Talk:Gravitational redshift

message: "Note from the formula above that the loss of energy of the photon is just equal to the difference in potential energy gh). You can't make a perpetuum mobile by having photons going up and down in a gravitational field, something that was, strictly speaking, possible within Newton's theory of gravity."
The "difference in potential energy" thing is a Newtonian calculation, see (Einstein 1911), or John Michell's 1784 paper on dark stars. I really don't think you can make a perpetuum mobile this way under Newtonian theory.
In fact, I think the Newtonian prediction is actually "lossy": if you first add and then subtract a fixed proportion of a photon's energy, the second operation is based on the photon's current energy, not the energy it had at the start of the experiment. You don't quite get back to where you started, e.g. 1*(1+0.5)*(1-0.5) is less than unity. 
We can calculate the exact gravity-shift predictions of a theory by working out the velocity-change associated with a gravitational gradient, calculating the conventional motion shift associated with an object receding or approaching at that velocity, and then saying that a light-signal then has to undergo the same shift. 
With special-relativity-based theory, we have the "relativistic Doppler" equaiton for motion shifts, so if the upper and lower observers can agree on the terminal velocity associated with a gradient, a photon passed downhill then uphill across the gradient returns with exactly the same energy it started with - this energy-conserved situaiton is possibly one of the reasons why Einstein may have felt that it was natural for the cosmology of GR to be pseudo-Euclidean, with lightbeams crossing large distances having their energies unchanged (on average) over large distances, and why he put in his gravitational constant to force that result.
But with Newtonian theory, a photon frequency-shifted by f'/f = (1+ v/c)(1- v/c) returns to its original height with a net energy-loss of (1- v^2/c^2), a Lorentz-squared redshift. 
So, Newtonian theory suggests that a photon travelling across the a reasonably uniform universe, encountering a switchback series of gravitational highs and lows, should actually be expected to show some sort of distance-dependent redshift, I think. ErkDemon 00:21, 30 July 2005 (UTC)
thread_title: Round-trip gravity shifts?

message: Just a few small points: 
* the prediction of gravitational shifts was made using Newtonian theory over a century before Einstein, as a consequence of light gaining or losing energy as it moves down or up across a gravitational potential gh (see, John Michell 1784, Laplace, dark stars).
* The "novel" effect that Einstein introduced in the 1911 paper was not so much the shift effect, but the idea that gravitational shifts led inevitably to gravitational time dilation. 
* Last I heard, Pound-Snider (1965) was still the best experimental assessment of the gravitational velocity-shift law that we had. I've seen it written that these tests stopped after Pound-Snider, when people realised that the accuracy of tests using currently-available technology was still not good enough to distinguish between GR and Newtonian theory. Perhaps another factor might have been that techniques for focussing x-rays (useful for producing a parallel beam for use in longer-drop experiments) would still have been classified at the time because of their applications for hydrogen bomb design.  
* Spaceborne experiments do produce larger shift effects than Pound-Snider, but also introduce larger uncertainties over the "real" heights of the satellites involved. Using light to measure the distances by making certain assumptions about the effect of gravity on light, and then using these presumed distances to test the exact effect of gravity on light, can get a bit circular. 
* For "starlight" verifications of gravitational shifts, the page needs some references: I know that the "starlight" experiments cited by Einstein in his relativity book no longer appear in modern textbooks, apparently they've since been "dropped" as unreliable, and without a citation or a name, I can't tell whether the author had these discredited results in mind, or some newer ones. 
So describing gravitational shifts as "the applied side of general relativity" is probably putting it a bit strongly. Proper textbooks and experimental writeups remember to put in a "caveat" that the gh/c^2 thingy is a Newtonian approximation that we use in these situations because it's very convenient and because in these situations we usually can't tell the difference between the diverging NM and SR-based predictions. I think that the sort of caveat used by these authors ought to also appear in the wiki page.  
FWIW, I'm not sure that Newtonian gravitational potential gives the correct gravity-shift relationships even for NM (I think that's more about the round-trip time dilation relationship than the one-way visible frequency change), but again, the divergences are so small in practice that we probably don't care if we are technically using the wrong set of math, it's still probably "close enough" to agree with the experiemntal data and to be counted as a usable first approximation. ErkDemon 02:38, 1 August 2005 (UTC)
thread_title:  This isn't just a GR result ... 

message: Please look over what you wrote and try to rewrite it to make more sense and to correspond better with mainstream.  For example, "redshift of a photon" doesn't make much sense as written.  Also, "it can be argued that star never collapses past horizon" is misleading in this context.  Either explain what you mean by this or write it out, please. ---CH  (talk) 00:57, 24 October 2005 (UTC)
thread_title: Recent edits by 24.201.168.190

message: This article is currently in awful shape and needs to be thorougly rewritten.  I agree with some points ErkDemon made (pleasant when this happens): it is important to stress that gravitational red shift is by no means a distinguishing feature of gtr among metric theories of gravitation, and applied side of gtr is a rather silly characterization which probably has no place here.  See the todo list at top of this talk page. ---CH 05:50, 23 December 2005 (UTC)
replies: text: I am no expert, but I believe I have redone most of the article and removed redundant unnecessary explanations.  I also cut out the stuff about the Pound-Rebka experiment and replaced it with a sentence and link to the seperate article which already talks about that.  I also add a graphic with an appropriate caption.  I think this will provide a better framework for the article.  I sincerely hope you like this revision! Kmarinas86 02:20, 26 December 2005 (UTC)
replies: text: It still won't satisfy all expert needs, however.Kmarinas86 02:21, 26 December 2005 (UTC)
replies: text: There would have to be a section the gives the causal explanation of Gravitational Redshift according to rigourous gtr, someone else may do that, not me.Kmarinas86 02:26, 26 December 2005 (UTC)

text: Experiments concering gravitational redshift could be done on different articles and linked to by this one.Kmarinas86 02:28, 26 December 2005 (UTC)
replies: text: I must say, this article look just fine.  Some of you guys like to go around to all these articles to make yourselves look intelligent by claiming the article is lousy / sub-standard (doesn't that put you in the instructor's chair?  Does that feel nice, to have the wisdom and expertise to grade all these papers?).  Does this refer to you, Hillman CH?  Kmarinas, you've probably done a great job, why do you need the approval of Hillman?  This is the most annoying thing about Wikipedia.  What's more, every mundane point does not need a citation.  This can drive readers up a wall, and away from Wikipedia.  BETTER YET - if your so smart and knowledgeable enough to know that a citation is need -- FIND IT YOURSELF!  Of course, many times such a notation is fitting and appropriate, but the way it is presently being used is overboard, petty and obnoxious. Nick  —Preceding unsigned comment added by 65.27.203.75 (talk) 02:17, 15 May 2008 (UTC)
thread_title: Expert cleanup desperately needed

message: I have no idea what the illustration is supposed to be showing.  It needs a better caption here and on redshift. -- Beland 13:22, 25 March 2006 (UTC)
thread_title:  Illustration problems 

message: As a courtesy, I have removed the "expert items" from the todo list. I am leaving WP and doubt anyone else will know how to implement the suggested improvements since this was mostly a note to myself.
This article concerns a topic dear to my heart, which unfortunately attracts many cranks.  I hadn't the heart to closely monitor it during my year as a Wikipedia, but I did write the original todo list.  As a courtesy, I have removed the "expert items" from the todo list. I am leaving WP and doubt anyone else will know how to implement the suggested improvements since this was mostly a note to myself.
Just wanted to provide notice that I emphatically do not vouch for anything you might see in more recent versions.  Given past history, I have some reason to believe that at least some future versions are likely to present slanted information, misinformation, or disinformation.
Good luck to all students in your search for information, regardless!---CH 00:15, 1 July 2006 (UTC)
thread_title: Todo list

message: V is not defined.DLH (talk) 13:03, 8 November 2016 (UTC)
thread_title: Define V in History

message: I've removed a reference to "Einstein shift", since there are oodles of different effects that might reasonably be referred to as an Einstein shift. The term doesn't identify the effect, and is only intelligible is the reader already knows the exact context of the phrase. I'm sure that the phrase sometimes gets used in essays or lectures where the subject being discussed is already known, but it doesn't identify the subject. SR velocity redshift? SR transverse redshift? I don't think anyone will object to its loss. ErkDemon 00:36, 3 September 2007 (UTC)
replies: text: I'm also deleting the sentence that says that this effect is sometimes referred to as the "Einstein effect" ... I'm sure it is, but there are probably 10-20 other effects that this loose phrase might refer to, covering anything from special relativity to general relativity to quantum mechanics, to the way that tealeaves move towards the centre of your teacup when you stir (seriously, Einstein wrote an excellent paper on that effect). Since the phrase doesn't seem to mean much unless the reader already knows which topic is being discussed, I'm hitting delete. Again, I'm assuming that nobody will object. ErkDemon 00:48, 3 September 2007 (UTC)
thread_title:  Deleting refs to Einstein shift, "Einstein effect" 

message: In the article it states: 
r is the radius of the star you consider. 
Does that mean the star that produced the light or the star that produced the gravitational redshift?
--Tommac2
:I tried to clear it up a little.  In the example, the star emitting the light is the same as the one that's redshifting the light.  In the example, r is the distance from the center of the star from which the light is emitted, but it was a little to complicated to rewrite it, so I just stated that r is the radius of the star. Geoffrey.landis (talk) 02:38, 25 March 2008 (UTC)
thread_title:  Clarification please 

message: This image has apparently been in the lede of the article since 2005, but I'm darned if I can see how it's supposed to represent gravitational redshift, even inexactly. It looks more like a new-age religious symbol than a scientific diagram to me. The image description page has no sources and no information about how it was calculated. Am I missing something? -- BenRG (talk) 14:19, 18 February 2009 (UTC)
thread_title:  Lede image 

message: This is just the first paragraph of the article:
----
This article is in need of attention from an expert on the subject. WikiProject Physics or the Physics Portal may be able to help recruit one. (November 2008)
In physics, light or other forms of electromagnetic radiation of a certain wavelength originating from a source placed in a region of stronger gravitational field (and which could be said to have climbed "uphill" out of a gravity well) will be found to be of longer wavelength when received by an observer in a region of weaker gravitational field. If applied to optical wave-lengths this manifests itself as a change in the colour of the light as the wavelength is shifted toward the red (making it less energetic, longer in wavelength, and lower in frequency) part of the spectrum. This effect is called gravitational redshift and other spectral lines found in the light will also be shifted towards the longer wavelength, or "red," end of the spectrum. This shift can be observed along the entire electromagnetic spectrum.
Light that has passed "downhill" into a region of stronger gravity shows a corresponding increase in energy, and is said to be gravitationally blueshifted.
----
Well, I'll be darned.  It's in need of a lot less intensive reworking than that available from an 'expert' on the topic.  Just look at the first paragraph:
'other forms of electromagnetic radiation'?  What would those be?  Sound waves?  Winter boots?
'of a certain wavelength'?  So at what wavelength is light no longer redshifted?
The grammar, clarity and thrust of this article obscure its very purpose.  —Preceding unsigned comment added by 142.1.134.54 (talk) 22:53, 12 March 2009 (UTC)
thread_title:  Holey Moley 

message: Substituted at 16:38, 29 April 2016 (UTC)
thread_title: Assessment comment

message: This article was quite nice in its original form as written by Chris Hillman in 2005 (  https://en.wikipedia.org/w/index.php?title=Gravitational_redshift&oldid=18056450 ), but seems to have suffered at the hands of less competent people in the years since CH left Wikipedia. I rewrote the first section, which in the recent version used cumbersome notation, belabored mathematical side issues, and didn't give even a sketch of the logical structure of the subject or where the formulas might have come from. This article is still in sorry shape and needs a lot more work. There is a lot of material that is disorganized and a lot that is written in an inappropriate style. I will bring back some of CH's nice descriptive material as well.--76.169.116.244 (talk) 18:27, 28 July 2018 (UTC)
thread_title: Rewrote first section

message: I deleted the sections. The first, titled "Important points to stress," was unencyclopedic, disorganized and out of place, and it included some wrong physics (a claim that the Planck relation was relevant). The second, "Exact solutions," was largely irrelevant, disorganized, and contained incorrect physics (such as a claim that the black hole solutions are the only exact solutions to the Einstein field equations).--76.169.116.244 (talk) 18:36, 28 July 2018 (UTC)
thread_title: Deleted two sections

message: I deleted two more sections. One, titled "Application," consisted of a single value and uninformative sentence. The other, "Gravitational redshift versus gravitational time dilation," was logically muddled and uninformative section.--76.169.116.244 (talk) 18:39, 28 July 2018 (UTC)
thread_title: Deleted two more sections

message: The historical section was a big block of dry historical and mathematical material that in my judgment would be of little interest to a modern reader who does not have a very strong interest in the grotty details of the early historical development of the theory. I've moved it down below the material on experimental tests. In my opinion this material should probably be deleted completely.--76.169.116.244 (talk) 18:43, 28 July 2018 (UTC)
thread_title: Historical section

message: I think a lot could be done to make this article more accessible to those of us who don't have higher math and physics degrees. I'll try to remember to come back to this article and not just tag and leave. 74.93.182.21 (talk) 15:04, 30 July 2018 (UTC)
thread_title:  Technical 

message: Can we verify the referenc to 1971 in the below statement?  The Hubble Space Telescope didn't exist until 1990.
"The redshift of Sirius B was finally measured by Greenstein et al. in 1971, obtaining the value for the gravitational redshift of 89±19 km/sec, with more accurate measurements by the Hubble Space Telescope, showing 80.4±4.8 km/sec."  SquashEngineer (talk) 15:25, 29 July 2019 (UTC)
thread_title:  Experimental Verification 

message: Thus I'm not following the reasoning in deriving the gravity effect on light by looking at an accelerated spaceship containing a light source vs. an free falling observer.
Let's assume that the spaceship has a negative relative speed compared to the free falling ob server and accelerates toward him with a constant acceleration.
When the spaceship has zero relative speed relative to the observer, there should be no doppler effect!
I'm also not getting the argument, that light looses energy when leaving a gravity well (E=h*f), as a photon does not have mass. I could understand this, if the photon would have a mass -- like a bullet. A bullet certainly looses energy when shoot straight up in a gravity field.
I understand, that once one accepts that a photon looses energy when climbing out of a gravity field, one can derive that space is curved, given that the scalar product of a 4 vector must be identical for different observers!
Please help me with above question!  — Preceding unsigned comment added by AchWasSoSo (talk • contribs) 23:31, 20 September 2019 (UTC)
thread_title:  doppler effect is caused by a speed (difference) not by acceleration 

message: Suggestion: Since the Schwarzschild metric is the foundation of gravitational redshift computations for spherical bodies, it should be shown in full:
ds2=(1-2Gm/c2r)dt2 - (1-2Gm/c2r)-1dr2 - r2dΩ2
71.32.47.34 (talk) 21:40, 19 March 2020 (UTC)
thread_title:  Importance of Schwarzschild metric 

message: The introductory paragraph is good. But the next section seems to be in the wrong order. It is confusing to read about the equivalence principle first and the Schwarzschild metric second. The standard method for calculating the gravitational redshift of spherical bodies utilizes the Schwarzschild metric. This should be shown first. The equivalence principle method is an approximation, and is of historic and heuristic interest only. It should be shown second.
71.32.47.34 (talk) 22:17, 19 March 2020 (UTC)
thread_title:  Schwarzschild first, equivalence principle second 

message: A photon falling into a gravity well is blue shifted, no argument.
Suppose the photon has zero energy mass, is it still blue shifted?  Why not?
The upshot is that gravity wells generate a constant supply of energy mass, the equivalent to that lost in inter massive space, to tenacious red shift.  If this were not the case, energy mass would either overwhelm or underwhelm the Universe.
This means stars can burn forever because we can abandon the fusion bonfire theory of stellar heat.  Earth's interior, for instance, is heated by the same process. GuildCompounder (talk) 03:32, 6 July 2022 (UTC)
replies: text: To clarify, a particle of energy mass can move from a compressed space to a decompressed space and redshift that way, or the space around the particle can decompress for the same effect (which I call tenacious since the particle need not move).  Mass generates a compression field, which compresses space within the mass, and decompresses space radiating without the mass.  The Doppler effect is null. GuildCompounder (talk) 02:59, 12 July 2022 (UTC)
thread_title:  Tenacious blueshift 

message: The illustration of the gravitational redshift is very misleading.
There is no observer who will judge a frequency of something travelling up in a gravitational potential to decrease due to GR.
If a supply rocket leaves the ground once a week, a rocket will arrive once a week at the space station. But the astronauts on the space station will judge the frequency to be lower (e.g. take-offs and arrivals once per month in an extreme case) than the frequency judged by the ground staff (once a week).
As a consequence, nobody will "see" an electromagnetic wave leaving the ground with a higher frequency than at some height. The entire wave looks red when viewed from the top and blue when viewed from the bottom. 147.147.166.106 (talk) 12:27, 9 August 2022 (UTC)
thread_title:  Illustration of redshift