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International System of Units overhauled in historic vote

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Re: International System of Units overhauled in historic vote

#141

Post by ska »

nafey wrote: post #68
reaperducer wrote: post #56

Earlier quoted context omitted.

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.

As opposed imperial constants? How is a foot standardized anyway? I suspect most manufactures rely on SI metrics for that.

Foot is defined as 0.3048 meters, you can't get away from the SI definitions.

Re: International System of Units overhauled in historic vote

#142

Post by jimktrains2 »

3pt14159 wrote: post #12

This is probably the dumbest thing I'll type on HN. In university I just gave up trying to understand why we even needed the Avogadro constant / mole as a fundamental constant. It still confuses me. Why have a difference between molar mass and mass? Why couldn't it just be "1" and everything else change around it?

Stoichiometry.

We need a means of converting mass to number of atoms so that we can predict how much mass of a specific product will be formed, what the limiting reägent will be, &c.

It also helps to define concentrations based on number of moles in a L of solvent (Molarity vs g/L) for the same reason.

Re: International System of Units overhauled in historic vote

#143

Post by zwkrt »

jcranmer wrote: post #114
3pt14159 wrote: post #12

This is probably the dumbest thing I'll type on HN. In university I just gave up trying to understand why we even needed the Avogadro constant / mole as a fundamental constant. It still confuses me. Why have a difference between molar mass and mass? Why couldn't it just be "1" and everything else change around it?

Understanding mass in molar terms is necessary to do a lot of chemistry correctly. The mole is effectively a count of the number of molecules kicking about, although the count is large enough that terms like "quadrillion" don't cut it. (One mole is about 600 sextillion molecules). Knowing how many molecules are around lets you actually compute how much stuff can react in a given chemical environment, and other aspect…

trying to use kg instead of moles when calculating a chemical reaction would be like trying to set up a speed dating event by taking the weights of all the men and women instead of matching them up pairwise. Sometimes it is much easier to calculate based on scalar quantity than it is to calculate based on mass.

Re: International System of Units overhauled in historic vote

#144

Post by School-Cotton »

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…

> I'm hoping we can solve this in my lifetime with everyone knowing one standard language through education as a child

It seems like this is rapidly becoming the case in the Western world, and that that language is English.

Re: International System of Units overhauled in historic vote

#145

Post by turdnagel »

So there was a definition of a kilogram based on an actual physical thing with an arbitrary amount of mass.

Does this new definition essentially try to approximate that mass as close as possible? What is the margin of error there?

I would love to know this for a second as well, although I don't know what we used to measure a second before we locked it to the vibrations of a cesium atom.

Re: International System of Units overhauled in historic vote

#146

Post by 3pt14159 »

leni536 wrote: post #31
3pt14159 wrote: post #12

This is probably the dumbest thing I'll type on HN. In university I just gave up trying to understand why we even needed the Avogadro constant / mole as a fundamental constant. It still confuses me. Why have a difference between molar mass and mass? Why couldn't it just be "1" and everything else change around it?

My understanding is that the cause is error propagation. We can measure certain kind of masses really accurately in unified atomic mass unit. With better relative precision than the Avogadro constant. But if we used kilograms to write down these quantities then they would carry the error of the Avogadro constant needlessly. Edit: I think your question boils down to "Why do we have two separate units for mass, u and k…

Right, I get that, and if anything I feel less dumb now that I've asked it publicly and people seem to think that it's a non-dumb question, but when talking about fundamental constants I understand there is a practical nature to it all, but just as we have a nano-meter and a light-year I figured we'd have the same for mass.

Why do I need both kg and u as fundamental constants?

I believe the comments here have answered it. It isn't something weird, like quantum gravity or some such. If I understand everyone correctly it's just a practical decision we made at some point because we didn't want mass to be in u and that's that.

I feel better about it now.

Re: International System of Units overhauled in historic vote

#147

Post by mherdeg »

orblivion wrote: post #26

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

The kilogram was the last one -- we're out of SI base units. This was the last one not defined based on some measurable universal constant, per https://en.wikipedia.org/wiki/SI_base_unit and https://en.wikipedia.org/wiki/Redefinition_of_SI_base_units .

The meter was originally an actual physical rod (you can see the prototype on display at the fascinating French Musée des Arts et Métiers). Now it's based on the speed of light in a vacuum.

The kilogram was a physical mass (and, yes, it was evaporating). Now it's finally defined in terms of the Planck constant, the second, and the meter.

The second was originally 1/86400th of Earth's day. Now it's based on the radiation of cesium-133 (I don't really understand how this is measured).

The ampere was originally defined based on depositing milligrams of silver from a solution of silver nitrate; it's now based on newtons and meters.

The kelvin was originally defined based on water; it's still defined based on water.

The mole hasn't really changed much? It's based on the number of atoms in a kilogram of carbon.

The candela was originally based on a literal standard amount of light emitted by candles made from dead whales; now it's based on radiation from light sources of a specific frequency and an intensity based on the watt (which relies on kilograms, meters, and seconds).

Re: International System of Units overhauled in historic vote

#148

Post by Fuzzwah »

ballenf wrote: post #35

Has a "metric" version of time ever been proposed? Seems odd to me that we're "ok" with divisions of 12 for time but not length. I guess for science seconds are basically the only measure of time that usually matters?

https://en.wikipedia.org/wiki/Swatch_Internet_Time

> Instead of hours and minutes, the mean solar day is divided into 1000 parts called ".beats". Each .beat is equal to one decimal minute in the French Revolutionary decimal time system and lasts 1 minute and 26.4 seconds (86.4 seconds) in standard time. Times are notated as a 3-digit number out of 1000 after midnight. So, @248 would indicate a time 248 .beats after midnight representing 248/1000 of a day, just over 5 hours and 57 minutes.

Re: International System of Units overhauled in historic vote

#149

Post by aaronblohowiak »

3pt14159 wrote: post #12

This is probably the dumbest thing I'll type on HN. In university I just gave up trying to understand why we even needed the Avogadro constant / mole as a fundamental constant. It still confuses me. Why have a difference between molar mass and mass? Why couldn't it just be "1" and everything else change around it?

The candela is a real abomination of a unit.

Re: International System of Units overhauled in historic vote

#150

Post by 3pt14159 »

AstralStorm wrote: post #17
3pt14159 wrote: post #12

This is probably the dumbest thing I'll type on HN. In university I just gave up trying to understand why we even needed the Avogadro constant / mole as a fundamental constant. It still confuses me. Why have a difference between molar mass and mass? Why couldn't it just be "1" and everything else change around it?

For the same reason count is not a mass. Avogadro constant is a count relating number of specified atoms to mass.

Isn't this something essentially tautological though?

When we discuss the mass of a neutron and we say "one neutron weighs one u" then we discuss the mass of an electron and we say "one electron is 5.4858×10−4 u" and "one proton is 1.0072764 u" then we add them up and say "one hydrogen atom is 1.00794 u while one helium atom is 4.002602" (forgetting some complications for a moment) are we not just summing likes?

Or is it just that since mass is defined in Planck and time / distance terms that we need to relate it to counts of things? Theres a gap there I don't understand. Can we not just say "we measured a proton's mass and it is u"? Am I making a jump there?

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