Many-worlds interpretation

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The quantum-mechanical "Schrödinger's cat" paradox according to the many-worlds interpretation. In this interpretation, every event is a branch point; the cat is both alive and dead, even before the box is opened, but the "alive" and "dead" cats are in different branches of the universe, both of which are equally real, but which do not interact with each other.

The many-worlds interpretation is an interpretation of quantum mechanics that asserts the objective reality of the universal wavefunction and denies the actuality of wavefunction collapse.


  • The “many worlds interpretation” seems to me an extravagant, and above all an extravagantly vague, hypothesis. I could almost dismiss it as silly. And yet…It may have something distinctive to say in connection with the “Einstein Podolsky Rosen puzzle,” and it would be worthwhile, I think, to formulate some precise version of it to see if this is really so. And the existence of all possible worlds may make us more comfortable about the existence of our own world…which seems to be in some ways a highly improbable one.
    • John S. Bell, "Six possible worlds of quantum mechanics", Proceedings of the Nobel Symposium 65: Possible Worlds in Arts and Sciences. (1986)
  • Fortunately, a minority of physicists, myself included, likewise side unequivocally with realism, by adopting Hugh Everett’s multiple-universes interpretation of quantum theory. According to this view, no particles exist where they have insufficient energy to be; it is simply that in some universes they have more energy than average, and in others, less. All alleged “paradoxes” of quantum theory are similarly resolved.

    So, while most accounts say that Bohr won the debate, my view is that Einstein, as usual, was seeking an explanation of reality, while his rivals were advocating nonsense. Everett’s interpretation doesn’t make Einstein a demigod. But it does make him right.

  • It is true that the MWI, in this realist form, avoids some of the paradoxes of QM. The so-called “measurement problem,” for example, is no longer a problem because whenever a measurement occurs, there is no “collapse of the wave function” (or rotation of the state vector in a different terminology). All possible outcomes take place. Schrödinger’s notorious cat is never in a mixed state of alive and dead. It lives in one universe, dies in another. But what a fantastic price is paid for these seeming simplicities! It is hard to imagine a more radical violation of Occam’s razor, the law of parsimony which urges scientists to keep entities to a minimum.
    • Martin Gardner, "Multiverses and Blackberries", Skeptical Inquirer (2001)
  • Some very good theorists seem to be happy with an interpretation of quantum mechanics in which the wavefunction only serves to allow us to calculate the results of measurements. But the measuring apparatus and the physicist are presumably also governed by quantum mechanics, so ultimately we need interpretive postulates that do not distinguish apparatus or physicists from the rest of the world, and from which the usual postulates like the Born rule can be deduced. This effort seems to lead to something like a "many worlds" interpretation, which I find repellent. Alternatively, one can try to modify quantum mechanics so that the wavefunction does describe reality, and collapses stochastically and nonlinearly, but this seems to open up the possibility of instantaneous communication. I work on the interpretation of quantum mechanics from time to time, but have gotten nowhere.
    • Steven Weinberg, in "Questions and answers with Steven Weinberg", Physics Today (2013)

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