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Technology

The Platonic Imperative: Reality and the Many Worlds of Quantum Mechanics

Reality BaseBy Adam FrankApril 6, 2008 9:44 PM
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Foundational studies of quantum physics hold a deep fascination for anyone interested in questions about the ultimate structure of the world. Quantum mechanics (QM) is now hovering around its 100th anniversary (depending on whether or not you take the work of Planck, Einstein, or Bohr to mark its true birth). Unlike other theories, quantum mechanics has proven to be remarkably elusive in terms of pinning down what truly, absolutely, no-kidding-anymore, really exists. With classical physics, things were easy—it was all just billiard balls. Not so with quantum physics. As Feynman famously quipped, "I think I can safely say that nobody understands quantum mechanics." Interpretations abound, but agreement does not. Given the central role QM plays in understanding what the world is made of, this situation causes a lot of consternation for physicists. The problem boils down to reality, what’s in it, and what access we have to it. Here at the University of Rochester, we’ve been running seminars on physics and philosophy. Last Friday, Peter Lewis, a philosopher from the University of Miami, visited and gave a great talk on the now famous "Many-Worlds Interpretation" of QM. His argument turned on probabilities in the Many-Worlds Interpretation. Rather than run through his reasoning on that topic, I thought it would be worth a note on the interpretation itself because it speaks so loudly to the central issue of what scientists think we are, ultimately, aiming for. The problem with quantum mechanics is that the basic entity of its mathematical machinery—the so-called wave function—does not give a single prediction for the outcome of experiments. Instead it provides a description of many outcomes with associated probabilities which all seem to exist simultaneously. It is not until a measurement is made that the wave function gets suspended (collapsed is the term) to yield a single answer. Or, at least, that is the way the standard interpretation of QM tells the story. This bothers lot physicists who like to take their mathematical descriptions of reality seriously. Why should a perfectly good equation that describes the evolution of the world (the wave function) go away just because someone made a measurement? To deal with this strange state of affairs, Hugh Everett proposed what would become the Many-Worlds Interpretation in the late 1950s (Bryce Dewitt did a lot of development on the idea, too). The Many-Worlds solution is, in a sense, a platonic one. The mathematical physics stays put, but our notion of what constitutes reality changes. Well, that is an understatement—it really, really changes. According to the Many-Worlds Interpretation, the wave function is never suspended. Every time a measurement is made, the world splits off into as many copies as there are pieces (terms) in the wave functions. If you are the lab technician making the measurement, you split off into multiple copies, as does the entire universe with you. In each copy, a different value of the measurement is recorded. After the measurement, each copy world goes on evolving and splitting as more quantum events occur. Sounds wacky, don’t it? Why would anyone believe in a universe that is endlessly splitting into (as far as we know) unobservable slightly-different versions of itself? Here is the point at which, as a physicist or philosopher, your biases will likely show themselves. People will line up behind the Many-Worlds Interpretation because of its consistency. Its advantage is that it keeps the math whole. There is no special pleading about consciousness intruding on the measurement. There is no sense that our access to the world is limited. You have a beautiful equation. It describes the evolution of physical reality, and that is that. People who favor the Many-Worlds Interpretation tend to be Platonists somewhere in their scientific souls. Recall that Plato argued that behind the world we see lies an ideal world made of purely mathematical forms. This idea dovetails with the worldview of many theoretical physicists. Timeless, immutable, mathematical laws govern the world. That is what makes physics so indescribably beautiful. Some people, like Max Tegmark, take their Platonism further—the world is not just governed by that math; it is that math (you can read more about Max’s views in an interview I did with him in the July 2008 issue of DISCOVER). The Many-Worlds Interpretation seems crazy to a lot of people, physicists and non-physicists alike. Personally, as a theorist of the astrophysical sort, I see its allure but remain suspicious of the enormous commitment it asks. What may be most interesting about it, however, is how, by taking things to an extreme, it raises two of the oldest and deepest questions we can ask: What truly exists, and what kind of access do we have to it?

Adam Frank is a professor of astrophysics at the University of Rochester who studies star formation and stellar death using supercomputers. His new book, “The Constant Fire, Beyond the Science vs. Religion Debate,” has just been published. He will be joining Reality Base to post an ongoing discussion of science and religion—you can read his previous posts here, and find more of his thoughts on science and the human prospect at the Constant Fire blog.

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