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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:

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

  2. There are not two but more participants.

  3. 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?

Anixx
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    This question contains so many logical fallacies and misunderstanding of quantum theory that it's hard to know where to start with an answer. Closing it seems like the best option. – Marek Mar 28 '11 at 11:19
  • If you see any fallacies please point them. Judging from what you wrote here: http://physics.stackexchange.com/questions/3871/is-causality-a-formalised-concept-in-physics/5586#5586 it is you who do not understand QM. – Anixx Mar 28 '11 at 11:25
  • @Anixx: I am not going to argue with you. And I am puzzled why you want me to point out anything to you if you've already decided that I don't understand QM. – Marek Mar 28 '11 at 11:38
  • Here is one point to consider: if there are initially N inhabited planets and at a later time T k planets have been destroyed, then N-k inhabited planets still remain to be detected and will be for many millions of years. Additionally new planets get formed... – Roy Simpson Mar 28 '11 at 11:38
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    This is a silly mixture made from poorly understood quantum theory and some multiple worlds theory and some more. Vote to close! – Georg Mar 28 '11 at 11:44
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    Thanks for closing it. But I am afraid - it doesn't prevent the OP from flooding this question with lots of new stuff, does it? ;-) – Luboš Motl Mar 28 '11 at 12:05
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    I'll take a crack at pointing to one concrete thing wrong here: "quantum immortality". That is not a term or phrase in widespread use, and you have not defined it, but I will assume that it is some variant of the ultimate anthropic principle. This idea suffers from taking too seriously the idea that a conscience observer is needed to disambiguate quantum systems (decoherence provides a viable alternative without mystic overtones) and from the sophistry of "I am the only proven observer". – dmckee --- ex-moderator kitten Mar 28 '11 at 15:30
  • Dear dmckee, I think your description of "quantum immortality" as the "ultimate anthropic principle" is right and Max Tegmark of the immortal fame would even proudly agree with it. ;-) It's "ultimate" because the number of conscious observers is used not only to calculate the probabilities of events and worlds; it is used to deny the existence of worlds (and regions?) without observers or the possibility that they may disappear. – Luboš Motl Mar 28 '11 at 15:51
  • "it is used to deny the existence of worlds (and regions?) without observers" - existence or not existence of such regions is quite irrelevant for this question, because the question deals expressly with the observable world. – Anixx Mar 29 '11 at 00:18
  • @dmckee "decoherence provides a viable alternative without mystic overtones" - not exactly. Decoherence is a process of interaction between the quantum system and the outside world, i.e., the observer. Decoherence is just collapse, stretched out in time. – Anixx Mar 29 '11 at 00:24

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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.

Luboš Motl
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  • Oh please, lightning was used in an example with physicists and philosophers. When we are speaking about different planets, we of course bearing in mind some natural disasters or other obstaclesthat can kill life on a planet. For example, all water evaporation, overheating, loss of the atmosphere, space collision etc. – Anixx Mar 28 '11 at 11:39
  • "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." - indeed. Where I said it is determined by the observer's feelings? Are you familiar with the quantum immortality/suicide thought experiments? – Anixx Mar 28 '11 at 11:43
  • "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." - No. this conclusion is not based on calculations of odds. If you thought so, you misunderstood. The conclusion is universal and not based on just counting the "lightning events". – Anixx Mar 28 '11 at 11:45
  • "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." - Only in classical, i.e., not quantum-mechanical world. – Anixx Mar 28 '11 at 11:46
  • "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." - It you attack Tegmark's interpretation of QM, you should take into account that in Copenhagen interpretation the observer cannot die either (at least without killing the whole Universe along with his death). – Anixx Mar 28 '11 at 11:49
  • Dear Anixx, it's irrelevant whether you used lightnings. The existence of the Earth for 4.7 billion years - with billions of years of life - proves that none of your bogus "catastrophes" is able to reliably exterminate life on planets with similar initial conditions as the Earth because if that were possible, it would have already happened here on Earth. Also, we know microscopically that none of these catastrophes can be "this" deadly. The very framework of your argument is wrong, not only some choice of "lightnings" which is an irrelevant detail. – Luboš Motl Mar 28 '11 at 11:49
  • In science, one can't prove that there is a reliable mechanism that does something - i.e. exterminates life - without having the slightest glimpse of the identity of such an effect. It is not enough in science just to handwave with "catastrophes", claiming that it is irrelevant what they are. It is damn crucial what those catastrophes are and the only way to study this scientifically is to say what they are and investigate their dynamics in detail. You are not doing science, you are just caught in a loop of silly logical fallacies. In this case, the "catastrophes" are just non-existent. – Luboš Motl Mar 28 '11 at 11:51
  • "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." In the paragraphs above you imply Classical logic, claim the it is "objective fact" whether Schroedinger's cat is dead or alive and deny the QM altogether. – Anixx Mar 28 '11 at 11:53
  • Annix: "Where I said it is determined by the observer's feelings? Are you familiar with the quantum immortality/suicide thought experiments?" The second question of yours contains the answer to your first question. You have used the assumption that physical effects are determined by observers' feelings because you have spoken about "quantum immortality", and the very point of "quantum immortality" is to measure the fraction of subjective feelings of an observer - which has nothing to do with the objective events around her. Yes, I am familiar with quantum immortality and it is rubbish. – Luboš Motl Mar 28 '11 at 11:55
  • "The existence of the Earth for 4.7 billion years - with billions of years of life - proves that none of your bogus "catastrophes" is able to reliably exterminate life on planets with similar initial conditions as the Earth because if that were possible, it would have already happened here on Earth" -this is an incorrect implication. At least it contradicts the Antropic principle. – Anixx Mar 28 '11 at 11:56
  • The comment that "observer cannot die" [generally, or in one interpretation or another] is just a meaningless sleight of hand. Whenever physics is used to study the fate of observers, the observers must be treated just like other physical objects, and it's damn likely that they will die in less than 150 years. No "interpretation" can change anything about it. Death is a physical (biological, to be more precise) phenomenon, a damn well-defined process, and whether it occurs or not is not given by subtleties in "interpretation of quantum mechanics". – Luboš Motl Mar 28 '11 at 11:57
  • "You have used the assumption that physical effects are determined by observers' feelings" - No. Quantum immortality/suicide does not imply the outcome is determined by the observer's feelings. – Anixx Mar 28 '11 at 11:58
  • "Whenever physics is used to study the fate of observers, the observers must be treated just like other physical objects." - This is desirable, but impossible, due to a special role of the observer in quantum mechanics. – Anixx Mar 28 '11 at 12:00
  • "Quantum immortality/suicide does not imply the outcome is determined by the observer's feelings." You have no idea what you're talking about. At least read the definitions of the pseudoscientific theories you're so eagerly defending. Second paragraph of http://en.wikipedia.org/wiki/Quantum_immortality contains the definition: "Quantum immortality refers to the subjective experience of surviving quantum suicide regardless of the odds." – Luboš Motl Mar 28 '11 at 12:01
  • Yes. But it does not mean that the outcome is determined by the feelings as you claim. – Anixx Mar 28 '11 at 12:03
  • Anixx: "This is desirable, but impossible, due to a special role of the observer in quantum mechanics." - There is no special role of the observers in quantum mechanics. Observers are physical systems, subjects to the laws of quantum mechanics just like any other physical system. They may also be able to "perceive" and/or "evaluate" things etc. but this extra ability can't make them able to avoid the laws of physics - for example, it can't make people immortal. I would argue that a person who disagrees with the previous sentence is not mentally healthy. – Luboš Motl Mar 28 '11 at 12:03
  • "There is no special role of the observers in quantum mechanics". There is. At least in Copenhagen interpretation, and constitutes the main problem of it. In MWI indeed there is no special role of the observer, but still the observer plays special role in the observable (by him) part of the universe. – Anixx Mar 28 '11 at 12:08
  • The observer in the Copenhagen interpretation is a concept that simplifies the process of making predictions, rather than creating new problems. By declaring certain objects to be "macroscopic" and "observers", the fathers of QM could have assumed that classical logic applied to them. This statement, while being an assumption at the beginning, can be deduced from full-fledged quantum mechanics that applies to all objects in the Universe. ... In MWI and Consistent Histories, no "systemic" rules depend on the presence of an observer. – Luboš Motl Mar 28 '11 at 12:20
  • Yes. That's why MWI is somewhat more symmetric than the Copenhagen interpretation. But still in the observable (by the observer) world the observer plays special role. That means he cannot deduct from the fact of his own existence (and existence of life on his planet) that life exists on other planets in his world as well. This does not imply that extraterrestrial life does not exist anywhere. It just may not exist in the world (in terms of MWI) that he observes. – Anixx Mar 28 '11 at 12:32
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    @Lubos: Annix is right--- an example of a possible catastrophe--- suppose that nearly all planetary orbits are unstable to drifting out of the habitable zone for their stars, with a half-life of 10,000,000 years. Then the Earth is just lucky, and only the boundary condition that we are here gives the probability of us observing a 4bn. stable solar system. The boundary condition that we are on Earth makes us special, and the question is "how special". – Ron Maimon Oct 24 '11 at 19:00