Re-examining the Hafele-Keating experiment

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Re-examining the Hafele-Keating experiment

Postby aramis720 on August 19th, 2018, 10:35 pm 

I'm writing a detailed paper examining various experimental bases for time dilation (for peer-reviewed publication), and finding that a lot of the papers have some serious problems. As my first discussion, I'd like to get some feedback on this discussion below of the now iconic Hafele-Keating experiment that sent cesium clocks on planes around the world in opposite directions in order to test time dilation. Here is a link for the 1972 Hafele-Keating paper and I've attached as a pdf Kelly 2000, a paper that presents a critical review of methods in the HK paper. ... -1972b.pdf

Thanks for your detailed and substantive feedback -- just stick to the points raised, please. I know this is a very large topic for discussion but I want to keep this very focused on the particular experiment and paper at issue.


The Hafele-Keating experiment had many serious issues from the outset, as their 1972 paper itself describes. The authors identify two main experimental accuracy issues: 1) the fact that they were measuring effects on the order of 0.1 microseconds per day and their machinery's accuracy was only within 1 microsecond per day; 2) in correcting the data for this issue they needed to also correct for unpredictability in expected drift in each clock, which they attempted to do with two different methods discussed.

With respect to the first method for correcting for naturally-occuring time drift in the four clocks employed for the experiment, "the average rate method," the authors state (p. 169): "Reliability of results with the average rate method, however, depends on the unlikely chance that only one rate change occurred during each trip and that it occurred at the midpoints. Furthermore, there is no obvious method for estimating the experimental error. Nevertheless, the average rate method does produce convincing qualitative results."

The last sentence is rather incredible given the first two sentences.

With respect to the second method, the authors state (p. 177): "An analysis of these data revealed the times and magnitudes for correlated rate changes during each trip. Thus significant rate changes were identified and ascribed to each clock. A piecewise extrapolation of the time trace for each clock relative to MEAN(USNO), with proper accounting for these identified rate changes, then produced the relativistic time differences [observed]."

We have to dig a bit deeper to find why this method, rather than being an appropriate adjustment, seems instead to be a strong example of cherry picking the data. Kelly 2000 looks at the original data collected by HK from the four cesium clocks used in the experiment (this data was not published in the original paper), after Kelly requested the original report from the US Naval Observatory, and concludes (emphasis added):

[The [US Naval Observatory] standard station had some years previously adopted a practice of replacing at intervals whichever clock was giving the worst performance. On a similar basis, the results of Clock 120 [one of the four used by HK] should have been disregarded. That erratic clock had contributed all of the alteration in time on the Eastward test and on the Westward test, as given in the 1971 report. Discounting this one totally unreliable clock, the results would have been within 5ns and 28ns of zero on the Eastward and Westward tests respectively. This is a result that could not be interpreted as proving any difference whatever between the two directions of flight.

Accordingly, under Kelly 2000’s re-examination of the raw data, it seems that we should accord little to no weight to this now iconic experiment purporting to find strong evidence of physical time dilation – that is, real differences in the elapsed time of traveling clocks.
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