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http://blogs.ngm.com/blog_central/2009/10/a-grander-k.html
--
~Mike
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Mike Raiford wrote:
> http://blogs.ngm.com/blog_central/2009/10/a-grander-k.html
...and here I was thinking that 12g of C12 was *defined as* one mole of
C12 atoms...
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Invisible <voi### [at] dev null> wrote:
> Mike Raiford wrote:
> > http://blogs.ngm.com/blog_central/2009/10/a-grander-k.html
> ...and here I was thinking that 12g of C12 was *defined as* one mole of
> C12 atoms...
Hence the definition of 'mole' depends on the definition of 'kilogram',
not the other way around.
--
- Warp
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Mike Raiford <"m[raiford]!at"@gmail.com> wrote:
> http://blogs.ngm.com/blog_central/2009/10/a-grander-k.html
Why can't the kilogram be defined as the weight of exactly 1 litre of
pure water at a certain temperature? After all, that has been the de-facto
definition for forever.
--
- Warp
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On 10/12/09 10:17, Warp wrote:
> Mike Raiford<"m[raiford]!at"@gmail.com> wrote:
>> http://blogs.ngm.com/blog_central/2009/10/a-grander-k.html
>
> Why can't the kilogram be defined as the weight of exactly 1 litre of
> pure water at a certain temperature? After all, that has been the de-facto
> definition for forever.
Perhaps they could _change_ the definition to that, but I'm willing to
bet that it's off by a bit from the current definition.
--
A tautology is a thing which is tautological.
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Neeum Zawan <m.n### [at] ieeeorg> wrote:
> Perhaps they could _change_ the definition to that, but I'm willing to
> bet that it's off by a bit from the current definition.
AFAIK kilogram *was* originally defined as the weight of 1 liter of water,
but for whatever reason they changed it to the weight of a specific object
(I really can't understand why).
According to wikipedia, one liter of pure water at 4 degrees celsius (the
standard temperature for measuring SI units) is 0.9999720 kilograms.
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?
--
- Warp
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Warp wrote:
> According to wikipedia, one liter of pure water at 4 degrees celsius (the
> standard temperature for measuring SI units) is 0.9999720 kilograms.
It may be that it's too difficult to measure "pure water" at "exactly 4
degrees celsius" for modern measurement precision.
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.
Given that a cm is defined in terms of the speed of light, and time is
defined in terms of a cesium atom at rest at 0K, I guess we already have
unusable base metrics, tho.
> 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?
Only for scientists working on 15 digits of precision in their scientific
experiments. Kind of like "would it really make a difference if we skipped a
leap-second every 3 to 5 years?"
--
Darren New, San Diego CA, USA (PST)
I ordered stamps from Zazzle that read "Place Stamp Here".
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>> ...and here I was thinking that 12g of C12 was *defined as* one mole of
>> C12 atoms...
>
> Hence the definition of 'mole' depends on the definition of 'kilogram',
> not the other way around.
Ah, I see.
Couldn't they just reverse it? I mean, just say that 1 Kg = the mass of
XXX C12 atoms?
--
http://blog.orphi.me.uk/
http://www.zazzle.com/MathematicalOrchid*
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Darren New <dne### [at] sanrrcom> wrote:
> Warp wrote:
> > According to wikipedia, one liter of pure water at 4 degrees celsius (the
> > standard temperature for measuring SI units) is 0.9999720 kilograms.
> 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).
> 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.
But a centimeter is not defined like that. It's defined from the speed of
light in vacuum. There's no circle.
> Given that a cm is defined in terms of the speed of light, and time is
> defined in terms of a cesium atom at rest at 0K, I guess we already have
> unusable base metrics, tho.
Unusable base metrics?
> > 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?
> Only for scientists working on 15 digits of precision in their scientific
> experiments. Kind of like "would it really make a difference if we skipped a
> leap-second every 3 to 5 years?"
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.
--
- Warp
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Warp wrote:
> But a centimeter is not defined like that. It's defined from the speed of
> light in vacuum. There's no circle.
Yes, I saw that.
> Unusable base metrics?
0K is unreachable even in theory.
> 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.
True. I don't know how they work it now.
--
Darren New, San Diego CA, USA (PST)
I ordered stamps from Zazzle that read "Place Stamp Here".
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On 12-10-2009 18:27, Warp wrote:
> Neeum Zawan <m.n### [at] ieeeorg> wrote:
>> Perhaps they could _change_ the definition to that, but I'm willing to
>> bet that it's off by a bit from the current definition.
>
> AFAIK kilogram *was* originally defined as the weight of 1 liter of water,
> but for whatever reason they changed it to the weight of a specific object
> (I really can't understand why).
Standards are defined on whatever can be most accurately measured. So if
you can measure distance and light speed more accurate than time you
define time as a the interval it takes light to cross a certain length.
If you increase the accuracy of your clock, you might want to define a
length as velocity times time, etc. At some point the accuracy of
quasars was approaching our best clocks, if they would have passed that
we would now have time defined in heaven.
The standard kilogram can be more accurately measured than a liter of
water. When we are able to count a specific number of atoms of a known
isotope and weight those with sufficient accuracy that will probably
replace the standard kilogram. ATM we are not that far, but only just,
as this article explains.
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Invisible schrieb:
> ...and here I was thinking that 12g of C12 was *defined as* one mole of
> C12 atoms...
No, the /mole/ is defined on this basis.
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Orchid XP v8 schrieb:
>>> ...and here I was thinking that 12g of C12 was *defined as* one mole
>>> of C12 atoms...
>>
>> Hence the definition of 'mole' depends on the definition of 'kilogram',
>> not the other way around.
>
> Ah, I see.
>
> Couldn't they just reverse it? I mean, just say that 1 Kg = the mass of
> XXX C12 atoms?
That's one attempt to it (except that they appear to intend to use
silicon instead of carbon).
However, at present (or at least until very recently) measuring the mass
of the Grand K can still be done with greater precision than counting
atoms, so they don't know /exactly/ how many atoms make up a mole.
Definitions of SI units change over time depending on how they can be
measured with highest precision.
For instance, the SI unit of distance (1 metre) was, like the kilogram,
initially (*) defined based on a physical entity - the "International
Prototype Metre". (* Actually, even before, for a few years the metre
was based on other properties, including being a 10,000-th of the
half-meridian through Paris, but those didn't last long.)
Later, scientists managed to measure wavelengths at precisions that
exceeded that of the Prototype Metre, and therefore it was decided in
1960 to redefine the metre as the vacuum wavelength of a particular
emission of krypton-86.
Some two decades later, both time and the vacuum speed of light could be
measured even better than wavelengths, and the metre was redefined in
1983 as the distance travelled by light in vacuum within a certain
period of time.
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Warp schrieb:
> Why can't the kilogram be defined as the weight of exactly 1 litre of
> pure water at a certain temperature? After all, that has been the de-facto
> definition for forever.
That was only the /initinal/ definition.
What reason would be there to stick to it - or choose a different
definition, for that matter?
There's only one: Reproducability.
If you can find a definition for a kilogram that /matches/ the initial
definition, but can be reprocuded with /higher precision/, then that
definition is both superior and "backward compatible".
It turned out that measuring the mass of 1 litre of water at a certain
temperature was subject to more error (possibly due to issues with
producing /really/ pure water, and reproducing /exactly/ the desired
temperature) than "copying" the mass of some sample entity made of a
robust, non-corrosive material.
Note that while it was found that the "primary copies" of the kilogram
exhibit a "drift" relative to the Grand K, but nobody is presently able
to tell whether the Grand K also exhibits an /absolute/ drift. If pure
water would provide a reference of adequate precision (or even anywhere
close), I guess scientists would already have checked for absolute
drifts using this method.
Also note that there is no such thing as a "de-facto definition"; there
are "practical realizations" that differ from the official definition,
but I guess none is based on the initial definition these days, except
maybe as makeshift references for low-precision applications.
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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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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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Warp schrieb:
> 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).)
I think #2 is just #1 taking into account the attractive forces working
simultaneously, so I see no difference there; so I'd say space is
appearing everywhere, but it's not noticeable at subatomic levels
because the attractive forces compensate for it.
In case the rate of "space growth" should be indeed increasing (as some
scientists claim it is), then it might be possible that (a) this
/acceleration of growth/ will cause the distance between subatomic
particles to increase (as there will be a new distance of equilibrium
between "space growth" and all the other forces), and (b) the rate of
"space growth" might some time in the future exceed the attractive
forces' ability to compensate, so that "space growth" itself (and not
just the acceleration thereof) might drive even subatomic structures apart.
> It also seems to be some kind of common misconception that the expansion
> of the Universe would somehow change units of measurements accordingly.
Well, obviously it cannot change units of measurements: Those are
/defined/. What it /can/ change is (a) practical realizations of
measurements, and (b) possibly the value of natural "constants" as
expressed in these units of measurement.
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clipka wrote:
> 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).
I think the 4C was because that's the temperature at which liquid water is
most dense.
--
Darren New, San Diego CA, USA (PST)
I ordered stamps from Zazzle that read "Place Stamp Here".
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Warp schrieb:
>
> 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.
Two spheres connected with a rubber band would be a better analogy:
While you inflate the balloon, the rubber band would keep the spheres at
a /constant/ distance, but it would be a different one than if the
balloon wasn't being inflated.
And the rubber band would be stretched even further if you'd increase
the rate of inflation.
>>> 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.
Not if you think of /changing/ parameters. In that case it starts
becoming a fundamental difference.
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Darren New schrieb:
> clipka wrote:
>> 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).
>
> I think the 4C was because that's the temperature at which liquid water
> is most dense.
Not exactly: They moved to 4 degrees C because it is the temperature at
which the density is /most stable/, so the influence of getting the
temperature wrong is minimized.
(Though of course this is closely related to the fact that at 4 degrees
C water is most dense, too.)
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clipka wrote:
> which the density is /most stable/,
>
> C water is most dense, too.)
Um, yes. We call that calculus. :-) But point taken.
--
Darren New, San Diego CA, USA (PST)
I ordered stamps from Zazzle that read "Place Stamp Here".
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Darren New schrieb:
> clipka wrote:
>> which the density is /most stable/,
>>
>> C water is most dense, too.)
>
> Um, yes. We call that calculus. :-) But point taken.
Yes, given the simplicity of the density-vs-temperature function,
calculus indeed correctly postulates that these temperatures must happen
to coincide :-)
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clipka <ano### [at] anonymousorg> wrote:
> Well, obviously it cannot change units of measurements: Those are
> /defined/. What it /can/ change is (a) practical realizations of
> measurements, and (b) possibly the value of natural "constants" as
> expressed in these units of measurement.
I don't think that's the case, given that units of measurements (at least
in the SI system) are defined by natural constants, not the other way around.
--
- Warp
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Mike Raiford wrote:
> http://blogs.ngm.com/blog_central/2009/10/a-grander-k.html
They've been talking about this since at least the mid 90s (when I first
heard about it), but this is the first time I've actually seen a picture
of the weight they use. Nice and shiny :)
...Chambers
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On 10/13/2009 12:59 PM, 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.
The gravitational force of the sun is also "way stronger" than the
gravitational force of the Hubble telescope. That doesn't mean it has
zero effect on us sitting here in front of our computers. :)
-Mike
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Warp schrieb:
> clipka <ano### [at] anonymousorg> wrote:
>> Well, obviously it cannot change units of measurements: Those are
>> /defined/. What it /can/ change is (a) practical realizations of
>> measurements, and (b) possibly the value of natural "constants" as
>> expressed in these units of measurement.
>
> I don't think that's the case, given that units of measurements (at least
> in the SI system) are defined by natural constants, not the other way around.
Well, that's actually far from the truth. For instance, the
gravitational constant is nowhere to be found in the SI system, nor is
the planck mass, electron charge etc.
As a matter of fact, the only /fundamental/ natural constant presently
used in the SI system is the speed of light; all other SI units are
based on physical properties that are only indirectly affected by
natural constants.
And no, units of measurement cannot change /by definition/, because it's
by them that we measure the properties of the world, including natural
constants.
This is counter-intuitive to what any sane person would think (which is
why you're disputing it I guess) - but who said that scientists are sane
in the first place?
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