Does quantum mechanics prohibit extraterrestrial life? There is a known thought experiment, connected to quantum immortality: a duel between physicist and a philosopher. 
Each turn physicist and philosopher fire at each other with a pistol. The quantum immortality predicts that each of the participants will find themselves alive and the opponent dead after a number of shots.
We can modify the experiment in the following ways:


*

*They do not fire at each other, but it is lightning that shots them. The outcome should not change.

*There are not two but more participants.

*They are located at separate islands in the ocean.
It seems that even if there are hundreds of participants, each of them will eventually find all others killed by lightning. If he could not observe other isles directly, after he discovers and explores other isles, he will find them uninhabitable.
Now pretend that the participants actually live on separate planets, and in each million years there happens disaster that kills all inhabitants on 1% of all planets. It seems that after years the physicist that lives on one of the planets should discover that all other planets are uninhabitable. Even more: there is even no chance that organic matter on each other planet could actually evolve into anything resembling actual life.
That said the physicist will find out that there is no other inhabitable planet in the Uinverse. On the other hand, this does not mean that extraterrestrial life does not exist. It actually cannot not to exist. But it exists in parallel universes (in terms of MWI) and thus unobservable. Both physicist on planet A and philosopher on planet B exist, but will never meet in the same universe.
Thus, QI it seems predicts one concrete observable consequence that extraterrestrial life can be never detected.
What is your opinion?
 A: Dear Anixx, we know very well from the observations - and theory - here on Earth that repeated lightnings are nowhere close to be able to exterminate the life on the planet. Because it's true here and because the laws of physics - including the emergent ones - are at least approximately translationally invariant, it's almost certainly true on similar other planets in the Universe: lightnings are not enough to exterminate life. Your "conclusion" - actually an "assumption" - that the life on other planets has to go extinct because of lightnings is therefore invalid. It is invalid in the whole Universe and this paragraph is a proof that can't be "unproved" by any silly logical game.
It is very obvious that none of those thought experiments can actually determine the real odds - or our rational expectations about them - of having some extraterrestrial life. It's because the emergence of life (whatever its definition is) and its extinction on every celestial body is controlled by the objective laws of physics rather than subjective feelings of an observer who is likely to die soon. So all calculations of an individual that simply "erase" the possibility that he will die from the calculation of odds are subjective and can't be used by other individuals as an objective calculation. So they cannot be used to count the extraterrestrial civilizations.
For this reason, the "quantum immortality" itself is a logical fallacy. From an objective viewpoint, whether an observer dies during a period of time or not is just a well-defined question that can have both answers and eliminating possibilities where he dies is objectively unjustifiable. By the way, the philosophies and religions that believe in "life after death" obtain much more valid figures for the probabilities than Max Tegmark's philosophy that simply denies the possibility that observers die.
No "interpretation" of quantum mechanics or physics, as long as it is consistent with the standard physical predictions, can change anything about the paragraphs above.
By the way, if you find my comment that the fate of life is given by the laws of physics and not games in philosophy controversial, let me mention that this battle between physics and philosophy has been settled since 1687. A general in this physics-philosophy war, philosopher Isaac Newton, published his aptly named "Mathematical Principles of Natural Philosophy" and declared a surrender. Since that time, it was clear that all natural phenomena have to be studied by physics (that he helped to establish) and not by meaningless and logically flawed philosophical games you have presented.
Also, I would respectfully disagree that the answer above is "my opinion". It is the objective answer to your question. Physics is not about personal opinions; it is the scientific method to search (and often find) the objective truth. Also, this server is meant to offer the right physics answers to questions that can be answered as of 2011, rather than an assorted flow of chaotic personal opinions, and yours was surely one of the questions that can be answered today.
I don't claim that the extraterrestrial civilizations have to exist or have to be absent - but I surely do claim that there exists no rational argument based on "quantum immortality" that could influence the odds.
