Clink Street, London, 8th February 2014
One of the inventors of the physics package in our No.10 atomic pocket watch was in town yesterday and asked if he could pay a visit. He and his organization aren't keen on publicity, so I'll just call him Doctor Jazz.
You have no idea how nerdy the world can get unless you witness the meeting of two physicists who have been working on the same thing, but from different angles. I had so many deep, technical questions for Doctor Jazz, and I think he was delighted someone finally wanted to ask him.
How to achieve accuracy; how to do so with minimum power; minimum cost; minimum size. The choice of rubidium or caesium; choosing exactly which electronic transitions to measure; the practicalities of vapour vessels, caesium fountains and optical lattices in a portable timepiece, and new adaptations that may come in future; the safety of such exotic devices. And exactly where the sources of inaccuracy come from: I knew about the effects of temperature, but never realized that as the atoms crashed into the sides of the tiny chamber, their resonances went slightly out of kilter.
We talked about so much, but three themes kept coming out.
First, temperature. Ever since water clocks were first used as timepieces thousands of years ago, temperature has been the bain of timekeeping. It affects water viscocity; candle burning rate; pendulum length; balance spring moment of inertia; quartz resonance due to thermal expansion; and, as I learned today, atoms going so fast that their crashing into vial walls makes them buzz like bees. There are only three solutions to this problem. You can compensate for it, which Graham devised and Harrison went on to use to take the chronometer prize for. You can dodge it, by holding the timekeeper at a constant temperature. Or you can delete it from the equation entirely, by laser cooling the system to (nearly) absolute zero. One day a watch with the like that will exist, just for the hell of it. Right now, the No.10 uses a combination of the first two methods.
Second - Military-tight GPS needs an accurate clock. It would be inappropriate to explain why online, but we delighted in the parallel with Harrison's mission to find longitude. The same remains true today - to know where you are, you need to know what time it is. Extremely well.
The third resonance was relativity. "Knowing what time it is, extremely well" is starting to lose its meaning, because everyone's reference frames are different. The best atomic clocks are so sensitive that you only need to raise one a metre and it can capably measure how much younger it is getting relative to another clock that stayed on the shelf. Defining the second will at some point need to be relative not just to the resonances of a caesium atom, but to a particular place with known gravity.
I'll leave you a fourth resonance. Top photo: Doctor Jazz's prototype atomic resonator. Bottom photo: 1947, Bardeen, Brattain and Shockley's prototype - the first transistor.