# How 'Faster Than Light' Neutrino Was Product of Loose Cable at CERN?

A "faster than light" neutrino discovery was actually the result of a loose cable. A fiber-optic cable in a GPS receiver at the European Center for Particle Physics ( CERN) near Geneva

Can someone show me the light here, What's Going On ?

how long was fiber-optic cable?

-
(-; The particles were so afraid of the loose cable that they ran away with a speed exceeding what the relativistic police allowed. When they finally were caught, the speed turned out to be slower than light, according to the rules ;-) – Arnold Neumaier Sep 22 '12 at 11:44

It's answered e.g. on page 4 of this talk:

http://agenda.infn.it/getFile.py/access?resId=2&materialId=slides&confId=4896

An analog circuit in the experiment is activated when the light intensity in the fiber optic cable exceeds a certain threshold. Because the connection was loose, the time measured several years ago, let's say in 2008, that did not include the neutrino propagation was measured about 73 nanoseconds longer than it should have been. That's because the circuit was activated later than it should have been – it was waiting when the erroneously weakened light intensity in the cable exceeds the same required threshold.

When the time including the neutrino propagation was measured with a more properly attached fiber optic cable e.g. in 2012, they got the right timing. The time needed for the neutrino to propagate is the difference $$t_{\rm travel} = t_{2012} - t_{2008}$$ Because the time measured in 2008 appears with a minus sign and it was spuriously increased by the loose connection, the time of neutrino travel appeared spuriously sped up by 73 nanoseconds. To "predict" 73 nanoseconds, one would have to describe the time dependence of the light intensity going through the fiber optic cable – the intensity is surely changing for dozens of nanoseconds – and he would have to know the exact weakening of the signal due to the loose connection.

Before the correction, the neutrinos appeared to arrive $60\pm 10$ nanoseconds too early. When the fix is applied, they appear $13\pm 10$ nanoseconds later than light which is right, within the error margin.

The collaboration also fixed another error but its effect on the result was negligible.

-
what was speed of neutrino by the way!@ – Neo Sep 22 '12 at 11:50
The speed of the neutrino was of course equal to the speed of light, 299,792,458 m/s, up to the relative error of $10^{-20}$ or so caused by the neutrino masses. In reality, only 5 or so significant figures of the neutrinos' speed may be measured. – Luboš Motl Sep 22 '12 at 12:47