Free will and the Many-Worlds interpretation of QM

[Flannel Jesus]

'It's just physics' is very broad. Why so many different theories that all seem to be equally plausible?

You mean why are there so many different interpretations of quantum mechanics?

When I said it's "just physics", I mean you aren't supposed to understand many worlds as a theory about human decisions, you're supposed to understand it as a theory about physics. Newtonian physics is a theory about how matter interacts with other matter - so is quantum physics, so is many worlds.

[accelafine]

But they all have to make sense to be understood. No one is going to question Newtonian physics, or Einstein's general relativity. Those are accepted without question. But what exactly are we being asked to accept regarding the quantum world?

[Flannel Jesus]

But they all have to make sense to be understood. No one is going to question Newtonian physics, or Einstein's general relativity. Those are accepted without question. But what exactly are we being asked to accept regarding the quantum world?

Unfortunately the quantum world uniquely betrays our common intuitions in ways Newtonian physics and even relativity don't. Real QM experiments force us to accept SOME kind of weirdness - the different QM interpretations just differ in what weirdness they'll allow themselves to accept.

Copenhagen interpretation weirdness means you have to accept randomness and apparent faster-than-light causality in the universe (in conflict with relativity).

Bohemian Mechanics means you have to accept retro causality.

Many worlds means you get to keep slower than light casualty, no retro causality, no randomness, but the weirdness you have to accept is that for every quantum measurement you make, there's a version of you that saw a different measurement result.

No matter what you do in QM, there's something weird you must accept. Its currently, apparently, a matter of taste which flavour of weird you're willing to go for.

[accelafine]

But they all have to make sense to be understood. No one is going to question Newtonian physics, or Einstein's general relativity. Those are accepted without question. But what exactly are we being asked to accept regarding the quantum world?

Its currently, apparently, a matter of taste which flavour of weird you're willing to go for.

Yes, I've gathered that

[Flannel Jesus]

You don't have to accept any interpretation. You can just plead ignorance, that's probably the most respectable option. "There's something weird going on, I don't know how to interpret it so I won't". Perfectly valid.

[Atla]

You don't have to accept any interpretation. You can just plead ignorance, that's probably the most respectable option. "There's something weird going on, I don't know how to interpret it so I won't". Perfectly valid.

Imo the instrumentalists who don't go for any of the weirdness, are the weirdest ones of all. How can you not want to make sense of anything? Like you wake up in the morning and say to yourself: gee, what a beautiful don't know what. I don't know who I am, I don't know what this place is, I don't know what's happening, nor would I want to. Now off to do something, but I won't know what.

[Flannel Jesus]

Imo the instrumentalists who don't go for any of the weirdness, are the weirdest ones of all. How can you not want to make sense of anything?

Yeah, the drive to want to understand what's going on, why these weird experiments have these weird results, is super strong in many of us - maybe they just feel helpless because it's pretty much impossible to be certain of an answer so, rather than deal with that uncertainty, they just give up on the question altogether.

[Flannel Jesus]

Yes, I've gathered that

As an example, what I think is one of the most important examples, of a weird experiment forcing us to accept some weirdness, is the Mach Zender Interferometer experiment. This, along with Double Slit and Bells Theorem, are my three go-to examples to illustrate the inherent weirdness going on in QM. The Mach Zender Interferometer experiment isn't super well known, not nearly as much as double slit, but I think it should be, I think it's a very simple experiment that proves the quantum stuff is necessarily weird. If you're not already familiar with it, would you mind if I spent some time laying out exactly why?

[accelafine]

Yes, I've gathered that

As an example, what I think is one of the most important examples, of a weird experiment forcing us to accept some weirdness, is the Mach Zender Interferometer experiment. This, along with Double Slit and Bells Theorem, are my three go-to examples to illustrate the inherent weirdness going on in QM. The Mach Zender Interferometer experiment isn't super well known, not nearly as much as double slit, but I think it should be, I think it's a very simple experiment that proves the quantum stuff is necessarily weird. If you're not already familiar with it, would you mind if I spent some time laying out exactly why?

I would be interested to hear that. Is it the one with mirrors?

[Flannel Jesus]

Yes, I've gathered that

As an example, what I think is one of the most important examples, of a weird experiment forcing us to accept some weirdness, is the Mach Zender Interferometer experiment. This, along with Double Slit and Bells Theorem, are my three go-to examples to illustrate the inherent weirdness going on in QM. The Mach Zender Interferometer experiment isn't super well known, not nearly as much as double slit, but I think it should be, I think it's a very simple experiment that proves the quantum stuff is necessarily weird. If you're not already familiar with it, would you mind if I spent some time laying out exactly why?

I would be interested to hear that. Is it the one with mirrors?

Yeah mirrors, or sometimes called beam splitters.

You get two "half mirrors" and two normal mirrors - the half mirrors have a 50% chance of reflecting a photon at a right angle, 50% chance of it just flying right through - and you arrange them in a particular way and something really weird happens. I'll illustrate it this evening.

[Flannel Jesus]

I would be interested to hear that. Is it the one with mirrors?

First you start with the ingredients

ingredients.jpg

A 'beam splitter' or half silvered mirror, maybe it goes by some other names. If you put detectors at the places indicated, then you see that 100% of the beam goes in, 50% comes out straight, 50% gets reflected at a 90degree angle.

A mirror, which simply reflects 100% of the photons.

So those are simple enough. However when you arrange it into this arrangement here, something funny happens:

funny-arrangement.jpg

Before I talk about the funny thing that happens, I want to set in stone what an intuitive thought process would lead someone to predict would happen. What would we predict would happen in the above scenario?

Well, at the first beam splitter, 50% goes on the upper path and 50% goes on the lower path, so to understand what happens, let's just separate them and analyze them individually:

If you remove the mirror from the bottom path, 50% of the photons go out into space, but the remaining 50% hit the mirror, then hit the beam splitter, and Output1 sees 25% of the photons sent in (50% of the remaining) and Output2 sees 25% of the photons sent in. Straight forward enough, right?
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top-only.jpg

(to be continued)

[Flannel Jesus]

And if you instead keep the bottom path intact and remove the top path, you get a really similar result: the 50% that go up top get sent out into space, and of the ones remaining, they hit the mirror at the bottom, then the beam splitter, and 50% of the ones remaining are seen by Output1 and 50% by Output2

bottom-only.jpg

Again, straight forward, doing exactly the things we would expect these mirrors to do.

So naturally, when you allow the photons to take both paths, what should happen? Well surely just the combined results of the ones that take the top path and the ones that take the bottom path. So 50% at Output1 and 50% at Output2, right? That would be the intuitive expectation.

But that's not what happens. What happens is:

weird.jpg

ALL of the photons go to one detector, and NONE of them go to the other detector.

And, crucially, this doesn't just happen when you're sending a stream of photons, it happens even when you send one at a time.

So what's so weird about that? Well, because most peoples intuition would tell them, either the photon took the top path or the bottom path - if it took the top path, we know 50% of them end up at Output1 and 50% at Output2. If it took the bottom path, we know 50% of them end up at Output1 and 50% at Output2. Every photon MUST have taken one of the two paths, so... why are they all ending up at Output1?

And the bizarre answer that seems to explain our observations is, the photon did not in fact either take the top path or the bottom path. There is some sense in which the photon kind of took both paths.

You can't really explain these results in any sensible way if you view photons as little balls that take one path or the other. Our only working models of quantum physics require modelling it as if the universe is calculating both paths simultaneously, and the "wave functions" of these paths can intefere with each other.

[accelafine]

I did watch an MIT lecture on Youtube on that experiment. I don't recall him mentioning its name though. I do recall noticing that there seemed to be NO females in the lecture hall. How can anyone not be interested in this? I just wish my maths/physics knowledge was better (and my brain in general)

[seeds]

And if you instead keep the bottom path intact and remove the top path, you get a really similar result: the 50% that go up top get sent out into space, and of the ones remaining, they hit the mirror at the bottom, then the beam splitter, and 50% of the ones remaining are seen by Output1 and 50% by Output2

bottom-only.jpg

Again, straight forward, doing exactly the things we would expect these mirrors to do.

So naturally, when you allow the photons to take both paths, what should happen? Well surely just the combined results of the ones that take the top path and the ones that take the bottom path. So 50% at Output1 and 50% at Output2, right? That would be the intuitive expectation.

But that's not what happens. What happens is:

weird.jpg

ALL of the photons go to one detector, and NONE of them go to the other detector.

And, crucially, this doesn't just happen when you're sending a stream of photons, it happens even when you send one at a time.

So what's so weird about that? Well, because most peoples intuition would tell them, either the photon took the top path or the bottom path - if it took the top path, we know 50% of them end up at Output1 and 50% at Output2. If it took the bottom path, we know 50% of them end up at Output1 and 50% at Output2. Every photon MUST have taken one of the two paths, so... why are they all ending up at Output1?

And the bizarre answer that seems to explain our observations is, the photon did not in fact either take the top path or the bottom path. There is some sense in which the photon kind of took both paths.

You can't really explain these results in any sensible way if you view photons as little balls that take one path or the other. Our only working models of quantum physics require modelling it as if the universe is calculating both paths simultaneously, and the "wave functions" of these paths can intefere with each other.

Nicely explained, FJ.
_______

[Flannel Jesus]

I did watch an MIT lecture on Youtube on that experiment. I don't recall him mentioning its name though. I do recall noticing that there seemed to be NO females in the lecture hall. How can anyone not be interested in this? I just wish my maths/physics knowledge was better (and my brain in general)

Yeah it's definitely super interesting, though maybe not that easy to talk about in a way that interests most people.

So I consider this to be almost a simplified version of the double slit experiment, because really the same things happen in both experiments: when the experimenter adjusts the experiment so that they know which path it took, it acts as if it took one path - when the particle can take both paths unperturbed, however, somehow the availability of both paths allows interference to occur.

So what does this all have to do with free will? Well, depending on how you view free will (which is a philosophical issue rather than a physical one), potentially nothing at all.

The two large schools of thought wherein people argue we have free will are libertarian free will and compatibilist free will. For compatibilists, how exactly physics functions doesn't have much bearing on the question of free will at all - a compatibilist free will is compatible with any physics that might underpin our world. For at least some, maybe most, libertarians, however, "randomness" is required for freedom.

https://www.informationphilosopher.com/ ... of%20atoms.

Whether or not randomness exists in the world at the quantum level is apparently a matter of interpretation. The particle goes through the slit, but what decides where the particle actually lands? Why did it land in this band instead of that band? There are deterministic ideas of quantum physics, like bohmian mechanics, where they say there's a specific bit unmeasurable or unpredictable reason why it landed here instead of there; there are interpretations that involve randomness, like Copenhagen, which say that the wave function decides the probability distribution but the actual result is kind of randomly selected by the universe among the available options; and then there's Many Worlds, which I would argue is kind of a hybrid view. It's meta-deterministic, but from inside the universe it feels random, it feels indistinguishable from Copenhagen.

Do you think randomness is required for free will?