I was brought to ponder on the matter yesterday, as I received the shocking news of the death of Gianfranco, my cousin Maria Cristina's husband. He was hiking on an easy mountain trail together with a couple of friends and their kids. They had stopped for a short break at the intersection of two trails; it was a sunny and warm summer day in the italian Alps, and nothing around appeared even remotely dangerous, leave alone life-threatening.
And yet, suddenly a big rock detached from the mountain above, acquired speed, and hit Gianfranco's head. His body was thrust off the trail and fell 100 feet downhill, where it was collected shortly thereafter by an airborne rescue team, by then already lifeless. Besides my cousin, Gianfranco leaves behind three lovely daughters, and a successful life as a business manager. It is tough to make sense of such a tragedy. Religious people may find comfort in their trust of a grander scheme of things; atheists like me, on the other hand, are left struggling for an explanation.
Of course, there is a very clear cause-effect relation to describe the incident. Its dynamics are clear; we can trace its occurrence to a precise trajectory of the stone, which intersected the victim's location; ultimately, the perpetrator is the force of gravity. But that is not the explanation we are somewhat unconsciously looking for: why Gianfranco, and not the friend's son nearby, who was missed by inches, or the others in the group? Why wasn't he nearly missed, too? Gianfranco was an avid mountain-goer. He loved the outdoors, and had climbed scores of mountains before yesterday's incident. Nothing extreme, and always safe. But mountaineering, it is well known, is an activity that retains some intrinsic danger, no matter how cautious you may be. So was it more probable that he would be hit during this excursion, considering how much more time he had spent below overhanging rocks during his lifetime?
Alas, no. The truth of the matter is that, as rare as the incident was (I would estimate the probability of dying that way, during an afternoon hike on a similar trail, as one in several tens of millions) it was _possible_ - so it could happen, and it unfortunately did.
Throughout our lives we are constantly exposed to a multitude of imperceptibly small risks; we do not need to go hiking on a mountain for that. Every morning, as we take a shower we risk slipping and banging our head on the floor; as we wait for the bus we may be hit by a swerving vehicle; as we drink our morning coffee at the bar we may be stabbed to death by a psychopath; or get choked to death by a piece of croissant.
If we consider our exposure to all the possible life-threatening accidents we subject ourselves to during the span of our lifetime, the word "chance" acquires more meaning: we may then quite aptly label one particular event that way because it belongs to a large class of possibilities, and it was the only one in that large class which manifested itself. The other events in the class would be similarly rare (some more, some less), and would similarly affect us; but only that one did occur.
If we reason that way, something remarkable happens: we are freed from the knee-jerk reaction of comparing the event that affected us so hardly - in Gianfranco's case, being mortally hit by a stray rock - with millions of other possibilities that would not affect us in any way, like the fall of a rock one hour before we cross the spot, or one week later, or its unharming tumbling away from us. We only have to reckon with the fact that we are very frail living creatures, and that our life hangs daily by a thread; we do not record, or even notice, the near misses we breeze through every day, but they do happen. And rarely, they do hit.
In exchange, we need not torment ourselves anymore with empty questions on meaning, causes, and the like. Did an evil God punish Gianfranco for challenging the mountains? Of course not. Was a similar accident bound to happen to him sooner or later? By no means. The most you can extort from me, in terms of meaningless statements, is that he was quite unlucky on his last hike.
And there, my fellow particle hunters, is a lesson for us, too. Your five-sigma peak in that mass distribution you have been showing around should be considered in the context of the class that includes all the other five-sigma peaks that could have shown up on your screen, on all the countless other times you hit the "return" key after typing that "Mass_Distr->Draw();" command in your Root console. That will give you a better perspective of reality than comparing the peaky histogram to all the other possible appearances, more background-like, that same mass histogram could have had. It is a fluctuation, not a new particle signal. And an unlucky chance, not something to which you should attach a lot of meaning.
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