Register to reply

Methods of measuring G.

by Starbles@Earthlink.net
Tags: measuring, methods
Share this thread:
Starbles@Earthlink.net
#1
Apr12-06, 04:00 AM
P: n/a
What are all the different methods being used to measure G? Why are the
results so different for more precise experiments?

Phys.Org News Partner Physics news on Phys.org
Physicists unlock nature of high-temperature superconductivity
Serial time-encoded amplified microscopy for ultrafast imaging based on multi-wavelength laser
Measuring the smallest magnets: Physicists measured magnetic interactions between single electrons
FrediFizzx
#2
Apr12-06, 04:00 AM
P: n/a
<Starbles@Earthlink.net> wrote in message
news:1144736590.371979.167020@e56g2000cwe.googlegroups.com...
| What are all the different methods being used to measure G? Why are
the
| results so different for more precise experiments?

Hmm... You second sentence indicates that must already know of some of
the methods. ;-)

F = G*m1*m2/r^2 so,

G = F/(m1*m2/r^2)

http://en.wikipedia.org/wiki/Gravitational_constant

Check out the references listed in that article for more info. But
mainly I would think it is hard to measure G precisely just because
gravity is very weak compared to other "forces". We can make a
gravitational coupling "constant" by,

alpha_G (m1,m2) = G*m1*m2/(hbar*c) in cgs units,

[(m1,m2) means "as a function of" mass 1 and mass 2.]

and plug in values of elementary particle masses to see how weak gravity
is compared to say the EM force. It is something like 39 or 40 orders
of magnitude weaker.

FrediFizzx
http://www.vacuum-physics.com

robert bristow-johnson
#3
Apr12-06, 04:00 AM
P: n/a

Starbles@Earthlink.net wrote:
> What are all the different methods being used to measure G? Why are the
> results so different for more precise experiments?


something to look at:

http://www.npl.washington.edu/eotwash/gconst.html

someone else (with better authority than me) can explain why gravity is
such a weak force (at least in the scaling of time, length, and mass
common in our anthropocentric experience) and why it's hard to measure.

if we knew precisely the Earth's mass, M, we'd have a good value for G,
but we only have a really precise and consistent value for the product
G*M. so we have to measure it with stainless steel balls of known
mass, but compared to the Earth, they're so damn small and their
gravitational effect on other objects is almost undetectable.

r b-j


Uncle Al
#4
Apr13-06, 04:00 AM
P: n/a
Methods of measuring G.

Starbles@Earthlink.net wrote:
>
> What are all the different methods being used to measure G? Why are the
> results so different for more precise experiments?


Science 288(5468) 944 (2000)
Phys. Rev. Lett. 85(14) 2869 (2000)
Phys. Rev. Lett. 89 161102 (2002)

Big G is a right proper pisser to measure for at least four reasons:

1) Gravitation's range is infinite.
2) Gravitation cannot be shielded.
3) Gravitation is by far the weakest interaction.
4) There is no theory - none whatsoever - for predicting its "true"
value ab initio.

If a fat girl walks past your lab, if it rains, when the moon and sun
go past overhead... the interactive surrounding mass distribution goes
all to Hell. The Adelberger group's quadrupole pendulum is then a
brilliant innovation for measuring Big G because its surroundings
cancel out.

They would get a better S/N if they replaced their planar fused silica
bob (2.20 g/cm^3) with a much denser homogeneous transparent body
(ease of testing during fabrication) carved from a single crystal,
e.g., lead tungstate (8.28 g/cm^3; though it is a bit soft).

--
Uncle Al
http://www.mazepath.com/uncleal/
(Toxic URL! Unsafe for children and most mammals)
http://www.mazepath.com/uncleal/qz3.pdf

Starbles@Earthlink.net
#5
Apr13-06, 04:00 AM
P: n/a

robert bristow-johnson wrote:
> Starbles@Earthlink.net wrote:
> > What are all the different methods being used to measure G? Why are the
> > results so different for more precise experiments?

>
> something to look at:
>
> http://www.npl.washington.edu/eotwash/gconst.html
>
> someone else (with better authority than me) can explain why gravity is
> such a weak force (at least in the scaling of time, length, and mass
> common in our anthropocentric experience) and why it's hard to measure.
>
> if we knew precisely the Earth's mass, M, we'd have a good value for G,
> but we only have a really precise and consistent value for the product
> G*M. so we have to measure it with stainless steel balls of known
> mass, but compared to the Earth, they're so damn small and their
> gravitational effect on other objects is almost undetectable.
>
> r b-j


What about using an inferometer? Is that better or worse?

Starbles@Earthlink.net
#6
Apr13-06, 04:00 AM
P: n/a
FrediFizzx wrote:
> <Starbles@Earthlink.net> wrote in message
> news:1144736590.371979.167020@e56g2000cwe.googlegroups.com...
> | What are all the different methods being used to measure G? Why are
> the
> | results so different for more precise experiments?
>
> Hmm... You second sentence indicates that must already know of some of
> the methods. ;-)
>

Yes. I've heard three theories for this, taking into account that G is
pretty weak, of course: 1. The instruments aren't exact. In which case
we need a 'dummy' experiment (forgot the more formal term) to calculate
the inexactness and factor it in as well as to repeat the experiment
multiple times and take the average. 2. The gravitational "constant" is
not actually constant. In which case we may need to conduct the same
exact test at several different locations. 3. The gravitational field
interferes with the experiment. The solution for this is to do it
further away from Earth. It could also be a combination of all three of
these problems.

I don't know exactly how feasible any of these theories are. 1 sounds
like sloppiness to me. 2. Really hasn't been proven. 3. Seems like the
most reasonable, unless of course there are experiments done in orbit
around Earth with the same problem. There are other theories, of
course, due to different models of gravity, but I won't go into detail.
Mostly because I don't know their explanation for the immeasurability
of G.

> F = G*m1*m2/r^2 so,
>
> G = F/(m1*m2/r^2)
>
> http://en.wikipedia.org/wiki/Gravitational_constant
>
> Check out the references listed in that article for more info. But
> mainly I would think it is hard to measure G precisely just because
> gravity is very weak compared to other "forces". We can make a
> gravitational coupling "constant" by,
>
> alpha_G (m1,m2) = G*m1*m2/(hbar*c) in cgs units,
>
> [(m1,m2) means "as a function of" mass 1 and mass 2.]
>
> and plug in values of elementary particle masses to see how weak gravity
> is compared to say the EM force. It is something like 39 or 40 orders
> of magnitude weaker.
>
> FrediFizzx
> http://www.vacuum-physics.com


Isn't it true the light bends twice as much as matter stationary WRT
the attractive mass? If so, then equations aren't exactly correct in
all instances, and only work for masses which are stationary WRT us and
each other.

(...Starblade Riven Darksquall...)

dvanbaak@calvin.edu
#7
Apr13-06, 04:00 AM
P: n/a

Uncle Al wrote:
> Starbles@Earthlink.net wrote:
> >
> > What are all the different methods being used to measure G? Why are the
> > results so different for more precise experiments?

>

.. . . The Adelberger group's quadrupole pendulum is then a
> brilliant innovation for measuring Big G because its surroundings
> cancel out.
>
> They would get a better S/N if they replaced their planar fused silica
> bob (2.20 g/cm^3) with a much denser homogeneous transparent body
> (ease of testing during fabrication) carved from a single crystal,
> e.g., lead tungstate (8.28 g/cm^3; though it is a bit soft).
>

With that last point I have to disagree; though raising the mass of the
'bob' would raise the gravitational force involved, it would also raise
its inertia (and by the same factor), and so the acceleration of the
bob would not change. And since the acceleration is what is in fact
measured, alas it doesn't help.

Now of course *some* density is required, to exceed the density of the
ambient air by a comfortable margin, and to load the fiber, and to form
a surface sensed for rotation -- but merely raising the density
wouldn't help here.

In a parallel example, you'll find that deducing G from a Cavendish
experiment doesn't depend on knowing the mass of the spheres on the
torsion pendulum, but merely on knowing that they dominate the
rotational intertia of the pendulum. So it follows that changing lead
spheres to lead tungstate or indeed to silicon would not make any
sizeable difference in the signal.

--D. Van Baak

robert bristow-johnson
#8
Apr13-06, 04:00 AM
P: n/a
in article 1144855095.294728.19850@v46g2000cwv.googlegroups.com,
Starbles@Earthlink.net at Starbles@Earthlink.net wrote on 04/12/2006 17:04:

> robert bristow-johnson wrote:
>>
>> if we knew precisely the Earth's mass, M, we'd have a good value for G,
>> but we only have a really precise and consistent value for the product
>> G*M. so we have to measure it with stainless steel balls of known
>> mass, but compared to the Earth, they're so damn small and their
>> gravitational effect on other objects is almost undetectable.
>>

> What about using an inferometer? Is that better or worse?


for what use? to detect minute movement of the little pendulum in the big G
setup? i imagine that they have some sensitive detectors of motion in that
experiment. otherwise i don't understand what you mean.

--

r b-j rbj@audioimagination.com

"Imagination is more important than knowledge."

Starbles@Earthlink.net
#9
Apr13-06, 04:00 AM
P: n/a

dvanbaak@calvin.edu wrote:
> Uncle Al wrote:
> > Starbles@Earthlink.net wrote:
> > >
> > > What are all the different methods being used to measure G? Why are the
> > > results so different for more precise experiments?

> >

> . . . The Adelberger group's quadrupole pendulum is then a
> > brilliant innovation for measuring Big G because its surroundings
> > cancel out.
> >
> > They would get a better S/N if they replaced their planar fused silica
> > bob (2.20 g/cm^3) with a much denser homogeneous transparent body
> > (ease of testing during fabrication) carved from a single crystal,
> > e.g., lead tungstate (8.28 g/cm^3; though it is a bit soft).
> >

> With that last point I have to disagree; though raising the mass of the
> 'bob' would raise the gravitational force involved, it would also raise
> its inertia (and by the same factor), and so the acceleration of the
> bob would not change. And since the acceleration is what is in fact
> measured, alas it doesn't help.
>
> Now of course *some* density is required, to exceed the density of the
> ambient air by a comfortable margin, and to load the fiber, and to form
> a surface sensed for rotation -- but merely raising the density
> wouldn't help here.
>
> In a parallel example, you'll find that deducing G from a Cavendish
> experiment doesn't depend on knowing the mass of the spheres on the
> torsion pendulum, but merely on knowing that they dominate the
> rotational intertia of the pendulum. So it follows that changing lead
> spheres to lead tungstate or indeed to silicon would not make any
> sizeable difference in the signal.
>
> --D. Van Baak


Are there any sites on this? If so, I'd like to see them. Is it
possible to create a network of light beams that has the same effect,
or are inferometers virtually useless in places where the gravitational
field varies?

(...Starblade Riven Darskquall...)

Oz
#10
Apr14-06, 04:00 AM
P: n/a
Uncle Al <UncleAl0@hate.spam.net> writes
>
>They would get a better S/N if they replaced their planar fused silica
>bob (2.20 g/cm^3) with a much denser homogeneous transparent body
>(ease of testing during fabrication) carved from a single crystal,
>e.g., lead tungstate (8.28 g/cm^3; though it is a bit soft).


Dear uncl'al

what was the result of the chinese experiment?

--
Oz
This post is worth absolutely nothing and is probably fallacious.



Uncle Al
#11
Apr16-06, 04:43 AM
P: n/a
dvanbaak@calvin.edu wrote:
>
> Uncle Al wrote:
> > Starbles@Earthlink.net wrote:
> > >
> > > What are all the different methods being used to measure G? Why are the
> > > results so different for more precise experiments?

> >

> . . . The Adelberger group's quadrupole pendulum is then a
> > brilliant innovation for measuring Big G because its surroundings
> > cancel out.
> >
> > They would get a better S/N if they replaced their planar fused silica
> > bob (2.20 g/cm^3) with a much denser homogeneous transparent body
> > (ease of testing during fabrication) carved from a single crystal,
> > e.g., lead tungstate (8.28 g/cm^3; though it is a bit soft).
> >

> With that last point I have to disagree; though raising the mass of the
> 'bob' would raise the gravitational force involved, it would also raise
> its inertia (and by the same factor), and so the acceleration of the
> bob would not change. And since the acceleration is what is in fact
> measured, alas it doesn't help.
>
> Now of course *some* density is required, to exceed the density of the
> ambient air by a comfortable margin, and to load the fiber, and to form
> a surface sensed for rotation -- but merely raising the density
> wouldn't help here.
>
> In a parallel example, you'll find that deducing G from a Cavendish
> experiment doesn't depend on knowing the mass of the spheres on the
> torsion pendulum, but merely on knowing that they dominate the
> rotational intertia of the pendulum. So it follows that changing lead
> spheres to lead tungstate or indeed to silicon would not make any
> sizeable difference in the signal.
>
> --D. Van Baak


Interesting and well-founded point. Material homogeneity and
perfection of fabrication are then paramount. A stiffer, harder, and
more perfect substrate might then trim a decimal, perhaps suggesting
single crystal fabrications of silica (z-cut), silicon, or alumina as
the bob rather than fused silica. They are all large volume
commercial products available in the necessary dimensions.

Either way, the determined value of Big G is the least accurate by far
of any measured physical constant. There is little optimism for
obtaining a substantially improved value in the forseeable future.
Mass is the only observable that retains a physical artifact as
primary calibrant. One might argue that both mass and gravitation are
not fundamental at all. One then only lacks a proposal for what is
fundamental.

--
Uncle Al
http://www.mazepath.com/uncleal/
(Toxic URL! Unsafe for children and most mammals)
http://www.mazepath.com/uncleal/qz3.pdf



Register to reply

Related Discussions
Numerical methods: Finite difference and spectral methods? Differential Equations 1
UN - methods Current Events 0
Methods of measuring concentrations of lead Biology, Chemistry & Other Homework 5
Methods of measuring G. General Physics 10
Experimental methods for measuring the size of nuclei High Energy, Nuclear, Particle Physics 1