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International System of Units overhauled in historic vote

npl.co.uk

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Re: International System of Units overhauled in historic vote

#131

Post by conbandit »

24gttghh wrote: post #118
zamadatix wrote: post #93

Earlier quoted context omitted.

This wikipedia section breaks down the new definition nicely https://en.wikipedia.org/wiki/Redefinition_of_SI_base_units#...

It is still not apparent to me. Is a kilogram of something now it's joules x seconds or something? Does the kilogram now vary depending on what the kilogram is of? Like, is a kilogram of Iron a different mass from a kilogram of Aluminum? >The kilogram, symbol kg, is the SI unit of mass. It is defined by taking the fixed numerical value of the Planck constant h to be 6.62607015×10−34 when expressed in the unit J⋅s, wh…

I believe it means something like:

Given the definition of metre and the definition of second, the kilogram is whatever value that makes the Planck constant h precisely 6.62607015×10−34 kg m^2 s^(-1).

Re: International System of Units overhauled in historic vote

#132

Post by biotech »

sandov wrote: post #127

>The definition of the kilogram for more than 130 years, the International Prototype of the Kilogram (IPK), a cylinder of a platinum alloy stored at the BIPM in France, will now be retired. It will be replaced by the Planck constant Maybe I interpreted it wrong (non-native English speaker), but does it say that the Kilogram will be equal to the Planck constant? Shouldn't it be that the definition of the Kg will be ba…

It's not just you, the wording is a bit weird. Yes, they are trying to say that the definition of the kg will be based on the Planck constant.

Further down, the article says: "The kilogram – will be defined by the Planck constant (h)."

Re: International System of Units overhauled in historic vote

#133

Post by bryondowd »

eutropia wrote: post #79
echodevnull wrote: post #52

Earlier quoted context omitted.

There has been a proposal during the French Revolution with 10 hours a day, 100 minutes an hour and 100 seconds a minute. But it has only been in use for a few years beginning in 1792: https://en.wikipedia.org/wiki/Decimal_time

(10/24) (100/60) (100/60)=1.1574 Our biggest revolution yet: extending the length of the day by 16%!

Presumably, the second was shorter, rather than the day was longer.

EDIT: More specifically, if you take an average rotation of the Earth as 86164.1 seconds, and divide by 10, then 100, then 100, you get 0.861641, which would be the conversion from our seconds to the metric second in that scenario. Likewise, the metric minute would be 86.1641 of our seconds or about 1.43607 of our minutes, the metric hour would be 8616.41 of our seconds or 143.607 of our minutes or 2.39345 of our hours. The day would be slightly shorter than our day, reducing the need for leap seconds to keep our time aligned with the sun.

Re: International System of Units overhauled in historic vote

#134

Post by ward »

jbay808 wrote: post #120
School-Cotton wrote: post #37

Earlier quoted context omitted.

Is there any compelling reason why the US should switch to the metric system for daily use? Metric is already used universally in science and engineering, which seem to be the main fields in which consistency across countries matters. I completely agree that in physics, chemistry, engineering, medicine, etc., it is important to use standard units. But what harm is it doing in practice if in everyday life, Americans s…

Why should there be any separation between science and daily life? Think about it not for the benefit of the current generation who would be forced to burden the switch, but for the next generation who could reap the benefits. I think one issue is that by growing up with US units and then using a second system for science and engineering, it can make American students studying those topics feel like they are in an un…

> Science becomes a Special Discipline requiring Special Language different than what your family uses at home. Like if all science were still done in French or Latin, and you needed to study those to read a paper.

You realize this is still a problem for most of the world and needing to learn English, right? :P I'm hoping we can solve this in my lifetime with everyone knowing one standard language through education as a child, but as it stands the struggle is real for many of my fellow non native English speakers.

That said I agree with you, all these differences are a pain and I'm confronted with them way too often in my daily life.

Re: International System of Units overhauled in historic vote

#135

Post by clord »

rubenhak wrote: post #117

Finally there is order in units for science community. Hopefully such transformation will come to US one day.

The US units are defined in terms of SI since the Mendenhall Order[0], so they also benefit from this change.

[0]: https://en.wikipedia.org/wiki/Mendenhall_Order

Re: International System of Units overhauled in historic vote

#136

Post by m1el »

sandov wrote: post #127

>The definition of the kilogram for more than 130 years, the International Prototype of the Kilogram (IPK), a cylinder of a platinum alloy stored at the BIPM in France, will now be retired. It will be replaced by the Planck constant Maybe I interpreted it wrong (non-native English speaker), but does it say that the Kilogram will be equal to the Planck constant? Shouldn't it be that the definition of the Kg will be ba…

> but does it say that the Kilogram will be equal to the Planck constant?

No, this statement means that Kilogram will give up its place as a fundamental unit to Planck constant. Earlier, Kilogram was used as a fundamental unit, defined by a physical object. Not anymore.

The kilogram will be defined by Planck constant. Namely, m = E/c^2 = hf/c^2.

> How can they measeure it with 0 error?

We can't, all measurements have errors. But we can measure things better than a physical object, which has changed with time. So we only have to measure Plank constant with precision higher than variation of physical object's mass for the purpose of replacing the definition of mass.

Re: International System of Units overhauled in historic vote

#137

Post by jbay808 »

ward wrote: post #134
jbay808 wrote: post #120

Earlier quoted context omitted.

Why should there be any separation between science and daily life? Think about it not for the benefit of the current generation who would be forced to burden the switch, but for the next generation who could reap the benefits. I think one issue is that by growing up with US units and then using a second system for science and engineering, it can make American students studying those topics feel like they are in an un…

> Science becomes a Special Discipline requiring Special Language different than what your family uses at home. Like if all science were still done in French or Latin, and you needed to study those to read a paper. You realize this is still a problem for most of the world and needing to learn English, right? :P I'm hoping we can solve this in my lifetime with everyone knowing one standard language through education a…

Absolutely! America is very lucky to have come out on the right side of that lottery. But then it unnecessarily goes and handicaps itself by making measurements into a foreign language.

Re: International System of Units overhauled in historic vote

#138

Post by ska »

Digit-Al wrote: post #119

Can anyone tell this poor ignoramus whether the kg is getting heavier or lighter - and how much by (approximately, I know it's not going to be an exact figure given the reason they are changing it)?

It's probably better to think about the other way around. Despite heroic efforts, the "old" kilogram was getting (a tiny big) lighter every time it was handled.

The "new" kilogram will finally be a constant. To a reasonable approximation it hasn't changed at all at this moment time.

Re: International System of Units overhauled in historic vote

#139

Post by ska »

reaperducer wrote: post #56
orblivion wrote: post #26

I guess the next redefinition will happen if and when there's detectable drift in "fundamental" constants?

I guess the next redefinition will happen if and when there's detectable drift in "fundamental" constants? And until then, all the metric fanbois will constantly tell you that metric is superior to imperial because its measurements are immutable. Until suddenly they are.

You are confused, arguments about the benefits or lack thereof of metric are based on internal consistency, not standards and metrology.

Nobody is arguing against more precise definition of standard values, which is why Imperial units these days are themselves defined in terms of SI units. So if the meter were to change, for example, so would the US foot.

Re: International System of Units overhauled in historic vote

#140

Post by ashwinikd »

sandov wrote: post #127

>The definition of the kilogram for more than 130 years, the International Prototype of the Kilogram (IPK), a cylinder of a platinum alloy stored at the BIPM in France, will now be retired. It will be replaced by the Planck constant Maybe I interpreted it wrong (non-native English speaker), but does it say that the Kilogram will be equal to the Planck constant? Shouldn't it be that the definition of the Kg will be ba…

I guess it means that we are fixing the value of the Planck constant and deriving measure of kilogram from it. Given the measure of second and metre, a kilogram is the mass so that the value of plank constant is observed to be exactly 6.626x10^−34.
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