[Boatanchors] SI units and the rest

[J. Forster]

The second is arbitrary also.

When I was working on SETI, the question came up about what frequency to
listen on. There are an infinite number of possibilities.

One sensible answer is the 1421 MHz Hydrogen line, as that is an atomic
transition and should be the same all over the universe. The second
depends on the rotation rate of the earth, which has been changing for
millions of years and will continue to change. (Hence "lea seconds")

-John

=================


> On Tue, Nov 16, 2010 at 07:21:30PM -0500, WA5CAB at cs.com wrote:
>> I'm curious as to why it isn't 1/(3*10^8).
>>
>> In a message dated 11/16/2010 11:39:45 AM Central Standard Time,
>> w7qho at aol.com writes:
>> > The original definition of a METER  was based on a measured standard
>> > bar.  The standard was redefined in 1983 as the distance traveled by
>> > light in a vacuum during a time interval of 1/299,792,458 of a
>> > second,   The standard kilogram is a a cylindrically shaped platinum-
>> > iridium slug and is the only SI unit still defined by an artifact
>> > rather than a fundamental physical property that can be reproduced in
>> > different laboratories.
>> >
>> > Dennis D. W7QHO
>> > Glendale, CA
>
> The second was deemed to be a more fundamental unit, since it could be
> defined directly from a quantum transition, and so the meter became a
> derivative unit defined in terms of the second.
>
> --
> Mike Andrews, W5EGO
> mikea at mikea.ath.cx
> Tired old sysadmin
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