8 ms·

2019 redefinition of the SI base units

85 points by tony 7y ago

Aardwolf 7y ago
Why is ampere a base unit instead of charge? Charge seems the more fundamental one to me, like mass is, and ampere feels more derived, as if volumetric flow rate were made a base unit instead of mass.
- anticensor 7y ago
  You cannot change the basic dimensionalities after the fact. Electric current was chosen at the time, rather than charge, hence it has been defined in terms of electron's charge.
- kriztw 7y ago
  Because the standards are made for people who makes measurement devices. It is much easier to measure electric current than charge. They also did make charge more fundamental, as they derive the Ampere by fixing the charge of an electron.
- raldi 7y ago
  Think of Charge as a collection of Amperage across a period of Time, or a supply of Charge that can yield a particular Amperage of current for a sustained amount of Time. https://electronics.stackexchange.com/questions/23449/why-is-the-ampere-the-only-si-fundamental-unit-for-electricity https://electronics.stackexchange.com/questions/23449/why-is...
- lopmotr 7y ago
  None of them are really fundamental. Some people feel that force is more fundamental than mass since force seems to be a "real" thing that happens while mass is just an abstract quantity that helps us predict forces. It's all human psychology, practical issues with measurement, and legacy. Quite a few of the base units are entirely redundant (candela, kelvin, mole) but we have them because of our human weaknesses.
anonu 7y ago
Seems like a positive change. But somehow (perplexingly) we still use inches, feet and miles in a major part of the world.
- Swizec 7y ago
  Funfact: US customary units are metric by definition. Everything is in grams and meters and liters, it’s just using weird numbers so it roughly fits with customary units roundedly. The switch happened in 1893 according to wikipedia. https://en.m.wikipedia.org/wiki/United_States_customary_units https://en.m.wikipedia.org/wiki/United_States_customary_unit...
- Someone1234 7y ago
  Except in science. Where the inconsistency itself can cause problems: https://en.wikipedia.org/wiki/Mars_Climate_Orbiter#Cause_of_failure https://en.wikipedia.org/wiki/Mars_Climate_Orbiter#Cause_of_... I have no idea how you'd convince a large country to convert to SI aside from just forcing it. There are quantifiable upsides (consistency, conversion, not needing two sets of tools, etc) but momentum is hard to overcome. Interestingly Metric is "the preferred system of weights and measures for United States trade and commerce" according to the 1975 Metric Conversion Act[0]. But it had no teeth, so everyone just continued using US Customary Units. [0] https://en.wikipedia.org/wiki/Metric_Conversion_Act https://en.wikipedia.org/wiki/Metric_Conversion_Act
  - atonse 7y ago
    Yup this is what I always tell people – in the US, all science is taught in Metric. (at least in my schooling in the DC area). All physics, chemistry, etc units were in metric. So where it truly makes a difference, it seems to all be metric. That's why I'm always surprised when I read a story about some scientific endeavor in the US that didn't use metric.
  - bobthepanda 7y ago
    The US uses some metric units for some liquids (e.g. a 2 liter of soda). For me a big stumbling block is cooking; I like using volume units for cooking, but metric recipes tend to use weight.
Willson50 7y ago
"The kilogram is the mass of a body at rest whose equivalent energy equals the energy of a collection of photons whose frequencies sum to [1.356392489652×10^50] hertz."
- effie 7y ago
  Glad that is a deprecated formulation. One never has collection of photons of exact frequency of net mass 1kg. Also 1kg weight can't be realistically entirely turned into photons.
  - saagarjha 7y ago
    Your sarcasm was a bit hard to distinguish ;)
    - effie 7y ago
      There wasn't any sarcasm intended.
  - deleted 7y ago
    [deleted]
  - marcosdumay 7y ago
    Isn't the new definition completely equivalent with the GP's one?
kevin_b_er 7y ago
So I'm glad they fixed the problems of depending on kilogram sample object for anything, but the definitions are now bizarre if you want to learn them in reference to real-world things. How will they be taught now? The Ampere went from being the thought experiment of two infinite wires 1m apart creating a certain force to being something nearly unintelligible to the average person. The Kelvin was just turning 0C into absolute units, carefully formalized as the triple point of water which was actually 0.01C, and making it into absolute units. Now its something nearly intelligible to the public. Another problem is I'm not entirely sure the atomic mass is quite defined anymore. That number at the bottom of each box of the periodic table of elements may not have a valid unit anymore??
- kriztw 7y ago
  The old definitions are still essentially correct, so for teaching nothing really changes except the word "approximately" being inserted somewhere.
  - hoten 7y ago
    Right. So in other words, the SI unit definitions are for scientists wishing to affirm the base units, but the old definitions are superior for education purposes (well, except maybe for the prototype kilogram...)
    - peter303 7y ago
      The pre-prototype kilogram is essentially valid- mass of liter of water. But now in terms of physical constants.
- anticensor 7y ago
  Charge of an electron is a concrete fact, unlike infinitely long imaginary wires inducing an electromagnetic pull.
  - Koshkin 7y ago
    The elementary charge is impractically small, while it is easy to approximate the "imaginary wires."
zzo38computer 7y ago
I am glad the definition of kilogram is corrected; I always thought it was no good, and now it is good.
droithomme 7y ago
For an international standard of universal constants they sure change a lot. Starting to get like the problem of OS updates changing everything, except what if it was the foundations of all scientific measurement constantly changing instead of some OS you don't even have to use. Some might say oh they haven't really changed. Well if they haven't really changed why change them.
zokier 7y ago
What does candela still do there? It is extremely unfundamental imho, and sort of vaguely based on human physiology which seems out of place for SI unit. It is not even fully defined because you need also a definition for luminous efficacy. Of course candela is utterly useless outside the visible spectrum which should be a huge alarm bell against it.
spindle 7y ago
Someone will save money by no longer having to store the prototype artefacts ... unless we're in any danger of running out of caesium atoms.
apricot 7y ago
Can anyone help me understand why the base SI unit of mass is the kilogram and not the gram? Why is the ×1000 prefix included in the base unit of mass, but not in any other units? When I asked that question to my science teacher, a long time ago, he said that was because it was easier to manufacture a precise prototype kilogram than a prototype gram, and that made sense at the time, but what's the reason now?

85 points by tony 7y ago

Why is ampere a base unit instead of charge? Charge seems the more fundamental one to me, like mass is, and ampere feels more derived, as if volumetric flow rate were made a base unit instead of mass.

anticensor 7y ago
You cannot change the basic dimensionalities after the fact. Electric current was chosen at the time, rather than charge, hence it has been defined in terms of electron's charge.
kriztw 7y ago
Because the standards are made for people who makes measurement devices. It is much easier to measure electric current than charge. They also did make charge more fundamental, as they derive the Ampere by fixing the charge of an electron.
raldi 7y ago
Think of Charge as a collection of Amperage across a period of Time, or a supply of Charge that can yield a particular Amperage of current for a sustained amount of Time. https://electronics.stackexchange.com/questions/23449/why-is-the-ampere-the-only-si-fundamental-unit-for-electricity https://electronics.stackexchange.com/questions/23449/why-is...
lopmotr 7y ago
None of them are really fundamental. Some people feel that force is more fundamental than mass since force seems to be a "real" thing that happens while mass is just an abstract quantity that helps us predict forces. It's all human psychology, practical issues with measurement, and legacy. Quite a few of the base units are entirely redundant (candela, kelvin, mole) but we have them because of our human weaknesses.

anonu 7y ago

Seems like a positive change. But somehow (perplexingly) we still use inches, feet and miles in a major part of the world.

Swizec 7y ago
Funfact: US customary units are metric by definition. Everything is in grams and meters and liters, it’s just using weird numbers so it roughly fits with customary units roundedly. The switch happened in 1893 according to wikipedia. https://en.m.wikipedia.org/wiki/United_States_customary_units https://en.m.wikipedia.org/wiki/United_States_customary_unit...
Someone1234 7y ago
Except in science. Where the inconsistency itself can cause problems: https://en.wikipedia.org/wiki/Mars_Climate_Orbiter#Cause_of_failure https://en.wikipedia.org/wiki/Mars_Climate_Orbiter#Cause_of_... I have no idea how you'd convince a large country to convert to SI aside from just forcing it. There are quantifiable upsides (consistency, conversion, not needing two sets of tools, etc) but momentum is hard to overcome. Interestingly Metric is "the preferred system of weights and measures for United States trade and commerce" according to the 1975 Metric Conversion Act[0]. But it had no teeth, so everyone just continued using US Customary Units. [0] https://en.wikipedia.org/wiki/Metric_Conversion_Act https://en.wikipedia.org/wiki/Metric_Conversion_Act
- atonse 7y ago
  Yup this is what I always tell people – in the US, all science is taught in Metric. (at least in my schooling in the DC area). All physics, chemistry, etc units were in metric. So where it truly makes a difference, it seems to all be metric. That's why I'm always surprised when I read a story about some scientific endeavor in the US that didn't use metric.
- bobthepanda 7y ago
  The US uses some metric units for some liquids (e.g. a 2 liter of soda). For me a big stumbling block is cooking; I like using volume units for cooking, but metric recipes tend to use weight.

Willson50 7y ago

"The kilogram is the mass of a body at rest whose equivalent energy equals the energy of a collection of photons whose frequencies sum to [1.356392489652×10^50] hertz."

effie 7y ago
Glad that is a deprecated formulation. One never has collection of photons of exact frequency of net mass 1kg. Also 1kg weight can't be realistically entirely turned into photons.
- saagarjha 7y ago
  Your sarcasm was a bit hard to distinguish ;)
  - effie 7y ago
    There wasn't any sarcasm intended.
- deleted 7y ago
  [deleted]
- marcosdumay 7y ago
  Isn't the new definition completely equivalent with the GP's one?

kevin_b_er 7y ago

So I'm glad they fixed the problems of depending on kilogram sample object for anything, but the definitions are now bizarre if you want to learn them in reference to real-world things. How will they be taught now? The Ampere went from being the thought experiment of two infinite wires 1m apart creating a certain force to being something nearly unintelligible to the average person. The Kelvin was just turning 0C into absolute units, carefully formalized as the triple point of water which was actually 0.01C, and making it into absolute units. Now its something nearly intelligible to the public. Another problem is I'm not entirely sure the atomic mass is quite defined anymore. That number at the bottom of each box of the periodic table of elements may not have a valid unit anymore??

kriztw 7y ago
The old definitions are still essentially correct, so for teaching nothing really changes except the word "approximately" being inserted somewhere.
- hoten 7y ago
  Right. So in other words, the SI unit definitions are for scientists wishing to affirm the base units, but the old definitions are superior for education purposes (well, except maybe for the prototype kilogram...)
  - peter303 7y ago
    The pre-prototype kilogram is essentially valid- mass of liter of water. But now in terms of physical constants.
anticensor 7y ago
Charge of an electron is a concrete fact, unlike infinitely long imaginary wires inducing an electromagnetic pull.
- Koshkin 7y ago
  The elementary charge is impractically small, while it is easy to approximate the "imaginary wires."

zzo38computer 7y ago

I am glad the definition of kilogram is corrected; I always thought it was no good, and now it is good.

droithomme 7y ago

For an international standard of universal constants they sure change a lot. Starting to get like the problem of OS updates changing everything, except what if it was the foundations of all scientific measurement constantly changing instead of some OS you don't even have to use. Some might say oh they haven't really changed. Well if they haven't really changed why change them.

zokier 7y ago

What does candela still do there? It is extremely unfundamental imho, and sort of vaguely based on human physiology which seems out of place for SI unit. It is not even fully defined because you need also a definition for luminous efficacy. Of course candela is utterly useless outside the visible spectrum which should be a huge alarm bell against it.

spindle 7y ago

Someone will save money by no longer having to store the prototype artefacts ... unless we're in any danger of running out of caesium atoms.

apricot 7y ago

Can anyone help me understand why the base SI unit of mass is the kilogram and not the gram? Why is the ×1000 prefix included in the base unit of mass, but not in any other units? When I asked that question to my science teacher, a long time ago, he said that was because it was easier to manufacture a precise prototype kilogram than a prototype gram, and that made sense at the time, but what's the reason now?