timmdeeg said:
You've been here long enough to know not to use Wikipedia as a reference, particularly for a topic like this.
timmdeeg said:
how do you define the "measurement problem"?
The measurement problem is the problem of why we observe single definite outcomes when we do experiments on quantum systems. In the basic math of QM, this is simply put in by hand as the projection postulate: there is nothing anywhere else in the math that predicts it. The rest of the math says that when you do an experiment to make a measurement you entangle the system being measured with the measuring device and end up with a superposition of different possible outcomes. The only way to get a single definite outcome out of that in the basic math is to put in the projection postulate by hand--i.e., just declare by fiat that we collapse the wave function in the math whenever we have to to make predictions for future experimental results come out right.
On a collapse interpretation, the projection postulate becomes an actual physical law and wave function collapses become actual physical events (instead of just mathematical devices to make correct predictions for future experimental results). But then they have to explain how these collapse events happen and how correlations between spacelike separated measurements on entangled particles can violate the Bell inequalities without actual faster than light signaling.
On a no collapse interpretation like the MWI, the projection postulate remains just a mathematical device, and the rationale for applying it is that, once decoherence happens after a measurement, the different branches of the wave function can never interact with each other again, so in each individual branch we can apply the projection postulate to get an "effective" wave function for that branch that works for predicting future measurement results in that branch, even though we "know" (if we accept the MWI as true) that there are other branches in the overall wave function. But this requires one to accept all of the other things that come along with the MWI and all of the other issues that have been raised with it.
On an interpretation like the Bohmian interpretation, while collapse of the wave function is not an actual physical process (the full wave function is always there), measurements have single outcomes because those outcomes are determined by the underlying, unobservable particle positions, and each particle always has a single definite position. But the equation of motion for these definite particle positions is highly nonlocal, which many people find very difficult to accept. Also, on this interpretation, when you dig into the details of how measurements of anything other than position actually work, you realize that what you are actually measuring when you think you are measuring, say, the spin of an electron, looks nothing like what you expect a spin measurement to look like.