The `Monster' blessed by the 5 sigmas

Rumors that the LIGO interferometers had most likely detected a gravitation wave (GW) were circulating in autumn last year. Personally, the direct information I got quite late, at the beginning of December, was ``we have seen a Monster'', without further detail. Therefore, when a few days before February 11 quantitative rumors talked of 5.1 sigmas, I was disappointed and highly puzzled. How could a Monster have only just a bit more than five sigmas? Indeed in the past decades we have seen in Particle Physics several effects of similar statistical significance coming and going, as Alvaro de Rujula depicted already in 1985 in his famous Cemetery of Physics of Fig.2[48].15
Figure: Alvaro de Rujula's Cemetery of Physics[48], with graves indicating `false alarms' in frontier physics, and not old physics ideas faded out with time, like epicycles, phlogiston or aether.
Therefore for many of us a five-sigma effect would have been something worth discussions or perhaps further investigations but certainly not a Monster.16This impression was very evident from the reaction many people had after seeing the wave form. ``Came on, this is not a five-sigma effect'', commented several colleagues, more or less using the same words, ``these are hundreds of sigmas!'', a colored expression to say that just by eye the hypothesis Noise was beyond any imagination.17

The reason of the `monstrosity' of GW150914 was indeed in Table 1 of the accompanying paper on Properties of the binary black hole merger GW150914[28]: a Bayes factor ``BBH merger'' Vs ``Noise''18of about $5\times 10^{125}$ (yes, five times ten to one-hundred-twenty-five). This means that, no matter how small the odds in favor of a BBH merger were and even casting doubt on the evaluation of the Bayes factor,19 the posterior odds would be extraordinary large, the probability of noise being smaller than Shakespeare's drop of water identically recovered from the sea.20

Giulio D'Agostini 2016-09-06