Re: redefining the standard kilogram

Paul Danaher wrote:
There was a pretty good (actually I though _excellent_ for a general
newspaper!) article about this in the Los Angeles Times last week:

http://www.latimes.com/news/nationworld/nation/la-sci-kilogram17apr17,0,7998161,print.story

physorg.com also had a pretty good short piece in 2005:
http://www.physorg.com/printnews.php?newsid=3178

Uh-oh - to quote:
"Electrical power can be related to the Planck constant, defined as the
ratio between the frequency of an electromagnetic particle such as a
photon of light and its energy. This experimental method of defining the
kilogram relies on selecting a fixed value for the Planck constant,
which is currently determined experimentally based on the fixed value of
the kilogram artifact."

The circularity is disturbing.

There are no logical circularity problems here:

The current definition of the mass and some other physical units produce
a value of the Planck constant. But the process can be reversed: fixing
the Planck constant and the other constants in order to define the mass
unit.

The problem is, using the current physical units, to get a value of the
Planck constant sufficiently exact that its fixing does not alter the
current mass unit too much. And there must be a measurement method that
can define the new mass unit at least as well as the current mass unit.

It seems me to be an interesting method of defining mass, as the Planck
constant is so fundamental in modern physics. But the main thing, for
now, is to find a measurement method that can fix the mass unit at least
as good as the current one, without reference to an artifact, but by a
generally accessible and invariant reproducible physical phenomenon.

Hans Aberg

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