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The Wikipedia article on Gravitational Redshift can be referenced to Lang "Astrophysical Formulae" Springer Verlag, second edition 1980 page 579 equation 5-279. I believe the first equation for z is correct, but the second equation is not correct unless a further explanation is given. Lang measures z at infinite distance from the mass center. Also in Lang the radius rs is the event horizon, and radius r is the location from which the photon is emitted. With these explanations the second equation is in agreement with Lang. These equations do not say what red shift would be measured at a finite distance inside the curved space of the same mass. |
"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)
Just a few small points:
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)
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)
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)
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)
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)
V is not defined. DLH ( talk) 13:03, 8 November 2016 (UTC)
I seem to miss a clear explanation on top of this page that Gravitational redshift is part of the
Relativistic Doppler effect. When I type
Gravitational shift I'm redirected to this page, but then I seem to miss a clear explanation of Gravitational shift itself as this is connected to light, time as well as space. A nice reference on this subject:
Harvard study
I might not be the expert needed, a generalist would be welcome here too for some general outline.
What if the reader wants to know about gravitational blue shift ? Maybe some redirections should be changed ?
Do I make a point ? --
Homy 11:36, 5 August 2006 (UTC)
Nobody here, let's make some changes --
Homy 17:16, 7 August 2006 (UTC)
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)
In the article it says that there must be a difference in gravitational fields to see a gravitational redshift.
My understanding is that gravity can bend space. If a great mass passes in front of light could one see a redshift since the distance now is farther than it was? the speed of light should be constant but I think that it relatively has to go father in the same time. Is this correct?
Also if the above is correct. What would be the effect of mass that was spread over great distances. Where I am going with this is :
Could dark matter spread out over billions of light years cause a red shift in a star that is not moving away from us in absolute terms? The thought is that the amount of dark matter that the light passed through could have quite a bit of gravity. Thus making far away objects appear that they are moving farther away from us quicker than they actually are.
-- Tommac2 —Preceding unsigned comment added by 69.249.66.67 ( talk) 04:15, 22 March 2008 (UTC)
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 am confused, not so much by what is stated in this article as by what is not. Nowhere in the theory is the redshift value, z, represented as being dependent upon the emitted frequency, f0. Yet if z is not dependent upon frequency, then a very simple mathematical model demonstrates that the resulting equation for predicting z should be exponential in form.
Suppose we set up a coordinate system with the x axis perpendicular to the earth’s surface. We place an observer at each of the points x1 and x2. Let d1 be the distance between x0 and x1. Let d2 be the distance between x1 and x2. A photon of frequency f0 will be emitted at x0. When it reaches x1, its frequency shall be f1; when it reaches x2, f2. Since over short distances the variation in g from x0 to x2 is negligible, we can consider g as a constant; and if the gravitational redshift is independent of frequency, we consider z = (f0- f1)/f1 as a function, F(d1), of the vertical distance traversed:
(f0 - f1)/f1 = F(d1).
Similarly, we obtain
(f1 - f2)/f2 = F(d2),
(f0 - f2)/f2 = F(d1 + d2).
Blending these equations so as to eliminate the fi, we get a functional equation F(d1 + d2) = F(d1) + F(d2) + F(d1) F(d2), whose only non-trival, continuous solution is
F(d) = (F(1) + 1)d - 1, or
f0/f1= kd.
This is the only solution that will predict gravitational redshift so that z is independent of frequency and the distance of separation between observers is additive. We can introduce b so that
f0/f1 = ebd.
To be linearly equivalent to current theory, as well as experimental results, b must have the value g/c2. Thus we end up with
f0 = f1 exp(gd/c2)
This result does not provide a reason for why the gravitational redshift occurs, but it does indicate the exact quantitative form for that redshift. If current principles do not yield such an equation, doesn’t that indicate something is amiss? Samdhatte ( talk) 18:55, 24 June 2008 (UTC)
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)
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)
The comment(s) below were originally left at Talk:Gravitational redshift/Comments, and are posted here for posterity. Following several discussions in past years, these subpages are now deprecated. The comments may be irrelevant or outdated; if so, please feel free to remove this section.
Talk:Gravitational redshift/Comments
This discussion results from an interesting article that contained the following premise: History The gravitational weakening of light from high-gravity stars was predicted by John Michell in 1783, using Isaac Newton's concept of light as being composed of ballistic light corpuscles (see: emission theory). FYI, the original link may still be active: http://en.wikipedia.org/wiki/Gravitational_red_shift |
Last edited at 20:47, 7 November 2006 (UTC). Substituted at 16:38, 29 April 2016 (UTC)
This article was quite nice in its original form as written by Chris Hillman in 2005 ( https://en.wikipedia.org/?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)
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)
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)
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)
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)
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)
Disregard - I reread the sentence. SquashEngineer ( talk) 12:56, 21 January 2021 (UTC)
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)
OK -- I found the answer at https://physics.stackexchange.com/a/86956 Mass is equivalent to energy and thus one can derive a mass from the photons energy: E=m*c^2=h*f A mass looses energy when dropped in an gravitational field: E=m*g*deltaY. One can derive the 'equivalent' mass from the first equation, use it to derive the change of energy by lifting/dropping this mass in a gravitational field and then derive the new frequency of the light. m=h*f/c^2. deltaE = h*f/c^2*g*deltaY=h*deltaF. deltaF = f/c^2*g*deltaY. voila! — Preceding unsigned comment added by 2605:6000:101B:8650:5C6F:D38C:62E8:B695 ( talk) 20:37, 21 September 2019 (UTC)
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) Kathleen Rosser
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) Kathleen Rosser
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)
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)
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The Wikipedia article on Gravitational Redshift can be referenced to Lang "Astrophysical Formulae" Springer Verlag, second edition 1980 page 579 equation 5-279. I believe the first equation for z is correct, but the second equation is not correct unless a further explanation is given. Lang measures z at infinite distance from the mass center. Also in Lang the radius rs is the event horizon, and radius r is the location from which the photon is emitted. With these explanations the second equation is in agreement with Lang. These equations do not say what red shift would be measured at a finite distance inside the curved space of the same mass. |
"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)
Just a few small points:
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)
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)
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)
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)
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)
V is not defined. DLH ( talk) 13:03, 8 November 2016 (UTC)
I seem to miss a clear explanation on top of this page that Gravitational redshift is part of the
Relativistic Doppler effect. When I type
Gravitational shift I'm redirected to this page, but then I seem to miss a clear explanation of Gravitational shift itself as this is connected to light, time as well as space. A nice reference on this subject:
Harvard study
I might not be the expert needed, a generalist would be welcome here too for some general outline.
What if the reader wants to know about gravitational blue shift ? Maybe some redirections should be changed ?
Do I make a point ? --
Homy 11:36, 5 August 2006 (UTC)
Nobody here, let's make some changes --
Homy 17:16, 7 August 2006 (UTC)
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)
In the article it says that there must be a difference in gravitational fields to see a gravitational redshift.
My understanding is that gravity can bend space. If a great mass passes in front of light could one see a redshift since the distance now is farther than it was? the speed of light should be constant but I think that it relatively has to go father in the same time. Is this correct?
Also if the above is correct. What would be the effect of mass that was spread over great distances. Where I am going with this is :
Could dark matter spread out over billions of light years cause a red shift in a star that is not moving away from us in absolute terms? The thought is that the amount of dark matter that the light passed through could have quite a bit of gravity. Thus making far away objects appear that they are moving farther away from us quicker than they actually are.
-- Tommac2 —Preceding unsigned comment added by 69.249.66.67 ( talk) 04:15, 22 March 2008 (UTC)
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 am confused, not so much by what is stated in this article as by what is not. Nowhere in the theory is the redshift value, z, represented as being dependent upon the emitted frequency, f0. Yet if z is not dependent upon frequency, then a very simple mathematical model demonstrates that the resulting equation for predicting z should be exponential in form.
Suppose we set up a coordinate system with the x axis perpendicular to the earth’s surface. We place an observer at each of the points x1 and x2. Let d1 be the distance between x0 and x1. Let d2 be the distance between x1 and x2. A photon of frequency f0 will be emitted at x0. When it reaches x1, its frequency shall be f1; when it reaches x2, f2. Since over short distances the variation in g from x0 to x2 is negligible, we can consider g as a constant; and if the gravitational redshift is independent of frequency, we consider z = (f0- f1)/f1 as a function, F(d1), of the vertical distance traversed:
(f0 - f1)/f1 = F(d1).
Similarly, we obtain
(f1 - f2)/f2 = F(d2),
(f0 - f2)/f2 = F(d1 + d2).
Blending these equations so as to eliminate the fi, we get a functional equation F(d1 + d2) = F(d1) + F(d2) + F(d1) F(d2), whose only non-trival, continuous solution is
F(d) = (F(1) + 1)d - 1, or
f0/f1= kd.
This is the only solution that will predict gravitational redshift so that z is independent of frequency and the distance of separation between observers is additive. We can introduce b so that
f0/f1 = ebd.
To be linearly equivalent to current theory, as well as experimental results, b must have the value g/c2. Thus we end up with
f0 = f1 exp(gd/c2)
This result does not provide a reason for why the gravitational redshift occurs, but it does indicate the exact quantitative form for that redshift. If current principles do not yield such an equation, doesn’t that indicate something is amiss? Samdhatte ( talk) 18:55, 24 June 2008 (UTC)
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)
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)
The comment(s) below were originally left at Talk:Gravitational redshift/Comments, and are posted here for posterity. Following several discussions in past years, these subpages are now deprecated. The comments may be irrelevant or outdated; if so, please feel free to remove this section.
Talk:Gravitational redshift/Comments
This discussion results from an interesting article that contained the following premise: History The gravitational weakening of light from high-gravity stars was predicted by John Michell in 1783, using Isaac Newton's concept of light as being composed of ballistic light corpuscles (see: emission theory). FYI, the original link may still be active: http://en.wikipedia.org/wiki/Gravitational_red_shift |
Last edited at 20:47, 7 November 2006 (UTC). Substituted at 16:38, 29 April 2016 (UTC)
This article was quite nice in its original form as written by Chris Hillman in 2005 ( https://en.wikipedia.org/?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)
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)
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)
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)
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)
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)
Disregard - I reread the sentence. SquashEngineer ( talk) 12:56, 21 January 2021 (UTC)
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)
OK -- I found the answer at https://physics.stackexchange.com/a/86956 Mass is equivalent to energy and thus one can derive a mass from the photons energy: E=m*c^2=h*f A mass looses energy when dropped in an gravitational field: E=m*g*deltaY. One can derive the 'equivalent' mass from the first equation, use it to derive the change of energy by lifting/dropping this mass in a gravitational field and then derive the new frequency of the light. m=h*f/c^2. deltaE = h*f/c^2*g*deltaY=h*deltaF. deltaF = f/c^2*g*deltaY. voila! — Preceding unsigned comment added by 2605:6000:101B:8650:5C6F:D38C:62E8:B695 ( talk) 20:37, 21 September 2019 (UTC)
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) Kathleen Rosser
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) Kathleen Rosser
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)
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)