On Tue, 4 Oct 2011, Zefram wrote:
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).
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 :
http://adsabs.harvard.edu/full/1994A&A...286..971P
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
considered sufficient.
--
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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