitation, by Misner, Thorne & Wheeler.

Sorry I am new to this game and forgot to sign the above. Charlie T, 19 August 06

This is correct, but that's exactly what the article says at this time. --ScienceApologist 09:11, 19 August 2006 (UTC)

[edit] Doppler redshift of source

The text notes that "A single photon propagated through a vacuum can redshift in several distinct ways". Doesn't a moving source appears to be a different mechanism since it requires a physical source in addition to said photon, and the redshift occur BEFORE said photon leaves the source surface and enters the hypothetical vacuum? --Iantresman 11:47, 28 August 2006 (UTC)

According to Galilean relativity, there is no "source" nor "observer" needed because you cannot distinguish between a moving source and a moving observer, therefore the source and observer are just place-holders in the equations and are not relevant to the mechanism. The definition of a reference frame abstraction is simply a change in perspective -- something which can occur while the photon is sailing through the vacuum of spacetime. --ScienceApologist 11:59, 28 August 2006 (UTC)
If you can't distinguish between a moving source or observer, then how can you infer a redshift? Perhaps there is a confusion in the text between "mechanism" and "transformation"; the former causes the latter. In practice, a moving sources CAUSES a redshift? --Iantresman 13:29, 28 August 2006 (UTC)
There is physically no way to determine whether a source is moving or whether an observer is moving. Trying to distinguish between the two is artificial to physics -- a weak form of the equivalence principle. The transformation is the mechanism. --ScienceApologist 13:40, 29 August 2006 (UTC)
  • So we can't determine whether stars and galaxies are moving away from us? --Iantresman 17:57, 29 August 2006 (UTC)
  • Only by triangulation averaging and assuming a stable spacetime metric.--ScienceApologist 19:57, 29 August 2006 (UTC)
  • It's just that I thought that the article tells us that we can calculate the radial velocity of an object from its redshift.
  • It occurs to me that "mechanism" might be the wrong word. The transformations are surely descriptions. The mechanism for the Doppler shift is surely the velocity of a source or observer; the mechanism of the cosmological redshift is surely the expansion of the universe. The transformation describe the effects? --Iantresman 22:18, 29 August 2006 (UTC)
This seems to be a matter of semantics rather than substance. I had an earlier discussion with User:Serjeant about the difference between cause and description and we never really resolved the issue. Mechanisms and descriptions to me are basically the same since science ultimately only answers the questions of "how" and not "why". --ScienceApologist 00:24, 30 August 2006 (UTC)
  • To me, and I should think most other readers, a mechanism should related to a real world analogy. The siren on an ambulance does not change tone due to the mechanism of "frame transformation"; it changes tone because the truck is moving. Likewise, the optical redshift of stars is CAUSED by the mechanism of stars moving. Mathematically and conceptually, this can also be described as a frame transformation. --Iantresman 08:30, 30 August 2006 (UTC)
You say "poh-TAY-toh", I say "poh-TAH-toh". The siren on an ambulance does change tone due to the mechanism of a frame transformation. In fact, such a physical description is more accurate that relying on the nebulous "truck is moving" observation. The CAUSE is the fact that the siren is in a different frame of reference from the observer. A stationary siren with a moving observer would therefore result in the same phenomenon. --ScienceApologist 13:28, 30 August 2006 (UTC)

[edit] Redshift transformations

I can find no refernces that describe redshift mechanisms:

  1. as a group of only three (or four), that are...
  2. due to Galilean, Lorentz, and general relativistic transformations, and...
  3. applying only to single photons in a vacuum.

Sure I can find references that describe a redshift as a certain transformation, but nothing which groups all three/four redshifts with all three transformations, and to single photons in a vacuum. This leads me to suggest that:

  1. transformations are not synonymous with mechanisms
  2. transformations are not necessarily the only type of mechanism
  3. that this summary of mechanism is highly selective and contrived.
  4. "single photons in a vacuum" is not a necessarirequirement of redshift mechanism.

--Iantresman 10:05, 30 August 2006 (UTC)

I would suggest you read some of the introductory physics and astronomy texts suggested. By the way, the "single photon" application is an idealization, not a requirement. You can read previous discussions about this. --ScienceApologist 13:29, 30 August 2006 (UTC)
  • OK, so give me a citation... not to any old textbook, but one which specifically describes redshift in the terms described above. --Iantresman 14:03, 30 August 2006 (UTC)
  • You can try any of the textbooks referenced in the article. --ScienceApologist 13:10, 31 August 2006 (UTC)
  • And if the "single photon" is an idealisation, then we should say so? --Iantresman 14:05, 30 August 2006 (UTC)
  • No, because it is an accurate statement. (Just like saying "in the absence of air resistance, all objects experience the same acceleration due to gravity"). --ScienceApologist 13:10, 31 August 2006 (UTC)

I've tried various textbooks and journals, and can find no citation that describes redshift as it is described in the article, and summarised above. --Iantresman 14:13, 31 August 2006 (UTC)

Look at, for example, Peebles, Principles of Physical Cosmology Chapter 5, where the first three are discussed in detail. Although, he doesn't explicitly call the first a galilean transformation -- since that derivation can be found in almost any introductory physics text book. Salsb 14:51, 31 August 2006 (UTC)
OK, I've had a look in the book and could find:
  • Nothing on Redshift mechanisms, (so there does not appear to be a "link" to transformations)
  • Nothing on photons in a vacuum
  • One mention of photons moving in free space, in relation to tired light (p.225)
So while I have no problems with the transformations per se, there does not appear to be a suggestion that these are mechanisms, nor any of the other points that I summarised above. --Iantresman 15:30, 31 August 2006 (UTC)
I am afraid you haven't read carefully then. The chapter, pages 94-99, specifically discusses doppler effects, and shows you specifically the transformations required to obtain the first two in detail, although you have to do some math to transform them precisely into the form in the table. Furthermore, the discussion and mathematics is specifically in the context of particles in vacuum. Salsb 15:42, 31 August 2006 (UTC)
  • OK, I have read through them, and am quite happy with the Doppler redshift being described as a Galilean transformation. But I can find nothing which describes the transformation as being the mechanism.
  • It seems to me that motion of the source or observer is the mechanism; the transformation is the description.
  • Indeed, all I find elsewhere are references that suggest that the "Doppler effect" is the mechanism,[1] [2]. And the Doppler effect is described as being caused by the motion of the source of observer.
  • Again, I can find no references that suggest that the redshift mechanisms are due to any transformation --Iantresman 16:02, 31 August 2006 (UTC)

Mechanistic descriptions are what physics does. There is no distinction in physics between mechanism and description because, to paraphrase Feynmann, physics asks "How?" and not "Why?" For example, the Doppler Effect is described fully by the Galilean transformation which means that the two are synonymous for waveforms. --ScienceApologist 16:10, 31 August 2006 (UTC)

  • So why do ALL the citation that I have found, tell me that one of the redshift mechanisms is due to the Doppler effect, and NO-ONE has been able to find a citation supporting the description that one of the redshift mechanisms is due to a Galilean transformation, or any other transformation? Again, I am not disputing that the Doppler redshift is described by a Galilean transformation.
  • Even if the description is technically correct, why should the Wiki article on Redshift be the ONLY ARTICLE ON THE PLANET, to describe redshift mechanisms in this way? --Iantresman 17:54, 31 August 2006 (UTC)
We've been through this already. Take it up with dispute resolution if you want. Protracted circular conversations like this should be avoided. --ScienceApologist 17:58, 31 August 2006 (UTC)
It's not circular. I've asked for a citation supporting the description, and no-one can provide one. The only conclusion is that the article statement is not verifiable. --Iantresman 18:47, 31 August 2006 (UTC)
Valid point - it's not hard to understand why it's hard to find such an unphysical description in the literature. Most literature distinguishes between mathematics (transformations) and physics (Doppler effect). Harald88 21:52, 31 August 2006 (UTC)
Untrue. Go ahead and support your claim with a citation that says that transformations are distinct from the Doppler Effect: or more precisely that the mathematical description of such a phenomena is "unphysical". The citations are provided, the naysayers have no evidence to back-up their claims. This discussion is over until the disputants can make a coherent, verifiable case for themselves. --ScienceApologist 14:22, 1 September 2006 (UTC)
Please don't put things on their heads. What is criticized on this Talk page without support from literature has to go. Simple as that. Harald88 22:38, 1 September 2006 (UTC)
  • The "disputants" could not be clearer; The statements in the article are not backed up by ANY citations. It is yourself that has no evidence. To DEMOMSTRATE that this is incorrect, all you have to do is provide a veriable quotation that describes redshift mechanisms in terms of transformation.
  • I will happily provide some verifiable quotes which unamibiguious describe redshift mechanisms in verifiable terms. --Iantresman 14:48, 1 September 2006 (UTC)
"This discussion is over." What kind of language is that? Just who the f*** do you think you are? Jon 10:11, 4 September 2006 (UTC)

[edit] Verifiable quotes on "redshift mechanisms"

No citations have been provide describing redshift mechanisms as frame transformations; this can be remedied by providing a single attributable quote containing the words "redshift", "mechanism" and "transformation" and any of "Galilean", "Lorentz", or "general relativistic".

I offer the following quotes describing redshift mechanisms, as described in the literature (emphases mine):

  • "Modern physics knows two experimentally verified mechanisms which produce redshifts that are independent of wavelength... the Doppler effect ... the Gravitational redshift. Besides these empirical and well understood redshift mechanisms, there is the theoretical phenomenon associated with the expansion of space. [...] Thus three redshift mechanisms act in nature." -- Discovery of Cosmic Fractals] (2002) by Yurij Baryshev, Pekka Teerikorpi
  • "For historical reasons, small redshifts are often quoted in velocity units: the radial velocity required to produce the observed shift strictly from the Doppler mechanism due to relative motions." -- "The Road to Galaxy Formation" (2002) by William C Keel
  • "Attempts to interpret the red shift by a gravitational mechanism ... efforts have been made to interpret the red shift by a Doppler mechanism", The Red Shift: A Different Mechanism (1972) Urbanovich, S. I.

--Iantresman 16:26, 1 September 2006 (UTC)

[edit] note for InvictaHOG

About all the fact tags you lefd thruout, don't the links fulfil the need for citations? Or, why must they be in the same page? -lysdexia 20:54, 23 September 2006 (UTC)

No. Otherwise we would not have a referencing system. A person who wants to verify a statement should be directed to either the page of a book or a scientific paper with that conclusion. A link is not sufficient. This may be basic physics knowledge, but most people do not have basic physics knowledge. I'm not sure I understand your second question about the same page. InvictaHOG 21:05, 23 September 2006 (UTC)

[edit] I still don't get it.

I'm happy to see that "recession" and "streching" are no longer called "completely different", but it is still not clear to me if they are different at all. To claim that this question is "clearly illustrated" by the Expanding Rubber Sheet Universe smacks of absolute Newtonian space. The statements toward the end of the section make it sound like the two points of view are related by a coordinate transformation and thus mathematically and physically equivalent, but the sentences before that suggest that it can't be recession because that would require relative velocities greater than c. That sounds to me like someone is using Galilean addition of velocities, obviously out of place here. ScienceApologist now brings in a new argument, the cosmological principle, but I still don't see why "real" recessional velocities would have to violate the CP, or indeed if there is any conceivable observation that could distinguish between the two. If I have trouble understanding this distinction, then I'm sure the average reader does, too. Is there any way to make this section more accurate and/or more understandable? Is it really one way or the other, or are they just two ways of looking at the same phenomenon? --Art Carlson 11:35, 6 October 2006 (UTC)

Thanks for pointing this out. Let's try to get an explanation in there that makes sense. Technically speaking, the expansion of space is all that is left if you assume both the Copernican Principle and the Cosmological Principle's claim of an isotropic universe. The easiest proof of this is to consider an equilateral triangle (ABC) with the observer at the apex (A). The redshifts of the two orther vertices (B and C) would be observed, if they were physically receding, to have the same redshift (z) for Hubble's Law to hold. However, if you considered what an observer at B would be observing, even though such an observer is the same distance from A and C, they would measure a redshift of (1+z)=1+v/2c-v^2/2c^2-v^3/4c^3+... for object B while measuring the redshift z for object A which does not correspond to the same redshift. Thus we have a contradiction and the proposition that we can describe a universe with isotropic, linearly increasing redshifts due entirely to physical velocities must be false. Traditionally, in cosmology classes the proof is done in the other direction where you assume an isotropic expansion and prove that the Hubble Law is linear. In a static three-dimensional space where there is no metric expansion it is simply impossible to observe an isotropic recession in all directions for all positions in space. (Caveat: this assumes that the universe is not infinite due to Obler's Paradox. If you have an infinite universe, there is a way to get an isotropic "explosion" of sorts that corresponds to an isotropic recessional velocity in all directions from all perspectives, but this assumes that general relativity is incorrect because such a universe would have a non-converging mass and a curvature that would warp the linear nature of the expansion. There are forms of Milne universes that propose such a Lorentzian (not GR-based) perpsective of an expanding universe. In fact, there is one cranky editor that keeps trying to insist that the Milne universe hasn't been disproven because of this -- even though this would require abandoning GR).
So the question is (assuming that Art Carlson understands my above explanation), how do we illustrate this to the reader? Please help!
--ScienceApologist 12:29, 6 October 2006 (UTC)
By the way, spacetime is absolute and insamuch as any observer in spacetime (save one travelling at the speed of light) can theoretically isolate space-like manifolds in Lorentzian spacetime, the view of "absolute Newtonian space" is not quite so problematic. So long as we remind ourselves that the rubber sheet's rulers are subject to modification depending on our choice of inertial frame, we still end up with the same situation (only with expansions, velocities, and the sizes of objects changed accordingly). --ScienceApologist 12:42, 6 October 2006 (UTC)
I beg to differ. All you have proven is that the equilaterality of a triangle is not an invariant in special relativity. On the contrary, if any observer sees a linear Hubble relationship, then all observers do. Imagine the first observer extrapolates all the motions back in time. He will find one time in which all the observered objects are on top of him. But such an event - simultaneous in time and co-located in spce - must be the same for all observers. But that can only be the case for other observers if they also see a linear Hubble relationship. (But you might have a point about the homogeneity. I'm not certain that all observers can see a uniform density.) --Art Carlson 20:06, 8 October 2006 (UTC)
I don't think so. Length contraction doesn't affect the sides of the triangle, only the rulers that are carried by the objects that are moving. Distances are equal, velocities are unequal, therefore Hubble's Law is contradicted. --ScienceApologist 00:34, 9 October 2006 (UTC)
Think again. I admit that my knowledge of cosmology and general relativity is rudimentary, but we are talking about special relativity here. A measures the distances AB, AC, and BC to be all equal. If you boost from the frame of A to the frame of B, the distance AB, which is parallel to the boost velocity, experiences the full Lorentz contraction, while the distances AC and BC, which are at an angle to the boost velocity, will be contracted less. Therefore, for B, C is farther way than A, which would correspond to a higher redshift. (Besides, it might help if you would look at the problem a different way, such as the one I suggested.) --Art Carlson 08:55, 9 October 2006 (UTC)
So the situation is even worse because B sees C has having a smaller redshift than A (roughly 1/2 in the low-velocity limit). --ScienceApologist 12:15, 9 October 2006 (UTC)
That's all you've got to say? No derivation of your formula? No critique of my argument? No verifiable citation? Looks like we need to throw out the claim that there is "a distinction between redshifts caused by the velocity of the objects and the redshifts associated with the expanding universe". --Art Carlson 07:40, 11 October 2006 (UTC)
Isn't it a case that there is a theoretical distinction, but no practical difference; a redshift is redshift, and there is no distinction between a cosmological, gravitational, Doppler, or (gasp) Wolf shift. I still don't from the article, how I work out from a measured redshift of a distant galaxy, the proportion due to each redshift mechanism... without making some assumptions.
I think I'm getting there, slowly. I still don't like Joshua's equilateral triangles. In addition to the fact that the observers are measuring with different rulers in different directions, the statement "ABC is an equilateral triangle" means "ABC is an equilateral triangle right now", but the three observers do not even agree what "now" means (simultaneity). I can't find any problem with my argument, so I firmly believe if one observer sees a Hubble relationship, then they all do. What I have realized is that the universe will not look the same to all observers in other respects. If an observer in the center of the explosion sees a uniform density of galaxies out to the limits of observation at v = c, then an observer near the edge, although he would still see a Hubble relationship with galaxies out to the limit, he would see an inhomogeneous universe. Looking toward the center of the explosion, he would see a higher density than looking outward. The argument thus goes like this:
Hubble: We see galaxies moving away in all directions.
Simplicio: Then we must be at the center of the universe.
Galileo: No. Calculate what the other worlds would see: also an expansion in all directions!
Einstein (special): But wait, if you take SR into account, the other worlds would observe an inhomogeneity in the universe.
Simplicio: So we are at the center of the universe!
Einstein (general): There is another possibility. With GR, the redshift could be caused by an expansion of space rather than an actual motion. Then everybody sees a uniform universe again.
Chorus of astronomers: Since we firmly believe that we do not have a special place in the universe, we must conclude that the cosmological redshift is the result of an expansion of space rather than recessional velocities.
Simplicio (coda): Well, there's no way you can prove that I'm wrong without traveling to a distant galaxy, so I still believe I'm special.
I know this is not the place to discuss physics in detail. I'm still hoping we can find a way to make the article more accurate and more understandable. I admit I haven't looked up the references, but that is partly because it is not clear from the text which reference will really answer my question - another opportunity to improve the article. --Art Carlson 07:42, 12 October 2006 (UTC)

Art Carlson points out another problem and that is the "Galilean vs. Lorentzian vs. Cosmological" view of velocities due to redshift. While it may seem that we are talking about a Galilean transformation when we discuss a velocity exceeding the speed of light, we are really talking about a general relativistic transformation and a definition of velocity that is rather brutishly (and some would say "non-physically") pulled out of fundametal kinematics. The problem is that the definition of "velocity" is muddled by the metric expansion of space. If we talk about the recession as the change in the proper distance between the two objects compared to the change in the proper time of either object, then the "speed" which is derived exceeds the speed of light. This means that if we took a space-like cut where dt = 0, there would be no chance for any light beam null-geodesic from either event to enter the future light cone of the other event assuming a linear and consistent expansion of space. In this sense, the objects have slipped past the other object's future horizons. We will continue to observe the distant object for all time, but we will never see it as it is "today". This is different from a Lorentzian redshift that exceeds z = 1 because in that case we will, in the future, see such an object as it is "today": at time t = x/c where x is the proper distance between ourselves and the object. --ScienceApologist 12:50, 6 October 2006 (UTC)

  • I think there is always going to be a problem explaining it to the reader, because expansion of space is a unique phenomenon for which there is no real analogy. The blowing-up of a balloon demonstrates "stetching", and dots on the rubber do actually move apart. The analogy with "sea floor spreading" and two people standing stationary either side of a rift, is similar. But I see the vague analogy, without being able to dinsguish the important bits, the "stretching" from the "expanion".
  • Personally, I believe that if you expand nothing (space) you end up with nothing, since "expanded" space is indistinguishable from unepxanded space. And the distance between objects due to recession is indistinguishable form that due to expansion.
  • Having said that, this is all interesting stuff that should be put in the relevant article, which is not Redshift. --Iantresman 13:32, 6 October 2006 (UTC)

[edit] Refs

I added a bunch of references. Someone needs to dig up a spectroscopy textbook to reference that section. Other stuff, such as using spectroscopy to study AGN and the rotation of stars, etc... should be easy to find on ADS. I may take a look later if I feel motivated. As for "Effects due to physical optics or radiative transfer", I don't know where to find most of that stuff, but I assume it has been hashed over about a million times on the talkpage, so perhaps it won't be that hard to reference. –Joke 16:12, 6 October 2006 (UTC)

[edit] The first two paragraphs

I find the first two paragraphs are a bit confusing:

In physics and astronomy, redshift is a phenomenon in which the visible light from an object is shifted towards the red end of the spectrum.

Ok, so this phenomenon deals with visible light, and causes a shift towards a specific range of wavelengths (red=620–750 nm).

It is an observed increase in the wavelength, which corresponds to a decrease in the frequency of electromagnetic radiation, received by a detector compared to that emitted by the source.

So it's no longer just visible light? And the shift isn't towards red, but rather a general shift towards longer wavelengths?

The corresponding shift to shorter wavelengths is called blueshift.

Ok.

The phenomenon goes by the same name even if it occurs at non-optical wavelengths (e.g. gamma rays, x-rays and ultraviolet).

I have no problem with this, but why state the opposite in the opening sentence?

At wavelengths longer than red (e.g. infrared, microwaves, and radio waves) redshifts shift the radiation away from the red.

Yes, but why this refrence to the colour red, I thought we just agreed that it was a lenghtening of the wavelength?

Well, I'm being a bit silly here, but I actually experienced a moment of confusion when reading these first two paragraphs. In my opinion, refrences to the colour red and visible light are only of historical interest. These were are probably relevant to the original experiments, but surely the term 'redshift' is now considered to apply to all EM radiation regardless of wavelength, and with no actual refrence to a specific colour (red)? O. Prytz 20:59, 10 October 2006 (UTC)

Tried to make it better. What do you think? --ScienceApologist 21:04, 10 October 2006 (UTC)
Ok, yes, it is clearer. But I would actually prefer to substitute the first two paragraphs with something like:
In physics and astronomy, redshift is a phenomenon in which electromagnetic radiation from an object is observed to have an increased wavelength compared to that emitted by the source. The corresponding shift to shorter wavelengths is called blueshift.
Which is totally unambiguous (but not very well written). Hm... on reading the opening paragraphs again I think it's pretty clear as it is now. Let's not change it furtherO. Prytz 21:12, 10 October 2006 (UTC)
The wording dealing with "red" up front was to avoid the confusion by certain editors (less savvy than yourself) who couldn't wrap their heads around why "red" was in the term. We address it up front and then knock down the misconception based on the assumption that the only forms of light that exist are those which are visible to the naked-eye. --ScienceApologist 21:15, 10 October 2006 (UTC)
I have very little problem with the opening paragraphs as they are now. If I were to make one final suggestion it would be to delete the sentence in par. 2 starting with "At wavelengths longer than red[...]". But I have no strong opinions on the matter. O. Prytz 21:20, 10 October 2006 (UTC)
  • Some of these "less savvy" editors, as you call them, perhaps realised that for most people who know little of physics, the description "increase in wavelength" was an abstract description that meant nothing to them. By mentioning the colour red in the definition, we at least give the reader something they are familiar with, that also related to the term "redshift".
  • As a guide, I would suggest reading the introduction to someone who is not familiar in phsyics. Our target audience is "people", not "physicists" --Iantresman 22:23, 10 October 2006 (UTC)

[edit] Redshift mechanisms

  • The text reads: "A single photon propagated through a vacuum can redshift in several distinct ways". The wording implies that there might be no redshift, or a different mechanism for (a) multiple photons, or (b) photons that don't travel in a vacuum? And since this is the practical case, it might be worth clarifying. --Iantresman 12:00, 11 October 2006 (UTC)
  • Isn't it the case that "redshift in several distinct ways" should be "the redshift can be caused in several distinct ways, because the resulting redshifs are identical? --Iantresman 12:02, 11 October 2006 (UTC)
I think these issues are more semantic than substantive. Indeed, it is correct that there might be no redshift because sometimes there is no redshift. I disagree that the wording implies a different mechanism for multiple photons not in a vacuum: this is just a common turn-of-phrase in physics when you idealize a situation so you can avoid the non-conservative abberations. For example, when describing inertia we need to eliminate friction. When describing conservation of momentum we need to eliminate impulse. When describing redshift mechanisms we need to eliminate geometrical and physical optics. I don't read any difference between the wording "redshift in several different ways" or "redshift can be caused in several different ways". One takes redshift as a verb, the other as a noun. Redshifting happens differently due to different mechanisms. --ScienceApologist 12:16, 11 October 2006 (UTC)
  • I understand it is "a common turn-of-phrase in physics". You and I understand its meaning. But I suspect that the majority of readers, who don't have a physcis background, may note the ambiguity... perhaps they will assume that we refer to photons travelling through a vacuum because space is a vacuum!
  • You wrote: "When describing redshift mechanisms we need to eliminate geometrical and physical optics."... but you don't say that in the description, which confirms that ambiguity. By explaining this, we could help the reader differentiate between these redshifts. Currently we don't.
  • As I noted earlier, you and I are not the best people to judge the explanation... someone who has no background in physics is. --Iantresman 12:42, 11 October 2006 (UTC)
There is a reason that common turns-of-phrase are common: they work. I don't think they are ambiguous. You are stating that you don't think it is ambiguous but are worried that someone else will think it is ambiguous. I'm not convinced of this. Let's wait and see what other people say, then. --ScienceApologist 12:48, 11 October 2006 (UTC)
  • Yes, common turns-of-phrase work... for phsyicists.
  • And still excludes the explanation for your using this particular turn of phrase: "to eliminate geometrical and physical optics" --Iantresman 14:22, 11 October 2006 (UTC)

It's not just geometrical and physical optics but also radiative transfer, energetic mechanisms beyond coordinate transformations, and scattering. It is far easier to say what the application is rather than what it isn't. --ScienceApologist 14:29, 11 October 2006 (UTC)

So while Doppler-like (z is independent of wavelength) is a requirement of redshift, is it a requirement or characteristic that a redshift involves either a Galilean, Lorentz, or general relativistic transformation? --Iantresman 15:09, 11 October 2006 (UTC)
It's semantics whether you call it a requirement or a characteristic. Characteristics are requirements in math and physics. --ScienceApologist 17:37, 11 October 2006 (UTC)
  • So are you saying that a redshift must be due to either Galilean, Lorentz, or general relativistic transformation? (ie. one of these transformations are a requirement that defines redshift)? --Iantresman 18:23, 11 October 2006 (UTC)
We've had this discussion before. Please read the archives. --ScienceApologist 18:34, 11 October 2006 (UTC)
  • Yes, I've read the archives, and can't find an answer, so I thought that for the sake of a quick answer, you might oblige. --Iantresman 19:06, 11 October 2006 (UTC)
  • Nope, the answer was given to you by FlyingJazz. Contact him if you are interested. --ScienceApologist 21:33, 11 October 2006 (UTC)

[edit] Citation request

So the introductory paragraph in "Mechanism" discusses "a single photon in a vacuum",

  • But we don't tell the reader why we have mentioned this restriction (you described it quite well above)
  • Applies also to multiple photons in a non-vacuum (ie. reality)
  • Mentions z independent of wavelenght, but we don't tell the reader its significance

I do not dispute that any of the individual mechanisms described by coordinate transformations can be considered to apply to a single photon in a vacuum, but I contend that this general description of redshift mechanisms is contrived and unique, and requires a citation.

I've restored the request by InvictaHOG for a citation,[3], and would appreciate a quote which involves "redshift mechanism" "single photon" and "vacuum" --Iantresman 11:09, 12 October 2006 (UTC)

I've included a reference. These works all define redshift in terms of the mechanisms listed. --ScienceApologist 12:01, 12 October 2006 (UTC)
  • Thanks for that. I have no problems with redshift and the mechanisms or transformations. I have a problem with its association with a "single photon in a vacuum".
  • Can you confirm that your citation to Kutner (2003) refers to "Astronomy: A Physical Perspective" by Marc L. Kutner?
  • Can you give me a page number that includes a definition of redshift, or discusses mechanisms, in terms of a single photon in a vacuum. For example, I can find only one page each:
  • Page 153: mentioning redshift and vacuum.
  • Page 150: mentioning redshift and "single photons". --Iantresman 12:17, 12 October 2006 (UTC)

As per Wikipedia:WikiProject Physics/Citation guidelines proposal, the material is easily inferred from the references you are looking at:

For reasons of notation, clarity, consistency, or simplicity it is often necessary to state things in a slightly different way than they are stated in the references, to provide a different derivation, or to provide an original example. This is standard practice in journals, and does not make any claim of novelty.[1] In Wikipedia articles this does not constitute original research and is perfectly permissible – in fact, encouraged – provided that a reader who reads and understands the references can easily see how the material in the Wikipedia article can be inferred.

--ScienceApologist 12:33, 12 October 2006 (UTC)

"it is often necessary to state things in a slightly different way". So please provide a page number where I can find the association of redshift mechanisms with "a single photon in a vacuum", even if it is not in those words. --Iantresman 12:41, 12 October 2006 (UTC)
Just look at the definition of redshifts found in these sources. They all apply to single photons in a vacuum by virtue of their choice of mechanisms. --ScienceApologist 12:50, 12 October 2006 (UTC)
  • They also apply to multiple photons in a non-vacuum, so I'd like a quote indicating the necessity to specify a "a single photon in a vacuum". --Iantresman 12:55, 12 October 2006 (UTC)
For reasons of notation, clarity, consistency, or simplicity it is often necessary to state things in a slightly different way than they are stated in the references, to provide a different derivation, or to provide an original example. --ScienceApologist 12:59, 12 October 2006 (UTC)
  • As an example, your source (Astronomy: A Physical Perspective", (2003) Marc L. Kutner) says (my emphases):
  • "Gravitational redshift The wavelengths of photons change as they pass through a gravitational field." (Page 145)
  • "One possible source of redshift is gravitational. We have already seen that photons are redshifted as they leave the surface of any object." (page 366)
  • "Cosmological redshift We can also see that the redshift (Hubble's law) fits ... (Fig. 20.8). As the universe expands, the wavelengths of all photons expand by the same proportion that cosmic distances expand." (Page 386)
  • "The redshift arises as a result of the increase in wavelengths of all photons moving through an expanding universe." (page 388)
  • These quotes demonstrate that "photons" (plural) are not excluded from the defintiion, and there is no reason to state a "single photon" nor state a vacuum. --Iantresman 13:15, 12 October 2006 (UTC)

There is no implied exclusion, only simplicity for the sake of clarity. --ScienceApologist 17:44, 12 October 2006 (UTC)

If it was simply and clear, we wouldn't be having this conversation, and I note that you have not provided any supporing quotes. --Iantresman 19:04, 12 October 2006 (UTC)
I disagree. I say we are having this conversation because you don't like it despite it being verified. --ScienceApologist 21:15, 12 October 2006 (UTC)

[edit] Eliminating geometrical and physical optics

You wrote above: "When describing redshift mechanisms we need to eliminate geometrical and physical optics"[4]. I note that you've included the Wolf effect which is consistent with your statement.

  • The article on the Wolf effect includes reliable sources stating that the Wolf effect is a redshift mechanism.
  • Can you provide a source that indciates that the Wolf effect should be excluded as you stated (I understand that many books might not include it for whatever reason, but this is not the same as excluding it) --Iantresman 19:26, 12 October 2006 (UTC)
  • Or how about a source excluding "physical optics" from redshift mechanism? --Iantresman 19:26, 12 October 2006 (UTC)

Absolutely! The basic astronomy sources listed do not include the Wolf Effect and therefore the Wolf Effect is not supposed to be included on this page in such a fashion since it is generally considered part of the other effects section listed below. --ScienceApologist 21:17, 12 October 2006 (UTC)

  • Why would you think that astronomers are the arbiters of the Wolf effect?. Optics sources include the Wolf effect, the very first sentence of the article says that redshift is something in physics and astronomy, and the article mention that "redshift has a number of terrestrial uses".
  • By the way, here as some astronomy sources that mention the Wolf effect:
  • "Gravitation and Cosmology: From the Hubble Radius to the Planck Scale"[5]
  • "The Universe: Visions and Perspectives"[6]
  • "Measuring the Universe"[7]
  • "Towards the Edge of the Universe: A Review of Modern Cosmology"[8] (Page 136)
  • "Gravitation and Cosmology: From the Hubble Radius to the Planck Scale"[9] (pp103) --Iantresman 22:23, 12 October 2006 (UTC)
Okay, so we rightly include the Wolf Effect in relation to its relevance to this page. Congratulations on using a search engine, Ian. Now maybe you can explain how the context of each of these sources mirrors the context of the Wolf Effect here in this article. --ScienceApologist 12:44, 13 October 2006 (UTC)

[edit] Poor notes

  • This note is rather poor: [2] "See Binney and Merrifeld (1998), Carroll and Ostlie (1996), Kutner (2003) for applications in astronomy"
  • There is no indication whether all three texts cover the information to which the note refers, or just one of them. There is also no indication where in the book, the information is discussed.
  • WP:CITE suggests that "When citing books and articles, provide page numbers where appropriate".
  • And there are many citations which give no titles, let alone a page number, including:
  • Feynman, Leighton and Sands (1989)
  • Misner, Thorne and Wheeler (1973) and Weinberg (1971)
  • Peebles (1993)
  • Odenwald & Fieberg 1993

The book references are at the bottom of the article. You may need to read substantial parts of the books to obtain the information you want to find out about. Wikipedia cannot teach you to be a competent researcher, they can only lead you to the water and then you can drink. --ScienceApologist 17:25, 26 October 2006 (UTC)

  • That's new, the reader needs to be a researcher in order to make use of the citations you've added to an article that I though you had researched.
  • Citing three books, and telling me that I "may need to read substantial parts" is a joke.
  • Peebles (1993) does not even tell me whether I am looking at a book or a paper.
  • WP:CITE suggests that "When citing books and articles, provide page numbers where appropriate". --Iantresman 18:03, 26 October 2006 (UTC)

Page numbers are not appropriate as explained above. If you want to know what book it refers to, check the book references at the bottom of the article. --ScienceApologist 19:06, 26 October 2006 (UTC)

  • The text says "It is believed that a yet unknown theory of quantum gravity would take over before the density becomes infinite"[51]
  • The reference is Peebles (1993), Weinberg (1971), Misner, Thorne and Wheeler (1973).
  • I can not find any reference to "quantum gravity" in Peeble's book, and the reader is not going to read through 700+ pages to try and find what your mean.
  • The citations need page numbers. --Iantresman 19:49, 26 October 2006 (UTC)

Please see: Wikipedia:Scientific citation guidelines. --ScienceApologist 20:03, 26 October 2006 (UTC)

Your "please see", like your references, tell me nothing. I have no idea what you are trying to point out. --Iantresman 20:18, 26 October 2006 (UTC)

I really can't see the point of this. Are you disputing this statement? Try pages 8 and 77 of Peebles. I don't have Weinberg or MTW handy to look up the page numbers. I don't think Peebles actually mentions quantum gravity, but he does mention that "unknown physics" becomes dominant near the singularity, which is one of the chief points of that sentence and the preceding two. –Joke 20:20, 26 October 2006 (UTC)

  • No, I'm not disputing the statements, so it's not a matter of verification. But like a curious reader, I go to the citations to find out more. If the statement tells me that "unknown theory of quantum gravity" is relevant, then I want to go the original source to find out what the author was trying to say about it, in more detail. Peebles is cited, so I want read what he has to say on quantum gravity. I can't find anything.
  • Most of the examples in your proposed Wikipedia:Scientific citation guidelines include page numbers. I agree that they are not always necessary. But if you mention the "aldol reaction", I can find the exact page numbers where the books talk about the "aldol reaction".
  • So where there is a reference to "reddening", I want to go the place in a book where "reddening" is detailed. Just as if I mention a concept you are unfamiliar, you'd want to find out exactly what the source said... a page number would help you get there. --Iantresman 21:03, 26 October 2006 (UTC)
- Ian asked me for my opinion on this topic on my Talk page, so here it is. I think the citations are lacking. "See Binney and Merrifeld (1998)" doesn't even tell me the title of the book, let alone the page number. I could just as easily say, "See Jones (2001)", and you would have absolutely no idea what I was refering to. After reading WP:CITE and the Scientific Citation Guidelines, they both show references like this: L. Hussein et al. (1999). "Nutritional quality and the presence of anti-nutritional factors in leaf protein concentrates (LPC)". International Journal of Food Sciences and Nutrition 50 (5): 333–343. or Wade, L. G. Organic Chemistry, 6th ed., Prentice Hall, Upper Saddle River, New Jersey, 2005; pp 1056-1066. ISBN 013187151
- While page numbers may not be absolutely necessary, the title of the book or article is. Footnotes 10 and 11 are good examples of acceptable citations in my mind. ABlake 20:27, 30 October 2006 (UTC)

While I agree that page numbers would be helpful for the book references in many cases, as noted above, the citation for the entire book is provided below the footnotes in the "references" section. This is, I admit, a slightly unusual mixture of citation styles but it seems to be accepted by the editors I've talked to about it. It has the advantage of seperating the references which are important sources that must be repeatedly cited throughout the entire article from sources of passing interest, cited for one statement (it also removes the need to repeat references in a redundant further reading section). –Joke 03:46, 31 October 2006 (UTC)

An example (meant to be so) is Wikipedia:Scientific_citation_guidelines.
I think that this article goes in the right direction. But the need for verification as in the footnotes (which may at some places refer to papers that are disqualified as general reference) is rather independent of the service of providing general references for further reading; in particular, there is no reason to call the general reference section "books", and IMO, mention of a book as general reference isn't an excuse for inaccurate referencing of debataed claims. However, the claim in consideration isn't debated, is it? Harald88 07:38, 31 October 2006 (UTC)
Yeah, I agree. When I was adding references to this article, I actually inserted a book reference (number 47) as a footnote. Perhaps "general references" or somesuch would be more appropriate than "book references". I'm doing something like that at cosmic inflation and intend to do so at cosmic microwave background when I get a chance. (Yes, I realize that it is incongruous that the general referencing I wrote for cosmic microwave background at the scientific citation guidelines is much better than that in the actual article.) I think it is usually helpful to provide page numbers for book references, even for claims that are not debated and particularly when things are not easily found in the TOC or index (as Ian pointed out above). –Joke 03:46, 1 November 2006 (UTC)
  • I don't mind so much that the references are below the citations, although if you are not aware that this style is being used, you wouldn't think to look below the citations. It's bad enough having to moving to the bottom of an article to look for a footnote, let alone look for a footnote to the footnotes.
  • While I agree that page number are not sometimes required for very general citations, the lack of a page number in a named book or paper, makes many of the citations in this article useless. As a reader, if the article talks about "reddening", I want the exactly page number in a book so that I can find out more. --Iantresman 09:33, 31 October 2006 (UTC)
I now see the books in the "Book References" section, but there is no intuitive link to the footnotes section. If I missed the connection, then a lot of others will too. I think there ought to be a line at the beginning of the Notes section that says, "Books cited in the Notes section are further referenced below in the Book References section." That would be clear up the confusion to my satisfaction. ABlake 11:08, 31 October 2006 (UTC)

As I mentioned above:

  • The text says "It is believed that a yet unknown theory of quantum gravity would take over before the density becomes infinite"[51]
  • The reference is Peebles (1993), Weinberg (1971), Misner, Thorne and Wheeler (1973).
  • I can not find any reference to "quantum gravity" in Peeble's book, and the reader is not going to read through 700+ pages to try and find what your mean, nor read Weinberg, nor Misner, to find out more.

The citation is inadequate, as are all references to footnote [2] --Iantresman 12:29, 2 November 2006 (UTC) --Iantresman 12:29, 2 November 2006 (UTC)

I fixed it. If you want, I will add the pages for Weinberg and MTW tomorrow, but it is easy to find in the index. Is this not making a mountain out of a molehill? –Joke 21:39, 5 November 2006 (UTC)
Many thanks. I'm not criticizing for the sake of it. As a featured article, I think we need to provide top quality citations, and I think that means giving people an accurate reference. If I write something in one of the minority scientific articles, an exact reference is expected, to help the reader verify, clarify and find out more information if necessary. --Iantresman 23:13, 5 November 2006 (UTC)