I'm not a particle physicist, but I do know quite a bit more about the actual numbers to start a calculation from than you do, because I bothered finding them out
So far the only number introduced here has been Rees' "one in fifty million". You've consistently avoided giving a number, using only the "but there's still a chance" thing, which in my interpretation you're using diametrically against its intended meaning (intended meaning is that you can't just use a binary "there is a chance" versus "there's not a chance", you actually have to worry about what the chance is). The only thing you've said that suggests any level of familiarity with the subject is mentioning the cosmic ray collisions, which were all over the newspapers during the necessary time period, and most of your comments make me think you're not familiar with the various arguments that have been put forward that the LHC collisions are in fact different from cosmic ray collisions.
But I don't actually think that matters. Assuming the prior for the LHC destroying the world before your cosmic-ray arguments and whatever other arguments you want to offer is non-negligible, you're saying you're certain to within one in (your probability of LHC destroying world/prior) that all your arguments are correct. Since you seem willing to give arbitrarily low probabilities, I'm sure we could fiddle with the numbers so that you're saying you think there's less than a one in a million chance there's any flaw in your argument, or that you're applying your argument wrong, or that you missed some good reason why LHC collisions don't have to be different from cosmic ray collisions, or that the energy of cosmic ray collisions has been consistently overestimated relative to the energy of LHC collisions, or that you're just having a really bad day and your brain is tired and you don't realize that the argument doesn't prove what you think it proves. I believe you're very smart, and I realize the prior is low, and I realize the arguments against the LHC destroying the world are very good, but predicting a novel situation that some smart people disagree upon in a field you don't understand to a level greater than one in a billion is just a bad idea.
The "but there's still a chance" principle only means that you shouldn't act as if you can keep on believing your argument even when the chance grows ridiculously low. It doesn't mean that you should never keep a tiny portion of probability mass on "or maybe I'm missing something" to compensate for unknown unknowns.
This discussion is not getting anywhere, so I will let you have the last word and then bow out unless you want to continue by private message.
Voted up for excellent points all around.
I have in fact completely given up on giving probability estimates more extreme than +/-40 decibels, or 50-60 in some extreme (and borderline trivial) cases. I haven't actually adjusted planning to compensate for the possible loss of fundamental assumptions, though, so I may be doing it wrong... On the gripping hand, however, most of the probability mass in the remaining options tends to be impossible to plan for anyway.
For background, see here.
In a comment on the original Pascal's mugging post, Nick Tarleton writes:
Coming across this again recently, it occurred to me that there might be a way to generalize Vassar's suggestion in such a way as to deal with Tarleton's more abstract formulation of the problem. I'm curious about the extent to which folks have thought about this. (Looking further through the comments on the original post, I found essentially the same idea in a comment by g, but it wasn't discussed further.)
The idea is that the Kolmogorov complexity of "3^^^^3 units of disutility" should be much higher than the Kolmogorov complexity of the number 3^^^^3. That is, the utility function should grow only according to the complexity of the scenario being evaluated, and not (say) linearly in the number of people involved. Furthermore, the domain of the utility function should consist of low-level descriptions of the state of the world, which won't refer directly to words uttered by muggers, in such a way that a mere discussion of "3^^^^3 units of disutility" by a mugger will not typically be (anywhere near) enough evidence to promote an actual "3^^^^3-disutilon" hypothesis to attention.
This seems to imply that the intuition responsible for the problem is a kind of fake simplicity, ignoring the complexity of value (negative value in this case). A confusion of levels also appears implicated (talking about utility does not itself significantly affect utility; you don't suddenly make 3^^^^3-disutilon scenarios probable by talking about "3^^^^3 disutilons").
What do folks think of this? Any obvious problems?