Re: How would we know?
That's some fine Anthropic sleight of hand there by Tegmark - what he's basically saying is that there's no need to worry about the world getting destroyed because if you can worry about it then it hasn't happened.
But really, my gripe over the CERN safety calculations is that in any risk analysis you are supposed to multiply the likelihood of an event by the potential damage such an event can cause. This is why we still drive cars, or even put the kettle on. So even if the probably of a catastrophic event is only 1 in a billion (a figure far lower than those proposed by various reputable scientists) but that event could cause 7 billion deaths, when you multiply them together that still gives you 7 deaths. Anybody found to be knowingly embarking on a course of action that would lead to 7 deaths would be arrested and charged with attempted murder.