Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Many Worlds Question

  1. May 7, 2005 #1
    My understanding of this theory is that instead of a superposition of states (for example, the Dead/Alive Cat in the box) you have two Universes, created from one, each satisfying one of the two outcomes (Dead cat in Universe 1, Live cat in Universe 2). It is also my understanding that, whenever such a situation arises (which is frequently) new universes are spun off, suggesting that there are some outrageous number approaching infinity of them.

    My question is, where does all the energy come from to create these new worlds? I would think that, to create an entire universe (or many), every time a particle is observed, would require a great deal of energy/matter.


    The Rev
  2. jcsd
  3. May 7, 2005 #2
    Hey Rev,

    Personally, I conceptualize the multiplicity of worlds the same way that I conceptualize the multiplicity of the cat's states. That is, in my mind, I imagine (a la MWI) that the worlds are in a superposition, similar to the way that I imagine (a la CI) that the cat is in a superposition. So if you ask the MWI: where does all the energy come from to make these different worlds? You should also ask the CI: where does all the energy come from to make all of these different cats?

    IOW, I do not think it is fair to shoulder this burden of explanation on the MWI without shouldering the same burden on the CI.



  4. May 8, 2005 #3


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    No, no, not at all. "Many Worlds" is a fancy name for "superposition".
    The only thing MWI (Many Worlds Interpretation) says, is that we should take the superposition principle, which every body uses on the microscopic level, seriously all the way up.
    And what happens, when you do that, is the following: the wavefunction of the universe (or at least a chunk of it, containing the system, the apparatus, you, the air, the earth, the moon...) will then quite quickly evolve (using Schroedinger's equation) into a sum of terms which will then not interact anymore amongst themselves. That's a result of decoherence. So you can then consider that each of these terms is, by itself, a "description of the world" and that the wavefunction of the world is then just a superposition of these descriptions. An observer associated with a physical construction (body) of which there is a state in each of the terms, is then supposed (postulated ?) to be only aware of ONE of these states, in one of these terms. So the term which is chosen is then his "world".

    That's a poetic description :-) What you simply have is that more and more terms occur in the superposition, just as it is microscopically the case.

    There are no "new universes" created: it is just an evolution of the wavefunction which splits in more and more terms ; the point is that we, as observers, are only aware of one single term.

  5. May 8, 2005 #4
    Your view of MWI also has some implications;

    IF you state that for every wave function that collapses into one of its eigenstates, all the other possibilities will become true in other universes, you cannot longer explain free will. For every you there's another you, doing what you don't do. Even if you accept free will for yourself, you're determining all the other you's, so they can never be indeterministic.
    That also leaves us with the question which of them universes is "in control". If an infinity of you's is influencing his environment independant from other universes, though directly determining how they can be, there would be chaos i think...

    Therefore I also try to look at it like Vanesch says
  6. May 8, 2005 #5


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    The objective part of MWI is fully deterministic, because all what can happen, also happens. Even in the hypothesis that there is something like free will for ONE of the you's (the you you are conscious of - the others are maybe just bodystates without a conscience: that's my personal view), at the NEXT split, because you do a next measurement, there are AGAIN new "yous" generated (and not even the same ones spinning off from your "you" and your old mates ; so you could then say that your old mates now also can make a choice).

    Look at it like this:

    |you0> |worldA> does a measurement, and turns into:

    |you1> |worldB> + |you2>|worldC>

    Now, imagine that you are consciously "you1", so you observe worldB.

    Next, you 1 does a measurement with 2 outcomes, and you2 does a measurement with 3 outcomes.

    So we now have:

    |you1a> |worldBa> + |you1b> |worldBb> +
    |you2x> |worldCx> + |you2y> |worldCy> + |you2z> |worldCz>

    Because you are consciously you1, and you "choose" you1b, you1a "has no choice" but to live in worldBa.

    But you2 has a choice: he can go into you2x, you2y or you2z.

    Personally, I don't need you2 to have a consciousness, and trace out only ONE path through all these possibilities (you1 and then you1b). But you can think of them as "equivalent" with created consciousnesses at each split.

    That's always a good idea :biggrin:

  7. May 9, 2005 #6
    I still have problems explaining MWI;

    1) If a measurement is made, a causality forward in time is made,(perhaps an immediate creation is possible, if you discern facts as causality and time, nevertheless, at that very instant of creation, world B will be dependant of world A) because due to measurement a new world is created spinning off from the one where a definate state is forced to take place.
    However, why then does all backward causality disappear? The universe B you created, will never have an influence on universe A.
    So in short, you keep on spinning off new worlds, and they keep on spinning off ever more new worlds, being backward independant from the other ones.

    2) If like some say, QM is on the base of everything, brainfunction and consciousness should also represent wave functions that on measurement choose an eigenstate, and by that, create new worlds with outcomes (omega - worldA). Therefore, if at this instant you have a thought

    about something, that thought will be a restriction to the new worlds spun off.
    In an experiment; You have the choice between 2 thoughts. By measuring (here thinking about A; you force off an eigenstate, rendering only thought B possible in the new world. So even if consciousness can evolve from world B, it can never occur at the moment of creation.

    3) It is not possible to duplicate an isolated system, cause in universal sense they don't fit the context. Every system that seems to be isolated, has been created from the universal wave function. Therefore, for any change whatsoever of the universal wave function, a new entire universal WF will be generated containing (omega - all eigenstates being present inour wavefunction). The question remains whether that is in any way useful.
    According to me, time is a serious problem in this evolving process, because new worlds are generated that are dependant from each others time of measurent and therefore new worlds wouldn't seem to have a history, mainly because as stated, all backward causality or dependance has gone.

    Last edited: May 9, 2005
  8. May 9, 2005 #7


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Well, take that as a fundamental postulate. The Born rule works forward in time (or better, defines what "forward in time" means on a time-symmetrical deterministic structure).

    First of all, I'm going to piss off certain people here, because this consciousness thing is roaring its ugly head again :-) But here we go.

    The way I see quantum theory:
    I think the universe consists of (at least) two antagonists: there is the objective, physical universe, which is described by its wavefunction. That's one part. Then the other element, outside of that physical universe, is (at least) ONE consciousness, namely mine. It is associated to a physical structure which is my body, and the link between both is given by the Born rule, in that my consciousness, at any moment, has to choose to go with ONE body state which occurs in a product state with the rest of the universe, and this choice is fundamentally random (I'm not *master* of it - no free will, a consciousness just passively observes). Each time my bodystate gets further entangled with something else, my consciousness is randomly, according to the Born rule, associated with ONE of the terms.
    That's a pretty solipsist view on things, and if you don't like it, you can postulate also other consciousnesses, like yours, which also follow their path through the terms of the wavefunction of the universe. You can also postulate that consciousnesses "split" each time. But all that is not really necessary to understand the observations of ONE SINGLE person, and for which the existence of only ONE consciousness, outside of the universe, but associated with a certain physical structure, is bluntly postulated:

    Postulate 1: the universe, containing my body and all, follows unitary evolution.
    Postulate 2: my extra-universal consciousness is, at any time (during a finite time lapse) associated with my body as a physical structure, and is successively associated with ONE term in the Schmidt decomposition of the wavefunction of the universe (H_mybody x H_restofuniverse). This association occurs randomly and according to the Born rule.

    Postulate 1 is MWI, and postulate 2 is my personal addition. If you don't like the word "consciousness" then just say "the observer associated with my body".

    That's not true: up to the split, both have the same history.

    You have:

    |psi_universe(t1)> = blabla + ... + |you0> |sys1> |stuff> + ...

    But as you happen to have your consciousness associated with "you0", you don't have to take into account all the other terms (it is an effective projection, as in Copenhagen QM).

    We then have:

    |psi_universe(t2)> = blabla2 + (|you1> |sys1a> + |you2> |sys1b>)|stuff> +...

    Now, you1 has the same history as you2 up to time t1, and then you1 observes sys1a (which just changes its last "update") ; you2 has the same history except for observing sys1b as last update. They are *almost* identical, except for the last thing happening to them. Your consciousness is now randomly associated to be "you1" or "you2".
    You both have still some stuff in common (far away stuff which has NOT been observed and evolved independently).

    Now this view is of course questionable. But I think it is the only view that respects the following things:
    - equivalence of all physical processes (no distinction between what constitutes a "measurement" and what is a "physical interaction") -> that's MWI.
    - respect of the symmetries (especially Lorentz invariance) and locality. EPR situations violate this whenever a "reality" is assigned to a measurement.
    - the fact that I'm convinced that the Born rule is independent of the unitary evolution.

  9. May 10, 2005 #8
    I have two things to say according to your question,
    First, Whichever view of MWI we hold, we should say that, as any theory, energy conservation is known from observation. As we can not measure things happenning in any other world except in which you live, so the defination of energy should be reconsidered. Energy is conserved only when it is properly defined.
    Second, My view about MWI is almost the same with Straycat. Any world is a state, all these states are in a superposition, the whole forms the universe. (I hold this point of view not because it is simplier in philosophy, but because it explains measurment better. ) So energy is not a problem on condition that QM is acceptable.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook