Talk:Gravitational redshift

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[edit] round-trip gravity shifts?

"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)

[edit] This isn't just a GR result ...

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)

[edit] Recent edits by 24.201.168.190

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)

[edit] Expert cleanup desperately needed

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 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)
It still won't satisfy all expert needs, however.Kmarinas86 02:21, 26 December 2005 (UTC)
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)
Experiments concering gravitational redshift could be done on different articles and linked to by this one.Kmarinas86 02:28, 26 December 2005 (UTC)
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)

[edit] Illustration problems

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)

[edit] 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.

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)

[edit] Doppler effect

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)

No, we shouldn't be trying to present this effect as somehow "belonging" to special relativity.
SR is a flat-spacetime, ~start-of-the-C20th theory, while gravity-shifts are usually reckoned to involve curved space(/time), and were suggested at the end of the C18th. The specific Doppler relationships referred to as "relativistic Doppler" are derived in modern relativity theory by assuming flat spacetime, and aren't neccessarily guaranteed to be correct when the geometry isn't flat. For instance, you don't often see the "relativistic Doppler" relationships mentioned in the context of cosmological redshifts, either ... if you ask about SR and Hubble shift, you'll likely be told that RD doesn;t neccessarily apply, because cosmological shifts are curvature-based, and are therefore a different class of problem. Similar objections could be applied to the "gravitational redshift" case, those are arguably curvature-based, too. ErkDemon 15:36, 16 September 2007 (UTC)

[edit] Deleting refs to Einstein shift, "Einstein effect"

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)

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)

In the article it says that there must be a difference in gravitational fields to see a gravitational redshift.

[edit] Can Dark Matter cause 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)

The simplest form of the gravitational redshift depends only on the difference in gravitational potential. So if a large mass is between you and a distant galaxy, the light would be blueshifted when it approaches the mass, and then redshifted by the same amount as it exits from the mass, and the total result would be that there would be no net redshift.
Dark matter does have gravity, so it causes gravitational redshift to exactly the same extent that ordinary matter does. Geoffrey.landis (talk) 02:26, 25 March 2008 (UTC)

[edit] Clarification please

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)