[LEAPSECS] preprint about timekeeping for neutrino experiment
fmeyer at obs-besancon.fr
Tue Oct 4 02:55:27 EDT 2011
On Tue, 4 Oct 2011, Zefram wrote:
> Rob Seaman wrote:
>> An interesting discussion of a difficult measurement:
> 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).
In my view the two clocks are just synced to each other as far as this experiment
is concerned. That does not change the point about gravitation though.
> <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.
This you cant say unless you estimate the order of magnitude of
gravitational effects. For example :
gives an exhaustive review of what terms should be accounted for.
Errors excepted, I found that the only term exceeding 1 ns is due to
sagnac effet and it is about 2 ns. The other terms are below 1 ns.
If those figures are correct then the newtonian approach could be
François Meyer Tel : (+33) 3 81 66 69 27 Fax : 3 81 66 69 44
Observatoire de Besancon - BP1615 - 25010 Besancon cedex - FRANCE
Institut UTINAM * Universite de Franche-Comte * CNRS UMR 6213 ***
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