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Parallel Universes and Quantum Computing

  1. Mar 1, 2007 #1
    If I remember right Paul Davis in his book 'Parallel Universes' suggested that each possible position of an electron represented its place in an atom of a corresponding parallel universe. Does this or a similar/associated principle apply to the operation of quantum computing?
  2. jcsd
  3. Mar 1, 2007 #2
    quantum computing is in essence a form of proof of parallel universes- but there are some stick-in-the-muds around here that won't hear it- they will in time

    In your book, The Fabric of Reality, you are challenging the single universe conception of reality. In Chapter II, you clearly explain quantum theory which tells us about the behaviour of microscopic particles. You also explain the ‘single particle interference’ experiment and argue that there are intangible shadow particles, and then that there are parallel universes each of which is similar to the tangible one. This is a difficult step for many of us. Could you please clarify how you proceed from intangible particles to many universes (or multiverse as you call it)?

    David Deutsch: Let’s start with the microscopic world, because it is only at the microscopic level that we have direct evidence of parallel universes. The first stage in the argument is to note that the behaviour of particles in the single slit experiment reveals there are processes going on that we do not see but which we can detect because of their interference effects on things that we do see. The second step is to note that the complexity of this unseen part of the microscopic world is much greater than that which we do see. And the strongest illustration of that is in quantum computation where we can tell that a moderate-sized quantum computer could perform computations of enormous complexity, greater complexity than the entire visible universe with all the atoms that we see, all taking place within a quantum computer consisting of just a few hundred atoms. So there is a lot more in reality than what we can see. What we can see is a tiny part of reality and the rest of it most of the time does not affect us. But in these special experiments some parts of it do affect us, and even those parts are far more complicated than the whole of what we see. The only remaining intermediate step is to see that quantum mechanics, as we already have it, describes these other parts of reality, the parts that we don’t see, just as much as the parts we do see. It also describes the interaction of the two, and when we analyse the structure of the unseen part we see that to a very good approximation, it consists of many copies of the part that we can see. It is not that there is a monolithic ‘other universe’ which is very complicated and has different rules or whatever. The unseen part behaves very like the seen part, except that there are many copies.
    It is rather like the discovery of other planets or other galaxies. Having previously known only the Milky Way, we did not just find that there are vast numbers of stars out there, far more than in the Milky Way. There are more galaxies out there than there are stars in the Milky Way. We also found that most of the stars outside the Milky Way are actually arranged in other little Milky Ways themselves. And that is exactly what happens with parallel universes. It is of course only an analogy but quite a good one; just like the stars and galaxies, the unseen parts of reality are arranged in groups that resemble the seen part. Within one of these groups, which we call a parallel universe, the particles all can interact with each other, even though they barely interact with particles in other universes. They interact in much the same way as the ones in our seen universe interact with each other. That is the justification for calling them universes. The justification for calling them parallel is that they hardly interact with each other, like parallel lines that do not cross. That is an approximation, because interference phenomena do make them interact slightly. So, that is the sequence of arguments that leads from the parallelism, which by the way is much less controversial at the microscopic level than the macroscopic level, right up to parallel universes. Philosophically, I would like to add to that that it simply does not make sense to say that there are parallel copies of all particles that participate in microscopic interactions, but that there are not parallel copies of macroscopic ones. It is like saying that someone is going to double the number of pennies in a bank account without doubling the number of Pounds.

    But couldn’t this interference phenomenon be due to a yet unknown law of physics within this universe?

    DD: Well, there are very sweeping theorems that tell us that no single-universe explanation can account for quantum phenomena in the same way that the full quantum theory does. Quantum theory explains all these phenomena to the limits of present day experiment perfectly, and it is, according to some measures anyway, the best corroborated theory in the history of science. And there are no rival theories known except slight variants of quantum theory itself. We know that an alternative explanation could not be made along single-universe lines, unless perhaps it is a completely new kind of theory. So, the answer is ‘no’.

    A few years ago, BBC Horizon did a documentary on time travel in which you explained the parallel universes theory and suggested that there was ‘hard evidence’ for it. Well, it is a controversial theory and is accepted only by a minority of physicists, as you yourself acknowledge in your book. Why do you think there is such a strong reaction to this theory in the scientific community? And how do you reply to their criticism?

    DD: I must confess that I am at a loss to understand this sociological phenomenon, the phenomenon of the slowness with which the many universes interpretation has been accepted over the years. I am aware of certain processes and events that have contributed to it. For instance Niels Bohr, who was the inventor of the Copenhagen interpretation, had a very profound influence over a generation of physicists and one must remember that physics was a much smaller field in those days. So, the influence of a single person, especially such a powerful personality as Niels Bohr, could make itself felt much more than it would be today. So that is one thing – that Niels Bohr’s influence educated two generations of physicists to make certain philosophical moves of the form "we must not ask such and such a question." Or, "a particle can be a wave and a wave can be a particle," became a sort of mantra and if one questioned it one was accused of not understanding the theory fully. Another thing is that quantum theory happened to arise in the heyday of the logical positivists. Many physicists – perplexed by the prevailing interpretations of quantum physics – realised that they could do their day-to-day job without ever addressing that issue, and then along came a philosophy which said that this day-to-day job was, as a matter of logic, all that there is in physics. This is a very dangerous and stultifying approach to science but many physicists took it and it is a very popular view within physics even to this day. Nobody will laugh at you if, in reply to the question "are there really parallel universes or not?", you answer "that is a meaningless question; all that matters is the shapes of the traces in the bubble chamber, that is all that actually exists." Whereas philosophers have slowly realised that that is absurd, physicists still adopt it as a way out. It is certainly no more than ten percent, or probably fewer, of physicists talking many universes language. But it is heartening that the ones who do tend to be the ones working in fields where that question is significant, which are quantum cosmology and quantum theory of computation. By no means all, even in those fields, but those are the strongholds of the many-worlds interpretation. Those also tend to be the physicists who have thought most about that issue. But why it has taken so long, why there is such resistance, and why people feel so strongly about this issue, I do not fully understand.

    Last edited: Mar 1, 2007
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