Why measuring time accurately matters

Justin Rowlatt and Andrea Sella, have continued their fascinating trek through the elements of the periodic table with a podcast and a web page on caesium.

They focused their article on the role caesium plays in atomic clocks, and much of the article describes the astonishing regularity with which caesium clocks ‘tick’.

However somehow they didn’t mention the fundamental reason why measuring time accurately matters: time is at the base of the International System of Units – the SI.

This means that almost all physical measurements are referenced to a measurement of time. In short:

The accuracy of clocks represents a physical limit
to the accuracy with which we can measure anything!

And this importance is only going to increase in future.

Let me explain.

Measurement is simply the comparison of the thing you are measuring (such as a length or a mass, or a temperature) against a standard ‘unit’ amount of that quantity (length, or mass or temperature).

So no measurement can ever be more accurate than the standard of the quantity being measured.

For example, if ‘one metre’ in the UK is not the same as ‘one metre’ in other countries, then we limit the accuracy with which we can compare measurements made in the UK with those made in other countries.

Similarly, if one metre now is not the same as one metre in 10 years time, or 100 years time, then measurements made now could ‘become inaccurate’ over time.

So at the heart of the system of measurement we want units that are universal, and do not change over time.

The problem is that all physical artefacts – even the international prototype of the kilogram which is made of platinum-iridium and kept in a safe for most of its life – are perishable: they change.

For that reason metrologists frown upon the use of physical artefacts as measurement standards. Instead we are trying to create a system of units in which we separate the definition of the unit from its physical realisation.

We then look for definitions of the units of measurement that – to the best of our knowledge – can never change.

This approach offers the advantage that if technology improves, perhaps in ways we cannot imagine, then there is the possibility of improved realisations of measurement standards, without ever changing the definition – and hence the actual value.

The present system of units, the SI, is the result of many historical anomalies and so below I describe pictorially the ‘New SI’ that many metrologists hope will be in place in 2018.

I hope you can see the system more or less makes sense, and the fundamental role that Caesium atoms play at the root of almost all physical measurements.

We begin with an atom of caesium-133:

 fundamental constant Css is used to define what we mean by one second.

1. A particular natural frequency of vibration of an atom of  Caesium-133  is used to define what we mean by one second, the unit of time. One second is defined as the time taken for just over 9 billion of these oscillations  (9 192 631 770 to be precise)

2. Next the unit of the length, the metre, is defined in terms of the second and the universal constant, the speed of light in a vacuum.

2. Next the unit of the length, the metre, is defined in terms of the second and the universal constant, the speed of light in a vacuum.

3. The most significant change in the new SI is that the kilogram will be defined in terms of the second, the metre and the fundamental constant, the Planck constant.

3. The most significant change in the new SI is that the unit of mass, the kilogram, will be defined in terms of the second, the metre and the fundamental constant, the Planck constant.

Together these three definitions form the core of the SI. This is because with definitions of these three units we can define the unit of energy, the joule.

The unit of energy, the joule, is defined in terms of seconds, metres and kilograms.

The unit of energy, the joule, is defined in terms of seconds, metres and kilograms.

4. The candela - the unit of 'luminous efficacy' is defined in terms of the joule, and a constant that describes the sensitivity of he human eye to a particular wavelength of light.

4. The candela – the unit of ‘luminous efficacy’ – is defined in terms of the joule, and a constant that describes the sensitivity of the human eye to a particular wavelength of light. By the way, ‘luminous efficacy’ just means ‘How bright it seems’.

5. The unit of temperature, the kelvin (and also the degree Celsius) is defined in terms of the joule and a fundamental constant, the Boltzmann constant.

5. The units of temperature, the kelvin and the degree Celsius, are defined in terms of the joule and a fundamental constant, the Boltzmann constant. Colloquially this describes how many joules of energy a molecule must have to increase its temperature by one degree.

6. The unit of electric current, the ampere, is defined in terms of the fundamental constant the charge on the electron and the second.

6. The unit of electric current, the ampere, is defined in terms of the fundamental constant the charge on the electron and the second.

And finally we come to the odd-one-out – the mole – the unit of amount of substance. This is defined in terms of a standard (tremendously large) number of basic entities called the Avogadro constant.

7. The unit of the amount of substance, is the only unit not fundamentally linked to the second. Instead it is defined as a certain number - call the Avogadro constant - of basic entities, typically atoms or molecules.

7. The unit of the amount of substance, is the only unit not fundamentally linked to the second. Instead it is defined as a certain number – called the Avogadro constant – of basic entities, typically atoms or molecules.

You can download a pleasingly-animated version of these pictures in this PowerPoint file.

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