How is measuring Redshift possible?

[timbreman]

I am very interested in physics but have no background education on it so forgive me if this question is amateur.

I am trying to grasp this redshift thing. It's the measurement of a shift in the wavelength of light. The only variable I can think of is the wavelength of the light received on our end. So how can one take a measurement with only one variable?

How can anyone tell how many gravitational fields the light passed thru affecting the redshift?

I have read in so many places how it's described as being so accurate but I don't understand how they know what wavelength the light had at point "A" in order to compare to what it is at point "B".

 

[Ich]

Welcome to PF,

I don't understand how they know what wavelength the light had at point "A"

You try to identify spectral lines, or other features of the spectrum. You know the respective wavelengths, because you can reproduce such lines in the laboratory. Read the http://en.wikipedia.org/wiki/Redshift#Measurement.2C_characterization.2C_and_interpretation".

 

[sylas]

I am very interested in physics but have no background education on it so forgive me if this question is amateur.

I am trying to grasp this redshift thing. It's the measurement of a shift in the wavelength of light. The only variable I can think of is the wavelength of the light received on our end. So how can one take a measurement with only one variable?

How can anyone tell how many gravitational fields the light passed thru affecting the redshift?

I have read in so many places how it's described as being so accurate but I don't understand how they know what wavelength the light had at point "A" in order to compare to what it is at point "B".

That's a really good question.

The answer is that light from a star has well defined spectral lines, where light is absorbed by neutral gases in the stars atmosphere. The redshift is measured by seeing how far those lines have shifted.

Here's a picture of how it works:
[PLAIN]http://stokes.byu.edu/redshift.jpg

The description of this, by Harold Stokes at BYU, is as follows:

BAS 11 is a super cluster of more than twenty dense galaxy clusters. In all, it contains over 10,000 galaxies. It is located just below (to the south) the handle of the big dipper. It is about one billion light years away.

The core of a star emits a continuous spectrum of light. As this light passes through the cooler outer atmosphere of the star, the atoms there absorb some of it. This light is absorbed by electrons which are excited to higher energy levels in the atoms. Since the energy levels are quantized, only photons with the corresponding amount of energy will be absorbed. Thus, the light is absorbed only at certain wavelengths. These appear as dark lines on top of a continuous spectrum of light emitted by the star and is called the absorption spectrum.

The image in the file, REDSHIFT.JPG shows a simulation of the absorption spectrum of the sun and of BAS 11. (Actual data is not taken with color photography.) The dark lines in the red and in the blue is from absorption by hydrogen atoms. The dark line in the yellow is from sodium atoms. The dark lines in the green are from magnesium and iron atoms. The dark lines in the violet are from hydrogen, iron, calcium, and potassium atoms.

The shift in the spectrum of BAS 11 from the spectrum of the sun is clearly seen. The lines from BAS 11 are all shifted towards the red end of the spectrum. This shift (called the "red shift") is toward longer wavelengths (smaller frequencies) and is caused by the Doppler effect. The shift in frequency is about 7%. This means that BAS 11 is traveling away from us at about 7% of the speed of light.

Quoted from http://stokes.byu.edu/redshift.html , by Harold Stokes, BYU[/size]​

PS. Crossed posts with Ich. The wikipedia article he recommends is a good one, and in fact that was how I tracked down the pages by Harold Stokes, which are resources intended for first year physics.

 

[timbreman]

Welcome to PF,


You try to identify spectral lines, or other features of the spectrum. You know the respective wavelengths, because you can reproduce such lines in the laboratory. Read the http://en.wikipedia.org/wiki/Redshift#Measurement.2C_characterization.2C_and_interpretation".

I don't quite understand this answer. The wiki article says they compare the spectral lines and compare the shift in wavelengths.

So what wavelength of light are they using as a baseline? How do they know what type of light the star emitted "before" going thru millions of years of travel and thru numerous gravitational fields that shifted it's wavelength?

How can anyone know what wavelength something that far away "started off as" in order to compare it to something in a laboratory?

 

[D H]

Look at the image in sylas' post. It looks kind of like a barcode, doesn't it? Each of those black lines corresponds to a specific transition between orbitals in a specific atom or molecule. Matching up those spectra doesn't involve looking at just one spectral line. It involves looking at several.

The image in post #3 is a simulated image. All of the spectral lines are shown as black. What is actually used is a spectrogram, a plot of intensity versus wavelength. The intensity of a spectral line is a function of the relative abundance of the atom associated with that line and the relative frequency with which the specific transition occurs. In those spectrographs, the hydrogen alpha line sticks out like a huge sore thumb. Hydrogen is by far the most abundant atom in the universe and the alpha line is by far the most frequently observed transition in the hydrogen Balmer series.

 

[timbreman]

Look at the image in sylas' post. It looks kind of like a barcode, doesn't it? Each of those black lines corresponds to a specific transition between orbitals in a specific atom or molecule. Matching up those spectra doesn't involve looking at just one spectral line. It involves looking at several.

The image in post #3 is a simulated image. All of the spectral lines are shown as black. What is actually used is a spectrogram, a plot of intensity versus wavelength. The intensity of a spectral line is a function of the relative abundance of the atom associated with that line and the relative frequency with which the specific transition occurs. In those spectrographs, the hydrogen alpha line sticks out like a huge sore thumb. Hydrogen is by far the most abundant atom in the universe and the alpha line is by far the most frequently observed transition in the hydrogen Balmer series.

Ok so the difference between the two varying sets of spectral lines in each chart is the shift. I get that.

But comparing the redshift from a distant galaxy compared to our own sun as a point of reference doesn't seem applicable. We don't know what type of wavelength the light started out as and we don't know how many gravitational fields the light had to pass thru and in turn further affecting the redshift. How wavelengths react in a laboratory in a controlled environmet compared to traveling light years thru the universe seem very different. I feel like there is a detail here I am missing or maybe I am just not grasping it. I'm tryin though. lol

 

[Upisoft]

Ok so the difference between the two varying sets of spectral lines in each chart is the shift. I get that.

But comparing the redshift from a distant galaxy compared to our own sun as a point of reference doesn't seem applicable. We don't know what type of wavelength the light started out as and we don't know how many gravitational fields the light had to pass thru and in turn further affecting the redshift. How wavelengths react in a laboratory in a controlled environmet compared to traveling light years thru the universe seem very different. I feel like there is a detail here I am missing or maybe I am just not grasping it. I'm tryin though. lol

You are missing one of the basic assumptions of the astrophysics, that the laws of physics are very similar everywhere in the observable Universe. That's called homogeneity. In other words we are not special. So, the stars there will behave more or less our Sun and the stars in our Galaxy.

Also, the assumption of homogeneity has a lot of evidence from the observations we make. So, it makes sense to assume that the light from the stars in the distant galaxies started quite the same as the light from the local stars.

 

[D H]

But comparing the redshift from a distant galaxy compared to our own sun as a point of reference doesn't seem applicable. We don't know what type of wavelength the light started out as ...

Assuming the laws of physics are universally true and the physical constants related to those laws truly are constants then we do know exactly what wavelength the light started out as. For example, the hydrogen alpha line as seen by an observer at rest with respect to some observed star will be at 6562.8 Angstroms here, there, and everywhere assuming that the laws of physics and the physical constants are universal.

That is a mighty big if, so scientists have made many tests of that assumption. One such test is to look at those spectral lines. Many absorption and emission lines are seen in the spectra from distant galaxies. Those remote spectra might look different from that of our the Sun if the laws of physics or the physical constants varied across space or time. That is not what we see. Instead those remote spectra match up quite nicely with that of our Sun (after accounting for redshift of course).

This is such a huge assumption that scientists are still probing it. Every once in a while news will come out that some particular test does make that assumption look suspect. One such test has come out very recently regarding potential variations in the fine structure constant. To date, it is the tests that look suspect on further investigation rather than this assumption.

 

[D H]

... and we don't know how many gravitational fields the light had to pass thru and in turn further affecting the redshift.

Sure we do: For the most part, the answer is essentially none. Even when it is not none, the net effect is zero. The light is blue shifted as it drops into a gravitational well but redshifted as it climbs out. The end result is a null effect.