Re: [Phys-l] New gravitational constant

[Todd Pedlar <pedlto01@luther.edu>]

Savinainen Antti wrote:
> Hi,
>
> have you noticed that the value of the gravitational constant has changed? The
> new value given by CODATA is:
>
> (6.67428 +/- 0.00067) * 10^(-11) m^3kg^(-1)s^(-2)
> <http://physics.nist.gov/cgi-bin/cuu/Value?bg|search_for=gravitational+constant>
>
> whereas the older accepted value was:
>
> (6.67259 +/- 0.001) * 10^(-11) m^3kg^(-1)s^(-2)
>
> I knew that this constant is very hard to measure accurately but it surprised
> me that the discrepance between the new and old value is so great that they do
> not agree within their uncertainty limits.
>
> Regards,
>
> Antti Savinainen
> Kuopion Lyseon lukio
> Finland
>
>
>   
Actually there's an interesting exercise here.  If this is
the way CODATA reported their previous value
(5 places after the decimal with an uncertainty in the
third place after the decimal) then they've not learned
basic rules of reporting uncertainties that I drum into my
students! :)

Second, though, you should note that the new value only
differs from the old by about 1.7 times the previous uncertainty.
(the previous sigma could in fact be as much as 0.0015 but is
rounded to .001... in that case, the discrepancy would
only be a tiny bit more than 1 sigma!)

This isn't unexpected at all.... if it were 4 or 5 sigma, then we'd
have something to talk about - but shifts of 1 to 2 sigma happen
frequently (and do not mean the uncertainty previously given
was wrong). 

Todd

--
________________________________________________________________________________
Todd K. Pedlar
Assistant Professor of Physics
Luther College
pedlto01@luther.edu
_________________________________________________________________________________