[time-nuts] Those pesky Neutrinos again...

Magnus Danielson magnus at rubidium.dyndns.org
Mon Nov 21 15:30:14 EST 2011


On 11/21/2011 09:49 AM, Hal Murray wrote:
>
> michael.cook at sfr.fr said:
>> I heard on the BBC the other day that a repeat experiment is planned,
>> firing neurinos from the US into Canada. The labs were not cited, but I
>> expect it would be Fermilab  to Sudbury Ontario. If this is the case,  then
>> there will still be the problem of not being able to run a parallel  fibre
>> as the Sudbury detectors are  also deep underground. ...
>
> I don't think that running a parallel fiber will help much.  I see two
> problems.
>
> First, fibers don't run in straight lines.  Even if you could accurately
> survey the pipes in the ground, there is slack in the fiber within the cable
> bundle so it doesn't break if somebody pulls on the cable.
>
> Second, the speed of light in fibers isn't known to the required level of
> accuracy.  (It's probably temperature dependent.)

It is. About 10% of the change is due to length-changes in the fibre and 
about 90% of the change is due to group-delay shifts in the fibre.

If you laser frequency (wavelength is the traditional value here) shifts 
then the dispersion effect also causes shift in delay.

> In the context of fibers, having the detectors located deep underground in
> not a problem.  They have to get power and data cables down to the
> instruments somehow.  It would be easy to run a fiber in parallel with those
> cables.
>
> There is also the delay in the amplifiers that you will need every 100 miles
> or so.  Or the low rise time if you try to avoid the amplifiers because you
> don't need much bandwidth...

Depends on the amplifiers you are using. EDFA amps has fairly high 
bandwidth and delay is like normal fibre of the same length.

Cheers,
Magnus




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