The founders of quantum mechanics knew there was something funny about its relation to consciousness. It seems that lesson has been lost through the decades, with many modern researchers believing that approaches like decoherence have solved the problem. They'll tell you that the idea that there's any relationship between consciousness and QM is just flapdoodle. But it's far from solved; we've just kicked the can down the road to ever-more esoteric ideas. If you don't believe me, here's the world's greatest physicist agreeing.
Why is there so much debate about the correct interpretation or formulation of QM? It's not just arcane maths. It centers around the measurement problem: why do we see only one result out of the sea of possibilities QM predicts?
This question is all too easy to conflate with a much more straightforward one: why does a spread-out wave function suddenly become localized (particle-like) when it interacts with measuring devices and/or the environment? For example, why does a light wave stop showing interference when a detector is placed at the two slits?
Well, when a particle encounters a measuring device, the device effectively works by becoming entangled with the particle. If it's a tiny measuring device (e.g., another single particle whose spin is made to match that of the first -- hence "measuring" it) the math is very straightforward, and clearly shows that the first particle won't exhibit "interference." Yet it also predicts that we can perform an interference experiment on the pair of particles. So the quantum-ness isn't gone; it can be found in the larger system, but requires that we are in careful control of all the involved particles.
So does the quantum-ness ever really disappear? Well, as our technology improves and we're able to do bigger and bigger (but still tiny) experiments, things keep looking quantum. But in a truly macroscopic system (such as one that includes an experimenter's body and brain) it is almost unthinkable to have control over all the relevant degrees of freedom that we'd need to confirm it. This is called "decoherence," and it's explained by roughly the same math before, showing that a system should "look quantum" as a whole if we could control everything, but we can't, and so we don't.
So is that it? Problem solved.
If you were following along, you might have noticed the sleight of hand I pulled on you. This whole thing is about measuring a particle. There could have been more than one outcome of that measurement. Everything I've told you so far is about why there's no interference between those outcomes, and nothing about why there's just one result instead of two (or many, depending on what property is being measured).
Here's where the different formulations collide. The orthodox ("Copenhagen") interpretation is that probably somewhere in that ill-defined micro-to-macro transition something magic happened and two became one. A genuine collapse, with each of the outcomes happening with 50% probability. And how could you disprove it? To show that there are still two, you need to do something like an interference experiment, and we already saw you can't at that scale. The system would be decoherent, and while there would still be two possibilities, each would behave basically like a collapsed result. So no experiment today can tell apart whether it collapsed or merely decohered.
This second approach can be extended such that a collapse never happens. Instead, every time there's more than one thing that can happen, the universe splits so that all the things happen. You know, each in its own parallel universe (this is called the Many Worlds Interpretation, or MWI). This neatly sidesteps the question of why we only see one outcome: we don't; we see all of them (in different universes).
Maybe that tickles your fancy, or maybe you'd rather retreat to the safe confines of Copenhagen, but in either case you've really cheated yourself.
Do you remember what question we were trying to answer? It was the measurement problem: why do we see only one outcome instead of many? Even if you have an answer ("we don't!"), it should make you think: why are we asking such a funny question? The easy answer is that we're wired to think primitively but I want you to look closer.
The reason we're asking this question is because the math predicted multiple results but we're seeing only one. We are experiencing only one world. If your solution to this conundrum is "you only think you're seeing one; that's just a trick," then I suspect I know your answer to the question: is consciousness a trick of the brain? Obviously the questions "are you really experiencing one world?" and "are you really experiencing something" are intimately related.
The reason the measurement problem won't go away is that it's intimately tied to the question of whether you "actually experience" anything.
What's the problem with just answering "yes?" Well, the math is supposed to describe everything that's happening. There's nothing there that could account for this strange idea that some real, external thing called "consciousness" somehow swoops into the equations at random points (human brains) and gives an "inside view."
Moreover, consider what things look like from the view of any embodied consciousness in this multiverse (if MWI is true): from his perspective, the rest of the world -- including other people -- obeys the "branching" laws of physics, but from his perspective it gets reduced to one. In other words, he's all alone with this magical power. Are the other people even conscious?
We can't have any of that.
But the problem is that you do really experience something. Pause for a moment, and look around. Doesn't it sure seem like something is going on? I emphasize "sure" here. If you check very simply and straightforwardly, you will notice an immediate and unequivocal certainty before your intellect kicks in and explains it all away ("This is just a trick of my brain! Nothing to see here; move right along!"). This certainty -- that something sure as hell seems to be happening -- is more profound than it may seem the first few times you encounter it. You can't be nearly as sure that time or space exist as that ... consciousness ... and even that doesn't touch the profundity I'm hinting at.
But we're not ready to face this possibility. You can confirm this yourself, as your mind comes up with one explanation after another to disprove the fact that you're really experiencing anything. Of course you could just check again, but who has time for confronting that they exist in an outrageously profound way?
And that is why the problem won't go away; not just because we haven't figured out the appropriate math, or haven't been willing to overcome our ape-like beliefs.
Why is there so much debate about the correct interpretation or formulation of QM? It's not just arcane maths. It centers around the measurement problem: why do we see only one result out of the sea of possibilities QM predicts?
This question is all too easy to conflate with a much more straightforward one: why does a spread-out wave function suddenly become localized (particle-like) when it interacts with measuring devices and/or the environment? For example, why does a light wave stop showing interference when a detector is placed at the two slits?
Well, when a particle encounters a measuring device, the device effectively works by becoming entangled with the particle. If it's a tiny measuring device (e.g., another single particle whose spin is made to match that of the first -- hence "measuring" it) the math is very straightforward, and clearly shows that the first particle won't exhibit "interference." Yet it also predicts that we can perform an interference experiment on the pair of particles. So the quantum-ness isn't gone; it can be found in the larger system, but requires that we are in careful control of all the involved particles.
So does the quantum-ness ever really disappear? Well, as our technology improves and we're able to do bigger and bigger (but still tiny) experiments, things keep looking quantum. But in a truly macroscopic system (such as one that includes an experimenter's body and brain) it is almost unthinkable to have control over all the relevant degrees of freedom that we'd need to confirm it. This is called "decoherence," and it's explained by roughly the same math before, showing that a system should "look quantum" as a whole if we could control everything, but we can't, and so we don't.
So is that it? Problem solved.
If you were following along, you might have noticed the sleight of hand I pulled on you. This whole thing is about measuring a particle. There could have been more than one outcome of that measurement. Everything I've told you so far is about why there's no interference between those outcomes, and nothing about why there's just one result instead of two (or many, depending on what property is being measured).
Here's where the different formulations collide. The orthodox ("Copenhagen") interpretation is that probably somewhere in that ill-defined micro-to-macro transition something magic happened and two became one. A genuine collapse, with each of the outcomes happening with 50% probability. And how could you disprove it? To show that there are still two, you need to do something like an interference experiment, and we already saw you can't at that scale. The system would be decoherent, and while there would still be two possibilities, each would behave basically like a collapsed result. So no experiment today can tell apart whether it collapsed or merely decohered.
This second approach can be extended such that a collapse never happens. Instead, every time there's more than one thing that can happen, the universe splits so that all the things happen. You know, each in its own parallel universe (this is called the Many Worlds Interpretation, or MWI). This neatly sidesteps the question of why we only see one outcome: we don't; we see all of them (in different universes).
Maybe that tickles your fancy, or maybe you'd rather retreat to the safe confines of Copenhagen, but in either case you've really cheated yourself.
Do you remember what question we were trying to answer? It was the measurement problem: why do we see only one outcome instead of many? Even if you have an answer ("we don't!"), it should make you think: why are we asking such a funny question? The easy answer is that we're wired to think primitively but I want you to look closer.
The reason we're asking this question is because the math predicted multiple results but we're seeing only one. We are experiencing only one world. If your solution to this conundrum is "you only think you're seeing one; that's just a trick," then I suspect I know your answer to the question: is consciousness a trick of the brain? Obviously the questions "are you really experiencing one world?" and "are you really experiencing something" are intimately related.
The reason the measurement problem won't go away is that it's intimately tied to the question of whether you "actually experience" anything.
What's the problem with just answering "yes?" Well, the math is supposed to describe everything that's happening. There's nothing there that could account for this strange idea that some real, external thing called "consciousness" somehow swoops into the equations at random points (human brains) and gives an "inside view."
Moreover, consider what things look like from the view of any embodied consciousness in this multiverse (if MWI is true): from his perspective, the rest of the world -- including other people -- obeys the "branching" laws of physics, but from his perspective it gets reduced to one. In other words, he's all alone with this magical power. Are the other people even conscious?
We can't have any of that.
But the problem is that you do really experience something. Pause for a moment, and look around. Doesn't it sure seem like something is going on? I emphasize "sure" here. If you check very simply and straightforwardly, you will notice an immediate and unequivocal certainty before your intellect kicks in and explains it all away ("This is just a trick of my brain! Nothing to see here; move right along!"). This certainty -- that something sure as hell seems to be happening -- is more profound than it may seem the first few times you encounter it. You can't be nearly as sure that time or space exist as that ... consciousness ... and even that doesn't touch the profundity I'm hinting at.
But we're not ready to face this possibility. You can confirm this yourself, as your mind comes up with one explanation after another to disprove the fact that you're really experiencing anything. Of course you could just check again, but who has time for confronting that they exist in an outrageously profound way?
And that is why the problem won't go away; not just because we haven't figured out the appropriate math, or haven't been willing to overcome our ape-like beliefs.
"I'm not going to attempt to define consciousness, in a way that's connected with the fact that I don't believe it will become part of physics. ... And that has to do, I think, with the mysteries that bother a lot of people about quantum mechanics and its applications to the universe. ... Quantum mechanics kind of has an all-embracing property, that to completely make sense it has to be applied to everything in sight, including ultimately, the observer. But trying to apply quantum mechanics to ourselves makes us extremely uncomfortable. Especially because of our consciousness, which seems to clash with that idea. So we're left with a disquiet concerning quantum mechanics, and its applications to the universe. And I do not believe that disquiet will go away. If anything, I suspect that it will acquire new dimensions." -- Ed Witten
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