Four of seven base SI units – the kilogram, the mole, the ampere and the kelvin – are set to be redefined. Delegates at the 24th Conference on Weights and Measures, held in France on 21 October, were persuaded that the existing definitions of these units were outdated and needed to be dragged into the 21st century.
Avogadro’s constant was named in honour of the Italian chemist for his contributions to the field
It was the definition of the kilogram, which has been controversial for decades, which prompted the changes. The original kilogram is defined by the international prototype, a cylinder of platinum-iridium that was cast in 1879. Over the years its mass has changed for reasons that are not entirely clear, but which may involve surface chemistry or the leaching of nitrogen or oxygen trapped during its manufacture.
For years the metrology community has disagreed about what form a new definition should take, but now the recommendations of the Consultative Committee for Units of the International Bureau of Weights and Measures have been accepted. The resolution to accept the new definitions represents something of a personal triumph for the committee’s president, Ian Mills, emeritus professor of chemical spectroscopy at the University of Reading in the UK.
‘There has been a good deal of argument about this and we have been working on it for some years,’ Mills says. ‘These things have to be agreed by everyone around the world.’
The committee recommended that the kilogram should be defined in relation to the Planck constant. ‘We need a definition that is a true invariant,’ Mills says. The new definition is: ‘the kilogram is such that the Planck constant is exactly 6.6260693 x 10-34 joule-seconds’. The kilogram can be defined in terms of the Planck constant given that the unit of the Planck constant is equal to the unit of action – J s = kg m2 s-1 – and the second and metre have fixed numerical values in terms of the numerical values of the caesium hyperfine splitting frequency and the speed of light in a vacuum, respectively. So fixing the magnitude of the unit kg m2 s-1 has the effect of defining the kilogram.
At the same time as redefining the kilogram, the opportunity was taken to propose redefining three other base SI units ‘to tidy up the system,’ Mills says. The mole’s current definition, based on the molar mass of carbon-12 being 12g per mole, has been changed to: ‘the mole is such that the Avagadro constant is exactly 6.0221415 x 10#23 per mole’. Mills says that the current definition is a bit obtuse. Changes were also proposed for the ampere and the kelvin.
The proposed definitions have yet to be ratified, which is expected to happen at a future General Conference of Weights and Measures. ‘It will be some years before these proposals are adopted, but we have crossed the major hurdle,’ Mills says. ‘It was a bit touch and go, but we got there in the end.’
Mills stresses that the new definitions will not affect the everyday work of scientists. But he adds that ‘defining base units to be used in all forms of fundamental science is very important for all sorts of reasons, for example in fundamental physics and in areas such as global positioning systems and other fields where timings, for example, are made to extremely precise limits’.
The definition of the remaining three base units, the second, the metre and the candela, will remain unchanged.
Simon Hadlington
When some units aren’t based on constants, but rather on arbitrary concepts, then if that concept ever changes, the unit has to change, something that shouldn’t happen, because these units are supposed to be constant. If these units aren’t constant, then some measurements made in past years would be rendered useless and obsolete, so this would bring place to confusion and discussion in the scientific world, since making some sort of calculation would be made difficult and in time would have to be replaced by a new one, with minor changes. This wouldn’t be progress, but instead it would be going back to the medieval ages, when the units of measurements were changed whenever the king changed, which brought great confusion and discontent among the people, so I’m in complete favor of making these units belong to constants instead of arbitrary concepts. The best about these changes is that the value of the measure doesn’t change, it’s just in terms of a constant to better make clear what it is.
me parece interesante que hayan decidido hacer esos cambios y actualizaciones que permite que la ciencia siga desarrollandose y que hayan mas cosas mejor definidas. tambien es bueno que algunas medidas las hayan cambiado o hayan impuesto nuevas cosas como lo del plank que no sirve para distintas cosas. es bueno saber que no se deja de lado estas cosas porque hacen progresar a la ciencia cada ves mas, aunque no se note pero estos pequeños cambios van formando cosas grandes en un largo plazo. tambien es bueno notar lo que dice mills en cuanto lo de que las nuevas definiciones no afectaran el trabajo diario de los cientificos ya que ellos tambien tiene que seguir avanzando con lo suyo realizando nuevos aportes.
sinceramente estos detalles no deberian ser una problematica en las decisiones, es una avance, es algo necesario estas modificaciones son necesarias para el desarollo cientifico,me parece bien que aprovecharan y redifinieran el mol el kelvin y si no me equeivoco el amperio. se podria decir que las explicasiones actuales de cada uno de estos sistemas de medidas son muy simples a comparasion de las nuevas propuesta y esto es algo bueno para el avance , no me extrañaria que dentro de un tiempo los vuelvan a cambiar y eso es algo bueno pues es necesario seguir avanzando y no conformarce
I do not think that they should change those SI measures, because it is something that you are used every day. They they should change those measures if the people of the world agree. Well I do not know what other people think about this. What it is important is that in the case the Conference of Weights and Measures are serious this is not going to happen inmediately it is going to take time. In a way because people will need to adapt. To get used to it.
The conference have some points in which they explain some benefits of modifiyng the those SI measures like that it would help on the fundamental physics, but I got my position about this.
They should not change the kilogram meassure! we’ve been using it for too many years and nobody have died because of it. So why it’s necesary to change something that will affect all the persons like us that use the kilogram? they should work on things that are more important in physics or biology or something else that will make things good.
It is a big problem that they want to change the kilogram meassure because it doesn’t have had any problem with the form of meassure. It will be very difficult for us to get used to a new form os meassure the wiegth. Also it is going to demerit the work of a big scientific person like Avogadro. This I think it isn’t a progress for the scientific studies.
Hay veces que es nesesario hacer cambios en este caso el kilogramo, aunque en el titulo parezca como si fuera ua tragedia en realidad es algo nesesario, la medida fue sacado dependiendo de otra cosa, si esa otra cosa cambia lo mas logico seria cambiarla medida de kilogramo el problema se da en que las personas ya estan acostumbradas a usar esta medida y supongo seria dificil para algunas personas que usan estas medidas para trabajar cambiarselo.