SpectraCat said:
Of course we are talking about a wavefunction .. what else would you use to describe the system in terms of standard quantum mechanics (which is what we are discussing here)? The "particle Z" system that you mention below *is* a wavefunction in terms of standard QM.
Yes, that is what we are saying. In standard QM, "real particles" are described by wavepackets, which have non-zero finite widths in both position and momentum space, and thus can only provide probabilities of observing particular eigenvalues of position and momentum. A measurement samples the probability distribution defined by the square modulus of the wavepacket, and thus prior to the measurement, it is fundamentally impossible to predict the definite values of observables that are implied by the context of your question. In other words, the proper way to frame the question in QM is, "Given particle Z, what is the probability that a measurement of position (or momentum) at time t, would have the value x (or p)?"
My argument does not depend on the type of prediction made by QM, but the fact that
a prediction is being made. So let us try again using your terminology:
Prediction 1: "A position measurement on particle Z at time t has a probability distribution ρ(x)"
Prediction 2: "A momentum measurement on particle Z at time t has a probability distribution ρ(p)"
Surely you admit that such predictions can be, and are made in QM. If you disagree, say so.
Both predictions are valid! Do you disagree? Now suppose, I decide to perform the position measurement at time t. I will in fact find that the observed position is in agreement with the QM prediction, confirming that QM made a valid prediction. However, it is no longer possible to do a momentum measurement on the same particle, but had I performed the momentum measurement instead, I would have definitely obtained a result consistent with QM.
In other words, QM predictions about previously possible experiments which are no longer possible are conterfactual definite statements just as well. If you deny this, the only alternative is to conclude that once the position measurement is performed, the momentum prediction becomes invalid and you run into a situation in which QM can not make a valid prediction at all.
Now in case the above point is still not clear, let me give another example closer to Bell. Bell's inequality is the following:
|P(a,b) - P(a,c)| <= P(b,c) + 1
Let us write down the meaning of the terms in plain English:
A stream of particle pairs are heading towards Alice and Bob each of whom will set their detectors to a chosen angle and measure an outcome"
1) What is the expectation value of the paired-product of the outcomes if Alice and Bob choose settings (a,b) respectively? --> P(a,b)
2) What is the expectation value of the paired-product of the outcomes if Alice and Bob choose settings (a,c) respectively? --> P(a,c)
3) What is the expectation value of the paired-product of the outcomes if Alice and Bob choose settings (b,c) respectively? --> P(b,c)
As you can see, before any experiment is performed, all three terms are possible and valid terms to be calculated. But clearly, if Alice and Bob have already set their devices to (a,b), in one experiment, the other two terms, are counterfactual definite (ie, "had Alice and bob set their devices to (b,c), they would have obtained the expectation value P(b,c)", same for (a,c))
It is already clear from this, why it is impossible to perform an experiment to verify Bell's inequality. Because it is impossible to recover the particles and measure them at a different pair of settings. If you measure three different streams of particles, the terms you get from those streams can not just be mixed in the inequality willy-nilly without paradoxes. It will be similar to mixing and matching terms from the following three inequallities, and expecting to get the same results:
|P(a1,b1) - P(a1,c1)| <= P(b1,c1) + 1
|P(a2,b2) - P(a2,c2)| <= P(b2,c2) + 1
|P(a3,b3) - P(a3,c3)| <= P(b3,c3) + 1
Although each one is a valid Bell's inequality which can never be violated, there is no mathematical or logical justification for expecting the inequality:
|P(a1,b1) - P(a2,c2)| <= P(b3,c3) + 1
to be valid. This is why you get a violation from experiments. If anybody thinks the above inequality is valid, they should be able to derive it easily.
What about QM. Same thing, the three terms calculated from QM are also related in a similar way. If one is actualized in a real experiment, the other two are counterfactually definite and therefore violate the inequality. The predictions from QM for P(a,b), P(a,c), P(b,c), although all valid, are not all actual. In fact only one can ever be actualized in a given experiment. So if you are suggesting that in my previous position/momentum example, the momentum prediction becomes invalid after the position measurement, you must also conclude in this case that P(a,c) and P(b,c) become invalid after the P(a,b) measurement, in which case using them in the same expression as Bell did will be a mathematical error! But if you admit as I do that they are all valid, but not actual, then you must admit also that, an experiment which can produce the three terms for Bell's inequality is impossible to perform.
As I hope is clear now, the violation of Bell's inequality by QM and experiment, has nothing to do with whether particles have objective properties at all times, or whether locality or non-locality is involved. It has simply to do with a misunderstanding of the difference between
possibilities and
actualities.
Everything that is actually true is possibly true; but not everything that is possibly true is actually true.
Everything that is necessarily true is actually true; but not everything that is actually true is necessarily true.