Home › Forums › 2015 International Workshop on Quantum Foundations › Retrocausal theories › Causality and quantum mechanics (Online 7/15 @ 10 p.m. to Midnight UTC-7) › Reply To: Causality and quantum mechanics (Online 7/15 @ 10 p.m. to Midnight UTC-7)
Thanks very much for your interest and comments. I agree with your reasoning entirely. You have raised valid points about temporal (ir)reversibility which is perfectly understandable because our submission is under the Topic “Time-symmetric theories”. But our submission, and I think some other of the submissions here, aren’t really purporting to be “time-symmetric” and we should have made that clear.
I think the term “time-symmetric” is being interpreted rather loosely in this Topic. Probably to most physicists, certainly those without an interest in foundations, it means time-(better motion-)reversal symmetry which is usually learnt in connection with Kramers theorem. I don’t think anybody means that here. Probably the next stage is something to do with time’s arrow(s). But I think ““Time-symmetric theories” has come to mean anything involving things like retrocausality, or states propagating backwards in time.
Anyway, to get back to your points, measurements A and B (and C involving Kraus operators) certainly remove any possibility of “time-symmetry”. A and B are there to record the “effects” of the “cause”. The aim is to see what it is in the algebra (rather than ideas about “causality”, etc) which prevents C changing A when it is able to change B. Also, at first at least, it seems surprising that C could change B when C was unitary and the initial state was maximally mixed (this is not dealt with explicitly in the short version in our submission – it’s rather obvious on reflection but it does need a larger Hilbert space where the initial state becomes not maximally mixed).
Finally, I agree that if one wants to talk about properties not directly the result of measurement, so time-symmetry can be properly talked about, then CH is the most thoroughly thought-out and justified way to go.