Monday, May 17, 2010

Quantum Darwinism. (Darwin Solves Schrödinger's Cat?)

We've discussed in this post and in its comments section that Schrödinger's Cat seems to be a paradox but only if you treat the system classically.  If you throw in quantum interactions, the paradox goes away.

I would like to discuss the (probably) best supportive case for how this happens. (Has experimental evidence as well.)  It is a recently proposed idea called Quantum Darwinism.  (See peer reviewed papers in Nature and PRL.)  This is how it works:

  1. A system begins "quantumly" in a superposition of states. (Like Schrödinger's Cat)
  2. After the system begins to interact with its environment, only certain states in the superposition remain stable. (Don't become destroyed.)  This is called decoherence.
  3. A measurement by an intelligent observer also is an interaction (Example, if you look at it you hit it with a photon).
  4. Of the initial stable states that survive, further interactions continue to destroy "unfit" states.
  5. As this process continues, the surviving states actually become parent states to even more states who sometimes resemble the parent.
  6. This is a form of replication.
  7. Thus you have a bona fide survival of the fittest situation with certain states (and their offspring) becoming favored over others.
  8. These different stable states become "orthogonal" to each other so there is no more superposition-like quantum interactions that happen anymore.
  9. What emerges is a classical system.  Ie.. one where it appears the wave function has collapsed. 
This is a lot like ideas expressed in Dawkins' The Selfish Gene. (For those who have read it.)  In that book he describes how some molecule configurations are stable and some furthermore have the added ability that they can replicate. (Make copies of itself.)  Over many many years, those with the best ability to "survive" (in both the stable and able-to-replicate sense) evolve into the complex structures we see today.

This is identically the same story with Quantum Darwinism.  After interactions with the environment are taken into account, only certain states are stable.  Furthermore, some of these states can influence what states are created and destroyed as we run the clock forward. (They are parent states.)  Those states that are best equipped for "survival" contribute to the collapsed classical structure we see today.

The PRL mentioned above cites experimental evidence for the idea.  They created systems where there would be preferred stable states (after taking decoherence into account) and showed that as time when on the stable states began to replicate and a classical state emerged.  See the picture.

So, Darwinian ideas may be solution to the measurement problem in quantum mechanics and therefore explain Schrödinger's Cat.  Stable quantum states after interactions and "survival of the fittest" is factored in determine if the cat dies or stays alive.


  1. Cool idea. Should we see this as we peer back in time to the early Universe? Or is our Universe the result of Darwinian selection? I'm thinking about the different constants that allow our Universe to be what it is. Having cohesive matter seems to be a stable state that can be selected upon.

  2. Stan,

    Good questions. I don't know enough to know what the large scale effects are other then this somehow would lead to our universe we see.

    "Or is our Universe the result of Darwinian selection?" Well, even if you don't believe this Darwinian stuff you have to admit the universe is the result of quantum probabilities. I guess now we know those probabilities are heavily influenced by the interactions of the system and that *perhaps* there is some "survival-of-the-fittest" states stuff going on in addition.

  3. So God created the Universe just by looking at it and collapsing the wave function(s). Dang that seems easy! =:)

  4. So there is no actual collapse of the wavefunction?
    Yet there is no Many Worlds?

  5. Anonymous,

    Good questions.

    There is still collapse of the wavefunction. The point is the wavefucntion collapses *both* by measurement as well as interactions with the system. Interactions with the system make some states in the superposition unstable and so they collapse.

    We still don't know about many worlds but most physicists think the many worlds interpretation is probably not true. That does not mean we may not live in a multiverse as you can have a multiverse without the many worlds interpretation being true.

  6. Thanks for replying.

    I really don't believe in "collapse of the wavefunction", never did.
    It just doesn't make any sense to me whatsoever, which is why I asked.
    I am a proponent of ontological wavefunction, but I guess I've always been a Bohmian as MWI didn't really make sense to me in the beginning.
    However it becomes clearer and clearer that decoherence is infact correct, which lead you with no other choice than to accept some sort of Bohm/MWI view or hold on to the 20's collapse interpretation.

    I guess this quantum darwinism doesn't really change anything after all?

    I think I got prematurely excited, but I think I realize now that this is not a answer to the Schroedingers cat paradox, it's still just a question of which interpretation you want to BELIEVE in...

    Am I right?

  7. "I realize now that this is not a answer to the Schroedingers cat paradox"

    No, in many ways it is the answer. Traditionally, the cat cannot be both dead and alive until the box opens. However, in the real world all the particles in the system interact, and based on those interactions either the dead or alive state becomes unstable.

    So, even without measuring the cat really does either collapse to a dead or alive state. (From decoherance based on those interactions.)

    The problem with the "Schroedingers cat paradox" is the original framers of quantum mechanics didn't understand Decoherance and so didn't properly account how interactions of all the particles in the system would influence each other.

    So, it is still a paradox if you turn off all particle interactions. Then the cat is still in a superposition of states as decoherance can't cause any states to become unstable if there are no interactions. But a world without interactions is sort of unphysical so in some respects this still isn't a problem.

  8. Yes, but let's view it from this point a mintue:

    The whole idea of collapse is wrong, there is no collapse, like Einstein, Schroedinger, de-Broglie, Bohm, Everett, Bell and most quantum physicists born after 1950 agree on.

    What does decoherence then? Except for give us a reason for why you personally only observe one outcome?
    The other outcomes are still real and occuring, but not to this version of you.
    Essentially pretty much a many world type of interpretation.
    That is the consequence of realism and decoherence, and I can't really say I see how quantum darwinism changes this at all?

  9. "(especially in the "does this man deserve help" category)"

    Okay, I see where you are coming from. You are saying that decoherence makes certain states unstable but maybe only in this world. You could still have "many worlds" where in those worlds other states become unstable and so you still have the many worlds scenario.

    Okay, I agree with that. To me the biggest problem was the cat was viewed as both dead and alive until the box was opened. This, I believe, is resolved in that *in my world* decoherance causes the cat to collapse to one of those states regardless of whether I open the box.

    But I guess there is still always quantum suicide. :) (Joke, don't try this anyone!)

    But I admit there could be another world where the cat meets the oposite fate. So yes many worlds is still possible.

  10. Thanks,

    Well thats just it.
    This doesn't change anything at all, because it's still just a question of whether you prefer collapse, many worlds, pilot waves and all the other interpretations.


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