[time-nuts] Neutrino timing

Tom Van Baak tvb at LeapSecond.com
Mon Oct 24 20:02:30 UTC 2011


> I have a more basic time-nut question.  Why is it a problem at all?
> How can the time uncertainty between two known and fixed locations be that 
> large?

In principal it's simple. But the logistics of equipment and cables
and rooms and labs is quite complicated. Read a few documents
at http://www.ohwr.org/projects/cngs-time-transfer/wiki and you'll
get a sense. All the numbers should add up. But it didn't.

The stakes are really high so everyone is double checking their
piece of the pie more carefully than a moon landing.

> If they know they have a 70ns uncertainty in time, that would suggest that 
> their time measurement is known to be varying at one or both places.
> Is this just from a spec or do they see a true variation in time between 
> something, and if so compared to what?

It's not varying. It was a 3 year experiment and there appears to
be a fixed discrepancy of 60 ns in time or 18 meters in distance.
Remember it's not certain that the error is in the timing at all; it
could just as well be in the distance. Or in the start trigger, or the
stop signal, etc.

> Is this time difference or variation between several difference timing 
> devices at each end or is it variation when compared to time of flight of 
> the supposedly same neutrinos?

The latter.

> I can not say anything about the accuracy of my absolute time, but the 
> difference and uncertainly comparing the phase difference between different 
> external  Osc Tbolts at the same location is way way under 70ns.

Yeah, this is true for most GPS receivers, which is why it's hard
to imagine the problem has anything to do with their GPS timing
set up. Their error budget is a couple of ns, stable over years,
which is why they use atomic clocks along with dual-frequency
carrier-phase receivers in common view, and calibration visits by
more than one national UTC lab, etc. It would appear they really
did their homework.

It's unfortunate that GPS even got brought into the whole neutrino
situation because GPS brings with it all sort of UTC and physics
and relativity baggage. The experiment could have done relative
timing without using GPS at all. It's not about GPS; it's about having
synchronized clocks at two locations. There are many ways to
achieve that. And when the stakes are high, then one must do it
in multiple independent ways.

So that's why there's talk of direct fiber links, radio links, satellite
links (non-GPS), traveling clocks, etc.

> Sure lots of BASIC things to do to make sure the two Tbolts are set the same 
> so that their oscillator's phase do they agree, such as using the same type 
> antenna and same cable and length, and getting the antenna's location 
> correct, etc, etc,
> but basic stuff and seems like if using the same basic GPS system at two 
> different locations, what would the additional problems be except to make 
> sure both ends are syncing on the same 100ns 10MHz cycle.
> 
> I was under the impression that getting down to ns uncertainly differences 
> (and staying there) at theses distances is old stuff using common view GPS.
> So what are the problems that cause their large timing uncertainty?

Yes, it is very old hat to those in the timing community. It just takes
time for the rest of the physics community to catch up. Many of us
amateurs have better timing at home than most physics laboratories.

/tvb





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