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I am a layman when it comes to physics and especially quantum mechanics. I have seen many documentaries on the subject, and often in these productions there is a physicist featured explaining the "many worlds" interpretation of quantum randomness. This individual will give an example of a person making a split second decisions (e.g. step right or step left). The physicist explains that this decision stems from quantum fluctuations in the particles composing the hypothetical individual's mind, and, if assuming the many worlds interpretation is correct, this person walks left and right simultaneously, leading to the existence of two realities: one where the person chose left and the one where he chose right.

At face value I have no qualms with this; however, this implies that all quantum fluctuations lead to multiple worlds where the position of the particle in question and every particle in existence assumes all points in its probability space simultaneously, branching into a mind-boggling infinity of realities in one "step" for lack of a better term.

Assuming the many worlds interpretation is correct, the number of branching of realities is stupendous for any given moment.

The implication of this is incomprehensible to me. Am I looking at this idea correctly? Or am I missing something that would make this idea a little more bearable?

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There are two misconceptions in your examples and there is another misconception in the reply to your question by anna v.

The first problem concerns what happens when a person chooses whether to turn left or right. If the person in question has a reason to go to a specific place and he has to turn left to do so, then he will turn left with extremely high probability, quantum fluctuations in his brain notwithstanding. Your brain did not evolve to be extremely responsive to random noise since that would get in the way of it helping you spread your genes.

The second problem is with the way you say the particle "assumes all points in its probability space simultaneously". This is not very clear. In the MWI the structure of the multiverse is determined by the flow of information. A universe is a structure within the multiverse in which information flows freely. So there may be a version of you that is sitting one millimetre to the left of where you are currently sitting but you can't interact with him owing to decoherence and so he is in a different universe. But at some point for two versions of you that are close enough, they can still interfere and so you and that "other version" are not in separate universes. For any particular purpose you have in mind there is some finite (but possibly large) number of versions of any particular system you might interact with.

The third problem is that you seem to regard the number of universes as relevant to judging whether the theory is true. It is not relevant. If we're just going to bar explanations with large numbers of entities then presumably we should dump the atomic theory and the theory that other stars exist since there are large numbers of both. And in any case, what is the standard for judging that a number is large? Is 100 a large number, or 1000, or 1 billion?

Finally, many people seem to think that the MWI is some optional add on to quantum mechanics. It is not. It is just what you get when you apply the theory to macroscopic objects. Now, anna v claims that the MWI makes no testable predictions but this is false. This can be seen by contrasting it to, say, the Copenhagen interpretion. The MWI claims that some quantum mechanical equation of motion will apply to any system you study including measuring instruments and your own brain. By contrast, the CI claims that somewhere, somehow, in some unspecified way, the equations cease to apply. The MWI makes precise predictions, the CI doesn't. The MWI has been tested, and will continue to be tested, in experiments on decoherence. And the idea that quantum mechanics applies to macroscopic objects solves problems that the CI does not, such as how correlations are established in EPR type experiments. See David Deutsch, Patrick Hayden, 'Information Flow in Entangled Quantum Systems', Proc. R. Soc. Lond. A 456(1999):1759-1774. available at And also David Deutsch, 'Vindication of quantum locality', Proc. R. Soc. A 468(2012), 531-544. available at

Further reading

I don't agree with everything David Wallace says but he does explain some issues correctly

See also many of the papers and books listed here

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Can you comment on counter-factual definiteness and the preferred basis problem? If my measurement choices are a quantum state evolving by the Schroedinger equation, counter-factual (could have chosen to measure a different non-commuting observable) arguments can't get started? – innisfree Mar 21 '14 at 10:46
as I replied to innisfree comment, my answer is not exploring physics papers, just the idea exposed in the question of the possibility infinitely many real worlds. I do not see any extra usefullness except a lot of semantics in the interpretation as far as experiments in the real world go. Experiments do ok with the standard interpretation and the clouds of new semantics are not proposing any new testable experiments. – anna v Mar 21 '14 at 10:51
inifree: A universe is a structure in the multiverse within which information flows. This requires that information is copied from one system to another - it is present in one system before the copying and in both systems afterward. This imposes constraints that imply that only information in orthogonal eigenstates of an observable can serve as a preferred basis W. H. Zurek, Phys. Rev. A 76, 052110 (2007) – alanf Mar 21 '14 at 11:37
inifree: Misunderstood the question. Quantum physics was not invented to solve the problems related to free will. Whether I could have chosen to make a different measurement is such a problem. If we are going to object that it doesn't solve such problems then we also have to throw out every other theory in the history of physics. And in any case decision making is a process that takes place at a higher level of abstraction than physical theories typically deal with. You should be studying something other than quantum theory if you want to solve that problem. – alanf Mar 21 '14 at 11:43
anna v: In order to interpret an experiment properly, you have to understand things like how the measurement apparatus works and what happens to it during the experiment. The MWI explains this, the CI does not. As a result the MWI is making testable predictions, not the CI. You say that experiments in the real world do okay with the standard theory. Really? Then why do so many physicists think wrongly that quantum mechanics is non-local? And why has so much ink been spilled in vain on this non-issue? – alanf Mar 21 '14 at 11:50

The many worlds interpretation of quantum mechanics takes the Platonic ideal to the extreme Pythagorean one "God always geometrizes": That it is mathematics that creates reality and not reality that is being described by mathematics.

They have taken the mathematics of quantum mechanics, a theory we believe describes the microcosm and from which the macroscopic world emerges and turned it into an "ideal" that generates reality. It is an interpretation with no tangible results, i.e., it can make no new predictions for experiments because it just reinterprets the successful mathematical description that does describe reality. It is a bit like the question : "what came first, the hen or the egg"?

Assuming the many worlds interpretation is correct, the number of branching of realities is stupendous for any given moment.

Mathematics is like that, there are infinities upon infinities, but theoretical physicists manage to reign them in and give specific quantum mechanical calculations that have not only fitted real experiments and predicted new results, but are also responsible for the great technological bloom since the 20th century, including this web page we are communicating with ( starting from the discovery of vacuum tubes and transistors to say the least).

The implication of this is incomprehensible to me. Am I looking at this idea correctly? Or am I missing something that would make this idea a little more bearable?

As this interpretation makes no testable predictions it is treated by physicists as just a funny taste of looking at mathematics. It just catches the imagination of people.

A real many worlds interpretation/model enters the realm of metaphysics, and there exist such models in the metaphysical literature, but these are not relevant on this site.

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i think you're too critical. although modern experts disregard the MWI, Everett's ideas influenced modern interpretations and treatments of QM. particularly the idea of a "history", that an observer was entangled with the system, and that the wavefunction need not collapse - we could have unitary evolution of the wavefunction by schroedinger's equation. – innisfree Mar 21 '14 at 8:49
@innisfree I am not judging the usefullness of the work of anybody in particular, just the idea as presented by the OP, that many worlds are alternate realities as we understand reality. – anna v Mar 21 '14 at 10:46

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