Rob Seaman wrote: >An interesting discussion of a difficult measurement: > > http://arxiv.org/pdf/1109.6160
I've found a description of the Time-Transfer Device that is the subject of that paper. The original OPERA paper doesn't actually say that a TTD was used for synchronisation, it says that the synchronisation was "independently verified", by PTB, using a TTD. <http://operaweb.lngs.infn.it/Opera/publicnotes/note134.pdf> is a note from PTB describing their part in the affair, and it's a very limited part. Their job was only to compare corresponding parts of the timing gear at the two labs, checking the delay between GPS signals and the resulting PPS signal at the input to a timestamping unit. PTB's note explains why they used a portable device, and it's got nothing to do with actual time transfer. There's a problem that units of the same species have individual variations, so measurements made with different units are not directly comparable at the finest precision. So they transfer one unit between the labs in order to perform corresponding measurements with the same unit, so that the unknown biases in the unit cancel themselves out. The unit does not maintain its own time scale, so the path it took between the labs is irrelevant. Contaldi's assumption that "this device [is] a transportable atomic clock" turns out to be wide of the mark. (He redeems himself by the footnote deploring the lack of sources on this point.) The synchronisation may still have gone awry in other areas not covered by PTB's work, which are not described by anything I've seen yet. There's also a clear error in that the OPERA paper treats time as Newtonian: the two labs are synched to GPS time, hence to TAI, and there's no discussion of the difference between this time scale (SI seconds on the geoid) and time along the neutrino path (varying between 1 km above and 30 km below the geoid). <http://operaweb.lngs.infn.it/Opera/publicnotes/note132.pdf> discusses the geodesy in reasonable detail. Down at the bottom of it, the endpoints are ascribed coordinates in ETRF2000, and Pythagoras's theorem is used to determine the path length. At the precision stated, gravitational length contraction must make this calculation invalid. Do the gravitational time dilation and the gravitational length contraction cancel each other out, when viewed from a suitable reference frame? I'm out of my depth here. -zefram _______________________________________________ LEAPSECS mailing list [email protected] http://six.pairlist.net/mailman/listinfo/leapsecs
