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clipka <ano### [at] anonymous org> 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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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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