[akvadrako]

No, he is basically correct. Traditional QM requires an observer who is outside the system being measured to cause non-unitary evolution.

You can always consider your automated apparatus to be part of the system and hence governed by unitary evolution.

[millstone]

Traditional QM requires an outside observer but has nothing to do with consciousness. Consider the collision events recorded at the LHC: the vast majority have never been looked at by a human.

[wongarsu]

> Consider the collision events recorded at the LHC: the vast majority have never been looked at by a human

What tells you those records aren't in a superposition?

The only way to determine anything about the records is to observe them, and at that point you have a human (= consciousness) in the loop. Anything you put between yourself and the experiment might be in a superposition until you observe it.

[ithkuil]

> What tells you those records aren't in a superposition?

This! Now substitute the record taking device and record keeping substrate with a human brain and this stays true. To rephrase your question:

"What tells you the state of your brain isn't in superposition?"

A human in the loop is not the end of the story, it's just yet another interaction in the quantum system.

When we experience decoherence of a quantum system, we interpret it as if something happened to the thing we observe, yet what actually happens is that something happened to us (and in turn to every system that observes us, and so on).

This is all utterly unintuitive for us, who experience the world through those brains, who feel being there, conscious in the moment. That feeling is one of our strongest direct perceptions of the world, and yet it's affected by the mechanisms of the physical reality. It's hard to accept though; it runs counter to many deep intuitions we have about ourselves, our inner lives, our identity, our values, our belief systems.

[rbanffy]

Wouldn't it be valid to consider the LHC and its unobserved data as a superposition of all their possible states?

Note that the recording of information has effects outside the system, even if no human looks at the data - recording a zero or a one will require different levels of power that, while they average to a mostly constant power consumption, are there nevertheless.

[akvadrako]

They have not been explicitly looked at, but they have been observed in the QM sense. That’s because decoherence has spread those records to conscious observers.

If the results had been kept completely isolated from all people, you could still say a measurement hasn’t occurred.

[millstone]

That's just not what traditional QM says. Traditional QM separates quantum systems (microscopic) from measurements devices (macroscopic). A measurement occurs when the measurement device interacts with the quantum system. You do QM by predicting the results of these measurements. Consciousness does not enter into it.

[TheOtherHobbes]

So why doesn't a measurement and/or decoherence occur when double slits or a half-silvered mirror interact with a passing particle? Are they not macroscopic objects which interact with the quantum information?

Perhaps it's only a "measurement" if the (alleged) particle has nowhere else to go after the interaction. But how does the (alleged) particle know which macroscopic interactions are terminal and should be counted as "measurements" and which are part of the rest of the experiment?

There is no answer to this in QM. You can calculate the probabilities and you will get predictable answers, but there are still >20 interpretations of what is really happening, and they all disagree with each other in important ways.

[BadThink6655321]

Until you get information from the measurement, it didn’t happen.

[rbanffy]

Can it be considered as until it causally affects something else (the observer), its state is not defined? Isn't it a causal relation that collapses the superposition?

[trenchgun]

Why from all people?

What if there is a person in an lab which is isolated from the rest of the people?

[kgwgk]

Wigner’s friend?

[akvadrako]

Then he will certainly be able to say a measurement happened.

That’s something only conscious beings can do.

[rbanffy]

But then isn't Wigner's friend also in a superposition until Wigner talks to them?

[akvadrako]

Traditional QM does not allow Wigner’s friend to be in superposition.

[rbanffy]

> You can always consider your automated apparatus to be part of the system and hence governed by unitary evolution.

But then wouldn't an external measuring apparatus (or person) observe the system in a single state instead of a superposition? Isn't that the same as a series of nested systems, each measured and having the state recorded by an apparatus that's part of a system that encapsulates it?