I would conclude instead that

“Users of quantum theory cannot consistently decide what quantum theory is.”

Once we are clear about what quantum theory is, then deciding about its consistency is easy. For instance, I think we agreed here that QBism and RQM cannot consistently describe their own use.

]]>I am surprised to see your “last word”. I thought that I’d asked a simple, clear question that cuts to the heart of this problem. And you have not answered it.

To take a geometric approach; this system can be drawn in 4 dimensional Hilbert space. In that space, there is a plane corresponding to the measurement z=+1/2.

In statement /W n:22, when you deduce that z=-1/2 is not the case, it implies that the state vector (relative to you) is on this plane. Since it’s a unit length vector, that leaves it with one degree of freedom. The intersection of the Bloch sphere with the plane is a circle.

Then you assume one specific state vector. And you carry on with that assumption.

What is your justification for ignoring the rest of this plane?

]]>“Users of quantum theory cannot consistently decide whether the theory can consistently describe its own use.” 🙂

(This includes professional users, see, e.g., https://www.quantamagazine.org/frauchiger-renner-paradox-clarifies-where-our-views-of-reality-go-wrong-20181203.)

]]>I also thank you for hosting this prolonged discussion. I wonder have you checked the analytics. Are we alone here, talking in an echo chamber?

Andreas #287: you said:

>> “it seems as if the future measurement of Wbar has an influence on which events do happen in the past”

I began to study this Gedankenexperiment without a clear picture of how QM works, and this experiment steered me into the RQM interpretation, precisely because it doesn’t require a rewriting of history.

In RQM there is no objective state of the system. Every “observer” holds a different version of the state, and this gets changed when they make a “measurement”, but there is no collapse and it changes for them only.

Consciousness is not involved. State information spreads via the Wave Function, and the region over which it’s available (e.g. the inside of a Lab) can be visualised as a “world”. Multiple “worlds” can coexist in the same region, and a “measurement” does indeed split them off as branches.

So, in this interpretation: Wbar and W exist in the same “world”, the external environment.

The system state *relative to that world*, is (at first):

( tu + td + hd ) / root(3)

and, as you said, we can rewrite this in other bases.

The beauty of RQM is that it allows F and Fbar to “see” different system states from their branches and worlds inside their labs.

For example, the state seen by F, in the branch and “world” where she measured “UP”, is:

(tu)

about which you wrote:

>> “Behind her back there is also the other half of “tails”, i.e. |tails>|down>”

It’s not really “behind her back” – it’s in another “world”, accessible to some agents but not to her.

]]>Andreas #288: I don’t think there is anything left to discuss. In my point of view the contradiction comes from the assumption that both Wigner’s and the friend’s perspectives are equally valid. Renato agrees that this assumption is necessary, but for him it is just quantum mechanics. I think this assumption is nonsense, and moreover only defended in QBism and RQM, so one can hardly claim that it is just quantum mechanics. In any case, I doubt that further discussion will resolve this disagreement.

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