Curbing the kilogram's weight-loss programme

Prototype kilogram mass (Image: NPL) The original prototype kilogram mass resides in a vault in France, seemingly losing weight

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"It's scandalous!"

Assent murmurs its way through the crowd as Nobel laureate Bill Phillips continues.

"The time has come to make this transition," he says. "It's a scandal we have this metal, sitting around changing its mass."

As scandals go, it's a subtle one. What Professor Phillips and a room full of those concerned with the measurement of things - metrologists, as they're known - are at this Royal Society discussion meeting to hash out is the ongoing scandal of the kilogram.

The kilogram as we currently know it is changing. The official standard kilogram that you might find, say, in the US weighs just a smidgen more than the original one kept under lock and key at the International Bureau of Weights and Measures (BIPM) in France.

Some of this story may be familiar - this drift of kilogram has been under scrutiny for years. But what is under discussion is something more radical than just solving the kilogram "scandal"; a fundamental change to the way units are defined is afoot.

"There's a lot to do and this is a very important conference," said Science Minister David Willetts as he opened the meeting.

"That painful debate on the climate change data involving the [University of East Anglia] researchers reminds us of the damage that can be done when - rightly or wrongly, fairly or unfairly - people lose confidence in measurement and the reliability of data."

Time and again

The SI system comprises seven units of measure, some of which may not be so familiar: the metre, kilogram, second, ampere, candela, kelvin, and mole.

The original push to define them did away with countless units of measure that were not standardised and did not cross international - or sometimes even local - borders.

Fast forward to today and the kilogram alone stands as the only standard unit that is still defined by a physical object - a lump of platinum kept under lock and key at the BIPM.

The metre-long lump became obsolete in 1960, and the metre has most recently been defined as the distance that light travels in a well-defined but tiny fraction of a second (1/299,792,458 of a second for those with great stopwatches).

Now, the push is to define the whole suite of SI units in terms of the fundamental physical constants - numbers straight out of some of the most precise experiments science has yet sought to devise.

In this way, the thinking goes, the national bureaux of standards in every country can build the same experiment and define their own units, without needing to refer to a single, global, lump-of-metal standard.

But getting a consensus together for the General Conference on Weights and Measures - the international affair that makes the final pronouncement on these matters - is no easy task.

"Whenever one wants to change things in a big radical way like this, people look at their own work: 'How's my work going to be affected? Are my experiments going to be important ones?'" said Terry Quinn, former director of the BIPM and organiser of this week's conference.

"They look for all sorts of arguments against it, but this is the way science advances, this is why we have the discussion meetings."

No effect

An underlying belief in the whole system seems to be a common theme among researchers struggling to explain how it will matter to the common consumer, measuring a shelf or weighing potatoes.

"In the first instance, it'll have no effect whatsoever," Dr Quinn told BBC News.

"But what it does do is it gives confidence in the system of measurement."

Watt balance (Image: NPL) The watt balance connects a kilogram mass with fundamental, repeatable natural phenomena

Ultimately, the goal is to create standards that, unlike the almost-a-kilogram of platinum in France, won't change with time. That is particularly relevant for measuring small changes that happen over the course of years or decades.

"One of the basic principles of metrology is that if you want to look at a change in any parameter over a long period of time, you have got be sure you measure it accurately at the end and at the beginning - but also that the measurements are linked to the same units and constants.

"It's very difficult now to say what was the concentration of heavy metals in the sludge at the bottom of the North Sea around England in the 1950s. We can measure it very accurately now, but if you go back to the data from the 1950s, it's difficult to be sure that the measurements are properly related to the SI units. This is the point.

"All these global climate studies depend on looking at small changes over long periods of time," he added.

In the balance

As the last bastion of "artefact-based" units, the effort to define the kilogram has a few knock-on effects. The measurements of force - weights and torques and so on - are based on the kilogram. Also, voltage and resistance have their units defined in terms of kilograms, so the numbers of merit in electronics are also implicated.

Even though the ultimate changes to the kilogram as we now know it may only be in the parts per million or even billion, the microelectronics industry deals in tiny distances and movements of charge. And these could be directly affected by any changes to the kilogram.

To pin it down once and for all, the way to define the kilo in terms of fundamental constants hinges on a simple but elegant experiment known as a watt balance.

This device, through a few relatively recently-discovered effects, turns the mass of an object into charges racing around a circuit - one of the many forms of energy.

The redefinition of the kilogram along these lines will be the last big step towards making SI units conform to an understanding of natural phenomena.

And how soon might that revolution take place?

"The first stage is the General Conference in October; we have a draft resolution in which the principles are laid out," explained Dr Quinn.

"Then at the General Conference in 2015, I hope the final decision will be taken. But this is science, maybe it won't. Or maybe it'll be quicker, maybe everything will fall into place next year. But that is the excitement of it."

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