[Colin2B]

Note: This has nothing to do with the original question so this is an off-topic discussion.

It is also moving perilously close to a new theory discussion, which could be split out and moved.
It must also be confusing the socks off the question poster!

[timey]

Doppler shift is function of the speed of the source and nothing else. The source of gamma rays is mounted in the center of the loudspeaker and is attached to the cone so that as a signal is sent to the speaker  the source moves back and fourth. Therefore there is a relative motion between source and receiver. There can only be a Doppler shift when the source is moving as is the case here.

Note: This has nothing to do with the original question so this is an off-topic discussion.

So Pmb - you are now telling me that the experimenters created a Doppler shift of gamma rays by a mechanism that is clearly obvious, and well documented in Pound Rebka link, (and indeed by most of the posters here, including myself)... and then used this man made Doppler shift to measured WHAT?
Because as far as I understand, they used this man made Doppler shift of the gamma ray, to measure a naturally occurring Doppler shift of light being shifted by the gravitational field!!!
True or false?

Although the man made Doppler shift was created by a slight variance in distance, caused by the speaker oscillations of 'back and forth', the naturally occurring phenomenon of gravitational shift that it was measuring occurred over a fixed distance of 22.5 metres.  There was no relative motion between the source and receiver with regards to the phenomenon being measured.
True or false?

With regards to the original posters question, you answered this question earlier this thread with the """correct""" answer already.  It's called invariance, and no one actually knows """how""" a photon travels at speed of light from a light source that is moving, it is only known what the light will be doing from an observers point of reference.

If you care to explore the parameters of the Pound Rebka experiment with me, as I am doing, I """can""" possibly actually answer the original posters question more precisely, because I think that perhaps I """do""" know how light moves across space in a gravitational field.

Btw...are you Pmb Pete who used to post before?  If so, I'm glad to see you back.

P.S.  Colin, if it's bothering the status quo, by all means split the thread and put it in New Theories.

P.S.S.  Joe - in answer to your post, please read the Wiki link on Doppler shift more carefully.  You will see that it is a documented fact that light undergoes a Doppler shift, that it is referred to as a Doppler shift, and is responsible for lights wavelength which is proportional to frequency.  The man made Doppler shift matched the frequencies of this naturally occurring phenomenon, cancelling out the gravitational shift of the gamma rays, more gamma rays were absorbed by the receiving sample and the number of gamma rays detected by the scintillation counter dropped accordingly, constituting the measurement, via the speaker 'phase', of the naturally occurring phenomenon of a Doppler shift in the gravitational shift of light.

[PmbNEP]

So Pmb - you are now telling me that the experimenters created a Doppler shift of gamma rays by a mechanism that is clearly obvious, and well documented in Pound Rebka link, (and indeed by most of the posters here, including myself)... and then used this man made Doppler shift to measured WHAT?
Because as far as I understand, they used this man made Doppler shift of the gamma ray, to measure a naturally occurring Doppler shift of light being shifted by the gravitational field!!!

If this thread was about the Pound-Rebka experiment then I wouldn't have responded to this thread because I'm not an experimental physicist. As you said, everything you want to know about that experiment is posted in Wikipedia. I only responded to the question I quoted. I simply don't know why they had to have a time varying Doppler shift in the signal. I do, however, know experts who'd know this. I'll ask them about it and get back to you.

[JoeBrown]

Note: This has nothing to do with the original question so this is an off-topic discussion.

It is also moving perilously close to a new theory discussion, which could be split out and moved.
It must also be confusing the socks off the question poster!

The original poster stated it was an enlightening argument with amusement, here:
quote author=NeT-HeaD link=topic=66096.msg483797#msg483797 date=1458670736

I've found the off topic premise concerning, confusing and aggravating.  Seems timey is enjoying itself inciting it.

[timey]

I've found the off topic premise concerning, confusing and aggravating.  Seems timey is enjoying itself inciting it.

IMO Joe - 'you' are finding 'on topic' discussion confusing!

It was Colin who, quite appropriately to the topic I might add, brought up the subject of a Doppler shift in light in response to the original posters question.

My contribution has been in response to Colin's commentary!

[timey]

If this thread was about the Pound-Rebka experiment then I wouldn't have responded to this thread because I'm not an experimental physicist. As you said, everything you want to know about that experiment is posted in Wikipedia. I only responded to the question I quoted. I simply don't know why they had to have a time varying Doppler shift in the signal. I do, however, know experts who'd know this. I'll ask them about it and get back to you.

Pmb - Thank you for your honesty... and, to say so, I'd be most grateful, and look forward to hearing back from you!

[Colin2B]

I simply don't know why they had to have a time varying Doppler shift in the signal.

My understanding is that this was the easiest way of creating a variable speed of the source towards and away from the receiver., change the frequency, change the speed, change the Doppler shift. They found the point at which this Doppler shift canceled out the gravitational shift and the frequency gave them the speed, hence shift. Sorry to labour this, but explanation for those who have not read the detail.

Timey, I find your use of Doppler shift to describe gravitational shift as seen by observers in the gravitational field to be confusing in this thread. Gravitational shift is only seen as a Doppler shift by the free falling observer, ground observers attribute it to the acceleration of the photons increasing photon energy, hence frequency, but due to clock rate differences can only measure std speed of light. Again sorry to labour this, but I'm aware a wider audience is reading this.

Can we separate the terms for clarity.

[timey]

Colin - To clear this up for any confused readers, the reason* why Pmb does not know why the Pound Rebka had to use a time varying Doppler shift in the signal, (this being the man made Doppler shift used to measure the natural phenomenon) is because the speed of light is constant, and over a constant distance there 'should not' be any such relative motion as to cause a Doppler shift (this being the naturally occurring phenomenon) in the gravitational shift in light over a fixed distance of the gravitational field.

As per GR and space time, redshift is indicative of an expansion of distance between light source and receiver, and blueshift is indicative of a contraction of distance between light source and receiver.

If anyone is confused by my observation here, really...don't worry, you have very good reason to be...

This being the point I am making!

*Pmb, if this is not the reason, I apologise for my presumptuous-ness, and invite you to correct me.

[alancalverd]

The Pound Rebka consists of a man made Doppler shift measuring a naturally occurring Doppler shift within the gravitational shift of light.
Is this true?

No, no, no. It uses Doppler shift to measure gravitational shift. They are not the same thing, but both have the same effect on wavelength.

Please read this very carefully

Gravitational shift is NOT Doppler shift.

I say again

Gravitational shift is NOT Doppler shift

which is what everyone else has been saying and Messrs P & R understood from the outset.

Please do not talk about "gravitational Doppler shift" - there is no such thing. Gravitational shift is due entirely to the presence of a gravitational field. Doppler shift is due entirely to relative motion.

There is no confusion in anyone's mind but yours, and that only arises because you have invented a term that nobody else uses and has no meaning! 

[PmbNEP]

Colin - To clear this up for any confused readers, the reason* why Pmb does not know why the Pound Rebka had to use a time varying Doppler shift in the signal, (this being the man made Doppler shift used to measure the natural phenomenon) is because the speed of light is constant, and over a constant distance there 'should not' be any such relative motion as to cause a Doppler shift (this being the naturally occurring phenomenon) in the gravitational shift in light over a fixed distance of the gravitational field.

That's not why I don't know why the experimenters used Doppler shift in there experiment. The reason is that I simply don't know the theory behind the workings of the experiment itself. For example; it may be that they got a decrease in experimental error by doing it that way. I simply don't know.

However there simply is no theoretical reason why they had to do it like that. Theoretically all they needed was a photon emitter and a photon detector which measures wavelength.

By the way it's not true that the speed of light is constant. That only holds in inertial frames, i.e. in gravitational fields. Here is a derivation for a uniform gravitational field/accelerating frame of reference and around a spherical body (i.e. Shwarzschild spacetime).

Go to www dot newenglandphysics dot org slash physics_world slash gr slash c_in_gfield.htm

[Colin2B]

Theoretically all they needed was a photon emitter and a photon detector which measures wavelength.

When I read the experiment, this is why I assumed they used the loudspeaker. They didn't have a detector accurate enough to measure the shift, so they used the go/no go absorption to see when the Doppler shift matched the Grav shift, neat.

....because the speed of light is constant, and over a constant distance there 'should not' be any such relative motion as to cause a Doppler shift.

Ok, now I understand why you keep stressing constant distance. Ignoring the loudspeaker, between the top of the tower there is no motion to cause a Doppler shift - that's because there is no Doppler shift as Alan says, just a gravitational shift, which is not caused by movement.

As per GR and space time, redshift is indicative of an expansion of distance between light source and receiver, and blueshift is indicative of a contraction of distance between light source and receiver.

Only in the case of Doppler shift

If anyone is confused by my observation here, really...don't worry, you have very good reason to be...

This being the point I am making!

The confusion is clearing. I am beginning to understand why you are making your claims and where the errors are. Thanks for the clarification.

The following from Pmb explains.

By the way it's not true that the speed of light is constant. That only holds in inertial frames, i.e. in gravitational fields.

Remember also that as a consequence rate of clock is slower deeper in the Grav field, which matches the blue shift - this plus energy change, etc are all part and parcel of an interlocked group of  phenomena with the same cause, as described in GR.

[timey]

Ok - in answer to the last 3 posts:

Alan, Pmb, Colin... good, good, now we have all understood that I am talking about a Doppler shift that the Pound Rebka identified in the gravitational shift, and that gravitational shift causes an 'expansion' and 'contraction' of wave'length'... Can you please tell me 'where' in the distance of 22.5 metres  the extra, or lesser length of the waves exists?

I understand that there is an aspect of time dilation that 'will' affect the journey of the light, however this time dilation is contrary to the direction of the contraction and expansion of the wavelength and 'cannot' explain the Doppler shift in the gravitational shift...

[puppypower]

Consider, as an analogy for a photon, a boat traveling on the water. The boat is analogous to the particle, while the wake created by the boat is analogous to the wave aspect of the photon. If we had two tunnels under the bridge, the boat; particle, can only go through one tunnel at a time, while the wake, if spread out, can go through both tunnels.

The boat needs a constant supply of energy to keep moving at velocity V. This energy is needed to overcome friction between the boat and water. If the boat cuts the engines, the particle will slow, coast and then stop, while the wake will decay and finally stop. The speed of light is needed to maintain the wake/wave of the photon particles.

The question becomes how does a photon move through the medium of space-time and not slow down or speed up? What is the source of the propulsion energy?

If we assume the speed of light is the ground state of the universe, this implies all inertial references will be at higher potential. In this case, the constant speed of light would be connected to photons being constantly induced into the ground state ay C. The photons are constantly induced to higher potential; less than C by inertial. They cyclically drop back to the ground state; speed of light, while the energy differential is given off; motor that generates the wake/wave. 

One analogous way to look at this is connected to a house with a variety of electrical circuits, all using the same ground; earth. The ground is common to all the circuits and is at lower potential than any voltage/current configuration we may use. The potential difference between the inertial states, and the ground, is the energy potential used to drive the particle, which creates an analogous wake in space-time. This wake can become modified with the medium of space-time; red and blue shift, but the ground never changes, so the particles are always moving at C.

Fair enough PuppyPower... BUT... in Pound Rebka """Where"" in space time does this wave LENGTH exist if the distance between light source and receiver is held 'static', with respect to each other, on the basis that the oscillations of the speaker are creating a Doppler shift that cancels out the gravitational shift of the light?

The oscillations of a speaker, can be traced back to the energy signals being added to the speaker. If we shut off the power to the speaker there are no waves. We need a source of power. The energy input propels the speaker forward, which then flexes back to the rest position, due to the potential energy stored within the elasticity of the speaker, caused by the original forward energy that was added.

If we assume C is the ground state, the power circuit for all energy analogy speakers, is part inertial, which can be any of a variety of inertial states, static or in motion. It is also connected to the C ground state which is the same for all speakers. The speaker is driven forward by the potential energy returning to the C ground state. The return wave of the speaker is connected to the elasticity of the space-time medium speaker. If we took away the C ground state, the speaker would stop and no waves will appear. Energy moving at the speed of light, in all references, means the speaker is plugged in.

The Doppler shift of the speaker is driven in one direction. The rebound, in the other direction, is more reactive and is caused by the potential energy stored in the speaker, due to the forward energy pulse. The wave motion is due to the flow of energy from inertial back to the C ground.

Another visual is a water wheel, where the movement of water from a position of potential back to a ground state, results in the water wheel defining a wave. The water analogy for energy waves is time potential. The stationary water wheel defines a circle in space. However, it will define a wave in time.

[Colin2B]

now we have all understood that I am talking about a Doppler shift that the Pound Rebka identified in the gravitational shift,

Ok, as long as you understand I'm not talking about a Doppler shift, because that's not what they identified, it doesn't exist.

and that gravitational shift causes an 'expansion' and 'contraction' of wave'length'... Can you please tell me 'where' in the distance of 22.5 metres  the extra, or lesser length of the waves exists?

They exist at every point in the 22m. The gravitational shift varies continuously from top to bottom so the frequency/wavelength also varies continuously, becoming increasingly blue shifted down the tower. You can view this as increasing energy as the photons accelerate, or you can look at the slowing of clocks - because each observer will measure the speed of light as constant, the frequency will be seen as increasing (shorter wavelength).

I understand that there is an aspect of time dilation that 'will' affect the journey of the light, however this time dilation is contrary to the direction of the contraction and expansion of the wavelength and 'cannot' explain the Doppler shift in the gravitational shift...

It's not a Doppler shift.
The time dilation - slower clock - explains the blue shift as I said above, how it is contrary?. The time dilation is what 'causes' the acceleration, it's all interlinked just as in SR - time dilation and length contraction are aspects of the same effect, you can't add both together and get an additional effect.

[timey]

Colin - For the man made Doppler shift to cancel out the blueshift and redshift frequencies, the man made Doppler shift must match those frequencies exactly!  You are wrong.  There is a Doppler shift in the gravitational shift of light.  The Pound Rebka proved it!  The wiki Doppler shift link states it!

If the time dilation aspect was responsible for relative motion within the gravitational shift of light, then you would see 'longer' wave lengths in blueshift, and 'shorter' wave lengths in redshift.

Furthermore:

https://en.m.wikipedia.org/wiki/Doppler_effect

Quoted from above link:
"For waves that propagate in a medium, such as sound waves, the velocity of the observer and of the source are relative to the medium in which the waves are transmitted. The total Doppler effect may therefore result from motion of the source, motion of the observer, or motion of the medium. Each of these effects is analyzed separately. For waves which do not require a medium, such as light or gravity in general relativity, only the relative difference in velocity between the observer and the source needs to be considered."

I repeat: "only the relative difference in velocity between the observer and source needs to be considered."

The Pound Rebka measured a Doppler shift in the gravitational shift over a distance of 22.5 metres.  There was no relative difference of velocity between light source and receiver with respect to the gravitational shift they measured over a distance of 22.5 metres.

Now I'm sorry, but physics cannot have its cake and eat it.  Either there 'must' be relative velocity between light source and receiver that expands or contracts the distance between them, in which a wavelength experiences changes in distance that affords the wavelength's expansion or contraction, or there must be a 'medium' in the gravitational field that explains the relative motion of a Doppler shift in the gravitational shift of light!

[Colin2B]

Colin - For the man made Doppler shift to cancel out the blueshift and redshift frequencies, the man made Doppler shift must match those frequencies exactly! 

Agreed

You are wrong. 

Thanks

There is a Doppler shift in the gravitational shift of light. 

No. as I've said before there are 2 different effects at work here

The Pound Rebka proved it! 

No it didn't

The wiki Doppler shift link states it!

Where

If the time dilation aspect was responsible for relative motion within the gravitational shift of light, then you would see 'longer' wave lengths in blueshift, and 'shorter' wave lengths in redshift.

The time dilation isn't responsible for any relative motion (because there isn't any relative motion) but it does affect the frequency.
Lets take an eg with simple numbers:
Observer high in weak gravity field measures time of 1 cycle as being 1s = 1Hz.
Further down the field clocks run slow (time dilation) so the same cycle could be measured as say 0.5s =2Hz.
This is an increase in frequency - blueshift - and a shortening of wavelength.
No movement needed, no doppler shift, just due to grav field and clocks running slow.

Furthermore:

https://en.m.wikipedia.org/wiki/Doppler_effect

Quoted from above link:
"For waves that propagate in a medium, such as sound waves, the velocity of the observer and of the source are relative to the medium in which the waves are transmitted. The total Doppler effect may therefore result from motion of the source, motion of the observer, or motion of the medium. Each of these effects is analyzed separately. For waves which do not require a medium, such as light or gravity in general relativity, only the relative difference in velocity between the observer and the source needs to be considered."

I repeat: "only the relative difference in velocity between the observer and source needs to be considered."

Although that is true it only applies to the doppler part of the experiment.
The presence of a medium is irrelevant, this is light not sound.

The Pound Rebka measured a Doppler shift in the gravitational shift over a distance of 22.5 metres. 

No, they used a doppler shift to measure the gravitational shift.

As I said before, between the top of the tower and the bottom there is a gravitational shift which they wanted to measure. As Pmb says, they could have used an accurate freq counter, but I don't think they had one accurate enough. So, my reading of the experiment is that they looked for a way of creating a null. Using a null reading is extremely accurate and commonly used eg an interference pattern or a bridge cct. In this case they decided to create the null by making a doppler shift using the movement of the loudspeaker, I suspect the other end was a simple scintilation counter. When the doppler shift equals the grav shift they cancel and the counter shows null. The value of the doppler shift = the value of the grav shift and they can work out the doppler value from the frequency fed into the loudspeaker.
At no point are they measuring a doppler shift in the grav shift, just using one to measure the other.

There was no relative difference of velocity between light source and receiver with respect to the gravitational shift they measured over a distance of 22.5 metres.

They didn't try to measure this.
Because of time dilation the observer at top of tower and the one at the bottom will measure the same speed of light.

Now I'm sorry, but physics cannot have its cake and eat it.  Either there 'must' be relative velocity between light source and receiver that expands or contracts the distance between them, in which a wavelength experiences changes in distance that affords the wavelength's expansion or contraction, or there must be a 'medium' in the gravitational field that explains the relative motion of a Doppler shift in the gravitational shift of light!

Physics isn't trying to have it's cake and eat it. You are looking for something that isn't there.

Anyway, that's it from me as I suspect you aren't going to believe me. Either Pmb or Alan will no doubt put me right as I've been winging it here.

[timey]

Colin - I cannot 'do' these multi quote posts on my phone easily at-all, and my phone is all I have at mo...so please excuse me, but can we do this one or two portions at a time?

It states in the Doppler shift link that Both light waves and gravity waves are subject to Doppler shift.  In lights case this is because wavelength is proportional to frequency.  It is the extra, or lesser length in the wavelength that is """synonymous""" to a Doppler shift.

Now - if the man made Doppler shift is cancelling out the natural frequency of the gravitationally shifted light, then perhaps the man made Doppler shift is creating a time variance in accordance with the time drift found in the gravitational field.  If it were then my question is:
Where in the fixed distance of 22.5 metres do the extra or lesser lengths of these wavelengths exist?

However, on the basis that a person living on top floor in a high rise, will, over the course of 79 years, only be 1 minute older than his downstairs neighbour on the ground floor, (or something similarly within this tiny region anyway), I think we can pretty much rule out the fact of the speaker cone creating such a 'subtle' vibration at between 10 and 50 hertz.  The vibration that the speaker vibrated at is more synonymous with a much greater factor of Doppler shift than this.  One that matches the frequency being proportional to the wavelength, I'd imagine.  So again, my question is:
Where in the fixed distance of 22.5 metres do the extra or lesser lengths of these wavelengths exist?
And:
What is causing the relative motion, that the man made signal cancels out,  in the gravitational field?

Btw, Colin, I'm not winging it.  I've had training in sound engineering and have been home studying physics for 7 years.  I appreciate that this does not mean that I'm correct in what I'm observing, I may well be mistaken, but I've racked my brain with it all for many years now, and really, I don't actually think I am mistaken

[timey]

https://en.m.wikipedia.org/wiki/Relativistic_Doppler_effect

[Colin2B]

Ok, we have some common ground. I worked as a sound/vision engineer (BBC trained) and studied Applied Physics day release/evenings until I went to study full time. When I say winging it, I mean reading the various reports of Pound Rebka experiment to make sense of it, most of it is fairly clear but you need a bit of guess work to fill in the gaps.

So, let's concentrate on one question at a time as you suggest.

"Can you please tell me 'where' in the distance of 22.5 metres  the extra, or lesser length of the waves exists?"

They exist at every point in the 22.5m. The gravitational shift varies continuously from top to bottom so the frequency/wavelength also varies continuously, becoming increasingly blue shifted down the tower. You can view this as increasing energy as the photons accelerate, or you can look at the slowing of clocks - because each observer will measure the speed of light as constant, the frequency will be seen as increasing (shorter wavelength). Remember these are nm wavelengths.

"What is causing the relative motion, that the man made signal cancels out,  in the gravitational field?"
That's easy, there is no relative motion, because the gravitational shift is not due to motion.

I don't know why you have quoted the 2 Wiki articles because neither of them suggest that Doppler is involved in gravitational shift, nor that PR found it as a cause of the shift.

[timey]

Damian and I did a lot of work for the BBC, back in the day, from Korner studios, Scrubs Lane, when I was in training as his tape hop.  Jingles, radio recordings, etc.  It wasn't just that we were so close to the White City BBC studios, Damian had the best live room ever!  I remember you posting that you are into sound... doing outdoor recordings as I recall, you play guitar as well if I remember rightly?

Ok - so, yes, of course the wavelengths exist at every point in the distance of 22.5 metres.  Now this is where it gets interesting!  Because there is no relative motion caused by changes in the distance in the phenomenon being measured, only a fixed distance of 22.5 metres, we can now 'peg' gravitational shift in light, exactly to a gravitational field, which provides us with an absolute reference frame.  The reason why GR cannot provide us with such, is because a relativistic Doppler shift in light is thought to be synonymous with expansion or contraction of distance.  The PR shows that this is not the case.  There is a relativistic Doppler shift 'effect' within a distance that is not expanding or contracting.

And:
I said - "What is causing the relative motion, that the man made signal cancels out,  in the gravitational field?"
You said "That's easy, there is no relative motion, because the gravitational shift is not due to motion."

Gravitational shift in light is due to an expansion, or contraction, of distance between light source and receiver.  The relative velocity between light source and receiver expands or contracts the gravitational field.  Hence redshift being indicative of an expanding universe.

In the PR, light travels a fixed distance of 22.5 metres, at the speed of light.  A time variance has been added to the test signal via the vibration of the speaker. This constitutes relative motion.  The signal matches the frequency of the gravitational shift.  Does this not indicate that the relative motion of the signal is reflected in the gravitational shift?

Of course, it could be that all these considerations are contained and addressed within the maths of the PR, as Pmb suggests.  I wouldn't have a clue being a non-mathematician, but the written in 'words' dialogue doesn't state this as being the case though, and usually it would...

P.S.  Not sure why you are asking that about the Wiki links.  The Doppler shift link mentions light and gravity.  The relativistic Doppler 'effect' link explains frequency and wavelength, and drops it back to the Lorentz transformations which I have addressed in my "4 a deeper discussion - is distance an absolute invariant" thread.  Are you sure I'm not just confusing you and the issue by my 'shoddy' use of terminology ?