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This is dependent on your point of view. The Copenhagen interpretation is probabilistic. However, there are reasons to believe that wavefunction collapse is an

This is dependent on your point of view. The Copenhagen interpretation is probabilistic. However, there are reasons to believe that wavefunction collapse is an illusion brought on by something called decoherence. The gist of decoherence is that the interaction of a quantum system with its surroundings couples that system to its environment, bringing about the appearance of wavefunction collapse, when the system + environment is still a single quantum system evolving together. This is a purely philosophical matter, though, and beyond the scope of science, since such things are not testable. However, there is a mathematical reason to prefer decoherence over Copenhagen, which is that wavefunction collapse is inherently non-unitary, which is to say that it's irreversible. The measurement process in Copenhagen is akin to applying a non-invertible matrix to the wavefunction, resulting in its appearance in a single state. The problem with this is that all known processes in quantum mechanics evolve according to unitary transformations. Interactions with electromagnetism and the strong and weak forces all involve unitary transformations. So where does this non-unitary behaviour come from? Decoherence theories suggest that when you're performing a measurement, this is a single transformation on the system + environment, which is still unitary, but when you examine the state of the system, for a given state of the environment, it appears to be non-unitary. In practice, however, measurements are still probabilistic, so in that sense it doesn't really matter; this is mostly just a philosophical question. However, in that sense, if somebody has this point of view, they could perhaps say that quantum mechanics is just as deterministic and not any more probabilistic than, say, classical statistical mechanics, where there's the appearance of randomness not because of non-deterministic laws of physics, but because of various issues like - even if they were classical particles, you couldn't hope to measure their positions and momenta - even if you could do so, calculation would be impractical to the point of impossibility - even if you had a supercomputer that was up to the feat, minuscule amounts of initial error (or accumulated rounding error over time) would result in drastically different long-term behaviour These combined would mean that only summary statistics and expectation values could be offered for such a system, even if all of the dynamics were fully deterministic with classical particles.

l33tman 5y ago
Decoherence is easiest to see in a MW setting. The basic observed fact is that at some point you need an observer that makes a clear, consistent observation. Decoherence is a concept about an isolated state (that can be multi-valued, like dead cat / alive cat) that leaks out into the environment by entanglement with more and more environment states, and proceeds to "pull in" versions of you (as the observer) one by one as the versions of you are also entangled with the environment. You can recurse this analysis and eventually you reach a point where the "leaf" quantum state only involves one outcome and is entangled with a single version of you, and at that point you don't need any collapse anymore. Also this process is completely deterministic. The Copenhagen interpretation is a shortcut that basically says let's ignore all those intermediate interactions between the original "weird" state and the observers, in a very real sense you "collapse" the chain of entanglements. You also ignore that you have multiple observer versions of yourself and instead explain it as a probabilistic distribution from the point of view of the "pre-split" observer. This has turned out to be a good enough strategy in practical physics since the 1920s! Note that none of this explains the quantitative aspect of it, the Born rule, or what it actually means that something's more probable than not. It just discusses how you can start with superpositions and end up with observers who don't see the superpositions.
simiones 5y ago
> However, there are reasons to believe that wavefunction collapse is an illusion brought on by something called decoherence. Decoherence itself doesn't explain the (apparent) wave-function collapse. Decoherence just tells you why the quantum states can't self-interfere after coming in contact with "the environment" (such as in a measurement). You still need to invoke some other explanation why the non-interfering states can't both be observed (or shut up and calculate). In MWI, this other explanation is the claim that "a classical observer" is an entity that only exists in one "branch" of the state space at one time, and can only perceive the solution to the wave equation they are entangled with. But this is an extra assumption entirely equivalent to the Born rule, though it is more intuitively acceptable in some way.

l33tman 5y ago

Decoherence is easiest to see in a MW setting. The basic observed fact is that at some point you need an observer that makes a clear, consistent observation. Decoherence is a concept about an isolated state (that can be multi-valued, like dead cat / alive cat) that leaks out into the environment by entanglement with more and more environment states, and proceeds to "pull in" versions of you (as the observer) one by one as the versions of you are also entangled with the environment. You can recurse this analysis and eventually you reach a point where the "leaf" quantum state only involves one outcome and is entangled with a single version of you, and at that point you don't need any collapse anymore. Also this process is completely deterministic. The Copenhagen interpretation is a shortcut that basically says let's ignore all those intermediate interactions between the original "weird" state and the observers, in a very real sense you "collapse" the chain of entanglements. You also ignore that you have multiple observer versions of yourself and instead explain it as a probabilistic distribution from the point of view of the "pre-split" observer. This has turned out to be a good enough strategy in practical physics since the 1920s! Note that none of this explains the quantitative aspect of it, the Born rule, or what it actually means that something's more probable than not. It just discusses how you can start with superpositions and end up with observers who don't see the superpositions.

simiones 5y ago

> However, there are reasons to believe that wavefunction collapse is an illusion brought on by something called decoherence. Decoherence itself doesn't explain the (apparent) wave-function collapse. Decoherence just tells you why the quantum states can't self-interfere after coming in contact with "the environment" (such as in a measurement). You still need to invoke some other explanation why the non-interfering states can't both be observed (or shut up and calculate). In MWI, this other explanation is the claim that "a classical observer" is an entity that only exists in one "branch" of the state space at one time, and can only perceive the solution to the wave equation they are entangled with. But this is an extra assumption entirely equivalent to the Born rule, though it is more intuitively acceptable in some way.