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Experimentally proving MWI

  1. Oct 8, 2007 #1
    make the following device:

    a stern-gerlach device oriented to the x axis, where +x particles enter a stern gerlach device oriented to the z axis. If a particle comes out of the +z end, the device shoots you in the head, otherwise it doesnt. Its like quantum russian roulette.

    Anyway, the thing that MWI has that other interpretations don't is that unobserved measurements actually exist in other universes. Since we can only be conscious of a universe we are alive in, if out universe splits, we can only go into one of the child universes where we are alive. Therefore, if we use this device many times, we could "prove" to arbitrary error that MWI is true. The problem with this proof is that we would only be able to prove this to ourselves and the lucky few universes where we remain alive. In most other universes we would be dead (dieing wouldn't disprove MWI tho). Any thoughts on this?
  2. jcsd
  3. Oct 8, 2007 #2
    I love this concept...quantum immortality. My question is what if in both branches of the split you remain alive? Which one gets your consciousness and how is that determined? Obviously if you could interview each version of you, each one would claim to be you, but which version will the current "you" experience? Also, do we KNOW that if in one branch you die that you will NECESSARILY experience the one in which you are still alive? What if in some cases you don't and you just end up dead?
    Last edited: Oct 8, 2007
  4. Oct 8, 2007 #3
    Your consciousness splits along with the rest of the universe. Like you said, both you's will think theyre you. So although it appears like they randomly picked a branch, there was really nothing random about it. They are conscious of being in that branch because they are in that branch.

    Similarly, if your dead in one branch, you cant possibly be conscious of the fact that your dead. This means that you will only be conscious of one of the universes in which you remain alive. Im having a hard time explaining this but it makes perfect sense to me... Its like asking why theres life assuming life is very improbable. Well the answer to that is just because life exists. We could never be conscious of the fact that there isnt life.
  5. Oct 8, 2007 #4
    But is there some factor determining which branch you are destined to experience, and is it determined before or after the split takes place? If before, then whether you will end up dead or not can't influence it. You might end up in the dead one and thus not experience anything. If after, then I could see how it might necessarily go to the one in which you are alive since as you said you can't experience the other one. What if time doesn't really exist? How does this influence it? What if your consciousness somehow goes to both simultaneously? This issue strikes me as one in which we are not asking the "right" question because the answers seem not to fit.
    Last edited: Oct 8, 2007
  6. Oct 8, 2007 #5
    well in a sense your consciousness goes to both. However from your point of view you only go to one. This can be verified by observing any quantum state. Your only conscious of one measurement, therefore "your" consciousness only goes down one branch.

    thats a good point of pre-determined branching. However, I find it hard to believe that its all predetermined. Even if time doesnt exist, and the future is already determined, that doesn't mean which branch your consciousness goes down is predetermined. And the fact remains, your consciousness splits along with the rest of the universe. If there is a branch where it exists, your consciousness will be there. Therefore you will be conscious of being in that branch. Since you cant be conscious of being dead, there will be a branch of your consciousness that survives any number of these russian roulette rounds.
  7. Oct 8, 2007 #6
    So are you saying that if you split now and in both branches you are still alive, that you will experience both at the same time from your own perspective? Or do you only experience one of those from your own perspective? If it's one, then which one? If you experience both from your own perspective, how is it that you don't see the branching happening? How is it that we don't see multiple outcomes from a given situation? Maybe we actually do somehow but I can't wrap my head around it. I understand that there would be another version of you in any case, but I am interested in your subjective experience.
    Last edited: Oct 8, 2007
  8. Oct 8, 2007 #7
    no thats not what Im saying. "You" would only go down one branch. You would only experience one branch. The point is, asking which one you go down isnt a valid question, since all versions of you are equal. There is no reason you go down a specific one because there is nothing special about the idea of you. You split, therefore your consciousness splits as well.

    Thinking its weird that you only go down one branch, and it seems random is just a by product of human nature. Its human nature to think that your special, and that your consciousness in your branch is somehow special when compared to the other branches. Im having a lot of trouble explaining this, but basically if you view consciousness as just an illusion of a complex system, it makes more sense. Its when you start to correlate consciousness with your "soul" that you run into these problems.
  9. Oct 9, 2007 #8
    hmm I think Ill have to think this through better. I just realized something fundamentally wrong with this theory. In the world where you die, you are alive for some finite time after the split. Therefore you could actually die...
  10. Oct 9, 2007 #9
    This is a variant of Tegmark's Quantum Suicide Experiment. I don't buy the conclusion of Tegmark that the MWI peredicts that you should find yourself alive with probability 1.

    Let's not consider the technical details that might prevent the experiment from working as intended, one can always solve such problems by thinking of ever more bizarre set ups, like e.g. making the person unconscious using some drugs before the experiment and waking the person up only if the spin is found to be in one state and killing the person otherwise.

    In the MWI, the wave function undergoes a unitary time evolution. The most natural assumption would be to assume that the wavefunction of the universe is just in an eigenstate of the Hamltonian of the universe and therefore does not evolve in time. But we find ourself in some isolated sector of the multiverse and that part of the multiverse does evolve under the unitary time evolution, however this does not contradict that the entire multiverse is static.

    Now in such a setting, you can only talk about an a priori proobability of finding yourself in some state. The notion of a conditional probability of finding yourself in such and such a state given that you were in some other state state one second earlier is not well defined in general. This conditional probability is not a fundamental quantity, but you can still define it by taking the ratio of the probability that you find outcome x in experiment y by the probability that you find yourself carrying out experiment y.

    Now, when death is a possibility in the experiment, then the total a priori probabability of finding yourself alive and having performed the experiment with some arbitrary result is less than the a priori probability of finding yourself alive before the experiment. So, the conditional probability becomes ill defined as the sum over all experimental outcomes do not sum to 1.

    This is not really a big deal as at any moment you should think of yourself as being sampled from the a priori probability distribution over the entire (static) multiverse. Experiencing the state that has survived the suicide experiment many times in a row is thus extremely unlikely.
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