3 ms·

So if I understand correctly, this is saying that we would be formally redefining the kilogram, mole, ampere and Kelvin units (or prototypes?) as some quantifia

So if I understand correctly, this is saying that we would be formally redefining the kilogram, mole, ampere and Kelvin units (or prototypes?) as some quantifiable relation to immutable, measurable constants instead of the arbitrary and self-referential "this is a kg because it's how much this block of metal weighs and we decided that's what a kg is". Is that correct? Not being a scientist myself, I'm struggling to understand the impact of these changes being made. Is it just a matter of increasing the precision of base units? Say they redefine the mole to be based on that 1kg sphere of silicon-28. What is the real-world impact of doing so? Presumably one mole will still be one mole and not all of a sudden e.g. 6.03x10^23. Does anyone have an example of where a change like this would be important?

lmm 9y ago
The big issue is that the standard reference kilogram (the block of metal) is losing mass. If we take the SI standards literally, this means e.g. distant galaxies now mass more kilograms than they did last year. Which seems obviously undesirable. The impact should ultimately be to make ultra-precise physics - and eventually, engineering - easier, because people reproducing an experiment in a different lab will have a closer consensus on exactly how much 1kg is.
- pdpi 9y ago
  > The big issue is that the standard reference kilogram (the block of metal) is losing mass. As per the article, there's several of them. Some are losing mass, others are gaining.
  - danbruc 9y ago
    But only one of them, Big K, defines the kilogram. So the ones losing mass might just gain mass more slowly than Big K. Or they might even all be losing mass but some more slowly than Big K making them apparently gain mass. I don't know if we know how the masses actually changed.
    - falcolas 9y ago
      We don't. And that's the problem. If it was a consistent loss, it would be much easier to work around.
Jedd 9y ago
> Not being a scientist myself, I'm struggling to understand the impact of these changes being made. Is it just a matter of increasing the precision of base units? My understanding is the big benefit is that if you are a scientist, you can now generate your own SI units accurately in your lab. Mass has been the trickiest one for a while, as noted in TFA. The insistence of spelling metre incorrectly is just a minor inconvenience in comparison.
pdpi 9y ago
> Not being a scientist myself, I'm struggling to understand the impact of these changes being made. Is it just a matter of increasing the precision of base units? It's not entirely unlike saying "we used to ship this as a binary blob, but we're moving to a source distribution". It buys us reproducibility and auditability.
skriticos2 9y ago
I guess the idea is stability. Seconds are defined as a constant pattern for some physical activity of an element. If you are on a colony on Mars, you can measure that with a complex scientific gizmo and the specification sheet and you are golden. With mass.. you have to get a copy of a reference mass or a tuned measuring instrument up with a shaky spacecraft, then down again and hope it didn't break too much. Having something that would allow to create a reference object on site with only the specification sheet and scientific gizmos is a better way to go about it. edit: spelling
marcosdumay 9y ago
About values changing, it took this long to change the definitions exactly because we needed experiments that could measure those things up to the same precision we got with the old system. That means both definitions agree up to current precision. The standards do certainly change the value of those unities, but not enough for we to be able to measure today.
falcolas 9y ago
> Does anyone have an example of where a change like this would be important? Well, it's important today, because the reference samples used across the world have drifted apart, and not in a consistent fashion. Which raises the question of whether the exemplar kg has drifted as well. So, we don't really know if the kg we're using today is the same kg we were using 10 years ago. Given how important mass is to many scientific calculations, a drifting standard could directly impact all the other work we do. I found the Veritasium video to be a bit more interesting for the "why" aspect of this change: https://www.youtube.com/watch?v=Oo0jm1PPRuo https://www.youtube.com/watch?v=Oo0jm1PPRuo

lmm 9y ago

The big issue is that the standard reference kilogram (the block of metal) is losing mass. If we take the SI standards literally, this means e.g. distant galaxies now mass more kilograms than they did last year. Which seems obviously undesirable. The impact should ultimately be to make ultra-precise physics - and eventually, engineering - easier, because people reproducing an experiment in a different lab will have a closer consensus on exactly how much 1kg is.

pdpi 9y ago
> The big issue is that the standard reference kilogram (the block of metal) is losing mass. As per the article, there's several of them. Some are losing mass, others are gaining.
- danbruc 9y ago
  But only one of them, Big K, defines the kilogram. So the ones losing mass might just gain mass more slowly than Big K. Or they might even all be losing mass but some more slowly than Big K making them apparently gain mass. I don't know if we know how the masses actually changed.
  - falcolas 9y ago
    We don't. And that's the problem. If it was a consistent loss, it would be much easier to work around.

Jedd 9y ago

> Not being a scientist myself, I'm struggling to understand the impact of these changes being made. Is it just a matter of increasing the precision of base units? My understanding is the big benefit is that if you are a scientist, you can now generate your own SI units accurately in your lab. Mass has been the trickiest one for a while, as noted in TFA. The insistence of spelling metre incorrectly is just a minor inconvenience in comparison.

pdpi 9y ago

> Not being a scientist myself, I'm struggling to understand the impact of these changes being made. Is it just a matter of increasing the precision of base units? It's not entirely unlike saying "we used to ship this as a binary blob, but we're moving to a source distribution". It buys us reproducibility and auditability.

skriticos2 9y ago

I guess the idea is stability. Seconds are defined as a constant pattern for some physical activity of an element. If you are on a colony on Mars, you can measure that with a complex scientific gizmo and the specification sheet and you are golden. With mass.. you have to get a copy of a reference mass or a tuned measuring instrument up with a shaky spacecraft, then down again and hope it didn't break too much. Having something that would allow to create a reference object on site with only the specification sheet and scientific gizmos is a better way to go about it. edit: spelling

marcosdumay 9y ago

About values changing, it took this long to change the definitions exactly because we needed experiments that could measure those things up to the same precision we got with the old system. That means both definitions agree up to current precision. The standards do certainly change the value of those unities, but not enough for we to be able to measure today.

falcolas 9y ago

> Does anyone have an example of where a change like this would be important? Well, it's important today, because the reference samples used across the world have drifted apart, and not in a consistent fashion. Which raises the question of whether the exemplar kg has drifted as well. So, we don't really know if the kg we're using today is the same kg we were using 10 years ago. Given how important mass is to many scientific calculations, a drifting standard could directly impact all the other work we do. I found the Veritasium video to be a bit more interesting for the "why" aspect of this change: https://www.youtube.com/watch?v=Oo0jm1PPRuo https://www.youtube.com/watch?v=Oo0jm1PPRuo