National Physical Laboratory

Time flies

In June 2010 one of NPL's atomic clocks was flown around the world as part of a rare experiment to test Einstein's theories of Relativity. The results demonstrate that Einstein's theories are correct, as NPL was able to measure a clear time-shift of 230 ± 20 nanoseconds between the two clocks involved in the experiment. This agrees with the time-shift predicted by Einstein.

NPL's Setnam Shemar with Bang Goes the Theory presenter, Dallas Campbell, and the atomic clock
NPL's Setnam Shemar with Bang Goes the Theory presenter, Dallas Campbell,
and the atomic clock. This photo was taken outside NPL before Dallas and the
clock set off on their round the world trip

Testing Relativity

The experiment was a repeat of the famous Hafele-Keating experiment and revealed the time dilation effects predicted by Einstein's theories of Relativity.

To reveal these effects, you need two highly accurate atomic clocks, calibrated to check that they are perfectly in sync with each other - ticking down the nanoseconds (that's one billionth of a second: 0.000 000 001 seconds) in complete unison. Then, one clock is taken on a trip around the world, whilst the other one stays at NPL.

Did you know?
NPL can measure time to the accuracy of
0.000 000 000 1 seconds


When the two clocks are reunited, according to Einstein's theories, they should no longer be in sync - the clock that travelled around the world should be ahead of the stationary clock by a significant amount. In this case, the time-shift was predicted to be 246 ± 3 nanoseconds after taking account of the aircraft speed and height for the different flights taken in the journey. The actual measured result using the clocks was 230 ± 20 nanoseconds, which agrees (to within the measurement error) with the prediction.

Time flies graph

The BBC1 science programme, Bang Goes the Theory, approached NPL in late 2009 to ask if it would be possible to use two of our atomic clocks to perform this experiment, which saw one of them fly from London - LA - Auckland - Hong Kong - London.

Dr Setnam Shemar, one of NPL's Time scientists, was pleased to oblige:

"This is a rare and exciting experiment - a full round-the-world trip like this hadn't been attempted since 1971, so we were absolutely delighted to be a part of it. It was great to have non-scientists [the BBC crew] so closely involved with the fieldwork aspect of the experiment - as they're the ones that accompanied the clock on the flights."

Einstein's theories are split into 'Special', and 'General' Relativity. Special Relativity deals with the effects caused by the relative motion of objects (e.g. the motion of the flying clock relative to the stationary clock). Whilst General Relativity deals with how gravity affects the shape of space-time (e.g. space-time curves around massive objects, like the Earth).

In this experiment, both Special and General Relativity effects are at work and contribute to the measured time-shift:

  • Special Relativity effects are caused by the speed of the flying clock as it travels around the world relative to the stationary* clock.

  • General Relativity effects are caused by the altitude of the flying clock - space time near the surface of the Earth is more steeply curved than at the height of the aircraft, so the airborne clock (and everything else on the aircraft) is travelling through space-time that is slightly less 'stretched' than it is at the Earth's surface. This stretching of space-time is what makes time run slower on the ground relative to on the aircraft.

NPL staff worked very closely with the BBC production team to make sure the experiment had the best chance of success.

NPL is the home of the UK's atomic Time Scale UTC(NPL), and the world's first working atomic clock was developed at NPL by Louis Essen in 1955.

The episode of Bang Goes the Theory featuring the atomic clock's trip also contains a street science feature based on an NPL weight perception demo – 'the dolls of confusion'. These are a series of Russian Doll-style objects of varying mass and size, which reveal that humans' perception of an object's weight is far from reliable when compared with physical measurements.

* Although, of course, the clock in Teddington was never stationary - it is rotating with the surface of the Earth once every day at a breakneck speed of 300 metres per second.

Find out more about NPL's Time research

Published: 8 September 2010

Last Updated: 1 Feb 2011
Created: 1 Feb 2011

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