## Nearly Over

Robin Underwood, Myself, and Gavin Sutton just after 'switch off'. The laboratory is genuinely not usually that untidy - we are half-way through a laboratory move. Click for larger picture.

Roughly four years ago I began planning an experiment to measure the Boltzmann constant, and today, together with my colleagues Robin Underwood and Gavin Sutton, we switched off the experiment. I would love to tell you more about the experiment, and indeed one aim of this blog had been to talk about some of the ups and downs of the experiment as they happened. But in the end, after eating, breathing and sleeping the experiment, I just couldn’t bear to blog it too! So most of this blog has been about other things. But I thought that at this point I would like to record answers to the three things about which I am asked most.

1. What is the Boltzmann Constant? The temperature of an object is a measure of the speed with which the atoms within a substance are moving. So in principle, instead of measuring temperature in degrees Celsius, we could measure temperature in terms of the speed of molecules.

It feels cold today darling, I think the average speed of the air molecules must be only 423 metres per second!

The Boltzmann constant is a measure of how much energy of motion – kinetic energy – of molecules corresponds to one degree Celsius. So measuring the Boltzmann constant allows to link our normal temperature scale, to the fundamental definition of temperature.

2. So what’s the Answer? I don’t know yet! I’ll tell you in a month or so.

3. How do we know that we’re right? Knowing that our answer is correct represents the hardest part of the experiment and is why the whole thing has taken so long. Along with our estimate of the true value of the Boltzmann constant, we need to produce another number: an estimate of the uncertainty in our estimate. To produce this uncertainty estimate we look at every assumption we make in the experiment, and evaluate the extent to which that assumption is true. In our experiment we do lots of things to check that our answer is correct, but the simplest thing we is that we do the experiment seven different ways!  Every different way tests our assumptions in a slightly different way, and by looking at the tiny disagreements between the results from different ways of doing the experiment, we can estimate how reliable our assumptions were. At this moment, we are closing in on the uncertainty value and without saying exactly what it is, we expect our answer to have an uncertainty estimate of close to (and hopefully less than) one part in a million.

• One part in a million? If we were estimating a length, we would be able to to measure 1 kilometre with an uncertainty of less than 1 millimetre.

One last thing. This experiment has been the hardest thing I have ever done. I have to learn about entire fields about which I had no prior experience, and in each field I had to work with world-leading experts and typically had to go from nothing to understanding the limits of the technology in a few months. For example, I had to learn about:

• Humidity measurements in ultra dry gases,
• The technology of mass flow controllers,
• Precision pressure measuring technology,
• Co-ordinate measurement technology for determining the shape of objects with amazingly low uncertainty,
• Precision grinding technology,
• Diamond-turning of objects to create objects with ultra-precision surfaces,
• The calculation of oscillating magnetic and electric fields in unusually shaped containers,
• The calculation of microwave fields in unusually-shaped containers,
• Ultra precision thermometer calibrations,
• Measurement of ratio of argon isotopes in our gas,
• Measurement of the amount of neon in ultra pure argon,
• Pyknometry,
• Anti corrosion coatings for copper.
• Ultra precision weighing
And in each field I worked the many fantastic colleagues that make NPL a national treasure. So I didn’t do all the work in all these areas. Far, far from it. But I had to understand what was happening, understand the consequences for the experiment, read scientific papers, and occasionally, driven by overwhelming anxiety, make a good suggestion or two. In this time I made lots of mistakes, but I also got lots of things of right. And now I am really looking forward, to finishing the analysis and finding out the uncertainty value. Oh yes, and working out the final estimate of the Boltzmann constant. I will keep you informed.