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

You have to measure the volume of the water at a specific temperature to 
know you have a liter. Then you can later measure it at other temperatures, 
and the change in mass from being hotter will probably be insignificant for 
quite some time in the future. :-)

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

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Warp wrote:
>   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.

Errr, not really. At zero, it's equally likely two molecules of water will 
stick or break apart. Ice stays frozen, water stays melted.

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

To repeatability of experiments?

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

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Darren New <dne### [at] sanrrcom> wrote:
> 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.

  There seems to be some confusion about what exact does it mean that the
universe is expanding. I have read two different explanations:

1) New space is appearing *everywhere*, making *all* distances larger over
time, including eg. distances between subatomic particles.

2) New space is appearing between galaxies, making only the distance between
galaxies grow larger over time. (The reason for this is that when you are
close enough to a galaxy, its gravitational pull is stronger than the
"outwards" movement caused by the expansion of the universe, which means
that gravity stops you from getting farther away from the galaxy due to
this expansion. In a way, you are "tied" to the galaxy and don't get farther
away from it (from the expansion phenomenon alone).)

  As far as I have understood, explanation #1 is a misconception. Distances
between subatomic particles is not growing because the forces keeping atoms
and molecules together is way stronger than any minuscule drift that the
expansion of the universe might cause.

  The same is true at macroscopic levels: The Earth is not expanding because
atomic bonds and gravity are strong enough to stop any expansive drift from
happening. The Sun's gravity is strong enough to stop planets from drifting
away due to the expansion of the Universe. All the way up to galactic sizes:
The gravity of a galaxy is strong enough to stop stars from drifting away
due to the expansion of the Universe.

  Only when we get to intergalactic space, very very far away from any
galaxy, is gravity so weak that it does not prevent galaxies from drifting
away from each other due to the expansion of the Universe.

  It also seems to be some kind of common misconception that the expansion
of the Universe would somehow change units of measurements accordingly.
Why would it? It simply means that distances between galaxies is growing.
It doesn't mean that units of distance are changing too. The diameter of
a proton will still be the same in 1 billion years than it is now. The
expansion of the universe only makes the universe bigger, it doesn't make
*everything* bigger, down to subatomic particles. It doesn't even make
galaxies bigger (AFAIK).

-- 
                                                          - Warp

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Warp wrote:
>   As far as I have understood, explanation #1 is a misconception. Distances
> between subatomic particles is not growing because the forces keeping atoms
> and molecules together is way stronger than any minuscule drift that the
> expansion of the universe might cause.

It doesn't make sense that new space would only appear outside the galaxies. 
The fact that things are falling back together again faster than new space 
is appearing doesn't mean new space isn't appearing.

On the other hand, I don't know that anyone is really quite sure wtf is 
going on out there. :-)

>   The same is true at macroscopic levels: The Earth is not expanding because
> atomic bonds and gravity are strong enough to stop any expansive drift from
> happening. 

It doesn't stop the drift. It compensates for the drift.

It's like saying a car on a treadmill can outrun the backwards drift of the 
treadmill, so the treadmill isn't actually moving.

> The Sun's gravity is strong enough to stop planets from drifting
> away due to the expansion of the Universe. All the way up to galactic sizes:
> The gravity of a galaxy is strong enough to stop stars from drifting away
> due to the expansion of the Universe.

Sure. That doesn't mean space isn't expanding.

>   It also seems to be some kind of common misconception that the expansion
> of the Universe would somehow change units of measurements accordingly.
> Why would it? It simply means that distances between galaxies is growing.

Well there's two ways to expand space. One is to add more space, the other 
is to make each bit of space bigger. We wouldn't necessarily know about the 
latter one.

> It doesn't mean that units of distance are changing too. The diameter of
> a proton will still be the same in 1 billion years than it is now. 

How do you know? How would you determine if everything in the entire 
universe suddenly got twice as big?  How would you know, for that matter, if 
everything in the universe suddenly started going at half speed?

> expansion of the universe only makes the universe bigger, it doesn't make
> *everything* bigger, down to subatomic particles. It doesn't even make
> galaxies bigger (AFAIK).

True, but there may be more than one way in which space is expanding. You 
can get more space (which is what seems to be happening between galaxies) or 
you can get space that's twice as big (by some absolute measurement we have 
no access to).

-- 
   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 11:45, Darren New wrote:
> Neeum Zawan wrote:
>> Who said you have to do it at 4 Celsius? Its density varies with
>> temperature, not its mass.
>
> You have to measure the volume of the water at a specific temperature to
> know you have a liter. Then you can later measure it at other
> temperatures, and the change in mass from being hotter will probably be
> insignificant for quite some time in the future. :-)

	I think Warp was talking about the solid object that is now the 
standard, not the liquid.

-- 
Ground yourself, THEN hug your motherboard!

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Darren New <dne### [at] sanrrcom> wrote:
> Warp wrote:
> >   As far as I have understood, explanation #1 is a misconception. Distances
> > between subatomic particles is not growing because the forces keeping atoms
> > and molecules together is way stronger than any minuscule drift that the
> > expansion of the universe might cause.

> It doesn't make sense that new space would only appear outside the galaxies. 
> The fact that things are falling back together again faster than new space 
> is appearing doesn't mean new space isn't appearing.

  I don't know if "new space" is appearing everywhere at the same rate or
at a faster rate far away from gravity wells (I'm in no way a physicist),
but as far as I can understand, forces like atomic bonds and gravity keep
masses and entire galaxies from expanding for the simple reason that they
are stronger than the (probably extremely minuscule) drift caused by the
universe expanding.

  Think about two small spheres connected by a wire on the surface of a
balloon: Even if you inflate the balloon, the wire will keep the spheres
at the same distance from each other, against the separating force caused
by the inflation of the balloon.

  If I have understood correctly, only galaxies are far enough from each
other to be drifted by the expansion of the universe. The gravity pull
between galaxies is not strong enough to stop them from recessing.

> >   The same is true at macroscopic levels: The Earth is not expanding because
> > atomic bonds and gravity are strong enough to stop any expansive drift from
> > happening. 

> It doesn't stop the drift. It compensates for the drift.

  Terminology.

> > The Sun's gravity is strong enough to stop planets from drifting
> > away due to the expansion of the Universe. All the way up to galactic sizes:
> > The gravity of a galaxy is strong enough to stop stars from drifting away
> > due to the expansion of the Universe.

> Sure. That doesn't mean space isn't expanding.

  But even though new space is forming all the time, that doesn't necessarily
change units of measurement. The only thing which is changing is the overall
size of the universe (and given that, as far as we know, the amount of mass
and energy is constant in the universe, the overall density of mass/energy
in the universe is correspondingly decreasing).

> > It doesn't mean that units of distance are changing too. The diameter of
> > a proton will still be the same in 1 billion years than it is now. 

> How do you know?

  Is there any plausible theory, backed up by measurements, that would say
otherwise? Is there any scientifical reason to think otherwise?

> How would you determine if everything in the entire 
> universe suddenly got twice as big?  How would you know, for that matter, if 
> everything in the universe suddenly started going at half speed?

  By measuring redshift? The speed of light doesn't change, and is
completely independent on the speed of the observer, as far as we know.
Redshift is thus a reliable measurement of speed changes between objects.

> > expansion of the universe only makes the universe bigger, it doesn't make
> > *everything* bigger, down to subatomic particles. It doesn't even make
> > galaxies bigger (AFAIK).

> True, but there may be more than one way in which space is expanding. You 
> can get more space (which is what seems to be happening between galaxies) or 
> you can get space that's twice as big (by some absolute measurement we have 
> no access to).

  But if you propose that everything is getting bigger in an absolute scale,
that would mean that also c is getting larger at the same rate (so that we
are unable to measure everything getting bigger). Is there any reason to
believe so?

-- 
                                                          - Warp

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Warp wrote:
>   Think about two small spheres connected by a wire on the surface of a
> balloon: Even if you inflate the balloon, the wire will keep the spheres
> at the same distance from each other, against the separating force caused
> by the inflation of the balloon.

Yes, and I'm saying "that doesn't mean the balloon isn't expanding between 
the spheres on wires. It just means the wires are then pulling the spheres 
close together again."

>>>   The same is true at macroscopic levels: The Earth is not expanding because
>>> atomic bonds and gravity are strong enough to stop any expansive drift from
>>> happening. 
> 
>> It doesn't stop the drift. It compensates for the drift.
> 
>   Terminology.

Yes. Just checking we're talking about the same thing. :-)

>>> The Sun's gravity is strong enough to stop planets from drifting
>>> away due to the expansion of the Universe. All the way up to galactic sizes:
>>> The gravity of a galaxy is strong enough to stop stars from drifting away
>>> due to the expansion of the Universe.
> 
>> Sure. That doesn't mean space isn't expanding.
> 
>   But even though new space is forming all the time, that doesn't necessarily
> change units of measurement. The only thing which is changing is the overall
> size of the universe (and given that, as far as we know, the amount of mass
> and energy is constant in the universe, the overall density of mass/energy
> in the universe is correspondingly decreasing).

Correct. That's a different kind of expansion than I was talking about. New 
space vs bigger space.

>>> It doesn't mean that units of distance are changing too. The diameter of
>>> a proton will still be the same in 1 billion years than it is now. 
> 
>> How do you know?
> 
>   Is there any plausible theory, backed up by measurements, that would say
> otherwise? Is there any scientifical reason to think otherwise?

No. The point is that you couldn't tell, without leaving the universe. :-)

>> How would you determine if everything in the entire 
>> universe suddenly got twice as big?  How would you know, for that matter, if 
>> everything in the universe suddenly started going at half speed?
> 
>   By measuring redshift? The speed of light doesn't change, 

The speed of light doesn't change in terms of space vs time. But if space 
gets twice as big *and* the speed of light gets twice as big, then space 
doesn't change.

Draw a ruler on your balloon, and put two dots an inch across. Now inflate 
the balloon. Does the ruler say they're still an inch apart?

>> True, but there may be more than one way in which space is expanding. You 
>> can get more space (which is what seems to be happening between galaxies) or 
>> you can get space that's twice as big (by some absolute measurement we have 
>> no access to).
> 
>   But if you propose that everything is getting bigger in an absolute scale,
> that would mean that also c is getting larger at the same rate (so that we
> are unable to measure everything getting bigger). Is there any reason to
> believe so?

Not that I know of. :-) I didn't know what "SHarkD" meant by "the density of 
space", so I was adding more technobabble. ;-)


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

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Warp schrieb:

>> 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.

The difference is that, with careful handling, the number of atoms of 
that one unique object doesn't change (not significantly, that is), nor 
does its chemical or isotopic composition, and so these aspects are 
perfectly irrelevant; and even the temperature is perfectly irrelevant, 
as it has no influence on the number of atoms of an individual object 
(unless you cool it down so much that some components of the air 
condense, or heat it up so much that part of the object evaporates) - 
while it does influence the results when trying to reproduce the same 
mass with a litre of water at given conditions.

>   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.

Which is actually the advantage in this case.

> 
>> 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.

Yes, but in that case you need to measure only the distance between two 
points. But trying to do that with a whole container is a good deal more 
complex, as you have to measure the relative position of quite a bunch 
of points.

That's also the reason why the attempt to define the kg as the mass of a 
certain number of Si-atoms - "counted" by measuring the crystalline 
structure and dimensions of a certain chunk of Si - uses a perfectly 
spherical shape for the chunk. Otherwise, computing the volume of the 
object would be infeasible at the desired precision.

Now while this is a quite well-understood process by now, creating a 
spherical /cavity/ in a solid object is yet a totally different thing.

>   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.

The accuracy of measurement even with state-of-the-art methods is quite 
poor though, which is precisely the point.

Also note that scientists /are/ working on the problem of getting away 
from the Prototype definition; they don't want to lose precision though, 
so water is /not/ an option for them.

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SharkD schrieb:
> 
> I wonder if a sphere is the optimal packing of silicon atoms. I doubt it.

At the atom level, the object those scientists are aiming for would 
indeed just be a rough approximation of a sphere, and instead keep its 
lattice structure (no "anti-aliasing" on that sphere, so to speak :-)). 
After all, it is a machined and polished Si monocrystal (like the stuff 
used in the semiconductor industry).

However, for the current target precision it will be good enough - and 
its dimensions will be easier to control than, say, a rectangular shape.

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Warp schrieb:

>> 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.

I was indeed slightly mistaken: The original definition was for water at 
the /melting point/, which is a bit above 0 degrees C (something like 
0.01 degrees C, IIRC).


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

To everyone requiring such a precision in /any/ physical unit derived 
from the kg.

I already mentioned how small the "drift" of the Prototype is - and yet 
scientists are worried about this, so /someone/ seems to be relying on a 
precision in that order of magnitude.

And after all, there is no reason whatsoever to redefine the kg in a way 
that "breaks backward compatibility": Whatever phyiscal properties you 
base it upon, you can always add some factor to the new definition to 
make it match the previous definition.

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