# Red shift of light from galaxies

Hi. In light of that ridiculous Ham / Nye debate I have someone trying to explain away the red shift of light from galaxies by suggesting it could be due to the photons interacting with electrons or other subatomic particles en-route, thereby loosing energy and increasing their wavelength.

Can anyone explain to me why this isn't a reasonable explanation for the red shift? (Please use small words without formulas if possible!)

As a supplementary question, can you explain why light is able to pass through glass but not opaque matter?

Thank you.

Drakkith
Staff Emeritus
The quick answer is that light of different frequencies does not interact the same way with matter. For example, glass is transparent to visible light but not UV light. When we measure red shift what we are doing is comparing the emission and absorption lines made by different elements. The way red shift works is that the entire spectrum shifts down in frequency a certain ratio. For example, an object moving at around 87% the speed of light would have the frequency of the light cut in half, thus doubling its wavelength. So blue light with a wavelength of 450 nm would be redshifted to 900 nm, while red light at 700 nm would be shifted to 1400 nm.

So when we compare the emission and absorption lines from the galaxy we find that they differ from our reference source here on earth. If the light were affected by the interstellar and intergalactic medium, we would expect to see something quite different from a simple doubling of wavelength across the entire spectrum.

Edit: note that this effect is identical to the Doppler effect that affects all waves. Since we know light is an EM wave, and we know how the Doppler effect works with other types of waves, it is reasonable to explain the red shift as being caused by objects moving away from us. Until we find evidence that supports another explanation better than that, we will attribute the red shift to the recession of galaxies. But so far our current explanation fits the data better than anything else.

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The quick answer is that light of different frequencies does not interact the same way with matter. For example, glass is transparent to visible light but not UV light. When we measure red shift what we are doing is comparing the emission and absorption lines made by different elements. The way red shift works is that the entire spectrum shifts down in frequency a certain ratio. For example, an object moving at around 87% the speed of light would have the frequency of the light cut in half, thus doubling its wavelength. So blue light with a wavelength of 450 nm would be redshifted to 900 nm, while red light at 700 nm would be shifted to 1400 nm.

So when we compare the emission and absorption lines from the galaxy we find that they differ from our reference source here on earth. If the light were affected by the interstellar and intergalactic medium, we would expect to see something quite different from a simple doubling of wavelength across the entire spectrum.

Edit: note that this effect is identical to the Doppler effect that affects all waves. Since we know light is an EM wave, and we know how the Doppler effect works with other types of waves, it is reasonable to explain the red shift as being caused by objects moving away from us. Until we find evidence that supports another explanation better than that, we will attribute the red shift to the recession of galaxies. But so far our current explanation fits the data better than anything else.

Thanks, Drakkith. Interesting: so the reason glass is transparent is the same as the reason why different elements absorb different wavelengths of light.

However.... I understand the basic working of red shift (having watched Brian Cox on "Night with the Stars" ) I am sure my antagonist does as well. The problem is he is suggesting the reduction of energy of photons as a result of interacting with "electrons and other sub-atomic particles" on their journey to us would also cause a red shift and could explain our observations. I am sure there is a good reason why this cannot explain what we observe....

Perhaps because the red shift wouldn't be so perfect? Perhaps because the random nature of such interactions would cause the "signature" of the elements visible in the light to become distorted?

Bill_K
Perhaps because the red shift wouldn't be so perfect? Perhaps because the random nature of such interactions would cause the "signature" of the elements visible in the light to become distorted?
Yes, this is a valid point. If the photons from a distant galaxy were colliding with dark matter or something, the randomness would not only spread out the spectral lines, it would also blur the image.

Thank you for your help - I think I have the answer now. Photon interaction with electrons en-route to us causes a "fuzziness" in the elemental "fingerprints" in the light, a "standard deviation" as a consequence of the random nature of such collisions. Additional interaction with electrons (or dark matter?) would simply cause additional "fuzziness" by increasing the standard deviation and could not increase the wavelength of the light while retaining the "focus" of the "fingerprint" of the elements in the observed light.

(Excuse the unscientific language!)

Drakkith
Staff Emeritus
Pretty much, Baggins.

But just as importantly, note the exact match between the way doppler shift works and the way the frequencies of the galaxies spectrum shift. So not only do we know that interactions between matter and light wouldn't result in the lengthening of the wavelengths in this manner, we also already have an explanation that fits this phenomenon exactly.

Had the spectrum been stretched differently, perhaps exponentially depending on the wavelength, then it wouldn't match what doppler shift does and we'd need a new explanation.

phyzguy
Theories of this type, which attempt to explain the red shift by photons losing energy through collisions or some other mechanism are called "tired light" models. This Wikipedia entry has a nice description of what these models must satisfy, and why they are no longer considered an option.

Bill_K
I once overheard someone ask Feynman about the tired light idea, and instead of the expected snappy "Hell no!" comeback, he got a very hesitant "probably not."

George Jones
Staff Emeritus
Gold Member
Gravitational time dilation in expanding universe models predicts that the rate at which photons leave a distant source is less that rate at which photons are received here. This leads to observable effects that are not predicted by tired light theories.

Weinberg in "Cosmology" (2008) said:
For instance, one important difference between "tired light" theories and the conventional big bang theory is that in the conventional theory all rates at the source are decreased by a factor ##\left( 1 + z \right)^{-1}##, while in tired light theories there is no such slowing down. One rate that is slowed down at large redshifts in the conventional theory is the rate at which photons are emitted by the source. This is responsible for one of two factors of ##\left( 1 + z \right)^{-1}## in ... apparent luminosity, the other factor being due to the reduction of energy of individual photons. On the other hand, if the rate of photon emission is not affected by the redshift, then in a static Euclidean universe in which photons lose energy as they travel to us, the apparent luminosity of distant source ... will be given by ... only a single factor of ##1 + z## in the denominator.

Lubin and Sandage have used the Hubble Space Telescope to compare the surface brightness of galaxies in three distant clusters ... quite inconsistent with the behavior ... expected in a universe with 'tired light'. ...

In the standard big bang cosmology all rates observed from a distant source are slowed by a factor ##\left( 1 + z \right)^{-1}##, not just the rate at which photons are emitted.This slowing has been confirmed for the rate of decline of light from some of the Type Ia supernovae used by the Supernova Cosmology Project ...

Modern cosmology is in a Golden Age of observations, and these observations have driven nails into the coffins of tired light theories.

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strangerep
Gravitational time dilation in expanding universe models predict that the rate at which photons leave a distant source is less that rate at which photons are received here. This leads to observable effects that are not predicted by tired light theories. [...Weinberg quote...]
Thanks for that reminder. (For other readers, that Weinberg quote is on p58, as part of section 1.7.) I've been reading Weinberg's "Cosmology" for the first time recently, and it's quite amazing how much can be deduced about competing models and compared with experiment.

Modern cosmology is in a Golden Age of observations, [...]
Indeed. I begin to grow more interested in cosmology than QFT these days.

George Jones
Staff Emeritus
Gold Member
Gravitational time dilation in expanding universe models predicts that the rate at which photons leave a distant source is less that rate at which photons are received here.

Oops. Should be the other way around.

Thanks for that reminder. (For other readers, that Weinberg quote is on p58, as part of section 1.7.) I've been reading Weinberg's "Cosmology" for the first time recently, and it's quite amazing how much can be deduced about competing models and compared with experiment.

Indeed. I begin to grow more interested in cosmology than QFT these days.

I am not sure, but you might also like the quite different advanced cosmology survey book "Relativistic Cosmology" by Ellis, Maartens, and MacCallum:

https://www.amazon.com/dp/0521381150/?tag=pfamazon01-20

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strangerep