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iantresman
2006-Mar-21, 10:05 AM
Idealistically, the Doppler, Cosmological and Relativistic redshifts produce distortion free (non-blurred, nor broadened) redshifts, which is a defining characteristic.

In practice though, I believe that thermal and mechanical motion of the source will distort the observerd redshift, and can be used, for example. to measure temperature. Does this process have a name?

Is such distortions of observed redshifts spectra ALWAYS present, and change solely on the temperature of the galaxy, star, Sun, candle?

Presumably, for a single photo to blur like this, it is essentially interacting with something; matter? other photons? This is due to scattering?

Regards,
Ian Tresman

antoniseb
2006-Mar-21, 01:04 PM
I believe that thermal and mechanical motion of the source will distort the observerd redshift, and can be used, for example. to measure temperature. Does this process have a name?
This is called "thermal broadening". Temperature does not change the apparent red-shift of an object, but it does make the spectral lines wider and shallower. It is a very important tool for measuring temperature. As you can visualize, temperature increases the range of velocities in all directions that atoms (ions) will have as they emit or absorb photons. Thus for any one photon, the event may be at the red or blue side of the line depending on whether the atom is moving toward or away from us.

iantresman
2006-Mar-21, 02:04 PM
This is called "thermal broadening". Temperature does not change the apparent red-shift of an object, but it does make the spectral lines wider and shallower. It is a very important tool for measuring temperature. As you can visualize, temperature increases the range of velocities in all directions that atoms (ions) will have as they emit or absorb photons. Thus for any one photon, the event may be at the red or blue side of the line depending on whether the atom is moving toward or away from us.

Thanks for that, do you know (a) if all observed redshift spectra show thermal broadening? (b) if stationary objects with no redshift show it (c) I presume it's due to scattering?

Regards,
Ian Tresman

antoniseb
2006-Mar-21, 02:20 PM
Thanks for that, do you know (a) if all observed redshift spectra show thermal broadening? (b) if stationary objects with no redshift show it (c) I presume it's due to scattering?

I'm not sure what you're asking. From other posts, I know you to be fairly well informed. All objects that are emiting or absorbing light should show some level of thermal broadening if you can get high enough resolution on the spectrum (There are some gas clouds that are very cold, and the broadening will be extremely narrow if you can look at just one small portion of the cloud). It doesn't matter if the object is being red-shifted or not. It is not due to scattering. It is due to the spread of the relative velocities of the individual atoms doing the absorbing or emitting. Note that there is also some built in thermal broadening in most detectors because the atoms capturing the inbound photons are also vibrating. Even Mossbauer spectra, which can be very high resolution, show some thermal broadening.

Nereid
2006-Mar-21, 02:30 PM
Thanks for that, do you know (a) if all observed redshift spectra show thermal broadening?The question is far too broad to be capable of being answered.

If you're referring to just astronomical bodies, then yes, all observed spectral lines, whether redshifted or not, have broadening.

However, there are many sources of broadening, and 'thermal' broadening is just one - there's rotational broadening, macro-turbulence, pressure, magnetic, ...
(b) if stationary objects with no redshift show it thermal broadening is inherent in all non-coherent emission processes, it has nothing to do with the (bulk) relative, line of sight motion of emitter and observer.
(c) I presume it's due to scattering?It's due to ... the doppler effect!

Take the case of emission (absorption is essentially the same). An atom, let's say H, emits an Halpha. The wavelength you detect it at depends on the line of sight difference in relative motion of the H atom and your detector - if the atom is moving away, it's redshifted, if towards, blueshifted.

In a cloud of H gas, with no net relative line of sight motion, approx half the H atoms will be moving towards you, and half away, at the time of emission. The spread of speeds translates to a spread of red- and blue-shifts, and is characteristic of the temperature of the gas (an ideal gas, in equilibrium, has its constituent particles going at speeds that follow a well-defined function - a result from classical physics). This translates to a characteristic line shape (or profile); from the observed profile, you can estimate the temperature of the gas.

iantresman
2006-Mar-21, 02:51 PM
Thanks all very helpful. I'm not out to ask any trick questions, and if my questions are broad, it's out of ignorance. But I am asking the question in relation to other issues -- the Wolf effect will produce redshifted spectra that shows broadening.

So I understand that idea Doppler, Cosmological and Relativistic redshifts produce distortion free spectra, but space is not ideal, and I read somewhere that these spectra may produce blurred spectra due to thermal broadening. Hence the clarification to see whether they ALWAYS will produce it, or only in some contrived circumstances.

Looking at the Wiki article on redshift (http://en.wikipedia.org/wiki/Redshift#Observations_in_astronomy), the text reads "distant objects are not blurred, and the lines are not broadened more than can be explained by thermal or mechanical motion of the source", which is true, but a strange way of saying that "distant objects are blurred, and the lines are broadened"... and this is due to thermal or mechanical motion of the source".

I had even asked for clarification over whether "... observed redshift be distortion free?" and told that "Yes (especially compared to reddened spectra).", but this must be in relation to the idealised component of the observed redshift.

Regards,
Ian Tresman

antoniseb
2006-Mar-21, 03:00 PM
Here's a few quick notes. The Wolf Effect causes line broadening, not red shift.
Second, when speaking of blurring, it is important to note the distinction between image blurring and spectral blurring. One of the main difficulties that some alternative explanations for Cosmological Redshift have involves all photons losing little bits of energy via interactions along the way, and usually this should cause some image blurring if it is really happening. The Wiki article may have been referring to this.

Nereid
2006-Mar-21, 03:01 PM
Thanks all very helpful. I'm not out to ask any trick questions, and if my questions are broad, it's out of ignorance. But I am asking the question in relation to other issues -- the Wolf effect will produce redshifted spectra that shows broadening.

So I understand that idea Doppler, Cosmological and Relativistic redshifts produce distortion free spectra, but space is not ideal, and I read somewhere that these spectra may produce blurred spectra due to thermal broadening. Hence the clarification to see whether they ALWAYS will produce it, or only in some contrived circumstances.

Looking at the Wiki article on redshift (http://en.wikipedia.org/wiki/Redshift#Observations_in_astronomy), the text reads "distant objects are not blurred, and the lines are not broadened more than can be explained by thermal or mechanical motion of the source", which is true, but a strange way of saying that "distant objects are blurred, and the lines are broadened"... and this is due to thermal or mechanical motion of the source".

I had even asked for clarification over whether "... observed redshift be distortion free?" and told that "Yes (especially compared to reddened spectra).", but this must be in relation to the idealised component of the observed redshift.

Regards,
Ian TresmanThere's another 'shorthand' that could get you into trouble here Ian, 'blurring'.

Line broadening is well understood; calling it 'blurred spectra' creates confusion, because it sets up a (false) relationship with 'image blurring' (which can arise from a wide range of physical effects, few if any of which have much to do - directly - with the physical effects that give rise to line broadening).

Your reference to the Wolf effect introduces another aspect - this is a coherence effect, which you need QM to discuss (cf thermal broadening etc, which you can talk about almost entirely in classical terms).

You also seem to be blurring the line between (astronomical) observation and theory - the physical effects are describable within theories; they can be demonstrated in the lab. In astronomy, you have only the observed spectrum - the extent to which you can account for ('model') it with a variety of physical effects depends on your assumptions about the source (and everything in between) of the photons that you detect.

iantresman
2006-Mar-21, 04:07 PM
Second, when speaking of blurring, it is important to note the distinction between image blurring and spectral blurring. One of the main difficulties that some alternative explanations for Cosmological Redshift have involves all photons losing little bits of energy via interactions along the way, and usually this should cause some image blurring if it is really happening. The Wiki article may have been referring to this.

Thanks, I'll make a note.


Here's a few quick notes. The Wolf Effect causes line broadening, not red shift.
Are you sure that the Wolf effect does not also produce a redshift? As James and Wolf note in their 1990 paper (p.169): "We see that just in the case when the shift is due to the Doppler effect, the relative frequency shift z induced by this mechanism is independent of frequency and can take on any value in the range -1<z<[infinity], even though the source, the medium, and the observer are at rest with respect to one another." PDF (http://www.physics.utoronto.ca/~dfvj/publications/06_Dopplerlikefrequencyshift.pdf)

Another quote on the Wiki Wolf effect article (http://en.wikipedia.org/wiki/Wolf_effect) also suggest that it produces a shift that "may imitate the Doppler effect .. [and] .. is independent of the central frequency".

Regards,
Ian Tresman

iantresman
2006-Mar-21, 04:13 PM
It is important to note the distinction between image blurring and spectral blurring.


Line broadening is well understood; calling it 'blurred spectra' creates confusion, because it sets up a (false) relationship with 'image blurring' (which can arise from a wide range of physical effects, few if any of which have much to do - directly - with the physical effects that give rise to line broadening).

So thermal blurring is image blurring? What other factors may give rise to image blurring, and which may cause line broadending?

If I was to guess, I'd go for atmospheric scattering, poor lenses, as contributing to images blurring... but I'm not sure about line broadening?

Regards,
Ian Tresman

antoniseb
2006-Mar-21, 04:25 PM
So thermal blurring is image blurring? What other factors may give rise to image blurring, and which may cause line broadending? If I was to guess, I'd go for atmospheric scattering, poor lenses, as contributing to images blurring... but I'm not sure about line broadening?

Ummm. No, thermal broadening does not cause image blurring. I was speaking (as the Wiki article was) of lack of image blurring as a reason we believe that cosmological red shift is not caused by lots of tiny interactions between photons and particles along the way.

antoniseb
2006-Mar-21, 04:29 PM
Are you sure that the Wolf effect does not also produce a redshift? Wolf effect produces both Red and Blue shift in equal amounts, effectively, taken in the large, and assuming many levels of Wolf effect in the coherent light coming from a source, producing a non-gaussian line broadening effect. Again, the idea of Wolf effect being used to cause cosmological, or perhaps "intrinsic" red shifts is something that alternative theory supporters have latched on to, but it has never been demonstrated to make sense for that context. Further discussion of that should be confined to the ATM section of this journal.

iantresman
2006-Mar-21, 05:31 PM
Wolf effect produces both Red and Blue shift in equal amounts, effectively, taken in the large, and assuming many levels of Wolf effect in the coherent light coming from a source, producing a non-gaussian line broadening effect. Again, the idea of Wolf effect being used to cause cosmological, or perhaps "intrinsic" red shifts is something that alternative theory supporters have latched on to, but it has never been demonstrated to make sense for that context. Further discussion of that should be confined to the ATM section of this journal.

Understood, I don't really want to discuss the Wolf effect as an explanation of cosmological redshift. But if the Wolf effect produces blueshifts and redshifts, then it does produce a redshift, and that has been demonstrated in the lab.

So yes, the Wolf effect produce redshifts, but its another discussion (for ATM) on whether it has anything to do with intrinsic redshifts.

Regards,
Ian Tresman

Ken G
2006-Mar-22, 02:37 PM
I think an important point to clarify are frequency shifts that occur in any frame you care to observe them, like thermal broadening, versus frequency shifts that depend on the frame you choose to observe them, like Doppler shifts. These two ideas are in danger of being confused in some of your questions, Ian. Cosmologically, we would then distinguish mechanisms that occur at the source, like thermal broadening, and mechanisms that appear when you propagate the light from a source to an observer, such as cosmological redshifts. It is the latter that must not blur either the spectrum or the image, to agree with observations-- the former can blur all it wants, because it is observed to be blurred due to those mechanisms. The problem that antoniseb is talking about is that other ways of getting redshift during propagation will cause further blurring, whereas the standard model of cosmological redshifts succeeds in avoiding that.

Tim Thompson
2006-Mar-22, 03:19 PM
There is a coomon theme here, and with other queries about redshift. The common theme is spectroscopy, and so I would like to make a general observation.

The emission & absorption spectra of just about everything you can think of are commonly measured in the "rest frame" of an Earthbound laboratory (not really a "rest frame" as far as GR is concerned, but the difference is too small to quibble over in this context). For atoms & molecules of astronomical interest, the laboratory spectra can be compared with the astronomically observed spectra, the differences noted, and conclusions drawn therefrom.

In the case of an astronomical redshift (or blueshift), we see that the entire spectrum is shifted, one way or the other, indepent of wavelength. So we measure a single, net redshift. But how the spectrum got that way is entirely a matter of interpretation, based on prior knowledge, accepted theory, new theory, or even a quick phone call to Aunt Martha, realizing that some interpretations may be more reliable than others. The argument invariably comes down to a discussion of that reliability factor.

Likewise, the astronomical spectral line widths may be wider or narrower than their laboratory counterparts, and what we measure is a single, net difference. And how it got that way is once again an equal matter of interpretation, to the same end.

What usually happens is that the laboratory spectra are "clean", in that we know exactly how they were produced in our laboratory, and we can control the environment. We can induce one change at a time. But astronomical spectra are not so "clean". They will simultaneously experience shifting & broadening (narrowing would be strange, though perhaps not altogether impossible). And the complicated environments of astronomical objects ensure that there will be multiple reasonable causes present for each & every effect. So we need to use the art & science of interpretation to decide which affect was responsible for what fraction of each effect.

Common experience tells us that line broadening is usually due to temperature & pressure, while line shifting is usually due to Doppler effects. A spectrum can be "blurred" by being measured with a spectral resolution that is too low, or simply by measurement noise. Or, it can be physically blurred, if excessive line broadening causes the wings of the lines to overlap. This is what happens in the Earth's atmosphere, where pressure (mostly) & temperature broadening of water absorption lines creates a contiuum absorption, leading to "greenhouse" effects. Images can be blurred by being measured with a spatial resolution that is too low, or simply by measurement noise. The usual condition is that all of the above, plus something you have not thought of yet, happen at the same time.

I am not even sure of my own point, except perhaps to note that one should avoid the pleasant trap of an over simplistic look at something which falls far short of simple, when seen in the wild.