# A basic question about redshifting

1. Jul 4, 2015

Hey everyone,

I get it - the faster the object is moving away from me, the more the light it emits moves to the red side of the spectrum.

But here's my question:
Here on earth, we watch galaxy A, which seems to have a very red light. How can we know it's not "originally" that red, and if it hadn't moved at all, it would've been a bit bluer? Where's that information embedded?

As an example, if I sent you a sample of a man talking with an extremely low voice, you wouldn't be able to say it's "originally" medium-pitched, and the reason it's low now is because the man has been moving away from me while recording?

Thanks.

2. Jul 4, 2015

### Orodruin

Staff Emeritus
You would have to know the usual pitch of the man's voice. Luckily, we do know in the case of redshifting galaxies, since we can look at different spectral lines and identify where they belong.

3. Jul 4, 2015

### Staff: Mentor

In the spectral lines! See the image below. The top spectrum is a spectrum showing absorption lines. These lines correspond to a specific element and only that specific element. As the spectrum is redshifted from either relative motion or expansion, the lines move towards the red end but, crucially, keep their exact same pattern. By studying the redshifted spectrum and comparing the lines to where they are in the unshifted spectrum we can identify how much the redshift is and from there figure out the recession velocity of the galaxy.

Image from here: http://coolcosmos.ipac.caltech.edu/cosmic_classroom/cosmic_reference/redshift.html

https://en.wikipedia.org/wiki/Absorption_spectroscopy
https://en.wikipedia.org/wiki/Emission_spectrum

4. Jul 5, 2015

Awesome!

Thanks!

5. Jul 5, 2015

### Jamie Harper

Note: it's not actually that the object is moving away from you which causes the red-shift. More correctly: it is not *exclusively* the fact that the object is moving away which causes the red-colouration.

Red-shift is caused by the expansion of the universe, sure. But a factor in the red-shift (a change of the wavelength of the EM radiation being observed/measured by a device that can measure wavelengths of EM radiation) is that the very universe (which is acting as the medium for the light waves) *itself* is expanding.

It's sort of like the police car driving past your house: the siren changes note. This effect (Doppler) is because the relative space of the air being affected is warped massively by the movement of the device creating the noise. i.e. the thing that is vibrating to impact the molecules of air is impacting them in a warped manner; as it moves. i.e. without the movement it might strike <this> air molecule, but with the movement, it strikes <this other> air molecule, in a different location. This gets "translated" by our ear to indicate a change in pitch, or an alteration in the note we "hear". This is merely just a weakness in our auditory system to lack the ability to account for the movement of the police car's siren, and the resulting warping of the audio signature being given out by the siren's speaker (which note is just an EM radiation device; using energy in the form of electricity to move the speaker unit, which is a collection of other atoms, moveable by the manipulation of an EM field).

So it is with the light waves.

With light waves received from distant objects: the very "space" or "nothingness" (or Higg's field/vaccuum of somethingness) (between us and the distant object) that is carrying the wave [carrying the energy pattern that we call "light"] i*itself* is growing.

It's like the universe is a police car driving rapidly past our house; but instead of one piece of it moving (the car), it's all moving: every part of it. Every part of the universe is growing outwards from each other part of itself.

Hence, exactly like the police car siren (which is trying to manipulate air molecules, which we call "sound"), the light from distant universes is also warped because of this movement.

Looking in the specific sense: the peak of the (light/audio) waveform impacts a slight different part of space than it would, without the expansion. Likewise the trough of the waveform impacts a slight different part of space that it would have, without the expansion. So with sound, as it is with light.

We "see" this light warping as "red-shift", but in reality, we could translate it back to "normal light" with a simple recalculation (one that assumed no expansion of space/or tried to counter-act what occurred). Indeed; the concepts of red and blue are just humanity's interpretation of the different wavelengths of light (EM radiation), and it's using these arbitrary units/colours which perhaps causes confusion.

Also - note that your statement "I get it - the faster the object is moving away from me, the more the light it emits moves to the red side of the spectrum" confuses the issue of the light being emitted. It is not correct that "red light" is being emitted by anything. What is being emitted is light (white, presumably, in this example), and as it travels to us, it is warped by the expansion of the universe: i.e. the expansion of the medium carrying the signal (relative to what we'd call "normal").

6. Jul 6, 2015

Hey Jamie,

Thanks for the subtle but significant correction.

I think I'm starting to really like that forum!

7. Jul 8, 2015

### Finny

"....It's sort of like the police car driving past your house: the siren changes note. This effect (Doppler) is because the relative space of the air being affected is warped massively ....... without the movement it might strike <this> air molecule, but with the movement, it strikes <this other> air molecule, in a different location. This gets "translated" by our ear to indicate a change in pitch, or an alteration in the note we "hear". This is merely just a weakness in our auditory system to lack the ability to account for the movement of the police car's siren........,

not so....The siren does not change note, it is the received signal that appears 'changed'. The change in pitch is due to the relative movement of the source and has nothing to do with any changes in bumping molecules except that's how the sound [wavelength or pitch] is transmitted. And that happens whether the source and receiver are stationary or in relative motion with each other. That is nothing like the transmission of light: Light requires no transmission medium as does sound.
"So it is with the light waves. "

As explained, no, it's very different than sound
"...With light waves received from distant objects: the very "space" or "nothingness" (or Higg's field/vaccuum of somethingness) (between us and the distant object) that is carrying the wave [carrying the energy pattern that we call "light"] i*itself* is growing...."

Space is NOT nothingness...it has vacuum [or zero point] energy for example. And includes spacetime of relativity.
"...It's like the universe is a police car driving rapidly past our house; but instead of one piece of it moving (the car), it's all moving: every part of it. Every part of the universe is growing outwards from each other part of itself...."

This is redshift [due to cosmological expansion] as contrasted with the doppler shift of peculiar [local] motion.
"Hence, exactly like the police car siren (which is trying to manipulate air molecules, which we call "sound"), the light from distant universes is also warped because of this movement."

no, it is quite different. And neither involves 'warping' nor 'manipulation'. Sound signals simply cause air molecules to vibrate and that's what the ear records..slight changes in pressure.
"Looking in the specific sense: the peak of the (light/audio) waveform impacts a slight different part of space than it would, without the expansion. Likewise the trough of the waveform impacts a slight different part of space that it would have, without the expansion. So with sound, as it is with light."

No, again, very different concepts between increasing distance in cosmology and air molecules carrying the energy of sound.
"Indeed; the concepts of red and blue are just humanity's interpretation of the different wavelengths of light (EM radiation), and it's using these arbitrary units/colours which perhaps causes confusion."

There is no confusion: The ear distinguishes different sound wavelengths and our instruments as well...as spectral shifts.
"...It is not correct that "red light" is being emitted by anything. What is being emitted is light (white, presumably, in this example), and as it travels to us, it is warped by the expansion of the universe: i.e. the expansion of the medium carrying the signal (relative to what we'd call "normal").

This is correct: the source emission characteristic, say a fixed wavelngth, is what it is. It is our measurement of the received signal that is changed by the relative cosmological expansion or motion of the source relative to us.

8. Jul 8, 2015

### Finny

A more reasonable analogy would be that as a police car recedes due to relative motion, it causes wavelengths of successive emissions to be extended.....a lowering of pitch, a 'redshift', analogous to cosmological expansion which also results in relative motion of emitter and receiver. And if the police car is accelerating away like cosmological expansion, the redshift in both continues to become greater and eventually fades to nothingness.
But even that is an over simplification.

9. Jul 11, 2015

### AC130Nav

I'd like a little clarification. I know the redshift caused by the constantly redefined distances in our part of space cause the redshift. That's OK.

I believe, however, that I've heard of redshifting of say the stars on the receding edge of a galaxy. The problem is that the speed of light is invariable and must always be measured the same. How then can the standard meaning of a Doppler shift be maintained?

10. Jul 11, 2015

### AC130Nav

I did not doubt that the frequency of light could be changed by the "expansion" of the universe, which even allows for observed speed of separation of distant object greater than c.

My point was that the Doppler theory proposes a frequency shift based on differing the speed of the wave by a relative motion the observer. This is forbidden where c is concerned.

11. Jul 11, 2015

### Bandersnatch

See here:
https://en.wikipedia.org/wiki/Relativistic_Doppler_effect
That's what's used for light.
(i.e., for peculiar velocities, not the cosmological redshift)

12. Jul 11, 2015

### Joe Ciancimino

Take for example the basic concept of putting a headlight on the front of a spaceship traveling at 80% of c. It all makes perfect sense when you look at the solution. But now place the headlight facing backwards and try to come up with the same answer. You are traveling one direction, light traveling the opposite direction. We know what the answer is, but it does not quite fit the conceptual model. Same goes with comparing the age difference from a stationary observer. If one person aged more than the other, then how is it they can still communicate? Should there not be a time delay when one person talks and the other one listens after the effect of the time dilation? Then when they reply the other person remembers them speaking in the past? Now we know there is not such a delay, but the theory really does not do well to explain why there is not one. Hence this is why they always refer back to time being in independent frames. If you try to make relationships with the frames them self, the theory breaks.

13. Jul 11, 2015

### AC130Nav

Thanks, Bandersnatch. That does look like the state-of-the-art answer.

14. Jul 11, 2015

### Bandersnatch

What seems to be the problem here? The relativistic Doppler shift works for both cases. For example, when measuring rotation of a galaxy or a star from Doppler shift of the opposite limbs, you're observing both 'a headlight facing forwards' and backwards w/r to motion of the emitter.

Why shouldn't they be able to? They will only disagree on time of emission and reception of signals. Have you ever tried drawing it out on a space-time diagram?
http://ibises.org.uk/Minkowski.html (tool made by PF member Ibix)
and select one of the preset twin paradox scenarios with signalling.
(I assume you meant communication in-flight here)

15. Jul 11, 2015

### Staff: Mentor

I disagree. It seems to fit the 'conceptual model' just fine in my opinion.

No, there's shouldn't be. Time dilation is not some lingering effect. Once the moving observer stops moving, then there is no longer any time dilation and everything works just like 'normal'.

Again, I disagree. It may not be obvious, but that doesn't mean something is wrong with the theory.

Last edited: Jul 12, 2015
16. Jul 12, 2015

### Staff: Mentor

This thread has undergone a massive cleanup and many of the later responses no longer make sense because of it. This thread will remain locked. Anyone interested in the original topic should feel free to create a new thread with their questions.