Talk:Pound–Rebka experiment

Physics: Relativity / History Low‑importance

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Are there missing square-roots here?

First, I would like to thank the authors of this page for a well written, clear article that was interesting reading. Second, being a newbie with regards to editing Wikipedia pages (or using Wikipedia in general) I apologize if this is an incorrect use of the discussion page, but I think there may be "square-roots" missing on the first 2 equations in this article which relates received to emitted light frequencies. —Preceding unsigned comment added by 70.28.56.144 (talk) 17:53, 24 February 2010 (UTC)[reply]

Yes, the square root is missing here

Special Relativity predicts a Doppler redshift of :

f_{r}={\frac {1-v/c}{1+v/c}}f_{e}.

Correct would be

f_{r}={\sqrt {\frac {1-v/c}{1+v/c}}}f_{e}.

The approximation is

{\sqrt {\frac {1-v/c}{1+v/c}}}={\sqrt {\frac {(1-v/c)(1-v/c)}{(1+v/c)(1-v/c)}}}\approx 1-v/c.

By far the difference of the approximation and Special Relativity is too small to be measured in this case. 95.222.228.77 (talk) 11:11, 23 March 2010 (UTC)[reply]

Someone appears to have fixed the missing square roots.--75.83.69.196 (talk) 01:54, 28 April 2011 (UTC)[reply]

Assessment comment

The comment(s) below were originally left at Talk:Pound–Rebka experiment/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 The comments may be irrelevant or outdated; if so, please feel free to remove this section.

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I've lost an hours work so this is the brief version. Apart from the discussion, at the end, of

importance, all comments are on one small paragraph:-

"The energy associated with gravitational redshift over a distance of 22.5 meters is very small. The fractional change in energy is given by δE/E, is equal to gh/c2=2.5x10-15. Therefore high energy (small wavelength) photons are required to detect such minute differences. (Think of wave length as the distance between marks on a ruler). The 14 keV gamma rays emitted by iron-57 when it transitions to its base state proved to be sufficient for this experiment."

0. The point that it is the change in height (distance from the centre of the Earth) that provides the required change in gravitational potential is not brought out.

1. Variables not explained.

2. Algebraic convention used is not explained (c2 [=c squared?])

2b. best to leave a space before and after the equals sign

3. Units not given for the equation gh/c2 = 2.5x10-15. If these are MKS units, should it not say so?

4. photon and wavelength sentences confused, apparently wrong and a disproportionate level of explanation - given the relative simplicity of linear measurement - compared to the rest of the article.

5. The particular state transition of iron57 would benefit from a little more explanation, although I suppose it can be taken as read and is not pivotal to the main thrust. What was the start state? (Or if the resultant state was not iron57, say so, and what it was).

6. I dare not look back now for fear of losing this text this time, but is the accuracy given? This is a strong point I believe.

7. Cannot teX or something like it be used to make the algebra and number clearer? (e.g. 2.5x10-15). If so, the TeX would instead be something like, if left as inline, $2.5 * 10^{-15}$. (And c2 if meaning squared, would be, if left as inline, $c^2$).

8. Bearing in mind the corrections required still for points 1, 2, 3 and 5 above, I suggest as an interim measure, the following for this paragraph:-

The energy associated with gravitational redshift over a vertical (i.e. Earth radial) distance of 22.5 meters at the surface of the Earth is very small. The fractional change in energy (δE/E), is equal to gh/c2 = 2.5x10-15 [MKS units?]. Where g=?, h=?(height, not e.g. Planck's constant), c=?. The 14 keV gamma rays emitted by iron-57 on a transition to its base state proved to be sufficient for this experiment.

Importance: Agreed, the importance to the Theory of Physics of the experiment may not be as great as the Michelson- Morley experiment for example, provided one is prepared to accept other, indirect evidence of the validity of this outcome of the General Theory of Relativity. But should we be prepared to do that? No I suggest. This was direct, Earth-based, evidence and therefore, even though 'just an experiment'!, it ranks highly in my view, not least because of its ingenuity, which I suspect does not fully come out in the article yet. We should not fall into the trap of denigrating temporal activities relative to THEORY that changes our mind-set just because they have 'merely validated' our understanding rather than changed it forever.

Now for those all-important (I hope) tildes:

1seek2find (talk) 00:42, 10 December 2007 (UTC)[reply]

Last edited at 01:46, 1 January 2012 (UTC). Substituted at 03:18, 30 April 2016 (UTC)

What's R?

$f_{r}={\sqrt {\frac {1-{\dfrac {2GM}{(R+h)c^{2}}}}{1-{\dfrac {2GM}{Rc^{2}}}}}}f_{e}.$

where $f_{r}$ ( $f_{e}$ ) is the frequency of the receiver (emitter), h is the distance between the receiver and emitter, M is the Earth's mass, G is Newton's constant and c is the speed of light.

I think the article should say what $R$ is. I don't think it does anywhere. --IslandHopper 973 12:17, 25 November 2016 (UTC)[reply]

A demonstration of pound-rebka experiment's formula (by akram louiz):

This is a demonstration without using einstein's relativity : https://twitter.com/TheDetective_L/status/1086677193110024192 — Preceding unsigned comment added by 160.177.134.132 (talk) 19:46, 19 January 2019 (UTC)[reply]

The gamma rays come from nuclear transitions, not electrons, but it mentions electrons SO MANY TIMES

When I read this sentence, I thought I could fix it:

"Consider an electron bound to an atom in an excited state."

Okay, that's how sodium vapor lamps work. The emitted photons are yellow. But this experiment uses gamma rays, this is much higher-energy and uses nuclear transitions, the movement of protons and neutrons within the nucleus. Electrons are not involved.

Then I saw the next two whole paragraphs, it says "electron" and "atom" so many times.

Unfortunately, it's "nucleus" and "nucleus", and they are both the same isotope, they are both iron-57. It's not even iron-57 and iron-57m, there is no iron-57m, the excited state of the nucleus is too unstable to get a name, the "m" means "metastable".

I just... I just don't know how to fix it. I don't know General Relativity, but this is chemistry - or close enough. Electrons transitioning between orbitals was discovered earlier and (in my opinion) has a lot more applications, but the photons are lower-energy and the line widths are wide. They even named the electron orbitals and have molecular orbitals, and they can explain why phenolphthalein changes color when the pH changes. I'm not sure people really know what is going on inside the iron-57 nucleus, and they certainly don't have pictures of both states, which they do for phenolphthalein. (To be fair, I haven't seen a picture of the two molecular orbitals in the colorful form of phenolphthalein, but they can draw those pictures, too. Showing that a large conjugated ring electron orbital forms or is broken depending on pH is the important thing.)

I want to just say "the iron-57 nucleus can exist in an excited state or the ground state, because quantum mechanics".

It's important to understand why laser interferometry works, but you need to use lasers, not fluorescent lamps.

This experiment needed to use nuclear transitions, not electrons. That's because the line width is very narrow. The resulting photons are very high energy, they are gamma rays.

Atomic clocks often use hyperfine structure, and transitions between hyperfine states, which gives extremely low energy photons, usually microwave or radio frequency. Just like a fluorescent lamp, the line width is not narrow. But just like lasers, they make the whole apparatus oscillate in phase.

I know that you can "drive" a microwave cavity at the wrong frequency, it is just less efficient, so I actually don't understand how atomic clocks are super precise. That probably involves electrical engineering and PID controllers, or something. People don't fall off of hoverboards, so they seem to be pretty good at PID controllers. If I can stand on a hoverboard for ten seconds, then an atomic clock can probably stay very near the maximum efficiency frequency for ten years. Apparently. Fluoborate (talk) 15:07, 29 November 2019 (UTC)[reply]

Exactly! The 14.4 kev gamma rays are due to nuclear transitions of Fe57 excited nucleus coming from the inverse beta decay of Co57 nucleus.No transitions of electrons in atoms involved. Juan lacruz (talk) 12:04, 12 October 2020 (UTC)[reply]

@Fluoborate and Juan lacruz: I made some changes to address your concerns and to clarify details of the experiment that were not described clearly. Hope you like my changes and find that the things that seemed confusing have been addressed to your satisfaction, especially how the extreme precision in measuring the gravitational Doppler shift was managed. Prokaryotic Caspase Homolog (talk) 22:45, 24 February 2023 (UTC)[reply]

Juan lacruz (talk) 21:50, 4 February 2024 (UTC) Looks much better now, txs.[reply]