vanhees71 said:
I don't need a collapse, I need Born's rule. That's it.
I don't see how it is possible to understand the Born rule without collapse unless one dives into tackling the MWI problem of how to interpret probabilities for a unitarily-evolving wave function.
Let's take the simplest case of measurement, which is just a spin measurement along some fixed axis. So suppose I have a device that measures spin. For definiteness, let's just say that there are two LEDs, one labeled "UP" and one labeled "DOWN"; if a spin-up electron enters the device, the "UP" light comes on, and if a spin-down electron enters the device, the "DOWN" light comes on.
So the Born rule says that for such a device, if we supply it with an electron whose spin state is a superposition of the form \alpha |u\rangle + \beta |d\rangle, then the "UP" light will come on with probability |\alpha|^2 and the "DOWN" light will come on with probability |\beta|^2.
To me, that way of describing the measuring device assumes that the state of the device after interacting with the electron is in a "collapsed" state of either one light being on, or the other light being on. If it were computationally feasible to analyze the device (and the environment, and possibly the rest of the universe) using quantum mechanics, you would not find that it has a state of definite value for which light is on. Instead, you would find that there is some amplitude for the device (plus the rest of the universe) to have one light on, and some amplitude for the device to have the other light on.
But we don't see that. We see that the device is in one of two possible definite states. That sure seems like "collapse" to me. The property "which light is on" takes on a definite value. To me, that seems inconsistent with the minimalist interpretation as applied to microscopic objects such as electrons. For an electron, you don't say (and
can't say, as Bell's theorem shows us) that it has a definite value for its z-component of spin at all times. So why does the measuring device have a definite value for the property "which light is on"?