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Quantum Darwinism!

Starting yesterday afternoon, I decided to begin doing some research into quantum physics and the Many-Worlds Interpretation thereof in preparation for Star Trek: DTI — since a book about time travel in the Trek universe has to deal with parallel universes to some extent.  True, the portrayal of temporal physics and alternate realities in ST is quite fanciful, but as my readers know, I like to do what I can to ground ST’s fanciful science in principles from real science.  I’m often surprised at how feasible it is.

I took quantum mechanics in my final year as a physics major in college, but it never really clicked with me.  Part of it was that the calculus was just too complicated for me, but I think part was that it was all so abstract.  I was interested in the concepts, the meaning, but all the class covered was the math.  I recall describing it to a friend as “variables performing unnatural acts on each other.” Nowadays, I still can’t follow the math in any detail, but I find it easier to study the subject now that I’m motivated to use it in fiction.  In fact, I got so caught up in it last night that I barely got any sleep.  It helps that I’ve found some good articles, starting on Wikipedia and including sites linked to in its articles on quantum theory.

So anyway, the basic idea of quantum physics is that a particle or other system can exist in multiple states at once — a superposition, it’s called.  The question is, how does that produce the classical world we observe where  everything appears to have one definite state?  The old idea, the Copenhagen interpretation, was that the wavefunction “collapses” into a single state when it’s measured.  Copenhagen didn’t explain how or why this happened; it was an ad hoc postulate that nobody was really happy with.  (The Schroedinger’s Cat thought experiment, often cited as an illustration of this phenomenon, was actually a critique of it, because it argued that there could be a scenario wherein a macroscopic object like a cat was forced into an impossible dual state because of the superposition of a single atom — the decay or nondecay of that atom determines whether or not the poison is released.)

These days, we understand it as a process called quantum decoherence, which is nicely discussed at this site.  The key is that we don’t really observe a particle directly; we observe its effect on the environment it interacts with.  If you measure the state of a particle, you’re actually observing the state it’s induced in the measuring device.  So the wavefunction’s multiple states never actually collapse into one; rather, they’re all still there, but we, as part of the environment interacting with the particle, only observe one state at a time.

So the idea that Schroedinger’s Cat is both alive and dead until the box is opened by an observer is wrong.  When the atom’s two states (decayed and undecayed) interact with the poison trigger, the interactions with the billions of particles in that trigger cause decoherence, isolating the states’ effects from each other, so the trigger will only react to one state at a time.  So the behavior becomes “classical” through the interaction with the trigger, and when the trigger interacts with the poison and the poison interacts with the cat, they all become part of that same combined quantum system and share the same single state.  The cat survives or dies before the observer opens the box, because the decoherence happens at the trigger.

This is why it’s a mistake to do what so many laypeople do and interpret wavefunction collapse mystically, as the result of observation by a thinking being.  In fact, observation is just one type of interaction between the local system and its environment.  Any such interaction causes decoherence, once the effects of the interaction propagating through the environment become thermodynamically irreversible (like a glass shattering — it’s all but impossible to make the atoms revert to being an intact glass and jump off the floor onto the table).

This idea was proposed in 1957 by Hugh Everett, in what he called the relative state formulation: when a system interacts with its environment, its state can no longer be described in isolation, but only as it exists relative to the environment it interacts with.  So the environment becomes correlated — or quantum entangled, in modern terminology — with the state of the particle.  But each of the multiple states of the particle is separately entangled with its environment, describing a separate system.  Instead of just the particle having a wavefunction that’s a superposition of states, the combined system of the particle and environment has a superposed wavefunction, the whole schmeer existing in two or more states at once.  And once the difference between those states becomes irreversible, they stop interfering and have no more interaction.  From that point on, the system has multiple independent histories.

This has become known as the Many Worlds Interpretation, and that’s often taken literally: each independent measurement history represents a parallel reality.  One world where the cat lives, another where the cat dies.  Naturally, this is the interpretation that applies in the Star Trek universe, with all its parallel realities.  Here’s a really good discussion of MWI, what it means, and what it doesn’t mean.  But although MWI is increasingly accepted among physicists as a mathematically valid approach to quantum physics, not too many of them believe that the other “worlds” literally exist as parallel universes; rather, they consider the alternative measurement histories to be simply alternative possibilities that are present but swamped within our singular reality, states that exist in a mathematical sense but don’t really split off into alternate universes.  This is the view I favor when I’m not writing a work of fiction that requires using MWI.

But either way, there are still questions about the actual physical mechanism behind decoherence.  What causes one state to dominate in the macroscopic system while the others fade away (or get shunted off into other realities)?  This is where Wojciech Zurek and his theory of Quantum Darwinism come in.  It’s pretty much just what it sounds like.  A Darwinian evolutionary process can happen — indeed, must happen — in any system that meets three conditions: 1) It has reproducing entities; 2) the reproductions are not exact; and 3) the environment favors the reproduction of some traits over others.  In such a system, some entities will have traits that let them reproduce more successfully than others.  That means there are more and more of them with each generation, until they inevitably overwhelm the competition.  (This is one of the many reasons why creationism is such BS.  The basic mechanism of evolution is so simple as to be inevitable.  Not only does it happen, but there’s no way to prevent it from happening in any system that meets those three simple conditions.)

In Quantum Darwinism, what’s being “reproduced” is information about quantum states.  The information about the original particle is encoded in the states it induces in the other particles/systems it interacts with, so each particle’s state becomes an “offspring” of the original state.  Basically, the process selects for states that can survive the decoherence process.  The states that survive are ones that are stable enough to survive interaction with other particles and thus get “copied” over and over by multiple interactions, so that they’re encoded redundantly throughout the environment.  Unstable states may be copied once or a few times before being destroyed, so their information doesn’t propagate as far.  The larger the environment becomes (i.e. the further the information spreads out into the universe), the more dominant the redundant information gets, swamping out the alternate states.

This is why reality looks classical.  We measure an object by measuring the environment it’s interacted with, and different observers measuring different parts of the environment wil see the same redundant information and agree on the reality they observe.  The original particle is still in multiple states, but the information about those other states has been swamped because it didn’t get reproduced redundantly enough.

So instead of a vast number of parallel realities, all carrying equal weight, what you have instead is a “signal” of classical reality on top of a faint “background noise” consisting of the unfulfilled potentials of all the other possible outcomes.  Which is how the universe can appear classical even while being entirely quantum-mechanical.  It’s not perfectly classical, which would require infinite redundancy, but it’s close enough to look that way for the most part.

Which doesn’t mean that Quantum Darwinism is incompatible with Many-Worlds.  It’s actually derived directly from Everett’s assumptions.  But as I said, it’s an open question whether MWI can be taken literally, whether the “worlds” are objectively real or just mathematical constructs.  Zurek doesn’t take sides on the question, and he says the following:

Objective existence can be acquired (via quantum Darwinism)
only by a relatively small fraction of all degrees
of freedom within the quantum Universe: The rest is
needed to “keep records”. Clearly, there is only a limited
(if large) memory space available for this at any time.
This limitation on the total memory available means that
not all quantum states that exist or quantum events that
happen now “really happens” — only a small fraction of
what occurs will be still in the records in the future.

If I’m reading this right, Zurek seems to be saying that Quantum Darwinism allows for there to be more than one “real” history to the universe, but rules out the interpretation of MWI stating that every possible outcome must be equally real.  So there could be a finite number of parallel timelines — maybe just those robust enough to stand out from the noise.  That fits the Darwinian paradigm, since a successful species can split apart into multiple coexisting species.  Not every genetic variation or mutation spawns a whole new species, but the most reproductively successful ones generally do.

That’s an interpretation I think I can live with.  The idea that every possible reality is real, that I’m splitting off at every instant into thousands of alternate selves, is one I find inelegant and kind of disturbing.  And from a dramatic standpoint it’s undesirable; if every decision actually happens more than one way, then any story’s outcome is arbitrary and meaningless.  However, if the number of realities is greater than one but still limited, it’s not so bad.  The number could still be quite large, though, large enough to accommodate the myriad universes (to coin a phrase) seen in Star Trek.

Note also that Zurek seems to be saying that a “real” alternate history won’t necessarily remain “real” forever.  The information could be lost, that state of the universe wiped from the cosmic memory.  That has interesting ramifications from a fictional perspective, particularly where a book about time travel is concerned.  And that’s all I’m going to say on the subject for now. 😉

  1. David
    May 2, 2010 at 4:55 am

    I’ve read about Zurek’s work before, but never really got it.
    If applying his hypothesis somehow changes MWI, doesn’t that mean it also somehow changes QM and should be testable?

    Also, would still some worlds split?
    Take the cat dead | alive, would there be a universe where 1 is dead and one where it isn’t?

    You say that most physicists that are proponents of MWI don’t believe the worlds are actually real, could you link me to that source?
    How can only one be real if wavefunction is all our universe is made up off? What makes the others math instead?

    • May 2, 2010 at 8:08 am

      Zurek’s quantum Darwinism doesn’t change MWI, it just clarifies some unanswered questions about it.

      Testable? Well, MWI itself may or may not be testable. It’s considered a metatheory, something that comments on a theory (and it’s independent of which actual theory of quantum physics you use, since it basically just describes how to interpret the results). That’s why it’s called an interpretation rather than a theory. In that case, QD is just a refinement of the interpretation. However, that MWI discussion page I linked to in the post offers a few suggestions of how it could be testable and would thus qualify as a theory.

      I wouldn’t swear to the accuracy of my reading of Zurek’s papers, but he seems to be agnostic on the question of whether the non-interacting “worlds” constitute existentially real alternate timelines. But as I said in that last part, I think he allows for the possibility of more than one timeline. But his work doesn’t require it.

      I think the source for what I said about physicists’ interpretations of MWI is Wikipedia’s article on same.

      As for the final question, how only one history can be “real,” I’m not sure what the various physicists might say. I gather a lot of them, like Hawking, don’t really care to address the existential questions at all; it doesn’t matter to them whether those other histories have some tangible reality, because they’re inaccessible anyway and it’s therefore irrelevant. But it seems that quantum Darwinism offers an answer: the “real” history is the one that wins the evolutionary competition, the one that becomes redundant enough to be objective, agreed upon by all observers.

      • David
        May 2, 2010 at 9:24 am

        What I mean about “changing” is that it seems applying QD to QM somehow changes QM, which ofcourse would require a new interpretation, which in return would mean QD would be a testable hypothesis.

        However, I guess we misunderstood each other, my bad.

        I know Zurek has stated before in some interviews that “He is not forced to accept that the other universes exists”, but he hasn’t said whether he believe they do or don’t either…

        You said that most MWI proponents only believe these other “universes” are nothing but mathematical constructs, not actual real things.
        Could you link me to the source of this statement?

        About Hawking, on wikipedia’s page on MWI it states that Hawking answered “Yes” when asked: “Do you believe the other worlds are as real as this one”.

        If Quantum Darwinism is true, would it mean that there is actually just one real world?
        Example: a dice has 6 different potential outcomes, but according to this only 1 really happens after some sort of quantum selection (like natural selection) ?
        What happens to the rest?

  2. David
    May 2, 2010 at 4:58 am

    By the way, very well written article.
    I could tell this wasn’t written by some amateur blogger;)

  3. May 2, 2010 at 10:51 am

    “What I mean about “changing” is that it seems applying QD to QM somehow changes QM, which ofcourse would require a new interpretation, which in return would mean QD would be a testable hypothesis.”

    As I said, QD is a refinement of MWI, which is a metatheory, a way of interpreting the results of a quantum theory. MWI can apply to multiple different quantum theories, because it’s only evaluating the results.

    If you want to learn more, please follow the links I provided in the post. They can explain much better than I can, which is why I linked to them.

    “About Hawking, on wikipedia’s page on MWI it states that Hawking answered “Yes” when asked: “Do you believe the other worlds are as real as this one”.”

    No, what Hawking said was that “according to Feynman,” that’s the case. He wasn’t saying it was his belief, he was reporting Feynman’s view. See paragraph 7 of the section of the article entitled “Reception.”

    “If Quantum Darwinism is true, would it mean that there is actually just one real world?”

    There’s no way of knowing. As I said, my reading of it seems to suggest that the number of “real” worlds might be limited, but there’s nothing in QD that says “There must be only one world” or “There must be more than one world” or anything of the sort. The point of QD is not to answer the question of how many universes may exist. That’s a side issue that Zurek only briefly touches on. The point is to elaborate on the mechanism by which quantum decoherence occurs.

  4. David
    May 2, 2010 at 11:01 pm

    I’ve read the FAQ and the article on decoherence by Andrew Thomas over a year ago.
    Which is why I don’t understand how *IF* the wavefunction is ontological, we can dismiss the others as math.
    That makes no sense what so ever.

    I’m sorry about the Hawking statement, I realize they have just updated the wikipedia page about that matter.

    However you haven’t provided me with the link to the statement that most physicists believe the other branches are mathematical constructs and not physical universes like ours.
    I know Martin Gardner said this in a article he wrote, The multiverse and blackberries or something was the title, but I have never seen this backedup by anyone.

    • May 3, 2010 at 7:00 am

      “Which is why I don’t understand how *IF* the wavefunction is ontological, we can dismiss the others as math.”

      I can’t say for sure. I’m far from being an expert in this subject. I’ve never even taken MWI seriously before, and I’m looking into it for the first time as part of my research for this novel. So I’m the wrong person to ask.

      However, as I already said, it seems to me — speaking as an amateur — that quantum Darwinism might provide an answer. See the last paragraph of my reply to your first comment.

      “However you haven’t provided me with the link to the statement that most physicists believe the other branches are mathematical constructs and not physical universes like ours.”

      Yes I have, twice. As far as I can remember, I read it in the Wikipedia article on MWI, and I specified the exact section and paragraph where you can find it. If that was reinforced in something else I read in that same night of obsessive websurfing, I’m sorry, but I just don’t remember where it was. This is a blog post, not a scholarly paper, and I don’t have comprehensive citations for you. I can’t give you any more answers to your questions than I’ve already given you, so it’s pointless to keep asking me the same things over and over. If the answers I’ve given you don’t satisfy you, then you need to find someone else to ask.

  5. Marc Hart
    May 6, 2010 at 9:51 am

    I wonder if the Nexus from Generations is like a hub for the many worlds that stand out from the background noise, and part of Guinan’s consciousness is quantum entangled in it, which allows her to “feel” that some things are wrong, like the TNG episode “Yesterday’s Enterprise” where she told Picard that this wasn’t how it was supposed to be, and how she knew about Tasha’s existence.

    • May 6, 2010 at 10:31 am

      I gather it was intended that Guinan’s Nexus experience was the reason for her time sensitivity, though the dialogue establishing that was cut from the final film.

      And I have my own thoughts on how Guinan was able to sense that the YE timeline was “wrong.” Or rather, what it was she was sensing, given that her Nexus link enabled her to sense it in the first place.

      • Marc Hart
        May 6, 2010 at 9:29 pm

        That would be a great idea for a Guinan novel, or does she make an appearance in DTI?

  6. May 7, 2010 at 6:31 am

    “That would be a great idea for a Guinan novel, or does she make an appearance in DTI?”

    Probably not. She’s already appearing in David McIntee’s Indistinguishable from Magic, which will be out in spring of next year. More on that here: http://www.trekbbs.com/showthread.php?t=118961

  1. May 12, 2010 at 9:59 am
  2. December 1, 2011 at 11:21 pm
  3. December 7, 2013 at 11:17 am

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