I'm currently writing an article on the philosophy of physics. Part of the article involves an experiment which would substantiate whether or not consciousness causes wave function collapse. Many philosophers and physicists define consciousness causing wave function collapse, to be the wave function reduced to an eigenstate, as a result of an observation made by a non-physical mind.

I believe this hypothesis could be tested, by performing an experiment on the brain. Here is an outline of the experiment below:

A volunteer has a superposition of multiple/different images sent to his brain. A scientist is monitoring whether these signals/images are in a state of superposition. During this experiment the decoherence effect should be prevented. This could potentially be achieved by modifying the brain.

Now if the wave function were to collapse down to one particular image (which is being monitored by an experimenter) when the signals enter into the volunteers brain, the only reasonable conclusion that one could reach is that this occurred due to an observation made by a metaphysical mind. This is because by ruling out the possibility of collapse occurring due to decoherence with the matter of the brain (by some how preventing it), the only other explanation, for the wave function collapsing to one particular state would be observation made by a metaphysical mind, since the other alternatives couldn't possibly be the cause (since they were prevented).

In principle, would this experiment work? (I understand that some of these ideas are not fully formed, such as preventing decoherence taking place in the brain, but my main concern is whether the ideas mentioned here, can in principle test the proposition that wave function collapse occurs or doesn't occur due to a metaphysical mind).

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    Which physicists precisely?
    – H Walters
    Jun 16 '19 at 18:55
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    I'm confused what you're actually testing here. If you're testing the effect of consciousness on something, isn't consciousness the dependent variable in an experiment? So why wouldn't you test the difference between having and not having consciousness present? (i.e., I'm not sure what you're actually testing with a working brain in the mix... why not swap out the entire person with a camera, which we presume wouldn't be conscious?)
    – H Walters
    Jun 16 '19 at 19:54
  • A metaphysical mind. Would it work in principle? Jun 16 '19 at 20:08
  • Still confused. Your dependent variable is consciousness being metaphysical causes it to cause consciousness to cause wavefunction collapse?
    – H Walters
    Jun 16 '19 at 20:31
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    "Being monitored by an experimenter" will produce all the decoherence one needs to collapse the wave function long before any metaphysical mind comes into play. This sort of thing is known as quantum Zeno effect. Moreover, no such experiment can work because quantum mechanical predictions are invariant under arbitrarily shifting the "moment" of collapse, as von Neumann showed. In other words, "consciousness causes collapse" is provably untestable if QM is true.
    – Conifold
    Jun 17 '19 at 6:01

As CriglCragl said, I think the key problem is with the idea of the scientist monitoring "whether these signals/images are in a state of superposition". Whenever a property of a system is measured, like the positions of particles in the system, the experimenter always gets a definite answer, up to the limits of the resolution of their measuring equipment. Superposition isn't directly measured, it's part of the mathematical model used to predict the results of later measurements, given known results of earlier measurements. The earlier measurement is used to define a quantum state for the system, which assigns "probability amplitudes" to all possible values of measurement variables like position and momentum. These amplitudes are then modeled as changing over time in a deterministic way governed by some fundamental quantum equation like the Schrodinger equation for non-relativistic quantum mechanics. Then if you perform another measurement later, the probability amplitudes at the later time would tell you the probabilities of getting different results.

If you initially measured a particle to be in some precise position at some time t0, then the quantum state at time t0 would assign all the probability amplitude to that position and zero amplitude to other positions, but over time the Schrodinger equation would cause this to evolve into a quantum state that assigns nonzero probability amplitudes to multiple different positions, and physicists would describe this by saying the particle is modeled as being in a "superposition" of different positions. But if you do another measurement at a later time t1, you will only find the particle at one specific position--again, the probability amplitudes at time t1 that you calculated by following this method after the first measurement give you the probabilities of finding the particle in different positions.

The evidence that these probabilities are correct comes from experiments where we have multiple particles or systems that start out in the same initial state, and then we can look at the statistics of different possible later states they end up in. A classic example is the double-slit experiment, where we have a bunch of particles that we know started out from a source at a particular fixed location, and then we can measure the frequencies that these particles are later detected at different positions on a screen facing the source, with a barrier containing two slits positioned between the source and the screen. If you use the method I outline to calculate how the quantum state evolves between the point the particle is emitted by the source and the time it's detected at the screen, there'll be some nonzero probability amplitude for the particle having passed through both of the slits in between, and this has consequences for the probability distribution for the particle to be detected at different positions on the screen due to a phenomenon called "quantum interference". But you only get this interference pattern if there is no measurement of which slit the particle actually went through--if you do measure which slit it went through, then according to the standard rules for calculating probabilities you have to model this as "collapsing" the quantum state at the moment of measurement, which changes the probabilities for finding the particle at different locations at the screen on a later measurement there, destroying the "interference pattern". If you're not familiar with the experiment, there's a short video on it here which is pretty good despite coming from an overall very New-Agey movie, and a more detailed explanation on this page.

Decoherence is a phenomenon that is modeled in terms of the deterministic evolution of the quantum state that happens between measurements, it doesn't involve any assumption of collapse in itself. The intriguing thing about it, though, is it can mimic some of the results of measurement when you just look at one subsystem of a larger more complex system. For example, in a double-slit experiment where you send an electron through the slits, you can suppose that the electron will interact with some other particles in the vicinity of the slits, like air molecules, and construct a larger quantum state for all these particles in combination (an 'entangled' system) to describe this. If you use this larger quantum state to predict the probabilities for just the electron at the screen, you find the same probability distribution that you would have got if you had performed a measurement at the slits. This despite the fact that you explicitly did not model any discontinuous collapse at the moment the electron was passing through the slits, and used the continuous Schrodinger equation to model the evolution of the whole quantum state between the electron leaving the source and the electron arriving at the screen. It's as if the interaction between the electron and its environment (the air molecules) was able to mimic the effect of a measurement in retrospect.

So we have two scenarios that produce exactly the same prediction about the statistical pattern of electrons on the screen; one scenario where the electron is measured going through the slits and this is modeled as collapsing the wavefunction, another where it interacts with air molecules as it goes through the slits but this is analyzed in terms of deterministic wavefunction evolution of an entangled state without a collapse at that moment. But to get back to your question, there is at least a theoretical type of experiment that would distinguish between these two ways of modeling things, illustrated by a variation of the double-slit experiment called the delayed choice quantum eraser, which I talked about in this answer on the physics stack exchange. Here, instead of an electron interacting with a bunch of air molecules as it goes through the slits, a photon going through the slits to a screen is entangled with a single other photon. The photon detected at the screen is known as the "signal" photon, and the other entangled photon is the "idler". The interaction guarantees that the total statistics of all the signal photons on the screen will not show interference, instead you'll get a pattern just like what you would have seen if the signal photon had actually been measured at the time it passed through the slits.

However, whether you can actually determine in retrospect which slit the signal photon went through depends on how you measure the idler. Referring to the diagram I posted on the physics stack exchange, if you direct the idler towards one pair of possible detectors D3 and D4, then a detection of the idler at D3 indicates the signal photon went through slit A, and a detection of the idler at D4 indicates the signal photon went through slit B. If on the other hand you direct the idler towards a different pair of detectors D1 and D2, then neither of those tells you anything about which slit the signal photon went through--this setup retroactively is said to "erase" the which-path information that you could have potentially measured with a different setup. And in that case if you do a large number of trials in the which-path-erasing setup, although the total pattern of signal photons on the screen won't show interference, if you just graph the positions of the subset if signal photos whose idlers went to one specific detector (either D1 or D2), you will see an interference pattern in that subset. This sort of interference pattern can only be seen if the which-path information has been erased this way, so if you modeled the initial interaction between signal and idler as collapsing the quantum state so the signal photon definitely went through one slit or the other, you wouldn't predict any interference pattern.

So although it would be totally unrealistic in practice, in principle you could imagine doing a similar experiment where instead of the particle traveling through the slits being entangled with a single other particle, it could be entangled with some complex intelligent being you believe to be conscious, and who is totally isolated from the outside world, like an AI running on a quantum computer at near absolute zero. If this AI makes a mental note of which slit it observed the particle to go through, but its memory is subsequently "erased" in such a thorough way that even an exhaustive measurement of its brain wouldn't allow outside observers to reconstruct what it had observed, then it should be possible to recover evidence of interference in a way analogous to the delayed choice quantum eraser. This has actually been proposed as a thought-experiment by David Deutsch, the physicist who first formalized the idea of a quantum computer, and who is also a strong advocate for the many-worlds interpretation (in which there is no real collapse of the wavefunction, whether due to consciousness or other proposed reasons in objective collapse theories). See this paper, which describes Deutsch's thought-experiment on page 15, where first a "quantum artificial intelligence" observes the spin of a silver atom, and then:

Step 3 – Having experienced this “state of consciousness,” the quantum observer makes a public record of whether it has observed a definite spin value or not without revealing what exactly it learned.

Step 4 – The next step is to undo, by reversing the dynamical evolution, Steps 2 and 1. This is in principle possible, because “the steps involve only the quantum computer which can effect any desired unitary transformation upon the state of a subsystem of itself” (p. 223)

Step 5 – Finally the horizontal component of the spin of the silver atom is measured

The paper also quotes Deutsch's comment that the quantum AI "experiences the splitting and remerging of its own consciousness by observing physical evidence for which there is no alternative realistic interpretation". (Deutsch originally proposed this thought-experiment in the 1986 paper "Three experimental implications of the Everett interpretation" which was published in Quantum Concepts of Space and Time edited by Roger Penrose and Christopher Isham)

  • Well done for such an exhaustive answer. Your Youtube link is broken, unfortunately.
    – CriglCragl
    Dec 22 '20 at 20:06
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    @CriglCragl -- Thanks, I updated the link with a different copy of the same video on youtube.
    – Hypnosifl
    Dec 22 '20 at 20:11

The experiment would work in principle, but it would amount to create a superposition of state of the observer+object system. If you don't include the observer, the state of the observed system considered separately will decohere because of the many degrees of freedom of the observer that gets entangled with it. After the observer has interacted with the object, you will only be able to observe interferences at the observer+object system level.

So basically, you're proposing to create superposition of states of a human being to see whether the laws of quantum mechanics (QM) break there. Of course, it's quasi impossible to achieve such an experiment, if only because living organisms are open systems (they need an environment to survive). At least you'd need a superposition of an even larger system, and it is quasi-impossible to monitor trillions of degrees of freedom to create such interferences.

Also note that if the experiment failed to show an effect, that is, if the laws of QM didn't break, that would not prove that consciousness doesn't provoke a collapse in general. For example, perhaps putting human beings in superpositions of states and measuring the interferences between the superposed states alter their consciousness. It's plausible that a human subject in such an experiment would not claim to have witnessed any definite state for the observed object afterwards (otherwise her/his experience would contradict the records), so the experiment would not correspond to a normal perceptual process, and the defender of consciousness collapse could argue that no collapse occurred because of the particular situation. In any case there's always room for metaphysical interpretations when consciousness and QM are involved...

  • 1
    I don't think this is right... you're not actually testing that consciousness did anything at all. Are you sure it would work in principle? "the only reasonable conclusion that one could reach is that" <- that phrase is insanely suspect. What if hemoglobin causes collapse and not consciousness? What if temperatures above 35C causes collapse? What if bilateral symmetry causes collapse? Just because one might suppose it's reasonable consciousness did something doesn't mean it did; there are an insane number of confounding variables in the mix.
    – H Walters
    Jun 19 '19 at 12:10
  • ...or think about it this way. What if you swap out the human with a steampunk robot, and find that this also results in decoherence. Did you prove that the robot was conscious?
    – H Walters
    Jun 19 '19 at 12:15
  • @HWalters you're right that it cannot be conclusive, but just as any scientific experiment is never strictly conclusive. Of course, various factors should be examined. Now if QM broke down when human or living organisms are involved, and not when only hemoglobin or some temperature are involved, that would still be an impressive and telling result (personally I doubt it would work). And if then robots give the same result, but nothing else, that would be good reason to think that robots are conscious. In any case my point was that such experiments are quasi impossible to do. Jun 23 '19 at 8:36
  • "Of course, various factors should be examined." Imagine this. A chemist has a chemical X, and he wants to know if it helps melt ice. So he sets up a solution of X, and drops an ice cube in it. Lo and behold, the ice cube melts. Great experiment, huh? Except for one thing. We have no idea if the ice melting has anything to do with chemical X at all. We're neglecting base rates; there's no control. That's exactly what I see here; there's a conscious entity with a complex brain, we "drop an ice cube" in it and see if it decoheres. Say it does. How can we conclude anything?
    – H Walters
    Jun 23 '19 at 13:25
  • As you point out the problem can occur in chemistry or any field. You're making a point about scientific experimentation (underdetermination of theory by experience) that is not specific to the topic under discussion. Surely we know that heat melts ice, because of various controls on plausible factors and I assume we could do the same here. I could have made this explicit but it's quite irrelevant to the original question: underdetermination by experience is pervasive, what makes you think it's more a problem here than for any type of experiment? Jun 23 '19 at 20:47

A scientist, is monitoring whether these signals/images are in a state of superposition

Easier said than done - creating a macroscopic state in quantum coherence takes it being extremely isolated https://www.ted.com/talks/aaron_o_connell_making_sense_of_a_visible_quantum_object/up-next?language=en

Explaining the limit of https://en.m.wikipedia.org/wiki/Coherence_(physics)#Quantum_coherence is a key target for physics. The 'Purification Principle' https://plus.maths.org/content/purifying-physics-quest-explain-why-quantum-exists looks to unite the decay of pure states into mixed states into a coherent picture of information flow. Susskind developed the idea of the conservation of information, and Noether's Theorem points to the link between conservation laws and spatial dimensions. We might expect this link to provide key insights into the implications of the multiverse, and evidence for the Many Worlds interpretation.

  • Can you give me a direct answer as to what you think of the principle behind it? Jun 16 '19 at 19:43
  • The whole objective of this was to demonstrate that in principle it is possible Jun 16 '19 at 19:44
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    @john taylor : It's rubbish. It doesn't deal with the coherence limit or the need for quantum systems in superposition to be isolated to maintain that state. More generally it fails to account for the tendency 9f pure states to become mixed states, which we call entropy. The 8nyerpretation of the measurement problem you seem to advocate is en.wikipedia.org/wiki/… And it is not taken seriously at all in modern physics
    – CriglCragl
    Jun 16 '19 at 20:09

It's going to be tricky to rule out decoherence and to be sure that you've observed a "collapse" that wasn't decoherence. But in principle it is possible if the test subject's brain is completely isolated from the environment.

But even if we manage to get that far, the more fundamental problem is how does the experimenter know that the "collapse" she observes in the test subject's brain isn't caused by her own measurement?

By the way the "observer observed" paradox is itself a big problem with interpretations of QM that involve wave functions collapsing.

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