# Thread: Gravitational redshift vice versa

1. Originally Posted by Trippy
No I'm not, I'm restating what he has already said without either agreeing with it or disagreeing with it.
What he's stating is nonsense , so why restate it?

2. Originally Posted by Tach

Originally Posted by RealityCheck

It STILL only EVER involves an emitter's and receiver's energy-time state affecting the photon AT EACH END respectively OF THE FLIGHT PATH, irrespective of whatever space path distance involved.

Nope. You can have a detector at every point along the path and you will measure a gravitational redshift at each location, as a continuum. Where do you get all these crackpot ideas? Do you make them up all by yourself? You certainly did not find them in any mainstream publications, did you?

Are you for real? How many times and in how many ways can it be said to you before you PAY ATTENTION and stop kneejerking and defaulting to some assumptive 'abstraction' not based on the reality involved.

Can't you see that if a photon is absorbed ONCE then FURTHER detectors don't come into play AT ALL. Each 'transaction' between emitter and ANY detector is a ONCE OFF transaction. Period.

The fact is, you actually DON'T see that this is ONLY a TRIVIAL SERIES of detections 'exercise'; and it says clearly that you ARE ASSUMING a continuum 'photon change in flight' BASED ON a SERIES of ONCE-OFF transactions where EACH TIME a photon is either absorbed or not.

No matter what series of positions/states for receptors at any distance, the path is traveled ONLY ONCE and ENDS at ONE detector and THAT's IT.

Get it now?

Here, I'll spell it out for you slowly and deliberately.....

ONE emitter determines a photon waveform.

The photon travels along whatever space distance.

It may or may not arrive AT ANY ONE of however many POTENTIAL receivers there may be some distance away on a particular space path.

You may TRIVIALLY REPEAT that transaction with receivers at different space distance locations, but it is only TRIVIAL REPETITION of one-off transactions as described above.

Merely trivial REPETITION.

No asumptions can be made about 'changing in flight' for each photon in any one-off transaction, however many times and places you may wish to repeat it.

Get it?

Cheers!

RealityCheck.
.

3. Originally Posted by Tach
What he's stating is nonsense , so why restate it?
:Sigh:

Didn't you once say you're a teacher of some kind?

What do you think might be gained by restating a problem that appears to be being cause, at least in part, through miscommunication?

I understand what you're saying, and I can understand what RC is objecting to. Often when a third party restates the problem in different terms, it can lead to a recollection of (for example) a specific paper or experiment that precisely addresses the question.

4. Originally Posted by Trippy
:Sigh:

Didn't you once say you're a teacher of some kind?

What do you think might be gained by restating a problem that appears to be being cause, at least in part, through miscommunication?

I understand what you're saying, and I can understand what RC is objecting to. Often when a third party restates the problem in different terms, it can lead to a recollection of (for example) a specific paper or experiment that precisely addresses the question.
There is no miscommunication, he's posting crackpottery, plain and simple.

5. Originally Posted by RealityCheck
you ARE ASSUMING a continuum 'photon change in flight' BASED ON a SERIES of ONCE-OFF transactions where EACH TIME a photon is either absorbed or not.
I am not assuming anything, the change in frequency is continuous and this is basic GR stuff. It happens whether the detectors are present or not. The fact that you are in denial is simply proving you to be a crank.

6. Originally Posted by Tach
There is no miscommunication, he's posting crackpottery, plain and simple.
He didn't accept your variation on the Pound-Rebka experiment.
RC's non-acceptance of your proposal, whether due to crackpottery or not was predictable to anyone who understood RC's objection in the first place.

7. Originally Posted by Tach
I am not assuming anything, the change in frequency is continuous and this is basic GR stuff. The fact that you are in denial is simply proving you to be a crank.
Read what I said AGAIN and THIS TIME see where your assumption lay. Try harder to see the other side of the discussion instead of just kneejerking like billy-o irrespective. Ok?

Here it is again answering your above kneejerking 'in denial' etc handwaving tactics.....

Originally Posted by RealityCheck
Are you for real? How many times and in how many ways can it be said to you before you PAY ATTENTION and stop kneejerking and defaulting to some assumptive 'abstraction' not based on the reality involved.

Can't you see that if a photon is absorbed ONCE then FURTHER detectors don't come into play AT ALL. Each 'transaction' between emitter and ANY detector is a ONCE OFF transaction. Period.

The fact is, you actually DON'T see that this is ONLY a TRIVIAL SERIES of detections 'exercise'; and it says clearly that you ARE ASSUMING a continuum 'photon change in flight' BASED ON a SERIES of ONCE-OFF transactions where EACH TIME a photon is either absorbed or not.

No matter what series of positions/states for receptors at any distance, the path is traveled ONLY ONCE and ENDS at ONE detector and THAT's IT.

Get it now?

Here, I'll spell it out for you slowly and deliberately.....

ONE emitter determines a photon waveform.

The photon travels along whatever space distance.

It may or may not arrive AT ANY ONE of however many POTENTIAL receivers there may be some distance away on a particular space path.

You may TRIVIALLY REPEAT that transaction with receivers at different space distance locations, but it is only TRIVIAL REPETITION of one-off transactions as described above.

Merely trivial REPETITION.

No asumptions can be made about 'changing in flight' for each photon in any one-off transaction, however many times and places you may wish to repeat it.

Get it?

Cheers!

RealityCheck.
.

Now Tach. Take a big breath and some time. Read it carefully. Try to see the point in question instead of just kneejerk repeating and dismissing.

Remember, you were wrong once before (as Trout) when you previously argued against the logical/physical observation that something sitting on a planet's surface is effectively under accelerative forces even though its position is not changing with respect to that surface. OK? It's NOT A CRIME to be wrong at times. It's only when we don't learn and continue to arrogantly dismiss (with similarly 'kneejerking' responses as you used then) that it becomes an unpleasant situation all round. Hey? So try to take it easy and to see what is being said as is rather than as you think it is.

Thanks.

.

8. Originally Posted by Trippy
He didn't accept your variation on the Pound-Rebkah experiment.
It is called Pound-Rebka, not Pound-Rebkah.
What I showed is basic GR, the effect is present whether the detector is present or not.

RC's non-acceptance of your proposal, whether due to crackpottery or not was predictable to anyone who understood RC's objection in the first place.
His objection is pure crackpottery , so how can it be understandable? What he claims is nonsense by definition.

9. Originally Posted by Tach
It is called Pound-Rebka, not Pound-Rebkah.
Congratulations, you found a typo and I've fixed it. Now what?

Originally Posted by Tach
What I showed is basic GR, the effect is present whether the detector is present or not.
I understand that. He doesn't.

Originally Posted by Tach
His objection is pure crackpottery , so how can it be understandable? What he claims is nonsense by definition.
It might be crackpottery, but at least it's no longer gibberish. All he wants you to do is explain to him why it's crackpottery without using discreet detectors.

10. Originally Posted by Trippy
Congratulations, you found a typo and I've fixed it. Now what?
Now you need to correct your more serious confusion between gravitational redshift and Hubble redshift. You have been mixing them throughout your posts, so you will need to go back through your posts and fix that as well.

I understand that. He doesn't.
Try explaining it to him. See if you can use a mathematical formulation, words don't make up physics.

It might be crackpottery, but at least it's no longer gibberish. All he wants you to do is explain to him why it's crackpottery without using discreet detectors.
Easy, have only one emitter, at altitude $H$ above the Earth and NO receiver. Write down the frequency at the ARBITRARY altitude $0. Explain why the effect exists in the absence of ANY receptor.

11. Hi Trippy, Tach, everyone.

May I remind all interested in this discussion that the SR case also involves this question.

The claim that the photon is affected 'in flight' between relatively moving emitter and receiver still involves photon travel through space.

So again, anyone claiming that the space itself affects a photon 'in flight' depending on the relative motions of emitter and receiver (in different energy-time states therefore) still must provide the mechanism that affects a photon 'in flight' whether or not a receiver is ever reached by that photon.

In other words, the same problem for such 'in flight' changes to photon energy-frequency state arises because the photon is determined at source and only ever 'changed' once it is affected by an energy-time state imprinted by a receiver at absorption.

The whole question of whether universal space is expanding/compressing etc is not clear because mainstream says that only more or less space/distance is involved, and not some direct affect like stretching/compressing. Otherwise we need the energy-frequency mechanism coupling that photon to 'flat' space as well.

Anyhow, I trust there is enough in the posts already made by all parties to this discussion to help us all reach a consensus as to what's really what. Eventually!

So I will leave it at that for a bit and see what Tach and Trippy thrash out between them.

Thanks for your interest, Trippy, Tach, everyone.

Cheers!

RealityCheck.

.

12. Originally Posted by Tach to Trippy
Now you need to correct your more serious confusion between gravitational redshift and Hubble redshift. You have been mixing them throughout your posts, so you will need to go back through your posts and fix that as well.

Try explaining it to him. See if you can use a mathematical formulation, words don't make up physics.

Easy, have only one emitter, at altitude $H$ above the Earth and NO receiver. Write down the frequency at the ARBITRARY altitude $0. Explain why the effect exists in the absence of ANY receptor.

Just caught this before signing off.

Your claim. Why don't you do the necessary?

And what is so explanatory about one emitter and no reciever maths model? It contains assumptions. Hence it's up to you to explain those assumptions and how they provide the mechanism that couples the photon to the space 'in flight' at any arbitrary location along the path to nowhere.

Thanks, bye.

.

13. Originally Posted by Tach
Now you need to correct your more serious confusion between gravitational redshift and Hubble redshift.
I've made one post in which I suggested that the Lyman alpha forest was analagous to what you were proposing with the variation of the Pound-Rebka experiment. When I bought it up, I was quite specific that I was referring to hubble redshifting in that post. The fact that I addressed the pound-rebka experiment in the same post does not imply I conflated the two.

Unless of course you want to suggest that gravity doesn't 'stretch' space time, that the difference between the source and receiver in the Pound-Rebka experiment isn't height, and that gravity isn't a function of height?

Originally Posted by Tach
You have been mixing them throughout your posts, so you will need to go back through your posts and fix that as well.
So then this is, at best, an exaggeration.

Originally Posted by Tach
Try explaining it to him. See if you can use a mathematical formulation, words don't make up physics.
Do you think he would understand it if I did?

Originally Posted by Tach
Easy, have only one emitter, at altitude $H$ above the Earth and NO receiver. Write down the frequency at the ARBITRARY altitude $0.
Address it to RealityCheck, not me.

14. Tach:

Originally Posted by Tach
I am not assuming anything, the change in frequency is continuous and this is basic GR stuff. It happens whether the detectors are present or not.
How can you possibly know what happens when no detector is present?

15. Originally Posted by RealityCheck
I read rpenner's statement to imply the case of a photon itself being still in transit (ie, still in free space not in contact with any object) when the 'in transit' photon itself is blueshifting/redshifting 'in free space'.

That would differ markedly from the usual photon wavelength/frequency doppler/smearing while being emitted/received by moving objects.

....

Did you mean to imply a case where some blueshifting/redshifting in a 'free flying photon' happens while it is still 'in transit' along the spacetime path between source/receiver?
Originally Posted by rpenner
It is incorrect to attribute Doppler shifting to a phenomenon at the source or at the receiver, because such a view imbues the frequency of a photon as an absolute quantity (i.e. one independent of state of motion or position in space and time) which requires the notion of absolute time which both special and general relativity wholly reject.
...
I believe that is exactly the GR picture in coordinate systems where source and receivers are "places" of constant spacial coordinates. Not only does the frequency change happen in transit but it happens smoothly in transit and one may interrupt the transition at any point by inserting a second receiver in a "place" intermediate between source and receiver. In the SR picture this corresponds to inserting a second receiver, intermediate both in position and in state of motion, between the original source and receiver.
For example, in Rindler coordinates where $A^2 + B^2 > 0$ we may have the following three geodesic world lines:
$x_1 = \sqrt{ A^2 + B^2} \textrm{sech} \left( \frac{k}{c} ( D - E ) \right) \textrm{sech} \left( \frac{k}{c} ( t - D) \right), \, y_1 = G + A \tanh \left( \frac{k}{c} ( D - E ) \right), \; z_1 = H + B \tanh \left( \frac{k}{c} ( D - E ) \right)
\\ x_2 = \sqrt{ A^2 + B^2} \textrm{sech} \left( \frac{k}{c} ( t - E) \right), \, y_2 = G + A \tanh \left( \frac{k}{c} ( t - E ) \right), \; z_2 = H + B \tanh \left( \frac{k}{c} ( t - E ) \right)
\\ x_3 = \sqrt{ A^2 + B^2} \textrm{sech} \left( \frac{k}{c} ( F - E ) \right) \textrm{sech} \left( \frac{k}{c} ( t - F) \right), \, y_3 = G + A \tanh \left( \frac{k}{c} ( F - E ) \right), \; z_3 = H + B \tanh \left( \frac{k}{c} ( F - E ) \right)$

Thus $t = D \; \Rightarrow \; x_D = x_1 = x_2 = \sqrt{ A^2 + B^2} \textrm{sech} \left( \frac{k}{c} ( D - E) \right) \, \wedge \, y_D = y_1 = y_2 = G + A \tanh \left( \frac{k}{c} ( D - E ) \right) \, \wedge \, z_D = z_1 = z_2 = H + B \tanh \left( \frac{k}{c} ( D - E ) \right)$ and $t = F \; \Rightarrow \; x_F = x_2 = x_3 = \sqrt{ A^2 + B^2} \textrm{sech} \left( \frac{k}{c} ( F - E) \right) \, \wedge \, y_F = y_2 = y_3 = G + A \tanh \left( \frac{k}{c} ( F - E ) \right) \, \wedge \, x_F = z_2 = z_3 = H + B \tanh \left( \frac{k}{c} ( F - E ) \right)$. In these coordinates, world line 1 is a time-like world line that is instantaneously motionless at time $t=D$ at place $(x_D, y_D, z_D)$. Likewise for time-like world line 3 at $t=F$ at place $(x_F, y_F, z_F)$. World line 2 is a null geodesic that connects $(x_D, y_D, z_D)$ at time $t=D$ to $(x_F, y_F, z_F)$ at time $t=F$.

Since the metric is $ds^2 = - \frac{k^2 x^2}{c^2} dt^2 + dx^2 + dy^2 + dz^2$ it follows that the time dilation ratio is $\sqrt{\frac{- \frac{k^2 x_D^2}{c^2}}{- \frac{k^2 x_F^2}{c^2}}} = \frac{x_D}{x_F} = \frac{\cosh \left( \frac{k}{c} ( F - E) \right)}{\cosh \left( \frac{k}{c} ( D - E) \right)}$ and this not only controls the Doppler shift from place to place, it also interpolates smoothly if a new place gets a receiver at an intermediate location.

Naturally, this GR view is already incorporated in the "pure gravitational redshift" of the Pound-Rebka experiment because the key ratio is $\sqrt{\frac{g_{00}}{g'_{00}}}$.

So now that I have explained again my position, that Doppler shift doesn't occur at the source or the receiver but instead can be viewed as a function of the source and receiver's position in a suitable GR coordinate system, let's just for fun return to the SR view via coordinate transformation to inertial coordinates.

$(t=D, x=x_D, y=y_D, z=z_D) \\ \Rightarrow \; \left( T = T_D= \frac{1}{c} \sqrt{ A^2 + B^2} \textrm{sech} \left( \frac{k}{c} ( D - E) \right) \sinh \left( \frac{k}{c} D \right), \; X = X_D = \sqrt{ A^2 + B^2} \textrm{sech} \left( \frac{k}{c} ( D - E) \right) \cosh \left( \frac{k}{c} D \right) , \; Y = Y_D = y_D = G + A \tanh \left( \frac{k}{c} ( D - E ) \right), \; Z = Z_D = z_D = H + B \tanh \left( \frac{k}{c} ( D - E ) \right) \right)$
$(t=F, x=x_F, y=y_F, z=z_F) \\ \Rightarrow \; \left( T = T_F= \frac{1}{c} \sqrt{ A^2 + B^2} \textrm{sech} \left( \frac{k}{c} ( F - E) \right) \sinh \left( \frac{k}{c} F \right), \; X = X_F = \sqrt{ A^2 + B^2} \textrm{sech} \left( \frac{k}{c} ( F - E) \right) \cosh \left( \frac{k}{c} F \right) , \; Y = Y_F = y_F = G + A \tanh \left( \frac{k}{c} ( F - E ) \right), \; Z = Z_F = z_F = H + B \tanh \left( \frac{k}{c} ( F - E ) \right) \right)$
$c^2 \left( T_D - T_F \right) ^2 = \left( A^2 + B^2 \right) \cosh^2 \left( \frac{k}{c} E \right) \sinh^2 \left( \frac{k}{c} ( D - F) \right)\textrm{sech}^2 \left( \frac{k}{c} ( D - E) \right) \textrm{sech}^2 \left( \frac{k}{c} ( F - E) \right)
\\ \left( X_D - X_F \right) ^2 = \left( A^2 + B^2 \right) \sinh^2 \left( \frac{k}{c} E \right) \sinh^2 \left( \frac{k}{c} ( D - F) \right)\textrm{sech}^2 \left( \frac{k}{c} ( D - E) \right) \textrm{sech}^2 \left( \frac{k}{c} ( F - E) \right)
\\ \left( Y_D - Y_F \right) ^2 = A^2 \sinh^2 \left( \frac{k}{c} ( D - F) \right)\textrm{sech}^2 \left( \frac{k}{c} ( D - E) \right) \textrm{sech}^2 \left( \frac{k}{c} ( F - E) \right)
\\ \left( Z_D - Z_F \right) ^2 = B^2 \sinh^2 \left( \frac{k}{c} ( D - F) \right)\textrm{sech}^2 \left( \frac{k}{c} ( D - E) \right) \textrm{sech}^2 \left( \frac{k}{c} ( F - E) \right)$

Thus $c^2 \left( T_D - T_F \right) ^2 = \left( X_D - X_F \right) ^2 + \left( Y_D - Y_F \right) ^2 + \left( Z_D - Z_F \right) ^2$ as expected for two points in a Cartesian coordinate system connected by a null geodesic.

Similarly, for world-line 1, translating it into the Cartesian coordinate system we have $\frac{\partial X}{\partial T} = \; c \, \tanh \left( \frac{k}{c} D \right)$ and for world-line 3, $\frac{\partial X}{\partial T} = \; c \, \tanh \left( \frac{k}{c} F \right)$ and by the Einstein velocity formula, the relative velocity is $c \tanh \left( \frac{k}{c} ( D - F) \right)$ with associated Doppler ratio: $\cosh \left( \frac{k}{c} (D - F) \right) \left( 1 - \tanh \left( \frac{k}{c} (D - F) \right) \cos \left( \cos^{\tiny -1} \tanh \left( \frac{k}{c} (D - E) \right) \right) \right) = \frac{\cosh \left( \frac{k}{c} ( F - E) \right)}{\cosh \left( \frac{k}{c} ( D - E) \right)}$

So the GR methodology of looking at the differential passing of time in "places" as defined by the coordinate system incorporates both the "relativistic" effect and the "geometric" effect of the Doppler shift in arbitrary directions.

Originally Posted by James R
Looking at this conversation/argument, I can't see any way to distinguish the two views that are being put. If you only make a measurement of a photon at the "end" of its flight (wherever that is), then you'll only measure a Doppler shift when you make the measurement.

RealityCheck: are there any experimentally observable differences between your view and those of Tach/repenner/Trippy? If so, please list them.
Originally Posted by RealityCheck
Hi James R, long time no see! You are well I trust?
Overdone appearances of civility can lead one to appear insincere. This is wildly out-of-line with the gratuitous bold face and capitalization.

Originally Posted by RealityCheck
It boils down to this:

rpenner/Tach have claimed that the photon ( $e=h \nu$ ) is changed by space along its 'flight'. I asked for the mechanism which couples the photon to space such that this can occur 'in flight' instead of the observed determination/modification to the photon ( $e=h \nu$ ) ONLY at either end during emission/reception by an object whose own relative/respective energy-time state is imprinted on the photon (during creation/absorption processes, as applicable).
I never said that "space" changed the photon. I said that there was a strong sense in which GR accounts for Doppler shifts in terms of a function of the "places" at the endpoints of the photons journey through space-time. Since coordinates are arbitrary in GR, these notions of "place" can be made up to suit the situation, but the physics predictions remain the same -- independent of the choice of coordinates.

Originally Posted by RealityCheck
There are 'mainstreamers' at physforum
That's the site where I am moderator.
Originally Posted by RealityCheck
who have had to correct another poster there regarding such issues.
Originally Posted by RealityCheck
The naive assumptions and imagery about expanding/stretching/compressing 'space' was contrary to the mainstream view that there is just more/less/same 'space'. Which means that photons merely travel along that space whatever its quantity/distance involved along the photon flight path between source and receiver.
I asked for and did not receive a source for this particular "mainstream view."

Originally Posted by RealityCheck
Based on that 'mainstream' view, it seemed only proper to ask rpenner/Tach to provide the photon-to-space coupling mechanism that would support and prove their claim there (which claim seems to go against mainstream view as expressed by those who should know over at physforum).
RealityCheck has an enormous sense of entitlement. It has long been my opinion that ordinary forum users do not have the right to demand a reply. It is this implied arrogance that makes the civil stylings feel awkwardly forced. Likewise, I feel it is bad form to let personal issues spread from forum to forum.

Originally Posted by RealityCheck
I have no observations/assumptions to put either way to support any particular view. I merely question and ask for the claimants (rpenner/Tach) to prove/support their claims about space changing the photon in flight rather than photon only being observed to be determined/changed at emission/reception due to emitter/receiver energy-time states applying during emission/reception processes.
I simply cannot endorse a statement that says that the photon's measured frequency is determined at emission, since it is a function of the relative motion of source and receiver in SR and a function of source and receiver positions in GR. The GR view subsumes the SR view.

Originally Posted by RealityCheck
I leave it to rpenner, Tach or anyone else to provide the necessary mechanism to counter the mainstream view that a photon, once determined at source as described, thereafter only travels at its accustomed speed along more/less free space along null geodesic paths etc whether there is a receiver waiting for it or not. And that's it.
Nothing I ever wrote implied that the speed changed or the path was not a null geodesic.

16. Originally Posted by rpenner
I simply cannot endorse a statement that says that the photon's measured frequency is determined at emission, since it is a function of the relative motion of source and receiver in SR and a function of source and receiver positions in GR. The GR view subsumes the SR view.
In fact if you consider mossbauer spectroscopy, and consider two samples of 57Fe, one moving, one stationary, and get two co-moving observers - one 'attached' to each sample to write down the frequency they measure from their own sample, and the frequency they measure from the others sample, they would neccessarily write down the same two numbers, but would disagree on the frequency of a specific sample.

17. Originally Posted by James R
Tach:

How can you possibly know what happens when no detector is present?
The "effect" is objective, i.e. it "happens" whether detectors are present or not. A tree falling in a forrest makes a sound whether there is a human present to hear it or not. The detector does not make the effect "happen", contrary to RealityCheck's claims. For a mathematical explanation, see either rpenner's post (motion between source and detector) or mine (source and detector stationary wrt each other).

18. Originally Posted by Trippy
In fact if you consider mossbauer spectroscopy, and consider two samples of 57Fe, one moving, one stationary, and get two co-moving observers - one 'attached' to each sample to write down the frequency they measure from their own sample, and the frequency they measure from the others sample, they would neccessarily write down the same two numbers, but would disagree on the frequency of a specific sample.
Try putting the above in math form. The verbiage is definitely incorrect.

19. Trippy

The problem lies with the choice of comoving coordinates. Look at the bottom page 27 through top page 28.

http://astro.uwaterloo.ca/~mjhudson/.../chapter02.pdf

Results of scientific measurements conducted by comoving observers would be identical. Tick rates are identical. Spacetime geometry is identical.

20. Originally Posted by Tach
The "effect" is objective, i.e. it "happens" whether detectors are present or not. A tree falling in a forrest makes a sound whether there is a human present to hear it or not. The detector does not make the effect "happen", contrary to RealityCheck's claims. For a mathematical explanation, see either rpenner's post (motion between source and detector) or mine (source and detector stationary wrt each other).
rpenner and Tach are correct.

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