On 7/3/2012 4:15 PM, Tony Finch wrote:
Vadim Antonov <avg@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.