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# The kilogram is dead! Long live the kilogram!

12 replies to this topic

### #1 ascii

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Posted 20 May 2019 - 09:50 AM

Starting today, Le Grand K is no longer the official kilogram.  The kilogram is defined now by fundamental physical constants, particularly the Plank constant.

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

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### #2 Dynan

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Posted 20 May 2019 - 10:06 AM

SCIENCE !!!

### #3 Jon Isaacs

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Posted 20 May 2019 - 11:57 AM

So.. Just what is Pluto's mass and does this change it's status?

Jon

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### #4 ascii

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Posted 20 May 2019 - 12:09 PM

So.. Just what is Pluto's mass and does this change it's status?

Jon

Perhaps we could get Orpheus to take up his lyre one last time to convince the king of the underworld to step onto his bathroom scale.

### #5 bobzeq25

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Posted 20 May 2019 - 12:13 PM

The mass of Pluto is still 27% less than the mass of Eris.

You can have a different number of planets than 8.  You just can't have 9, that's would be a completely arbitrary number.  <smile>.  Unless you want to make discovery date a criteria.

Oops.  Ceres.

Starting today, Le Grand K is no longer the official kilogram.  The kilogram is defined now by fundamental physical constants, particularly the Plank constant.

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

Cute, esthetically.  The idea that any lump of anything was a definition never really appealed to me.

As a practical matter I'd take an even money bet it has no practical effect.  I would guess almost no one measures anything to that degree of precision, except people calibrating weights for the sake of calibrating weights.

Edited by bobzeq25, 20 May 2019 - 12:15 PM.

### #6 brave_ulysses

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Posted 20 May 2019 - 12:32 PM

it sounds like you are saying this won't have mass appeal

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### #7 Mister T

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Posted 20 May 2019 - 02:19 PM

it sounds like you are saying this won't have mass appeal

Do you realize how expensive this will make bathroom scales??

it is not gonna be easy to fit a Planc length measuring device in to something you can fit in your bathroom.

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### #8 BillP

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Posted 20 May 2019 - 08:53 PM

Starting today, Le Grand K is no longer the official kilogram.  The kilogram is defined now by fundamental physical constants, particularly the Plank constant.

*Phew*  One less thing to worry about now moving the definition from one arbitrary assignment to another.  I will definitely sleep better tonight

Edited by BillP, 20 May 2019 - 08:53 PM.

### #9 ascii

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Posted 20 May 2019 - 09:36 PM

All kidding aside, at least the kilogram is now reproducible without relying on a physical artifact.  Prior to today, it was the last of the seven base SI units to be defined by an artifact.

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### #10 Mister T

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Posted 21 May 2019 - 07:46 AM

All kidding aside, at least the kilogram is now reproducible without relying on a physical artifact.  Prior to today, it was the last of the seven base SI units to be defined by an artifact.

But who is in charge of the Planc length unit??

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### #11 ascii

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Posted 21 May 2019 - 08:31 AM

But who is in charge of the Planc length unit??

I don't know who is in charge of the Planck length (ℓP), but it is not involved in the kilogram definition like the Planck constant (h) is.

On 16 November 2018, the International Bureau of Weights and Measures (BIPM) voted to redefine the kilogram by fixing the value of the Planck constant, thereby defining the kilogram in terms of the second and the speed of light. Starting 20 May 2019, the new value is exactly

h = 6.626 070 15 × 10-34 J⋅s.

### #12 Mister T

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Posted 22 May 2019 - 06:08 AM

I don't know who is in charge of the Planck length (ℓP), but it is not involved in the kilogram definition like the Planck constant (h) is.

Length, constant whats the diff??

you science types get all caught up in "Units".

### #13 ascii

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Posted 22 May 2019 - 07:03 AM

Length, constant whats the diff??

you science types get all caught up in "Units".

I'm not sure if you're being facetious or not, but for the sake of argument, I'll take the statements at face value.

The Plank length and Plank constant are two different things that are used for entirely different purposes.  The Planck length is a length.  It can be expressed in meters.  The Planck constant is a physical constant, expressed in joules • seconds, that expresses the relationship between the frequency and energy of a photon.

Few things are as vitally important to science (or to my field, engineering) as units.  A mistake in units is what led to the loss of the \$327.6 million Mars Climate Orbiter mission.  I am quite certain that there have been countless other failures in science and engineering that could be attributed to a mistake in units.  Who would want to drive over a bridge that was designed with a unit error in its engineering?  Who would want to take a medication whose dose had been calculated with the wrong units?

Le Grand K served its purpose, but it and it's replicas have been drifting apart in mass for years due to not entirely understood changes.  Those changes meant that the definition of the kilogram was not stable.  The new definition fixes that problem.

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