The Hafele-Keating experiment was performed 50 years ago. Starting October 4th, 1971, Joe Hafele and Dick Keating took 4 hp cesium clocks on an airplane trip, a very long trip, all the way around the world, twice. The purpose of the trip was to see if they could detect predicted relativistic effects using "flying clocks". At that point in history portable hp cesium clocks were just accurate/stable enough to try this bold experiment. Subsequent work during the 70's confirmed and refined their results. For a brief description of the experiment: https://en.wikipedia.org/wiki/Hafele%E2%80%93Keating_experiment Here's the detailed technical paper Hafele presented at PTTI in 1971: "Performance and Results of Portable Clocks in Aircraft" https://apps.dtic.mil/sti/pdfs/ADA489971.pdf Links to their two scientific journal articles in 1972: "Around-the-World Atomic Clocks: Predicted Relativistic Time Gains" https://www.science.org/doi/10.1126/science.177.4044.166 "Around-the-World Atomic Clocks: Observed Relativistic Time Gains" https://www.science.org/doi/10.1126/science.177.4044.168 For a collection of PDF and photos see my 50th anniversary Hafele-Keating page: http://leapsecond.com/museum/HK50/ I've also attached a photo of their clock setup in the back of my car. Two vintage 5061A cesium clocks, each with 1PPS and Patek Philippe clock option. Between them a K02-5060A NiCad battery pack. Combined it's about 200 lbs of equipment. Two of these racks were used for a total of 4 clocks. In addition they had a hp 5360A computing counter with time interval plugin to make 4-way clock consistency comparisons periodically during the experiment. Nearly 500 lbs of "carry-on". You won't be able to do that today... For those who want a time nut introduction to cesium clocks and relativity here's a talk at Stanford a few years ago: http://web.stanford.edu/group/scpnt/pnt/PNT18/presentation_files/I08-VanBaak-GPS_Flying_Clocks_and_Relativity.pdf /tvb

Hi > On Oct 5, 2021, at 11:33 AM, Tom Van Baak <tvb@LeapSecond.com> wrote: > > The Hafele-Keating experiment was performed 50 years ago. Starting October 4th, 1971, Joe Hafele and Dick Keating took 4 hp cesium clocks on an airplane trip, a very long trip, all the way around the world, twice. The purpose of the trip was to see if they could detect predicted relativistic effects using "flying clocks". > > At that point in history portable hp cesium clocks were just accurate/stable enough to try this bold experiment. Subsequent work during the 70's confirmed and refined their results. For a brief description of the experiment: > > https://en.wikipedia.org/wiki/Hafele%E2%80%93Keating_experiment > > Here's the detailed technical paper Hafele presented at PTTI in 1971: > > "Performance and Results of Portable Clocks in Aircraft" > https://apps.dtic.mil/sti/pdfs/ADA489971.pdf > > Links to their two scientific journal articles in 1972: > > "Around-the-World Atomic Clocks: Predicted Relativistic Time Gains" > https://www.science.org/doi/10.1126/science.177.4044.166 > > "Around-the-World Atomic Clocks: Observed Relativistic Time Gains" > https://www.science.org/doi/10.1126/science.177.4044.168 > > For a collection of PDF and photos see my 50th anniversary Hafele-Keating page: > > http://leapsecond.com/museum/HK50/ > > I've also attached a photo of their clock setup in the back of my car. Two vintage 5061A cesium clocks, each with 1PPS and Patek Philippe clock option. Between them a K02-5060A NiCad battery pack. Combined it's about 200 lbs of equipment. Two of these racks were used for a total of 4 clocks. In addition they had a hp 5360A computing counter with time interval plugin to make 4-way clock consistency comparisons periodically during the experiment. Nearly 500 lbs of "carry-on". You won't be able to do that today... My recollection is that even back then, they bought one or more seats for the clock(s). The ideal location apparently was at a bulkhead (by the galley ???). It was the only place they might be able to get power to charge the batteries in flight … Bob > > For those who want a time nut introduction to cesium clocks and relativity here's a talk at Stanford a few years ago: > > http://web.stanford.edu/group/scpnt/pnt/PNT18/presentation_files/I08-VanBaak-GPS_Flying_Clocks_and_Relativity.pdf > > /tvb > > <HK50-tvb-IMG_8802-640x640.jpg><HK50-tvb-IMG_8801-640x640.jpg>_______________________________________________ > time-nuts mailing list -- time-nuts@lists.febo.com -- To unsubscribe send an email to time-nuts-leave@lists.febo.com > To unsubscribe, go to and follow the instructions there.

On 6/10/21 2:50 am, Bob kb8tq wrote: > Hi > >> On Oct 5, 2021, at 11:33 AM, Tom Van Baak <tvb@LeapSecond.com> wrote: >> >> The Hafele-Keating experiment was performed 50 years ago. Starting October 4th, 1971, Joe Hafele and Dick Keating took 4 hp cesium clocks on an airplane trip, a very long trip, all the way around the world, twice. The purpose of the trip was to see if they could detect predicted relativistic effects using "flying clocks". >> >> At that point in history portable hp cesium clocks were just accurate/stable enough to try this bold experiment. Subsequent work during the 70's confirmed and refined their results. For a brief description of the experiment: >> >> https://en.wikipedia.org/wiki/Hafele%E2%80%93Keating_experiment >> >> Here's the detailed technical paper Hafele presented at PTTI in 1971: >> >> "Performance and Results of Portable Clocks in Aircraft" >> https://apps.dtic.mil/sti/pdfs/ADA489971.pdf >> >> Links to their two scientific journal articles in 1972: >> >> "Around-the-World Atomic Clocks: Predicted Relativistic Time Gains" >> https://www.science.org/doi/10.1126/science.177.4044.166 >> >> "Around-the-World Atomic Clocks: Observed Relativistic Time Gains" >> https://www.science.org/doi/10.1126/science.177.4044.168 >> >> For a collection of PDF and photos see my 50th anniversary Hafele-Keating page: >> >> http://leapsecond.com/museum/HK50/ >> >> I've also attached a photo of their clock setup in the back of my car. Two vintage 5061A cesium clocks, each with 1PPS and Patek Philippe clock option. Between them a K02-5060A NiCad battery pack. Combined it's about 200 lbs of equipment. Two of these racks were used for a total of 4 clocks. In addition they had a hp 5360A computing counter with time interval plugin to make 4-way clock consistency comparisons periodically during the experiment. Nearly 500 lbs of "carry-on". You won't be able to do that today... > > My recollection is that even back then, they bought one or more seats for the clock(s). The ideal location > apparently was at a bulkhead (by the galley ???). It was the only place they might be able to get power to > charge the batteries in flight … There's a photo (sadly I've not got a reference, appears to have been lost to easy searching) of how dailies were shipped from New Zealand to Los Angeles for the Lord of the Rings films which predated New Zealand having affordable international connectivity. The photo shows an Air New Zealand plane with a (business or first class) seat removed and a Netapp NAS along with a few disk trays sitting in its place, so even in the early 2000's this was a thing. These days the bigger trouble is finding an airline that'll let you take a cesium clock on board, when I wanted to bring a Symmetricom cesium back from the US I asked Qantas via their appropriate query path (at the time I was flying QF73/74, their SYD->SFO/return, every few weeks) but they refused to accept it. At some point I'll want to get my Fluke 732's done which probably means getting a 3458 to act as a transfer, or flying down to Melbourne with those running, as one of the few cal labs in Australia I'd trust is Keysight's which is conveniently just down the street from family in Melbourne. >> For those who want a time nut introduction to cesium clocks and relativity here's a talk at Stanford a few years ago: >> >> http://web.stanford.edu/group/scpnt/pnt/PNT18/presentation_files/I08-VanBaak-GPS_Flying_Clocks_and_Relativity.pdf >> >> /tvb >> >> <HK50-tvb-IMG_8802-640x640.jpg><HK50-tvb-IMG_8801-640x640.jpg>_______________________________________________ >> time-nuts mailing list -- time-nuts@lists.febo.com -- To unsubscribe send an email to time-nuts-leave@lists.febo.com >> To unsubscribe, go to and follow the instructions there. > _______________________________________________ > time-nuts mailing list -- time-nuts@lists.febo.com -- To unsubscribe send an email to time-nuts-leave@lists.febo.com > To unsubscribe, go to and follow the instructions there. >