 |
|
|
|
|
|
| |
| |
|
|
|
|
| |
| |
|
|
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".
Post a reply to this message
|
|
| |
| |
|
|
|
|
| |
| |
|
|
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.
Post a reply to this message
|
|
| |
| |
|
|
|
|
| |
| |
|
|
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.
Post a reply to this message
|
|
| |
| |
|
|
|
|
| |
| |
|
|
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.
Post a reply to this message
|
|
| |
| |
|
|
|
|
| |
| |
|
|
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.
Post a reply to this message
|
|
| |
| |
|
|
|
|
| |
| |
|
|
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".
Post a reply to this message
|
|
| |
| |
|
|
|
|
| |
| |
|
|
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.
Post a reply to this message
|
|
| |
| |
|
|
|
|
| |
| |
|
|
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.)
Post a reply to this message
|
|
| |
| |
|
|
|
|
| |
| |
|
|
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".
Post a reply to this message
|
|
| |
| |
|
|
|
|
| |
| |
|
|
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 :-)
Post a reply to this message
|
|
| |
| |
|
|
|
|
| |
|
|
