Warp schrieb:

>   According to wikipedia, one liter of pure water at 4 degrees celsius (the
> standard temperature for measuring SI units) is 0.9999720 kilograms.

That's because the /initial/ definition of the kilogram was one liter of 
pure water at /zero/ degrees celsius.

>   If they changed the definition of kilogram back to its original form, the
> change would be less than 0.003% from the currently accepted value. Would
> that be a huge catastrophe?

There would be no change at all, because they'd define it as something 
along the lines of:

"1 kilogram is the mass of 1.000028 liters of pure water at 4 degrees 
celsius"

And yes, a /change/ of 0.003% would be a /tremendous/ catastrophe. Note 
that they're currently worried about /imprecisions/ of less than 100 
micrograms (0,0000001%).

See also 
http://en.wikipedia.org/wiki/Kilogram#Importance_of_the_kilogram for all 
the units of measurement that would be affected by such a change.

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Warp schrieb:

>> It may be that it's too difficult to measure "pure water" at "exactly 4 
>> degrees celsius" for modern measurement precision.
> 
>   Not more difficult than measuring the exact speed of light (which is needed
> to define the unit of length) or an exact magnetic force (which is needed to
> define the unit of electric current).

It's not a matter of /exact/ measurement, but of /precise/ measurement 
instead.

Vacuum speed of light is (comparatively) simple to measure at high 
precision; it seems to me there are not many sources of errors in 
experiments except the vacuum (scientists are pretty good at that 
nowadays), the stability of the distances in your experiment (which will 
be what you'll measure), and the quality of your time reference (note 
that the second is the SI base unit that can presently be measured with 
the highest precision of all).

With a liter of pure water at exactly 4 degrees celsius, one problem 
you'll have is to exactly hit the 4 degrees celsius. Another problem is 
to /get/ really pure water, and /keep/ it pure. Yet another problem is 
that you'll have to define the exact isotopic composition of the water.

Pressure is another thing, as I guess even pure water is only 
theoretically incompressible.

Then there's the shape of the container. You need to make sure that it 
/precisely/ holds 1 litre when it is at 4 degrees celsius /and/ filled. 
Note that you must also account for any openings used to fill it with 
water: If you want them to be completely filled with water when 
measuring, you need to know their precise dimensions up until the valve, 
while making sure you have no residual water outside the valve. If you 
choose to not have such openings to be completely filled, you must take 
into account capillary effcts to compute the actual volume of the water, 
which then depends on the material and even surface structure of the 
container...

Sounds like quite some fun.

>   Given that the object which is currently the measurement of 1 kg changes
> weight every time it's measured, does it really matter? It would simply be
> one more change, but then it would be fixed, and that's it.

Nobody /knows/ whether if it actually changes weight. All they know is 
that the "primary copies" do change weight with respect to one another 
and the Prototype.

And then look at the order of magnitude of those changes.

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Darren New schrieb:

> 0K is unreachable even in theory.

It may provide just as good a base for actual experiments as specifying 
any other temperature; I guess the systematic error introduced by a 
deviating temperature is comparatively easy to "compute away", and I 
also guess that it might be easier to measure an actual temperature than 
to achieve a certain reference temperature, so the math to compensate 
for the deviating temperature might be needed anyway, making 0K just as 
good as any other reference temperature (maybe making the math even 
slightly easier).

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On 10/12/2009 12:57 PM, Darren New wrote:
> Plus, of course, if you make *everything* circular, then you have
> nothing. If a kilogram is the weight (well, mass) of a liter of water,
> and a liter is 1000 cubic centimeters, and a centimeter is the length of
> one gram of carbon atoms lined up (or some such) then the whole thing
> falls down.

I wonder if a sphere is the optimal packing of silicon atoms. I doubt it.

-Mike

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On 10/12/2009 10:45 AM, Mike Raiford wrote:
> http://blogs.ngm.com/blog_central/2009/10/a-grander-k.html

"They’ve gone into high gear to redefine the kilo as a universal 
constant based on nature instead of an object vulnerable to distortion."

I bet that the new unit will also be proven susceptible to "distortion" 
over very large amounts of time, like with the density of space changing 
over thousands of millennia or something. (Just a hunch.)

-Mike

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clipka <ano### [at] anonymousorg> wrote:
> With a liter of pure water at exactly 4 degrees celsius, one problem 
> you'll have is to exactly hit the 4 degrees celsius. Another problem is 
> to /get/ really pure water, and /keep/ it pure. Yet another problem is 
> that you'll have to define the exact isotopic composition of the water.

  I don't see how that is different from the current method, ie. measuring
the weight of that one object at 4 degrees celsius.

  Except that with water you don't have to rely on one specific object which
is unique and there exists only one in the world.

> Then there's the shape of the container. You need to make sure that it 
> /precisely/ holds 1 litre when it is at 4 degrees celsius /and/ filled. 

  Not much different from defining length in relation to the speed of
light. If you want to measure it, you need precise timing and precise
length measurements.

> >   Given that the object which is currently the measurement of 1 kg changes
> > weight every time it's measured, does it really matter? It would simply be
> > one more change, but then it would be fixed, and that's it.

> Nobody /knows/ whether if it actually changes weight. All they know is 
> that the "primary copies" do change weight with respect to one another 
> and the Prototype.

  At least with the water the definition would be fixed to one single
weight. Then it's only a question of how accurately it can me measured,
which is no different from all the other units.

-- 
                                                          - Warp

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clipka <ano### [at] anonymousorg> wrote:
> Warp schrieb:

> >   According to wikipedia, one liter of pure water at 4 degrees celsius (the
> > standard temperature for measuring SI units) is 0.9999720 kilograms.

> That's because the /initial/ definition of the kilogram was one liter of 
> pure water at /zero/ degrees celsius.

  At zero? I don't think so. At zero celsius water freezes, after which one
litre of it weights a whole lot less than one kilogram.

> And yes, a /change/ of 0.003% would be a /tremendous/ catastrophe.

  To what?

-- 
                                                          - Warp

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clipka wrote:
> Darren New schrieb:
> 
>> 0K is unreachable even in theory.
> 
> It may provide just as good a base for actual experiments as specifying 
> any other temperature; 

Sure. It's just something you can't *even in theory* measure perfectly. :-) 
You could, in theory, get a liter of water so pure there are no other 
molecules in it. Theory says you can't get a cesium atom down to 0K. I just 
found that amusing, is all.

-- 
   Darren New, San Diego CA, USA (PST)
   I ordered stamps from Zazzle that read "Place Stamp Here".

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On 10/13/09 10:29, Warp wrote:
> clipka<ano### [at] anonymousorg>  wrote:
>> With a liter of pure water at exactly 4 degrees celsius, one problem
>> you'll have is to exactly hit the 4 degrees celsius. Another problem is
>> to /get/ really pure water, and /keep/ it pure. Yet another problem is
>> that you'll have to define the exact isotopic composition of the water.
>
>    I don't see how that is different from the current method, ie. measuring
> the weight of that one object at 4 degrees celsius.

	Who said you have to do it at 4 Celsius? Its density varies with 
temperature, not its mass.

>    Except that with water you don't have to rely on one specific object which
> is unique and there exists only one in the world.

	Yes, but clipka's points still stand. It's likely harder to get the 
right purity, volume, temperature, and pressure. That one object may 
have variations, but perhaps those variations are smaller than the 
accuracy of purity, volume, temperature and pressure all put together.

>> Then there's the shape of the container. You need to make sure that it
>> /precisely/ holds 1 litre when it is at 4 degrees celsius /and/ filled.
>
>    Not much different from defining length in relation to the speed of
> light. If you want to measure it, you need precise timing and precise
> length measurements.

	We have had precise length measurements for a long, long time. Precise 
timing may be another story.

>> Nobody /knows/ whether if it actually changes weight. All they know is
>> that the "primary copies" do change weight with respect to one another
>> and the Prototype.
>
>    At least with the water the definition would be fixed to one single
> weight. Then it's only a question of how accurately it can me measured,
> which is no different from all the other units.

	True.

-- 
An unbreakable toy is useful for breaking other toys.

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SharkD wrote:
> I bet that the new unit will also be proven susceptible to "distortion" 
> over very large amounts of time, like with the density of space changing 
> over thousands of millennia or something. (Just a hunch.)

I wonder if that counts, tho. If space gets twice as big over time, I'd be 
worried if the measuring sticks didn't.

-- 
   Darren New, San Diego CA, USA (PST)
   I ordered stamps from Zazzle that read "Place Stamp Here".

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