Hi Ken
As a classical example of something sometimes being a cause and sometimes not, I think of the proverbial butterfly in the Amazon jungle causing a hurricane – sometimes it does but mainly it doesn’t.
I think you can only go so far with classical intuitions about quantum causes because as we have found out from nonlocality, etc “common-sense” is not always a good guide, that is why it is good to just calculate the quantum answer. Take the EPRB case and let Alice rotate her spin about the z-axis before she measures. That doesn’t change anything that either Alice or Bob alone can possibly get as a measurement outcome so if it is a “cause” of anything, that thing or effect can’t be Alice’s or Bob’s results alone. (I’m assuming a “cause” must at least under some circumstances be capable of producing an “effect”.) However Alice rotating her spin can change the correlations between Alice’s and Bob’s results. The “effect” is a change in the correlations (joint probabilities) but not the individual probabilities. This doesn’t necessarily involve entanglement because the same thing happens in the corresponding SEPRB cases. Unlike a cause, it is at least arguable there is no way of determining which direction the correlation takes place, ie Alice to Bob and vice versa is the same “effect” (joint probability), including in the two possible SEPB cases. So it still seems to me, for the time being at least, that there is a difference between a cause and a correlator.
To remove any doubt, I don’t think ‘a “cause” could be dependent on what other external observers can *know*’.
Thanks for your interest.
David
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