In article <4aa861fb.0308011045.5cb5df4@posting.google.com>,
Starblade Darksquall <Starblade13@Yahoo.com> wrote:
What is the most accurate value for the gravitational constant? And
what are the current means of measuring it? Have they ever gotten to
or past six significant digits? And exactly how much attention is
being devoted to it?
I would like to know these things. I hope to one day be able to
determine it to perhaps as much as eight significant digits, perhaps
using a device of my own making. I want to make sure that my method is
unique, for one thing.
http://physics.nist.gov/cuu/Constants/
G = (6.673 +- 0.010)e-11 m^3 / kg s^2
If you can add some more sig-figs to that, I think you can't help but
have a unique method. And even if it's just a variation on what has
been done before, adding a sig-fig or two is worth doing anyway.
The usual method is the Cavendish balance. I can think of a few
improvements on the usual freshman demonstration, like throwing in
interferometers rather than projecting a laser beam onto a peice of paper
taped to the wall, but I don't know what the state of the art is.
You need to know the geometry very well, you need to know masses and mass
distributions very well. That requires materials and machining of the
most expensive sort, because even a solid ball of lead will have probably
non-uniform gas voids and impurities within it. The point-mass formula
only works for point masses or spherically symmetric masses, other mass
distributions will have multipole moments. Send the parts to a
metrology laboratory for characterization. Once the apparatus is built
and working, plan to spend a very long time on calibration and
systematics, the amount of time spent taking actual data is somewhere in
the noise. Try to generate a realistic error budget before spending much
money on it. E.g. mass of material X can be determined to xx%,
homogeneity to yy%, dimensions to zz%... giving overall uncertainty of
such-and-such.
"There's no such thing as an easy measurement." -- Jeff Nico
--
"A good plan executed right now is far better than a perfect plan
executed next week."
-Gen. George S. Patton
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