# Information for the uninformed

• James23
In summary: Doppler Effect is only one of several things that can cause redshift. Other things include the expansion of space itself, the recessional velocity of the source, and the gravitational pull of a larger object on a smaller object.
James23
I am in college and have no formal training in physics, but i really enjoy the field. I was reading about redshift and am currently stumped in regards to its relation to light. I have read that light is constant, but in redshift the wavelengths of the light lengthen. I was wondering if light is constant shouldn't the wavelengths remain the same length when traveling through space? Or do the different wavelengths of light still travel at the same speed? Again i am a layman so please help to shed some light on this topic. i would greatly appreciate it.

I'll try and have a go at answering your question, but feel free to jump in if I've not answered the correct question! Perhaps you recall from some introductory physics course (if you've taken one) that the wavelength, velocity and frequency of light are related by c=fl, where f is the frequency, l is the wavelength and c is the speed of light. Thus, when we talk about light being red-shifted, its wavelength increases, but its frequency decreases, thus leaving c constant.

James23 said:
{snip} I was wondering if light is constant shouldn't the wavelengths remain the same length when traveling through space? Or do the different wavelengths of light still travel at the same speed?

Only the speed of light is constant.

But its energy can change.

And energy is dictated by wavelength (=1/frequency).

So different photons of light can have different wavelengths, but they do all travel at the same speed!

cristo said:
I'll try and have a go at answering your question, but feel free to jump in if I've not answered the correct question! Perhaps you recall from some introductory physics course (if you've taken one) that the wavelength, velocity and frequency of light are related by c=fl, where f is the frequency, l is the wavelength and c is the speed of light. Thus, when we talk about light being red-shifted, its wavelength increases, but its frequency decreases, thus leaving c constant.

If l/c =T, in what frame does "T" apply, and what does it apply to?

Regards,

Bill

tiny-tim said:
Only the speed of light is constant.

But its energy can change.

And energy is dictated by wavelength (=1/frequency).

? I thought wavelength = c/f.

Regards,

Bill

I think that the redshift of distant galaxies is caused primarily by cosmological expansion. That might be part of the confusion. Essentially it's something like the lightwave has gotten physically stretched over astronomical lengths of time as it traveled (because the spatial dimensions of the universe itself have gotten stretched since the time the light wave began traveling.)

CaptainQuasar said:
I think that the redshift of distant galaxies is caused primarily by cosmological expansion. That might be part of the confusion. Essentially it's something like the lightwave has gotten physically stretched over astronomical lengths of time as it traveled (because the spatial dimensions of the universe itself have gotten stretched since the time the light wave began traveling.)

I don't think so.

I think the redshift is more commonly attributed to the recession velocity of the source(s) - which, consequently, is the basis for the "cosmological expansion".

Regards,

Bill

Antenna Guy said:
I don't think so.

I think the redshift is more commonly attributed to the recession velocity of the source(s) - which, consequently, is the basis for the "cosmological expansion".

Regards,

Bill

The reason why distant galaxies are receding isn't because they're moving, like they were flung away from our galaxy by the Big Bang or something; it's because space is stretching. See this http://www.astro.princeton.edu/~aes/AST105/Readings/misconceptionsBigBang.pdf" article that marcus frequently posts:

Scientific American said:
WHY IS THERE A COSMIC REDSHIFT?
WRONG: Because receding galaxies are moving through space and exhibit a Doppler shift.
RIGHT: Because expanding space stretches all light waves as they propagate.

Sorry, I should have quoted that in the first place.

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CaptainQuasar said:
Sorry, I should have quoted that in the first place.

That would imply that absorbtion bands shift as a function of time!

Regard,

Bill

Antenna Guy said:
? I thought wavelength = c/f.

Sorry!

Some books measure time and space in units in which c = 1 - and some don't - and I'm used to the ones that do!

Antenna Guy said:
I think the redshift is more commonly attributed to the recession velocity of the source(s) - which, consequently, is the basis for the "cosmological expansion".

Yes, red-shift (and blue-shift) are non-relativistic effects that Newton knew all about, and depends only on the recession velocity of the source.

Antenna Guy said:
That would imply that absorbtion bands shift as a function of time!

Absorption bands? Do you mean frequency and wavelength shift as a function of time? I think that would be a yes, unless the author of that Scientific American article was wrong.

tiny-tim said:
Yes, red-shift (and blue-shift) are non-relativistic effects that Newton knew all about, and depends only on the recession velocity of the source.

Cosmological expansion isn't a relativistic effect. Are you saying that the Doppler Effect is the only thing that can cause red-shift, even on a cosmological scale? I must respectfully point out that the above article I linked to seems to disagree with you.

How not to explain pool!

CaptainQuasar said:
Cosmological expansion isn't a relativistic effect. Are you saying that the Doppler Effect is the only thing that can cause red-shift, even on a cosmological scale? I must respectfully point out that the above article I linked to seems to disagree with you.

(Scientific American articles are far too long.)

I'm saying that the original post in this thread was asking about the relationship between wavelength and velocity.

That relationship is redshift.

It's a straightforward effect, familiar to us even before relativity.

Introducing cosmological expansion to explain redshift is like introducing nuclear physics to explain pool!

tiny-tim said:
I'm saying that the original post in this thread was asking about the relationship between wavelength and velocity.

Yes…

tiny-tim said:
That relationship is redshift.

Redshift isn't a relationship, it's a phenomenon. The relationship between wavelength and velocity is v=λf or some other formulation of that like Bill mentioned.

tiny-tim said:
It's a straightforward effect, familiar to us even before relativity.

I don't understand why you keep mentioning relativity. No one else in this thread has mentioned relativity.

tiny-tim said:
Introducing cosmological expansion to explain redshift is like introducing nuclear physics to explain pool!

Unless James23 is reading about astronomy and that's why he's asking about light “traveling through space.” In which case the important redshift, the one that let's us gauge how old the universe is and how distant astronomical objects are from us, is the one caused by cosmological expansion.

In that case it's particularly unhelpful for you to make statements like saying that red-shift “depends only on the recession velocity of the source.” He's trying to understand how red-shift relates to the speed of light, so avoiding a frequently-mentioned cause of red-shift that involves no change in the speed of the wave seems more confusing than anything to me.

If there's anything he doesn't understand he can just ask questions.

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CaptainQuasar said:
I don't understand why you keep mentioning relativity. No one else in this thread has mentioned relativity.

You introduced "cosmological expansion" - that is relativity!

… so avoiding a frequently-mentioned cause of red-shift that involves no change in the speed of the wave seems more confusing than anything to me.

Are you saying that cosmological expansion changes the speed of light?

tiny-tim said:
You introduced "cosmological expansion" - that is relativity!

I don't think it is, I thought it was cosmology. Do you think it's part of Special Relativity or General relativity?

tiny-tim said:
Are you saying that cosmological expansion changes the speed of light?

No, it changes the wavelength and frequency at the same time without changing the velocity, that's exactly why it's relevant to the question.

He asked,

James23 said:
I was wondering if light is constant shouldn't the wavelengths remain the same length when traveling through space?

Cosmological expansion is the only way I know of that the wavelength might change while the light is traveling through space. The wavelength of light red-shifted due only to Doppler effects would remain constant.

CaptainQuasar said:
I don't think it is, I thought it was cosmology. Do you think it's part of Special Relativity or General relativity?

Cosmology is definitely part of General relativity!

No, it {cosmological expansion} changes the wavelength and frequency at the same time without changing the velocity, that's exactly why it's relevant to the question.

wavelength = c/frequency anyway, and you agree c is constant, so it's the same effect, but a far more obscure example.

tiny-tim said:
Cosmology is definitely part of General relativity!

Well technically everything physical is affected by GR, including the wave mechanics of light we're discussing. But you don't need to understand relativity to understand cosmological expansion or the relationship between wave velocity and wavelength and frequency.

tiny-tim said:
...it's the same effect, but a far more obscure example.

As a non-physicist that is not my experience. I hear about astronomical applications far more frequently than I ever hear about redshift in a moving terrestrial source of light. And the use of redshift to measure the age of the universe and the distance of galaxies and quasars - the redshift caused by cosmological expansion - is by far the most frequent mention of redshift within astronomy that I hear of.

And as I pointed out, the Doppler effect does not change the wavelength of light as it's moving through space, which is what he asked about.

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Everyone seems to be taking this discussion a little too far, if you ask me. I saw a question that asked about the redshift of light, not about cosmological expansion, or relativity. Perhaps we should wait and see what James thinks before guessing which question he want answering.

CaptainQuasar said:
The wavelength of light red-shifted due only to Doppler effects would remain constant.
This isn't correct: the doppler shift is defined as a change of wavelength and frequency of the wave (see http://archive.ncsa.uiuc.edu/Cyberia/Bima/doppler.html).

tiny-tim said:
Cosmology is definitely part of General relativity!
I would say that, today, general relativity is a part of cosmology, in that cosmologists use GR since it is our current theory of gravity, but cosmology is not a subset of GR.

cristo said:
Everyone seems to be taking this discussion a little too far, if you ask me. I saw a question that asked about the redshift of light, not about cosmological expansion, or relativity. Perhaps we should wait and see what James thinks before guessing which question he want answering.

Good point, I agree.

I just think it's important to mention various possible sources of red-shift, particularly since, as I said as a non-physicist (and non-astronomer), it appears to me that one is much more likely to encounter mention of red-shift as a consequence of cosmological expansion. I think it was for something like ten years that I had the faulty understanding that the primary source of astronomical redshift was due to velocity.

cristo said:
This isn't correct: the doppler shift is defined as a change of wavelength and frequency of the wave (see http://archive.ncsa.uiuc.edu/Cyberia/Bima/doppler.html).

Oh, I didn't realize that - so even red-shift due to cosmological expansion rather than the velocity of either of the sources is considered doppler shift? If so I apologize.

Thank you all for taking the time to answer my question. I was just curious about the relationship between the speed of light and redshift, which you answered, so again thanks.

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