Quantum Physics Debate, 2: The Many Worlds Interpretation rebutted

Further to our debate on the Many Worlds Interpretation of Quantum Physics begun here at Not PC by Brian Scurfield, doctoral student Mark Sadgrove offered this response which has been lightly edited to appear here as a post (thanks, Mark):

I probably shouldn't wade in here, but it is nice to see this topic being discussed on a New Zealand oriented site, simply because, academically speaking, there are some fine minds in this country when it comes to research into fundamental quantum physics, and it would be nice if that translated into wider public interest.

Disclaimer: I'm a Quantum Physics PhD who has published work with Rainer Leonhardt and Scott Parkins mentioned in comments in the original post. Specifically, my own work could be said to err more towards supporting the notion that interaction with the outside world is the only thing that's needed to explain the quantum measurement problem away... Having said that, the Many Worlds Interpretation is as far as I'm aware still an interpretation rather than a theory with new, testable predictions, so I'm really no more qualified than anyone else or for that matter any more biased by my use of established quantum theory when it comes to commenting on this stuff.

I have a few points to make:

1) The "Many worlds interpretation" is still called an interpretation. Is anyone aware of any specific predictions that Many Worlds theory makes which are outside of standard quantum theory? In this case there might be hope for confirming it one way or another...

I seem to recall that some (perhaps prominent?) physicists have said that a successful quantum computer would force people to believe the many worlds theory because, well, that super-classical computing power must be coming from somewhere , the idea being that we share the possible computing power of a huge number of somehow existent universes when we use a quantum computer. Personally I still don't find this convincing. From a very utilitarian point of view, that computing power arises because nature, as embodied in the laws of quantum mechanics, allows it to occur. Quantum mechanics just works that way and that's that.

The reason people are groping for interpretations is because they find it difficult to form a clear, intuitive picture of the natural process that is occurring [in observations of Quantum phenomena], unlike in classical mechanics where little balls colliding with each other is to a crude degree all the mental imagery you'll ever need. But an interpretation is just window dressing until it actually leads to intuition which makes NEW predictions. Smarter people than me are backing 'many worlds,' but I'd like to know if they've made any progress on the prediction front.

2) Occam's Razor. I always thought that the most compelling reason for being suspicious of the Many Worlds Interpretation was Occam's Razor which states that given the infinite possible explanations that one can give for a physical phenomenon, the simplest possible explanation (that is the one with the fewest parameters) should be preferred. [N.B., it can be simply stated as the law of succinctness, in Latin: "entia non sunt multiplicanda praeter necessitatem," or, "entities should not be multiplied beyond necessity."

'Many Worlds' theory multiplies Universes ad infinitum just so we can have a cosy mental picture of the natural processes described by QM. A lot of people think that this is a dear price to pay! This is a capitalist-oriented site right. [Oh yes - Ed.] So do you really think that Nature would be this wasteful of resources when cleaner, more efficient single-universe models can explain things just as well? (Of course, the point is that some people think that ONLY Many Worlds theory can explain all the observed phenomena in which case it doesn't matter how "wasteful" it seems, because it's the only game in town.)

Another way to look at it is through the testability lens which a lot of people have also brought up. If you claim that there are multitudinous other universes which are created by quantum "splitting" or "differentiation" events but, oh hang on, you can't ever reach them or even feel their influence except via a rather unspectacular interference experiment, then why should I believe you? People who do believe such things are, for example, good candidates for believing that there is a very specific God in Heaven - a bearded man in the Judeao-Christian tradition for example, exactly as described in the bible [sorry: The Bible].

While we have no real evidence of such specifics, believers might say, if exactly such a god did exist it explains a few things about the world. Well perhaps, but why believe all these things that you can't verify along with the very general idea of a creator which does not imply any specific form for god.

==> Do we really have to swallow a multiverse teeming with infinite slightly differentiated universes all equally "real" just to explain the quantum measurement problem? Are there leaner versions of many worlds without infinite versions of each of us floating around in them?

(3) Is QM an interpretation? Remember that Physicists don't have to buy into any interpretation of quantum mechanics to use it effectively. In a comment on the previous post, Brian S took Scott Parkins' to task about a quantum jump "interpretation". But I don't think Scott was using any interpretation. The jump issue in quantum mechanics, or the collapse of the wave function, ra, ra, ra, whatever you wish to call it, is I think the bare minimum theory needed to predict naturally observable events. I don't agree that there is any interpretation going on here.

The facts are that when you look at the distribution of photons from a slit experiment on a detector screen, each one makes a "dot" at just one point - that's your measurement. To describe the distribution of points you use quantum mechanics. Quantum mechanics describes the photon as a "delocalised" or non-point-like entity - a wave as such which gives the requisite interference pattern. It also provides a probabilistic rule telling you how likely a given measurement is given the form of this wave. What you do to connect the wave model of the pre-measurement photon to the measurement you actually make - your interpretation of the observed physical phenomenon - is up to you, but it doesn't change the basic description of what happens which is standard quantum mechanics.

Some rather vocal proponents of the de-coherence project think that they've got the quantum measurement problem - jumps and all - ironed out by just considering the interaction of a closed quantum system with it's environment. I favour de-coherence myself (remember I'm biased - I invoke it a little in my thesis) because it seems clean. The idea is that the idea is that the nice probability waves that don't have any particular "position" are in reality rapidly converted into particular "classical" states in any real system because there is interaction with the environment. I don't think many people believe that de-coherence theory is enough to explain the measurement problem away completely, but it IS a theory and it does help make sense of certain experimental observations to some degree.

Personally, de-coherence looks to me like Quantum Mechanics without the difficult jump, but with the Born Rule explained (see W. H. Zurek, Probabilities from Envariance, 2004 - still controversial!!!) and no multiple Universes required.

Reading the recent Wheeler preprint ('100 Years of the Quantum' -Tegmark & Wheeler [pdf]) referenced in the previous post, the De-coherence theory is in fact presented as being an important addition to Everett's Many Worlds Theory and not an explanation in competition with it. I am not convinced after a cursory review of that paper that there is really anything left to explain if dec-oherence works as well as Tegmark and Wheeler imply in this preprint, but even so the "Many Worlds Theory" they describe does not seem as grandiose as the usual idea of infinite simultaneously existing Universes being almost identical to our own. Rather, a far more subtle "Many Minds" idea is suggested which seems rather different to me, or at least is far less suggestive in a sci-fi sense.

* * *

Okay sorry to go on. Maybe my overall point is that Many Worlds Interpretation seems to excite a lot of people when it's really a subtle idea, for which refinement might remove a lot of the sci-fi elements -- this is even if it survives as an attempt to provide a picture of the undoubtedly successful quantum mechanical laws. It seems to me that the successful implementation of Shor's algorithm alone is not enough to irrefutably prove the existence of multiple "me's" in Universes of which I can never otherwise observe the effect.

As a final note, I attended the Quantum Physics of Nature last year and saw Zeilinger's labs in Vienna. Very nice! I also heard debates about precisely this stuff and I can assure you that the physics community is nowhere near a consensus...

LINKS: Quantum Physics Debate, 1: The Many Worlds Interpretation - Brian Scurfield at Not PC
Introduction to Quantum Physics - Wikipedia
Many Worlds Interpretetation - Wikipedia
Born Rule - Wikipedia
Probabilities from Envariance - Arxiv.Org
Copenhagen Interpretation - Wikipedia
The Quantum Aristotle - Peter Cresswell, SOLO

RELATED: Philosophy, Science