Why is Everett (many worlds) interpretation so unpopular?
Frequently is not even mentioned, as it was not so plausible, or so implausible, as Copenhagen.
Frequently is not even mentioned, as it was not so plausible, or so implausible, as Copenhagen.
It's appealing for its mathematical simplicity, giving unitary wave function evolution with the reasonable assumption that the universe has a wave function. It eliminates wave function collapse from the axioms, and possibly eliminates the Born rule.
Talking about its 'complexity' in the 'creation' of parallel worlds isn't much of an issue, you can look at simple systems like the von Neumann measurement model to see how they naturally arise through superposition.
The main issue is how the Born rule arises. That is, why the probability of an 'observer' ending up in a particular branch of the wave function is |psi|^2 isn't clear, but then it isn't clear what probability means in this context...
Question 12: What is your favorite interpretation of quantum mechanics?
Top Three:
Copenhagen: 42%
Information-based/information-theoretical: 24%
Everett (many worlds and/or many minds): 18%
"Many worlds" suggests the opposite: That physical quantities such as mass are forever increasing, albeit in a way we can't experimentally detect.
I think that's a little uncharitable. In classical terms you could say that each "universe" is associated with some weight or probability, and those probabilities always sum to one.
I guess you could say that the "number" of things increases (provided that number is countable and finite, I guess, which seems a bit silly), but the probability-weighted mass of things going on stays the same.
Of course, it isn't terribly useful to talk about all of the inaccessible universes taking up the majority of the probability, so we condition on our observations (i.e., we do a Bayesian update, or "collapse the wavefunction".)
Then if we talk about "our universe" having probability 1 given our observations, and surmise that people in another parallel, inacessible universe would also say that theirs has probability 1, we might say "Aha! There is now a total of 2, where once there was a total of 1!" It should be clear how this is an error, though.
The other one is about the observer affecting the universe in an unknown way.
there is no pure "observation" in QM like the one we can imagine in classical case. Any observation in QM is actually an interaction. While i'm not a fan of Copenhagen, there is nothing un-materialistic that an interaction with the system affects its state, like a coin balancing on its edge finally would finally fall on one side upon you touching it.