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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.

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.

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?

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.

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.

> 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.

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”.

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.

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.

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.

Why from all people?

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

Wigner’s friend?

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

That’s something only conscious beings can do.

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

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

Until he talks to them, how can Wigner tell? ;-)

Wigner can’t tell, but the friend knows. If he thought he was in a superposition he would believe in many worlds.

But shouldn't every friend in every possible state observe a single state that's consistent with their own internal state?

The friend only exists in many worlds from Winger's perspective, and only as long as their state has no causal effect on Wigner's own state.

What's wrong with many worlds?

It’s not what this thread is about.

I guess I got confused at "If he thought he was in a superposition he would believe in many worlds."

Do you mean "he can either think he can be in superposition or he cannot be, if he thinks he can be then he would believe in many worlds, which is incompatible with 'traditional' QM, which thus implies 'Traditional QM does not allow Wigner’s friend to be in superposition.'" ?

I don't think the Traditional QM ever explicitly disallowed a person to be in superposition; being in superposition it's just not something "decent people" do, I suppose.

Thus we needed to find other ways to come to terms with what we observe. But the math is the same. The rest is only metaphors that help us reconcile what we observe with how we feel inside.

Thus, many worlds is not antithetic to traditional QM.

Do you think that many quantum physicists during the 21st century accepted the possibility of living in a splitting multiverse?

I don't think that's historically accurate. If you read about how Everett's ideas were treated it's pretty clear traditional QM is antithetic to many worlds.

Sure, I agree with you that most people vehemently rejected that interpretation.

But the QM theory is not its interpretation.

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.

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

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?

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.

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.

> 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.

> 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.

> 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?

In quantum computing, I can implement important effects using the measurement operation. For example, I can reduce the number of Toffoli gates used during an addition by measuring at particular places in the circuit. From this we can easily see that measurement must matter a lot in quantum mechanics, since its presence allows you to reduce the cost of certain tasks.

No one has postulated a "conscious measurement operation" with additional useful effects beyond the measurement defined in textbooks. (Well, okay, Roger Penrose says things that kinda sound like that, but that's pretty fringe and beyond the scope of quantum mechanics.) In fact, there's literally no known experiment that a person or a machine could perform that would distinguish a "conscious measurement" from the usual mechanical measurements quantum computers use. That is the sense in which consciousness has nothing to do with measurement.

IMO quantum mechanics really doesn't have anything to say about consciousness that wasn't already present in classical mechanics. Instead of saying "but how can an assemblage of gears experience an outcome" we're saying "but how can an assemblage of superposed gears experience an outcome". It's just the same hard-problem-of-consciousness confusion dressed in new clothing.

> Consciousness comes indirectly into quantum mechanics through the measurement postulate. You have to assume something special about measurement which breaks unitarity. Not all physicist are convinced, but it is the traditional way quantum mechanics is taught (all of the "transition probabilities" are really measurement probabilities). The problem is that you can't tell if your apparatus caused the collapse or if it was you that measured the apparatus that then caused the collapse. And because you can only know that something was "observed" when a conscious physicist makes that final readout you end up with a solipsistic situation where the only thing you can be certain of is that it was that last conscious observer that could have for sure collapsed the wave function.

There are experiments that show decoherence in absence of conscious measurement. For example, https://www.physics.upenn.edu/~pcn/Course/250/Week.of.02.21/...

> Here we report matter wave interferometer experiments in which C70 molecules lose their quantum behaviour by thermal emission of radiation. We find good quantitative agreement between our experimental observations and microscopic decoherence theory. Decoherence by emission of thermal radiation is a general mechanism that should be relevant to all macroscopic bodies.

> We observe that at temperatures below 2,000 K the emission rate is negligible, whereas at higher temperatures the molecules may emit photons whose wavelengths are comparable to (or even smaller than) the maximum path separation of ,1 mm. They transmit (partial) which-path information to the environment, leading to a reduced observability of the fullerene wave nature. Around 3,000 K the molecules have a high probability to emit several visible photons yielding sufficient which-path information to effect a complete loss of fringe visibility in our interferometer.

I don't think there is consensus that decoherence = measurement. This is a sophisticated experiment that controls the level of position information being "radiated" to the environment. But Born's rule and the measurement postulate are still baked into the fact that you are then measuring the interference pattern.

One thing I never quite understood about the decoherence argument for resolving the measurement problem is that it forces the "collapse" to happen with local interactions. This of course makes sense since you want to think of a measurement as a "normal", unitary process. This means the environment has to "enact" the projection operator through interactions. If you have the most basic EPR pair, how can this happen? How can you decohere the wave function of the spacelike separated counterpart (B) by locally measuring the other spin (A) and then letting the environment project out the state of B through local interactions? In this experiment https://arxiv.org/pdf/1511.03190.pdf they have two detectors 60 meters apart in a subbasement of a castle. What are the interactions in those hallways that are carrying the correlation from the decoherence/measurement on one side of the experiment to the other, faster than light to maintain the correlations. This seems too basic of a question so I don't know if I'm just missing something obvious, but to me it seems like a pretty straightforward argument that the collapse cannot happen through decoherence. I've only dug out one random paper that mentioned this argument with very little subsequent referencing of it.

Certain experiments have shown that quantum mechanics is incompatible with local realism, and this means either that locality is wrong, and the world has faster-than-light communication, or that realism is wrong, and that there are no single outcomes to events. Assuming that locality is true and realism is wrong, when you measure particle 1, it's not that the result is being instantly communicated to particle 2 and causing it to come out that way, but all possible outcomes for each particle measurement happen separately, and all versions of things affected by particle 1's measurement only interact with versions of the world where particle 2 has or will have a compatible measurement. No collapse ever happens, though from each of the perspectives of humans inside the superposition, they will see results that look like their own measurement caused a collapse, because they each will eventually see a single consistent outcome for the results of the particles' spins.

The results of nonlocal and nonrealism theories come out the same, though nonlocal theories imply a lot of mysteries around the contradiction between instant communication and relativity of simultaneity, while nonrealism just paints a picture of a MWI universe that's surprisingly larger than our expectations.

This is true in the same way for any observable phenomena, so what's strange here that qm is seems special to scientists in this regard.

Even so... do you consider the cat to be conscious? Or does the observer have to be human? Or does the observer have to be you?