[time-nuts] Neutrino timing

Magnus Danielson magnus at rubidium.dyndns.org
Mon Oct 24 20:45:04 UTC 2011


Hi!

On 10/24/2011 10:02 PM, Tom Van Baak wrote:
>> 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.

I think I have essentially all the equipment needed for both sides. 
Double frequency receivers with antennas, counters, cesiums etc. Lacking 
5071As but...

What I don't have is the time to build two complete sets with software 
etc. Not that I am not trying, as being the time-nut that I am.

I think a few other time-nuts such as Tom would also be able to provide 
the equipment. Again the time to build a pair of functional sets.

>> 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.

Just re-validating the distance, time and time-distribution would be fun 
and time-consuming.

>> 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.

This is also why the preliminary PTB report is a bit unfortunate, as it 
leaves bits and pieces out which the professional should need, but also 
the larger picture of bias compensations intended for the 
non-professional on GPS based time-transfer systems. There are many 
enhancements to be done to be able to present to physics people. The 
necessary backlog of articles to read is quite high.

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

Indeed. PTB should have used a traveling clock alongside their GPS 
receiver system.

>> 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.

Indeed. It's strange that a few private time-nuts have the capability to 
do this, but it is fully possible. There is a few things like recording 
RINEX data, get it post-processed alongside some other measurement stuff.

So, to come back to the original question, I agree with TvB that the way 
to go would be to get the assistance from NIST. I think we are a few 
time-nuts that would be "happy to assist" as a fun treat. :)

However, I think that one should combine traveling clock (traveling both 
directions) with double frequency GPS receivers exercise. It would 
assist in providing consistency.

Cheers,
Magnus



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