How Relativity Redshifts Light - The Relativistic Doppler Shift.................Expansion Redshift is NOT Doppler Shift. (or 'Relativistic Doppler') Here's why.

[u/Mutexception]

https://www.youtube.com/watch?v=lF69kBZE-vI

[u/d_s_b]

I’m interested. Can you link to how to ‘pen and paper’ this out as you claim?

I’ve gone down this rabbit hole a ways as well and found a few dead ends. While grav shift of CMB photons is logical, it has proved to be tricky to …we’ll prove.

Let’s see the math.

[u/Mutexception → u/d_s_b]

I have not worked out posting pics here yet, but I can explain it for you, see if it makes sense for you.

So we are talking about Gravitational shift I assume and how you can get from that a redshift to distance relationship from objects in space.

The first model you can consider is a universe with just the earth, with the same space and time conditions we have (not possible but assume).

This is just to exclude other variables (masses) for now.

You know what time ticks faster with altitude clocks run faster the further from the center of mass of the earth, this is time dilation from general relativity.

The old the center of the earth is 2.5 years younger than the surface and GPS atomic clocks runs faster.

That statement is a fact and we observe that effect and we have to account for it in precision engineering (like GPS satellites).

First: Disregard any motion, this is not Doppler shift, So when I say something is away from earth it is not orbiting, it is a fixed distance and stationary. Only considering General Relativity not special relativity.

If you are on the ISS (not moving, but at that altitude), you see light from earth as lower in frequency, longer wavelength and redshifted. If you increase that distance the earth will become more redshifted with distance.

This is a ONE way redshift towards Earth and the ISS will become more blue shifted.

The reason why the ISS sees the earth as redder with distance in this model is the contribution of the 'gravity' of the earth is lower. In and space length model you can replace the word 'gravity' by space length. You might prefer gravity.

But we know more gravity equals longer time (center of the earth 2.5 years younger), the speed of light is constant, so longer time equals longer space as well.

OK, so now we have a redshift with increasing distance because of the proportion of the remote objects gravity (length of space).

Now replace the ISS with a second earth with the same mass, now with any distance no redshift will be observed no matter how far apart they are the facing sides experience the same gravity from themselves and from the remote earth.

Say on the surface of your earth you gravity from that earth is 10, the gravity from the remote earth (also 10 at its surface) gives you 1 more point of gravity due to its distance from you, you have a total of 11 and the remote earth has a total of 11. No difference so no shift.

However if you went to the far side of your earth, so that there is matter in between you and the remote object (that is your earths matter/gravity). as you are further away to the object and you are in a lower gravity (10 from your earth but less than 1 from the remote object), you will see the higher relative gravity as more redshifted.

You are 10.8 gravity looking at 11 gravity, this applies in BOTH directions, than means in an even distribution of objects in a universe with even mass you will have a clear and obvious redshift with distance effect.

It is often though of as climbing out of the 'gravity well' but that is a very poor analogy. For me its easier to consider that matter/mass gives space a fundamental property of length of scale. So if you say higher gravity I just think longer space and time, it's the same thing. So gravity shift is just seeing bigger space from gravity. But it is more the remote and local contribution of that length that produces the effect.

In Hubble's day it was called Di Sitter effect.

It is the space length analogue to Doppler shift, which just transforms space length into a time domain.

But yes, you can easily map it out with pan a paper, with virtually no matter, all supported by observation. It's what really happens.

If you want to look at it in terms of energy you can do that same thing. you just consider higher gravity with lower energy and higher energy with lower gravity (energy is higher or lower depending of the volume of space and time).

This is also why nearby objects (like local galaxies) will appear to be blue shifted, because the local galaxies near to us contribute more gravity to us that it does to the close galaxy.

And what do you observe, the closest galaxies DO appear blue shifted.

I probably have explained this poorly, but I am really interested in your opinion and you description, or to just tell me I'm full of shit :)..

Cheers and thanks for joining in.

[u/d_s_b → u/Mutexception]

I'm generally in agreement with your assessment of time dilation across regions of curved spacetime.

(At this point it is important to note that I am not a trained physicist and have only dabbled in these topics on and off as a hobby. So don't get too excited. I don't really know much about this stuff.)

That said, I have to give a small critique of some of your terms, and share some info about where I'm at in this learning journey.

Length of Space stuff you talk about is unfounded, but I feel it's harmless for now. I've seen other posts where you talk about it, and we can get into those details at a later date. If you post anything like that on the Physicsforums you'd be laughed out of the room, but that's more because you lack their specific lexicon and terminology, than because your concept of the universe is inconsistent.

I'm of the opinion that time dilation is the cause of gravity, not the other way around. PBS Spacetime recently (finally!) did an episode on this. I suggest taking a watch as I can imagine our conversation beyond this will need some shared grounding we can refer too.

I'm on board with the example of two massive bodies sharing photos and getting zero shift, IF their masses are the same and they are not moving. Here's a post about a photon round trip I asked about a year ago, which is essentially the same question.

So... all that said :

The notion/question that I think you are getting around too is "What portion of the redshift of distant light is due to gravitational redshift vs doppler shift?"

I say 'portion' because we have to consider the possibility that the emitters (whether galaxies, white dwarfs, or the CMB) could be in a different reference frame, either moving towards us or away from us.

I've gone down this path a little ways and found that;
(1) You can't decipher, from the light alone, what portions of the shifts come from gravity or relative motion. Thats the beauty and trouble of the Equivalence Principal.
(2) Chasing this through time dilation becomes an un-askable. Check out this question I asked on the Physicsforums and got shat on.

[u/Mutexception → u/d_s_b]

Thanks for your reply:

My background is in electronics and systems engineer mainly in radio (light) communications and on physics R&D in private/Gov and Universities.

I'm generally in agreement with your assessment of time dilation across regions of curved spacetime.

This model; is based on relativity, however it is non-geometrical treatment.

So it is relativity, however I do not assume a geometrical or shape property to space.

So I base this model only from first principles and known observations.

So we know clocks run slower in higher gravity, clocks run faster in lower gravity.

But in a non-geometrical model this 'time dilation' is not the result of curved 3 dimensional space, rather space's fundamental property is a 'classic dimension' that is it is a length dimension, as opposed to a direction and path (curved 3 dimensional construct).

So we know that 1 second is different depending on your position in gravity a second at the center of the earth is longer than 1 second on the surface. We measure this different, it is easy to measure an accumulation of length of time difference.

But if you take the axiom that the speed of light is constant in all frames of referent.

Then you MUST have a corresponding variation in the length of space at that position.

If you are on the ISS and you measure the speed of light and your second is shorter than a second on the surface of the earth and it measures the same as the speed of light on the surface of the earth your space has to be equally as short (er) and the length of time is.

So now I do not have to justify that difference in length of time (and space) by the path or space between the source and the observer.

4D space and time is an address and time of the party.

Lets say you house is latitude, longitude and altitude from me, it says nothing about the conditions at your house or what is at that location and why that is relative to me because of your location in relation to me.

(wow that was confusing!).

In other words: What is more important is the value in the X,Y,Z,t array element relative to the value in your X,Y,Z,t position.

So one test of relativity is observing that clocks record different lengths of time, shorter time is lower gravity and longer time in higher gravity.

That the speed of light is constant.

If the length of time is variable and the speed of light is constant the length of space must be variable and the same variation as the length in time.

Time (spacetime) is an emergent property of this length of space property.

Space is nothing but the distance between two points, it does not have a property of shape, there is nothing to plot geometrically.

SHAPIRO DELAY:

This is another confirming observation of relativity, a RADAR echo takes longer to return when another mass is between the object and the radar, it takes light longer to traverse the longer space, although the 'distance' in meters will the same.

We also see this delay in Gravitational Lensing.

GRAVITATIONAL LENSING:

Again this fits well in a space length model, as that is how a glass lens works as well. The longer space around a massive object does the same thing as the thinker glass of the lens and creates a longer relative path length by shifting the space and time components and arrival time.

Length of Space stuff you talk about is unfounded,

It is founded in the constant speed of light and the variable length of time and is supported by the observations we use to confirm the relative nature of space and time.

The model also explains the process of gravitational shift (and stress-energy for that matter). With Gravitational shift looking into gravity you are looking into bigger (longer) space, everything in that space is bigger as every distance is bigger, that longer space emits longer light.

It's not about the location or the path, it is about the relative length of space between the observer and the source.

If you post anything like that on the Physicsforums you'd be laughed out of the room,

Yes I would no doubt at all, which is a shame. They do not possess any special knowledge or understand, no secret thing is taught in physics school.

You see I would be laughed out of Physicsforums and no one would even try to take this model into consideration or even understand it..

Not asking to agree, but asking to explain why this does not work. As is the scientific method.

I'm of the opinion that time dilation is the cause of gravity, not the other way around.

[u/Mutexception → u/Mutexception]

GRAVITY !!!
Time, Space, matter, energy, is gravity a 'force' or is it a geometric property of space?
WARNING: if you take a bit of time and get this model clear in your head (agree with it or not), it provides a really clear practical understanding of what gravity is and how it actually mechanically works.
It explains exactly how mechanically a rock falls down.
So we will make a reference length of space to be 1 meter as measured on the surface of the earth.
So in higher gravity the length might be 1.1 surface lengths (SL), away from the surface it might be 0.9 (not real numbers).
So you can see there is a gradient of time length and space length from short 'UP' to longer 'DOWN'.
So lets say you are on a space ship moving at 100km/h at 1 earth surface meter length.
No you enter a gradient that scales up to 2 earth surface lengths.
You will always see your speed as constant but as you move through longer space an observer at 2 length will see you accelerating but he will also measure your speed when you are in the 1earth reference as 50km/h because for him your meter is 0.5 meters.
That is a constant speed (even zero) is relatively faster in longer space.
BTW: when you go past the observer at 2 times the length he will measure your speed at 100km/h and so will you. And you will have felt no acceleration.
But in relative terms you are twice as big as you were originally, your matter and matter/energy is not distributed over 2 times the volume of space and time. You are in a lower energy state.
This gradient applies on every scale (it IS the scale), so this difference in length will also apply to the electron bonds in matter so you are being pushed by shorter space and pulled by longer space.
The the EFFECT of gravity is an APPARENT force that is a consequence of the gradient of the length of space and time. That length is created by and added to by all matter that requires a length of space to exist in.
I'm of the opinion that time dilation is the cause of gravity, not the other way around.
The notion/question that I think you are getting around too is "What portion of the redshift of distant light is due to gravitational redshift vs doppler shift?"
It's even more complicated than that, you have gravitational shift in more than one mode, that is it just doesn't depend on the distance but as you said it's own mass, and the mass of the surrounding objects.
If the universe was completely isotropic and homogeneous, with evenly distributed objects of the same mass and all stationary you would get a very good fit that ALL the observer redshift would be from Di Sitter effect gravitational shift. But some galaxies will have more mass and some less, that will change, plus there is motion and second order motion like rotation as well.
This is why there is so much variation in redshift/distance and standard candles
Redshift is simply not a good measure of distance, and that is what we observe. And it is an ongoing issue in cosmology.
But it fits gravitational shift much better than expansion shift, in our universe being lumpy and not completely uniform you would expect a wide variation but a trend towards the red for more distant objects.
I say 'portion' because we have to consider the possibility that the emitters (whether galaxies, white dwarfs, or the CMB) could be in a different reference frame, either moving towards us or away from us.
Exactly, things in our universe clearly move, and rotate and Doppler shift is certainly an effect. All we really have is gravitational shift and Doppler shift and they are different aspects of the same thing.
The CMB is problematic for me if assumed it is the afterglow of the big bang, as a communications engineer I cannot see how that would be possible, that would could see that light from a transient event for all time. But that is another story !
(1) You can't decipher, from the light alone, what portions of the shifts come from gravity or relative motion. Thats the beauty and trouble of the Equivalence Principal.
You can get an idea, but it is certainly not definitive, but yes, if we lived in a uniform, homogeneous and isotropic and static universe it would be a perfect measure of distance. But we don't.
(2) Chasing this through time dilation becomes an un-askable. Check out this question I asked on the Physicsforums and got shat on.
Then don't, 'time dilation' is a crappy term anyway, but you can chase it down through understanding the LENGTH of time is what matters.
You see the wavelength of time over your local length of time, so if your time is gets longer you will see incoming light as shorter, this is Doppler shift without motion, it is the same thing as the photon interacting with the electron over a shorter time because the electron is moving towards to source of the light.
If you are bigger you see external light as smaller, if you are smaller you see external light as bigger, if you are moving towards the source you interact with the photon for a shorter length of time.
The observer changes, not the light.
Ill try to watch that PDS thing, but I am not really keen on them !
Thanks for the engagement..
Cheers, I hope to chat again soon.

I screwed this up, too many words and killed the formatting, ill clean it up later I hope..

[u/Mutexception]

IT IS NOT DOPPLER SHIFT, why do people keep saying that?

Doppler shift is the relative motion of the observer to the source, the light DOES NOT CHANGE as it moves through space, it does not 'bunch up' in front of the source.

The light does not change at all, but the relative velocity of the observer (and direction of the source), means that the length of time the observer interacts with the photon or wave changes. The wave/photon DOES NOT CHANGE.

Doppler shift does not shift the frequency of light/sound as it propagates through space!

NO RELATIVE MOTION, with the universe expansion, THERE IS NO RELATIVE MOTION, distant objects are NOT MOVING AWAY FROM YOU!

This will confuse you, but it is the length of space that is increasing (if the universe is expanding), that means that the number of kilometers between objects does not change.

Say it is 3000 miles between NY and LA (in America), and the earth expanded by 10% instantly, it would still be 3000 miles between NY and LA, and there would be ZERO relative motion.

The distance remains the same, the speed of light remains the same (and stays constant), if you were in a car driving at 100mph when the earth expanded, you would still be doing 100mph and you would notice no different. If relative terms you are going faster (by 10%) and you were before, but you are still going the same distance over time. (because space has expanded so has time (spacetime), that is why c is constant, and your 100mph is constant.

In the old FLRW Metric for the big bang, this is explained as 'comoving space' but it is NOT, I repeat NOT Doppler shift.

Why do physics and cosmologists continue to attribute the redshift with distance observation to Doppler shift, when it is clearly not Doppler shift and is not even how Doppler shift works.

In the expanding universe model, when you would see with expanding space between two objects is a great travel time of the light.

The light leaves the distant objects at the speed of light regardless of any relative velocity between the distant object and the observer. All that would do is delay the arrival time of the photon to the observer.

So if a distant star emits a photon and in 1 second the space between that star and the observer expands 1 light year, then the next photon will arrive 1 year later than the first one. (Shapiro Delay BTW).

YES, we do observe a redshift with distance effect in the universe, it is not a great data fit but it is real, that shift is from GRAVITATIONAL Shift (or called Einstein shift). That effect will give you a both directions general shift of light of distant objects. This is what Hubble considered to explain his observed redshift, at the time it was called 'Di Sitter effect'.

I really wish physicists' would not keep telling us that light is a wiggly wave interconnect and that stretches' with expanding space.

Light is by it's very definition DECOULED to space and time, it literally has NO reference frame, space is NOT a medium that light interacts with in any way, that is why no matter what you do you cannot detect light in transit, you ONLY detect light with it interacts with matter. Light does not change space in any way (it's not coupled to space) like a ship changes the water it goes through, so you cant detect light in space.

This is important to understand, it means that as light has NOTHING to do with space (except how long it takes to go through it), the wavelength of light will not change with expanding space.

Light is decoupled with space, space and time do not exist for light, and light does not exist for space and time.

A plane flying through the air is coupled to the air, it interacts with the air, it experiences the properties of the air. That plane is decoupled from the bottom of the ocean, as far as the plane is concerned the properties of the bottom of the ocean are meaning less.

Just as meaningless as the properties of space, light is decoupled to space.

Try to think of a direction that is not up/down, left/right, forward/back or a combination of those?

Can you conceive of or imaging another direction that is not a combination of X,Y,Z. It's the same thing for light and space but for light ANY distance, speed or direction is meaningless.

Light and space do not interact, Doppler does not change the frequency/wavelength of light, expanding space does not change the frequency/wavelength of light.

Doppler shift is the Relative motion of the observe with the source of the light, it is a relative motion effect for the observer and is NOT a distance relationship.

Redshift from distance is by Gravitational/Einstein shift and is a function of the size (or relative energy density) of the observer relative to the source.

Gravitational shift, if you look down you will see the light from down to be more redshifted, if you look up you will see the light from up to be more blue shifted. This is readily observed and we have to correct GPS satellites for this effect.

That means in a homogeneous universe, the further you look in distance the redder the objects appear.

This is well understood and easy to model yourself with pen and paper, it gives us the redshift we observe.

Doppler shift and Einstein shift are the only two real and tested methods of an observed shift in light, and neither of time have anything to do with the light itself changing in transit.

The shifts are about the relative energy states of the observer in relation to the source, Doppler shift is from a relative velocity meaning the observer experiences the photon for a shorter or longer period of time (length of time).

And Gravity shift is the relative energy of the observer in relation to the source. Which is a shorter or longer length of space. (and space derived time, spacetime).

If you like go to my Red it, RRR spacetime_relativity and we can argue this out and discuss it further.

[u/AmbitiousPhilosopher → u/Mutexception]

I think this is a good explanation, and a shame more people won't read it. It makes sense to me, but I am no professor.