[brummm]

This is misleading at best. A measurement has nothing to do with a conscious observer taking the measurement. An automated experiment that could note down the result of an experiment would note down the result without any conscious observer having to check in and the result would be the exact same.

[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?

[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?

[elektropionir]

How do you know it would be the same? That's an assumption. The measurement problem is a problem because quantum mechanics postulates two types of processes "normal" unitary evolution and "measurements" that project out the wave function into eigenstates of the measured operator. This is obviously inconsistent since there is no definition, formal or operational of what process is a measurement and what process isn't a measurement. How do you know that your measurement apparatus isn't just evolving unitarily (which is what you would expect if it was a "normal" system) until someone looks at it to read out the results. Consciousness enters only in the fact that for anyone to know what happened to the experiment someone has to do the readout (otherwise you're just writing equations). At the point of readout you can't tell who or what did the collapse.

[trenchgun]

To take it further: how do you know that the people checking the results are not just evolving unitarily until you hear about the result?

[jbay808]

Many worlds interpretation of QM is that everything continues evolving unitarily, even you when you hear about the result. It's just that "being in a superposition" doesn't feel like listening to the garbled sound of someone simultaneously telling you that the result was positive and that it was negative, or looking at a blurry instrument screen reporting two results at once. Each component of you in the superposition feels like it got a single clear definitive measurement result. It feels the same as not being in a superposition.

[ithkuil]

It appears as if one of the main reasons for almost a-priori rejection of the Many Worlds interpretation lies in the words "many worlds". More often this metaphor gets in the way instead of helping. It seems as if you have to accept something "more", yet, at its core, the Everettian interpretation is the simplest pure consequence of QM. We just have to grapple with the psychological consequences if being a cog in the machine and we devise further metaphors to help us talk about how it would "feel" to be in superposition.

Is there a better way to build intuitions how an information processing system would behave while being in superposition?

Things get complicated when we throw humans in the mix, perception and consciousness and all. But modelling even simpler machines and their "point of view" can be insightful.

In this day and age it shouldn't be hard to imagine the working of a computer that uses computer vision algorithms to perceive its world and take action as a result, acting as a "causality amplifier". For example image we feed it the output of some QM experiment and instruct it output a description of what it "sees" (as we routinely do with cat pictures classification).

I assume it would be far less controversial to think about the unitary evolution of the wave function if the system being described is a QM experiment plus a computer rather than the same QM experiment and a human.

Yet, there are many similarities. The output of this computer (the classification) would be in superposition, and when measured by us it would appear as if the wave function collapsed. But we could add another such computer in the mix and ask ourselves "does it also see the wave function collapsing"? Well can program it to take the measurement and record the answer and then convey the answer to us (or to another computer down the chain). These "answers" are the "point of view" of the computer. It will "observe" decoherence yet it won't be decohered itself, as its own statement about whether it observed decoherence is itself in superposition and thus can be used as a further input to other machines witnessing subjective decoherence.

[AgentME]

> It's just that "being in a superposition" doesn't feel like listening to the garbled sound of someone simultaneously telling you that the result was positive and that it was negative, or looking at a blurry instrument screen reporting two results at once.

It seems like there's regularly articles saying stuff like "QM implies that both outcomes happen, but it's a longstanding mystery why we only see one outcome", as if they seriously expect your hypothetical to be the consequence of QM. I'm so frustrated at that, because as you say, seeing one outcome is exactly what you'd expect to see from inside of a superposition.

It's almost as frustrating to see as it would be to see an article saying "Newton's theory of gravity says mass attracts mass, but it's a longstanding mystery why we haven't all fallen into the sun". The theory already has an answer for that if you follow the chain of consequences from it.

[Xcelerate]

I’m not sure why the measurement problem is so difficult to understand for some; your explanation of it is very clear. You don’t have to believe in anything supernatural or mysterious to recognize there is a clear inconsistency here with “quantum systems sometimes evolve unitarily”.