On 7/3/2012 4:15 PM, Tony Finch wrote:
Vadim Antonov <a...@kotovnik.com> wrote:
But in theory, if you can get the technical wrinkles worked out, you can
derive the same frequency standard in your lab with a single instrument.
(One more issue is that non-relativistic time is not only the frequency of
oscillators, but also a reference point).
Your parenthetical point explains why TAI does not tick at the same rate
as the SI second in your lab, expecially if your lab is (for example) in
Colorado. You have to adjust the frequency depending on your difference in
gravitational potential from the geoid.
Tony.
I'm afraid I didn't express my thoughts clearly... I means besides
agreement of what a second is there is also an agreement on when the
zeroeth second was, a fixed reference point in time. *That* cannot be
recreated in a lab. (You can correct for relativistic effects of local
gravity and moving frame of reference, though, to match conditions on
the Earth and thus the SI definition of second).
However, the whole concept of universal standard of _time_ (as opposed
to standard of second) is thoroughly non-relativistic because it claims
to have clocks at different locations ticking simultaneously. The
special relativity, of course, makes it clear than simultaniety is in
the eye of the observer:) In the end, you can only do limited
Einstein-Poincare synchronization within a chosen reference frame.
An interesting factoid: the notion of synchronized time differs if you
synchronize clocks from East-to-West and from West-to-East, due to
Sagnac effect:)
--vadim
PS. I would vote for using TAI instead of UTC as the non-relativistic
time base in computer systems. The idea of expressing UTC as a single
number (instead of <minute, second within minute> tuple) is silly
because it creates aliases or gaps. You cannot do simple interval
arithmetic over UTC, no more than you can do that over local daylight
savings time; and doing accurate time computation for events in the
future is impossible in both because they depend on unpredictable
factors (Earth rotation rate, politics, etc).
TAI is also not a fixed given, because the standards are being refined,
but at least the refinements tend to be predictably in the direction of
improved accuracy, so they don't break things.
