From what I’ve read, according to the Many Worlds theory, every time I make a choice, a new universe springs into being in which I make a different choice. But what constitutes a choice? Does it have to be a conscious, binary choice? For example, there are a large number (infinite?) of things I’m NOT doing right now, including running around my workplace naked except for clown makeup. Does that mean that there’s a universe in which I am doing that? And am I just getting lucky to keep ending up in the universe in which I’m behaving well? Or does the theory only apply when I have to actually consider what to do next? (Until now I’ve never considered running around naked at work with clown paint on).
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These questions are actually good questions…against other interpretation of quantum mechanics, but not Many World itself. Many World is basically the interpretation that said “I don’t have to care what ‘choice’ really mean”.
The better description of many world interpretation would be “there are no trimming of worlds, particles don’t make choices, and there are no special physical process called ‘measurement’ that has new mechanism”. The reason why many world interpretation often was described as “a new world spawn whenever a particle make a choice” is because they’re trying to contrast it against other interpretation when the particle do make choices. Unfortunately, that gives the wrong impression that many world interpretation tackle on an ad hoc hypothesis (many worlds) on top of poorly defined concepts (measurements and choices), when in fact, it’s the opposite, it removes an the poorly defined concepts (measurements and choices) commonly seen in other interpretation, and let the rest of the math happen.
Let me elaborate.
Quantum mechanics, on its own, already lead to many world. Any system has many classical states (technically speaking this is not quite correct either, but simplicity for the sake of ELI5). If you have a small system, these different states can interfere with each other easily. If you have a huge system, it’s astronomically unlikely that these states can interfere with each other. When you have a huge system, its states are so unlikely to interfere with each other, these states are called “world”.
But there is no clear dividing line between what’s a state and a world. When you have a small system – that was previously isolated – interact with a large system, the states of the small system rapidly separate from each other until they become unlikely to interfere, this process is called decoherence. This is a “measurement”. From the perspective of one state in the big system (one world), the small system made a choice. But there are no clear dividing line here. The small system gradually (but rapidly) has its states separated from each other, but there are no single instance of time where you could say it had made a choice, no single instance of time where you could say they are now in different world. The bigger your system, the more separated the states, and the less likely the interference, but there are no exact dividing line where the system is big enough that the states are now world, just a gradual decline in interference. There are no precise instance in time when the small system interact with the big system either, the amount of interactions can vary which affect how fast these states get separated.
As an analogy, asking exactly when a new world is spawned would be like asking the exactly when a baby is born: is it when its head pop out of the womb, is it when the entire body is out, is it when it takes the first breath? There are time when we are sure the baby is not yet born, and there are time when the baby is clearly already born, but there is a time gap in between when there are no clear answer. Our concept of “the baby is born” is just an imprecise description of what happened physically.
The above situation are true in quantum mechanics, and hence is true in all interpretation of quantum mechanics.
What Many World Interpretation said, is that all of the term above, “world”, “measurement”, “choice” are just our imprecise description of the underlying deterministic physics. It’s the one interpretation that does not care about what these word means exactly.
But most other interpretation of quantum mechanics care about what these word mean exactly, because they also suppose that there is only 1 world in the entire universe. Somehow, despite all of the spawning of world that keep happening, these extra world get trimmed away, somehow, so that the entire universe has only 1 world. This trimming of worlds is called a collapse, and it happens during measurement. The difficult problem of determining exactly what is a measurement – so that we know exactly when a collapse occur – is called a measurement problem.
That’s why the issue of when the particles make choices is a question for other interpretations of quantum mechanics, but not many world itself.
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