# Red shift of light from galaxies

1. Feb 9, 2014

### Baggins101

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.

2. Feb 9, 2014

### Staff: Mentor

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.

Last edited: Feb 9, 2014
3. Feb 9, 2014

### Baggins101

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?

4. Feb 9, 2014

### Bill_K

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.

5. Feb 10, 2014

### Baggins101

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!)

6. Feb 10, 2014

### Staff: Mentor

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.

7. Feb 10, 2014

### 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.

8. Feb 10, 2014

### 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."

9. Feb 10, 2014

### George Jones

Staff Emeritus
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.

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

Last edited: Feb 10, 2014
10. Feb 10, 2014

### strangerep

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.

11. Feb 11, 2014

### George Jones

Staff Emeritus
Oops. Should be the other way around.

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/Relativistic-Cosmology-George-F-Ellis/dp/0521381150/ref=pd_sim_b_1

Last edited by a moderator: May 6, 2017
12. Feb 11, 2014

### strangerep

Thanks. It turns out I had casually perused a copy of that book a few weeks ago, but then forgot about it.

I'll take another, closer, look.

Cheers.