In QM, how can all people see something and all report the same thing? Wouldn’t 1 person’s observation cause their reality to branch off?

Quantum physics, without any additional “interpretation”, is just a tool for predicting the probabilities of various possible future observations from knowledge of other observations we have made in the past. To do so, it summarizes the observer’s previous observations (up to the point of the observer’s last interaction with the system) in what is called the “state” of the system relative to that observer. Any new observation ends the period of isolation of the system from the observer and so requires that a new relative state be defined taking into account the result of the most recent observation.

(Actually the “observer” of the system here doesn’t have to be a person or any other conscious entity. Any other physical system that it could interact with will do – with observations just corresponding to changes of the state of the observing system relative to any other “external” observer.)

It turns out that all observers who are isolated from the system during an experiment, and who start with the same information about the system, can use the same mathematical object to represent its relative state and for making predictions about the outcome; and this has led to the idea that the state is somehow completely independent of the observer – with various convoluted “interpretations” being added to “explain” what is “really” going on. But none of these adds anything in the way of useful predictions, and they all lead to various kinds of seeming paradox which get seriously multiplied if you mix different “interpretations” (as pointed out in Johann Holzel’s answer ).

Actually, if some friend, or other observers, (or just other physical systems) observe (or just interact with) the system before you do, then the states of the system relative to them “collapse” in the sense that after the observation (or other interaction) the probabilities of future observations are changed (with some becoming no longer possible and others more likely). But the state of the system relative to you does not collapse until you interact with it – either directly (eg by observing it yourself), or indirectly (eg by observing or communicating with your friend).

Usually it is quite hard to keep things isolated, and so just by being in the same room and sharing contact with the same air and ambient radiation you are effectively always interacting with your friend; so even without consciously learning what the friend has observed you have access to that information and so the collapse occurs for you too at the same time as for the friend. But if we were to keep the friend completely isolated in a pure quantum state (which is not possible for a real person, or even a cat, but might be possible for another microscopic system as the “observer”), then the combination of experimental system and “friend” could be in a pure state relative to you which remains uncollapsed until you actually learn the outcome (either by observing the system directly or by checking with your friend).

But as soon as we have been in contact with one another, the you that I see will agree with me about the experiment, and the me that you see will agree with you.

Source: (250) Alan Cooper’s answer to Quantum physics question If reality or superposition is fixed at observation or measurement, how can all people see something and all report the same thing? Would 1 person obseving something different cause their reality to branch off? – Quora

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