Talk:Pound–Rebka experiment/Archive 1

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Does this experiment actually provide evidence for general relativity? My GR lecturer made a point of saying that it doesn't. Instead it is an experiment to see if the local inertial frames of Newtonian theory and special relativity coincide.

Pages 101~103 here : http://people.pwf.cam.ac.uk/gjw33/Lecture%20Notes/Mathematics%20Tripos%20Notes/Part%20III/General%20Relativity%20(III)/Sections%209~13.pdf elaborate on what his views are. --AlphaNumeric 12:20, 21 June 2006 (UTC)


 * Of course it is evidence for General Relativity. If the experiment had shown no gravitational redshift, then GR would've been up the creek.


 * That said, what the experiment really measures is gravitational redshift. Special relativity as usually formulated does not try to predict the effects of gravity, but if you were to guess based on SR and Newton's gravity, you'd pretty much have to guess the same amount of redshift that GR predicts.


 * So although the experiment gives evidence for this (rather striking) effect of GR, it doesn't really distinguish between SR and GR. Nor is it intended to.  Fortunately, other experiments -- such as the deflection of light by the sun -- fill that role adequately enough.


 * - 18.252.6.96 06:18, 6 September 2006 (UTC)

Why the Pound Rebka et al experiment ought to be repeated. First of two reasons. SdeHJ
The original experiment did not monitor the polarization (spin) states of the light entering and the light leaving the experiment. It can be argued in analogy to the known interaction of a spin 1/2 electron with a spin 1 photon wherein the electron changes its spin state from (say) -1/2 to spin state +1/2 that the spin 1 photon might interact with the (argued elsewhere) hypothetical spin 2 graviton by "reversing" its spin 2 units (h), which would show up as a change of the light's polarization state. Refer to http://www.bosonics.net How does Gravitation Really Work?
 * Wikipedia is not a platform for your original research. If you believe there is a flaw in the experiment, you need to find a verifiable source for your statement. Otherwise your claim does not belong on WP.--75.83.69.196 (talk) 01:49, 28 April 2011 (UTC)

great job
fantastic and well thought out/written, nice work! —Preceding unsigned comment added by 66.66.127.44 (talk) 06:42, 3 November 2007 (UTC) Well, good work, but a reservation, SineBot at sign of the 66es: We are told that " When an atom of the same species in its base state encounters a photon with that same frequency and energy, it will absorb that photon and transit to the excited state. If the photon's frequency and energy is different by even a little, the atom cannot absorb it (this is the basis of quantum theory). "

This (this is the basis of quantum theory)  is cute. It has that feyness that is oh so fashionable in writing about all things quantum, but what does it mean? For that matter, what does "absorb" a photon mean? Are we back at plums flying from one boiled Christmas pudding to another? "Basis" could mean one of two things with respect to quantum theories, it seems to me: it could be a reference to the problems which made the quantum explanations necessary, or it could mean a cornerstone phenomenon which made the rest of the edifice stand. If the first is meant here, the basis would be ridiculous, there being so many such problems. If a cornerstone is meant, then surely the claim is a little premature.

And a minor quibble: "If the photon's frequency and energy is different" it sez here. Well if either one is different the other one would have to be. I don't know whether this use of the singular verb is a new standard for the recondite, a pointer to a cute tautology, or just offhandedness like that "absorb" stuff. David Lloyd-Jones (talk) 16:45, 14 September 2014 (UTC)

references are behind paywall
Find new ones that aren't or remove links. —Preceding unsigned comment added by Crusty007 (talk • contribs) 23:14, 7 February 2008 (UTC)
 * I checked, and none of the current external links appeared to be behind a paywall. I did find an external link that was broken, so I deleted it. As far as the references, they're under copyright, so we can't help the fact that they're paywalled. WP does not have a policy that references are to be deleted unless they are free online -- such a policy would eliminate the vast majority of print-based references on WP. --75.83.69.196 (talk) 01:51, 28 April 2011 (UTC)

Time Dialation vs. Doppler Effect
From the article: "It is a gravitational redshift experiment, which measures the redshift of light moving in a gravitational field, or, equivalently, a test of the general relativity prediction that clocks should run at different rates at different places in a gravitational field."

This is not quite correct. Clocks running at different rates exhibit Time Dialation. This is a distinct phenomenon from gravitational redshift. It is easy to confuse these two phenomena because this distinction is somewhat subtle, and unfortunately missed in most popular books such as "The Black Hole War" by Leonard Susskind. Both phenomena correspond to the perception of time "slowing down" from the point of view of an observer. The difference is that time dialation corresponds to an actual difference in the passage of time according to two (differently accelerating) frames of reference. Gravitational redshift, on the other hand, only affects a perceived difference, which one can compensate for by considering the time it takes for the light to travel to the observer.

A similar distinction exists in special relativity. If I look at Alpha Centauri in a telescope, what I see forms my opinion of what was taking place there 4.5 years ago. In any problem involving disagreement between two frames of reference, we always take into account the time that it takes for light to travel between two points. After compensating for that travel time, we then measure the relativistic effects.

Likewise, in the case of gravitational redshift, I can take the travel time of light into account, and form my opinion of what is actually happening. Thus gravitation redshift only affects my sensory perception, but not my opinion of what is happening. After compensating for gravitational redshift, I will still find evidence of time dialation. —Preceding unsigned comment added by Kenseehart (talk • contribs) 18:43, 10 October 2008 (UTC)
 * The article is correct. There is no empirically testable distinction between a gravitational redshift and a gravitational time dilation. They are not distinct phenomena.--75.83.69.196 (talk) 01:48, 28 April 2011 (UTC)

Doppler Effect - not Relativistic Doppler Effect
I appreciate the author taking the time to put this article together. I've never read such a clear explanation of this experiment.

Please let me bring what I believe is an error to your attention --

The article says "Pound and Rebka countered the gravitional blueshift by moving the emittor away from the receiver, thus generating a relativistic Doppler redshift"

As I understand it, Relativistic Doppler Effect can only be created by having emitter and receiver separated by distance greater than the size of a galaxy cluster. (Cosmological constant effects are claimed not to exist at smaller distances.)

I suspect the experiment's emitter and receiver were within a few kilometers of each other and receding at velocities under a few kph.

Unless the emitter and receiver were approaching or receding at relativistic velocities at redshifts approaching 1.0 -- isn't this is a demonstration of simple Doppler Effect - not Relativistic Doppler Effect?

Thank you.

PS Please also consider correcting the spelling errors: gravitional should be "gravitational", and emittor should be "emitter" —Preceding unsigned comment added by 98.234.21.242 (talk) 06:35, 24 February 2010 (UTC)
 * No, the article is correct. The source and receiver were 22.5 m apart. This is a gravitational Doppler shift, not a cosmological one.--75.83.69.196 (talk) 01:46, 28 April 2011 (UTC)