NPL is the home of some of the most accurate clocks in the world. But getting our ‘super-clocks’ to tell the time to the more humble variety in my office is more difficult than one might think.
My office is only 70 metres from the clock which is the source of UK time: that is the black box containing a hydrogen maser at the back of the room in the photograph below.
[ASIDE: My colleagues in NPL’s time team were very keen to let me know that the maser isn’t really a clock, but a frequency standard. I think that means that it is just the ‘tick-tock’ part of a clock. But since they’re never going to read this, let’s just call it ‘the NPL Clock’.]
My office clock is tuned to a radio signal which tells it, for example, to change from British Summer Time to Greenwich Mean Time. This signal is broadcast throughout the UK from a clock in Antorn, Cumbria which is regularly checked against ‘the NPL clock’. So in effect, clocks all over the UK can be synchronised with ‘the NPL clock’.
Unfortunately, the radio signal can’t make it through the steel frame of the NPL building into my office. And so twice a year I have to take my clock for a walk so that it can pick up the radio signal and re-synchronise.
While chatting with some of the folk from the ‘time team’ the other day, I learned that there are similar geographical leaps involved in the synchronisation of the NPL clock itself.
It is only a few metres away from an even more accurate clock based on a ‘Caesium Fountain’. This fountain clock doesn’t run all the tim,e so it is not itself `the NPL Clock’, but it is used to periodically adjust the rate at which ‘the NPL Clock’ ticks.
Additionally, the ‘Caesium Fountain’ clock also links to clocks across the world via satellite, and a comparison is computed monthly at the International Bureau of Weights and Measures (BIPM) in Paris.
This international timescale, called `Coordinated Universal Time (UTC)’ is then sent back from BIPM and used to periodically adjust ‘The time from NPL’. This periodic adjustment prevents clocks around the world from drifting slowly out of synchronisation.
I was slightly surprised at how complicated all this was, but after a moment’s reflection, I realised that this problem is not new, and that the difficulty in synchronising clocks is not a mere detail.
Synchronising clocks in different places has always been difficult.
It was the railways that made us aware of the differences between the times told by local clocks throughout the UK. Trains travelling at 30 metres per second made us aware of differences of many minutes between local clocks.
Now we use light travelling at 300,000 kilometres per second – ten million times faster– and we worry about differences ten million times smaller.
But the conceptual difficulty remains because – and this was perhaps Einstein’s greatest insight – there is no universal ‘Now’.
The concept of ‘Now’ relies on the concept of ‘simultaneity’.
When we say that “something happened at a particular time” we mean that it occurred simultaneously with a particular ‘tick’ of a clock.
But as we consider events more and more distant, the delays involved in observing the event mean that events which some people see as simultaneous, are not seen as simultaneous by other observers.
Which means that events which some people consider are happening ‘Now’, will have happened ‘Then’ for other observers.
I am so glad I don’t work in the time team – it would hurt my head!
As a counterpoint to all this ‘behind-the-scenes’ complexity NPL, has recently installed a new decorative clock in its Foyer.
It looks nicer than the ‘black box’ and, pleasingly, it does more than just ‘tick’, but is also effective at ‘telling the time’.
And, yes, it is linked directly to the NPL clock, so hopefully no one will need to take it for a walk twice a year!