[rcxdude]

Such 'objective reference time' doesn't really exist at the precisions we are talking about (both in terms of the precision to which you can define them and effects like relativity). And the difference between 'objective time' and 'keeping time with another copy of yourself' is basically just a scaling factor, which is irrelevant a lot of the time, when the far more relevant parameters for high-precision clocks in actual applications are stability, noise, and bandwidth.

[thaumasiotes]

> And the difference between 'objective time' and 'keeping time with another copy of yourself' is basically just a scaling factor, which is irrelevant a lot of the time

This assumes that your own divergence from objective time is linear in the amount of time that passes.

[puzzledobserver]

See for example the experiment conducted by Tom Van Baak about 15 years ago: http://leapsecond.com/great2005/.

He drove three Cesium clocks up Mount Rainier and returned after a week. He compared them to a clock he left at home for the journey. The graph that he shows on the page and his associated commentary is interesting.

Ultimately, a clock is simply an abstract device that goes tick-tick-tick at some regular rate. Once one starts measuring the phenomenon itself---mechanics of the human heart, tidal forces on the Earth, friction in a pendulum, or the uncertainty principle in atoms---it is no longer feasible to treat it as objective time.

International Atomic Time (TAI) itself consists of an ensemble of 400 atomic clocks, with the collective being more accurate (the proper word is "stable", I think?) than any of its constituents.