Light from distant stars is Compton scattered many times by free electronsin the outer space before reaching us.This shifts the light toward the red.How can this shift be distinguished from the Doppler red shift due to the motion of receding stars???


When visible light [of colour other than red---like blue/yellow/green etc] is scattered off free electrons,the Compton shift is negligible...and what we observe is very nearly the original wavelength, somewhat reduced in intensity...

But when the Doppler Effect is the concern,the colour (wavelength) is increased with respect to us due to relative motion.If we were flying at the same rate with the star, there would be no red-shift...In this case,light that reaches us, has long wavelength (red). Compton scattering does nothing but to change red to red itslef...So, from colour observation, we get the idea...

Please let me know if you support this idea.

There's not nearly as much Compton Scattering as you imply, and some of it will be Inverse Compton Scattering - a source of blueshift.

But I did not use Compton Scattering in my argument.Question did it...

And later I mentioned of it...Even if we consider Compton scattering for red-shifted light, the effect is negligible for red light...

Hi Kolahal_b,
I always found you to be more knowledgeable than me, so take my thought with a grain of salt.
The red-shift we usually associate with expansion is actually cosmological redshift (due to the stretching of space time, not your typical doppler effect).
From there I have to ask you why would compton scattering redshift rather than just absorb energy and make the light less intense?
Thanks K_B,
Ron

I basically know nothing about cosmology...But it is easy to see that if there is a relative motion between two moving objects...and if one object emit waves, there will be a Doppler shift in frequency from the original...Exactly this happens here...

And it is questioner's choice to frame this question this way...

Compton scattering makes some photons go other way...they do not reach our eyes..that results in decrease in intensity...



I am more lucky than many people...less lucky than many people...

Hi K,
I just wanted to point out the subtle difference between "Doppler shift" and "cosmological expansion". Any star moving away from us is in cosmological expansion, and , although they are basically the same, cosmological expansion needs relativity to calculate the expansion (and this type of expansion is not limited by the speed of light).
Just being dweebish,
Peace Bro,
Ron

If you are assuming a large amount of Compton scattering, then the answer should be easy. The light from a star will have spectral lines in it, and a Doppler-type redshift will merely shift the wavelengths of these lines.

Compton scattering however, would add a random wavelength change and directional change to each photon. Not only would this tend to smear spectral lines but the direction the photon appears to come from will not be where the star is but where the photon was last scattered.

To distinguish Doppler from Compton:

- Compton scattering deviates the photon, so if you get the photon from the right direction, it's Doppler. OK, putting figures would be better.

- Doppler changes the whole spectrum with a uniform relative frequency variation, Compton not.

This should be enough to preclude any significant Compton effect.

Edit: sorry, already written in a previous answer. Didn't find how to suppress mine.

On your invitation with the question without a question mark, please let me know if you support this idea.

In deep field Hubble views, which were reported as protogalixies, what happens, is that light obtains an identity.
This must be an assigned identity, as if the character of this received light does not have an identity, then what is translated from the Hubble Observatory plates, would not agues into any sort of picture.

So light must have an assignment, then a maintained character, then a download point, to where what is being seen, translates into the objects viewed.

This is all I’m going to say on this subject. The rest is for you to translate?

Note that a while back, NASA had what was known as the GRC invitational, or the proposed development of a NASA based spaceship, which could exceed the speed of light.

In order to do this, one must know the handling character of light.

Hi All,

If there is significant Compton Scattering the information content (qubit) of the light is linked to the scattering region of space and is no longer linked to the original source information. This means that instead of seeing the star the light originally came from you see only the scattered light from a Nebula (light scattered by interstellar dust) somewhere between that star and us.

Enthalpy is correct in what he/she said. Red and Blue Doppler shift in spectra move all lines uniformly without any distortions. This effect is used to measure the velocity of the sources. Light actually "travels" over great distances without any technical "losses". This is because light is not so much a wave but composed of quanta or packets which are not altered except for Hubble Shift (frame dragging of "everything" due to universal expansion), high speed proper motion or due to gravity wells. Quanta can only be absorbed "whole". Scattering does occur but in space it is extremely seldom. Gravitational Red Shift and Special Relativity which do not "sap" energy from the quanta but change the energy content in a non-invasive "parametric" way. The actual distance photons travel is no consequence to them and they can encounter a rather large number of particles on the way without actual any effect because scattering is wavelength dependent and wavelength depends on the scattering particle wavelength or size. The clarity of Hubble deep field photographs should be enough to show everyone that most distortions in pictures we see of objects in deep space are due to 'stuff" out there. All that you see "changed" is usually due to Relativistic Doppler of one sort or another. The Hubble Deep Field Photos indicate "quite clearly" that light is not actually scattered very much at all en route to Earth and the fantastic clarity and crispness of the distant sources indicate that little of no scattering per se occurs, not enough to matter. There are certain technical reasons for this but they are "all good" for "far seeing".

Most of the scattering occurs in our own atmosphere, where there is far more matter to be found per unit of volume of atmosphere than almost anything encountered "out there".. That nearby atmospheric turbulence is what causes "twinkling" and has nothing to do with the stars distance, stars do not twinkle in space. Space is really quite benign to light passing through it. It is far clearer than the best optical fibers... it is nearly "perfect" and light can travel for billions of years without encountering anything that disturbs it's passage. If scattering dominated "out there" the sky would not be "transparent" and we would not be able to see the stars at all. We would be lit up at night by all the scattered light, probably never get to sleep... he he he!

The other fact is if light was a classical wave we would also not see anything either since the distances being so great to even the nearest space objects that all photons should have fallen to an "intensity level" below the level of detection due to inverse square law spreading in only the first few tens of kilometers from the respective sources. It is fortunate that photons spread and are absorbed only as "packets" with a fixed conserved energy size that is dependent on the source.The reason we see the stars is directly related to the fact that light is certainly "non-classical" and each "packeted photon" retains the entire "punch" that it had when if first left the star... we know this because we can see the same spectra in stars billions of light years away reflecting the same "chemistry" we see in our own star.. the sun. The wavelengths of all emission lines are unaffected by "space travel" and "mostly" all that is changed is related to the relative velocity of the distant objects (sources) and the intervening gravity fields. The little scattering that is out there leads to those pretty pictures of Nebula, so most scattering is there in pockets around stars being born or dying where there is incandescent dust.

Cheers and comments welcome