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RIP Le Grand K

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    Lewri [1762864] Lewri
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    Thread created on 21:08:58 - 19/11/18 (6 years ago)
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    Last replied 11:12:49 - 23/11/18 (6 years ago)
    The International System of Units was made official in 1960, however the kilogram has been around since 1795. Originally defined as the mass of 1 litre of water at the melting point of ice, the definition was soon changed to the mass of a prototype weight, later to be the International Prototype Kilogram (aka Le Grand K). Other units within the SI were then defined using this definition of the kilogram.

    Over the years, the definition of the kilogram based on the IPK caused problems, such as its deviation from its original mass and that of its sibling prototypes, as well as its inaccessibility. This of course made life difficult for all out there who needed to make precise measurements involving mass. Even using the most precise measurements possible with the IPK, we could only then determine the values of physical constants to within a certain degree of accuracy, in other words we had uncertainties associated with many important constants of nature.

    But how else could we have defined the kilogram? Could we have gone back to the original definition based on the mass of water? Well this brings up several issues, the mass that we now take as 1kg has varied by a small amount over the years and no longer matches that original definition, not to mention issues with the pressure and temperature at which these measurements are taken. Going back to the original definition would still rely on artifacts, the arbitrary chosen pressure and temperature, but what way allows us to get rid of artifacts?

    On the 16th November 2018, the General Conference on Weights and Measures approved a new definition, alongside 3 other units. As of the 21st May 2019, the kilogram will be defined in terms of Planck's constant, which will now be (rather appropriately) taken as a constant value. This can be done as Planck's constant is equal to energy divided by frequency, energy can be given as E=mc2 and so m=hf/c2. 

    The kg is dead, long live the kg

    Do you think this was a long overdue necessity, or purely circular logic? With the new system what do you think we should strive to do next, or do you think that the new system takes us further from or maybe closer to reality? Please share your thoughts on these new changes to the SI units and their possible impacts upon the world.

    In loving memory - Mars Climate Orbiter (1998-1999)
    Last edited by Lewri on 23:11:11 - 21/11/18 (6 years ago)

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    WiseTheRumGone [2078276] WiseTheRumGone
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    Posted on 22:06:21 - 20/11/18 (6 years ago)
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    I remember reading about this earlier. i found it pretty funny how the definition of a kilo was a metal in some french basement lol.

    "Refusing to help a neighbor who's house burned down is shitty. Refusing when you helped start the fire is monstrous."

    ttv: vincento111

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    Sepulchrave [2092631] Sepulchrave
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    Posted on 21:54:31 - 21/11/18 (6 years ago)
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    Fascinating subject, but I'm not a physicist, so I can only follow it so far before the maths begin to boggle my mind. Planck's constants do appear to be accurate to a high degree, but we live in a relativistic world, and all values are variant. (https://physicsworld.com/a/can-gps-find-variations-in-plancks-constant/) Coming at the subject from another angle, the French penchant for insisting on exact measures may relate more to their Platonic philosophical lineage than to any enduring attributes of space, time and energy (this is my own hobbyhorse - I am a little suspicious of French thinking in general). There is another, obviously small, flaw in using Planck's constant - since its value is possibly variable, or at least debatable, it will need to be "fixed" to make meaning of its relationship to the meter. Once fixed, its value becomes dogmatic, not empirical, and again we are back where we started, without any real surety of exactitude. To my mind, relativity is the big kicker, it will always throw an onto- or epistemo- logical spanner into the works of an idealist.

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    Lewri [1762864] Lewri
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    Posted on 22:56:32 - 21/11/18 (6 years ago)
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    Sepulchrave [2092631]

    Fascinating subject, but I'm not a physicist, so I can only follow it so far before the maths begin to boggle my mind. Planck's constants do appear to be accurate to a high degree, but we live in a relativistic world, and all values are variant. (https://physicsworld.com/a/can-gps-find-variations-in-plancks-constant/) Coming at the subject from another angle, the French penchant for insisting on exact measures may relate more to their Platonic philosophical lineage than to any enduring attributes of space, time and energy (this is my own hobbyhorse - I am a little suspicious of French thinking in general). There is another, obviously small, flaw in using Planck's constant - since its value is possibly variable, or at least debatable, it will need to be "fixed" to make meaning of its relationship to the meter. Once fixed, its value becomes dogmatic, not empirical, and again we are back where we started, without any real surety of exactitude. To my mind, relativity is the big kicker, it will always throw an onto- or epistemo- logical spanner into the works of an idealist.
    An interesting argument. I may be wrong but I dont think this is actually a problem as Planck's constant should be constant in all inertial reference frames, it is only when you look at another frame moving relative to you that it may appear to you that their value of Planck's constant is any different (I will have to have a proper think about this tomorrow and read over your source once my exam is out of the way!). If Planck's constant wasn't inertial in all reference frames, this would seem to me to be a violation of Einstein's postulates of relativity which would allow you to determine some sort of absolute rest frame.
    Last edited by Lewri on 22:58:11 - 21/11/18 (6 years ago)

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    Nekys [538682] Nekys
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    Posted on 13:15:29 - 22/11/18 (6 years ago)
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    No matter how they redifine it, fat will still be fat

     
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    Sepulchrave [2092631] Sepulchrave
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    Posted on 11:12:49 - 23/11/18 (6 years ago)
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     https://arxiv.org/abs/astro-ph/0511440
    An interesting paper suggesting in several ways that the possibility of "Varying Constants" ought to be taken seriously...dealing in particular with the keck/VLT data suggesting variant alpha. An interesting consequence of a variable fine structure constant would be variance of related constants...such as e. Wouldn't that be a kick in the head? Or would it...I suppose if all the constants varied proportionally it might work out, as long as they avoided any messy variables. Actually, I can't think of any reason why the entire universe couldn't double in magnitude, constants, variables and all, with no one the wiser. (Cue the penis-size jokes.)

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