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Experimental rejection of observer-independence in the quantum world (arxiv.org)
187 points by lisper 7 days ago | hide | past | web | favorite | 111 comments

Here's a dumbed down version: https://outline.com/7Ckkqa

Also not directly related video if you like your mind being blown: https://www.youtube.com/watch?v=8ORLN_KwAgs

Especially given how our understanding of the world changed during the last 200 years, I would say we are still very much clueless. I find that exciting.

Btw, no idea why did researchers get "freedom of choice" involved.

Here is what I don’t get, and never did as a physics undergrad: why is the information transfer important here?

Let’s modify the thought experiment: Wigner’s friend measures the photon’s polarization and determines that it is horizontal. He then calls Wigner on the phone and tells him the result. Does this affect the result that Wigner sees when he verifies superposition? What if the friend lies? The post you linked seems to specifically call out that the friend cannot reveal the result of his measurement, lest the experiment get spoiled.

This all seems to point to some philosophical/theological Mumbai jumbo with regard to consciousness being a necessary ingredient here, which doesn’t hold water. Yet with every thought experiment like this that I have seen, this kind of clause is included. Can anyone explain it like I am 25?

"Mumbai jumbo"?

On the road to Damascus and enlightenment, stunned by a bystander's use the term "Mumbai jumbo", Paul fell off his horse and into the stones beneath a set of railroad tracks. Upon reaching consciousness again he saw an old man who extended a hand, smiling, nodding and, speaking the phrase "mumbo jumbo" re-assuredly and repeatedly, helped Paul to his feet.

I think it’s more of a new dance move but I like your parable nonetheless.

The video in the comment you replied to is excellent I think. And it doesn't involve any sort of hand waviness (theological implications) in the final description. Basically just having machines accurately measuring the photons changes the results (e.g. vs. the machines measuring the photons, but scrambled so they can't recover the information). So it's definitely a property of attempting to recover the information, regardless of how it is measured or reported, which is weird, but doesn't have any sort of consciousness involving implications, because it happens to machines as well.

It's definitely worth the 10 minute watch if you want it explained to you.

The field's (quantum physics') moving back and freely between: Serious Math; ill-advised specialist terminology that borrows then twists common words in just the worst ways; and seemingly metaphysical stories or explanations that are... literal? Metaphorical? Full of borrowed BS words from point two that fall out of the math but don't "mean" anything in everyday terms? Definitely one of those until you ask a question then you're an idiot and it's another, until it's not again? Has put me off any childlike interest I once had in it. It seems fun. But I can't tell when they're serious or screwing with me. Sometimes one person claims something's serious, and they do so with authority, and another says they're screwing with me with just as much authority. I gave up.

Somewhat relatedly[0], I can't find an explanation of the Oberth Effect that makes any damn sense. Every so often I try again and leave disappointed. People are happy to provide them. There are lots of them. They attribute it to all sorts of things. Not one of them I've read makes sense.

[0] i.e. is physics, but is not quantum physics. So far as I know, anyway.

To be fair, "quantum physics" is much too broad a brush here, your issue is with "quantum interpretations" or something -- I'm not sure this subfield has a really firm label. But it's fully of people tripping over paradoxes they invented and encouraging you to likewise stumble and admire how cleverly they tried to disguise their handiwork. In content it's unclear we've learned anything new here in the last 50 years.

But straight "quantum physics" covers lots of other, more solid, areas. Including understanding how lots of things you can actually manufacture work, like semiconductors and superconductors.

>I can't find an explanation of the Oberth Effect that makes any damn sense.

It's simpler than people make it. When a spacecraft is moving at a high velocity, the amount of kinetic energy embodied by its fuel mass is a significant fraction of the chemical energy already present. And kinetic + chemical is greater than chemical alone. That's all there is to it.

> I can't find an explanation of the Oberth Effect that makes any damn sense

I'll take a crack at it:

Kinetic energy plus gravitational energy is conserved when you aren't actively accelerating[0]. Rocket fuel has kinetic and/or gravitational energy in addition to it's chemical energy. If you burn the fuel while going very fast[1], you can extract it's kinetic energy by bringing the exhaust to(wards) a stop. When going slowly[2], you end up spending energy to accelerate the exhaust away from you, and you also can't really do anything to get at its gravitational energy[3].

0: Ie, thrusting your engines or crashing; things that involve g-forces; not freefall.

1: That is, when most of its gravitational+kinetic energy is in kinetic form.

2: With energy mostly in gravitational form.

3: Gravitational energy is a function of where mass is at that exact moment; if you're high away from a gravity well, you have lots of energy tied up as gravitational, and no quick way to turn it into a more useful form.

If Wiegner knows the outcome he has no reason to use the entire superposition formalism of quantum mechanics as there is no uncertainty to constrain.

For me "freedom of choice" argument always seemed completely philosophical with no practical implications. But still, philosophical arguments are fun.

The most important thing is to acknowledge that "freedom of choice" or "free will" or any concept like that isn't a binary.

Addiction and marketing both prove beyond a shadow of a doubt that we don't have "perfect" free will, and this is both (1) true regardless of what you think about determinism and (2) more relevant than any consequences of whether the universe is deterministic or not.

The pragmatically relevant aspect of "free will" is to what extent our actions can be predicted and/or manipulated by others today and in the future. This necessarily involves e.g. measurement and computational limitations that others have, and it's something that occurs on a scale rather than as a binary.

> For me "freedom of choice" argument always seemed completely philosophical with no practical implications.

The "free will" debated in philosophy is not the same as the "free will" physicists talk about. The former is about moral responsibility, the latter is about being able to set experimental parameters independently of the system you're measuring.

I'm personally a fan of the theological-style "everything is perfectly predestinated, and also you have full free will, and both are true at the same time" arguments.

The argument isn’t that “everything is predestined” but there is still free will. That’s illogical by definition (and theologians are not intellectual weaklings). Rather the question is wheather, despite God’s omniscience, we still have free will.

The question therefore is: is God’s omniscience equivalent to predestination?

Some think yes (no free will. some Protestants and Muslims). Some think not (therefore we are free. Catholics, ppl who reject omniscience.)

Personally I like Tolstoy’s take on the whole thing in WnP, because it betrays how deep his understanding of physics is (he even scratches at chaos theory).

I definitely believe in both but not because God is omniscience. We experience time because that's how God designed our universe and we progress along or timeline exercising free will. God on the other hand is outside our universe and can see our entire timeline. So he hasn't predestined us to make one decision or the other but is rather able to see every decision we've made in or future.

Like the series Quantum Leap?

I like to use a record player analogy for this concept.

We are the needle. The needle responds to the environment of the record but can't see outside it's little perspective.

God on other hand sees the record from above all at once.

We are a consciousness needle scratching a path through 4D space-time.

Furthermore, our consciousness is harmonic in nature. Every moment experienced reflects a bounded bitrate state of the brain.

And death is when the needle has scratched the record so much that it has become worn flat, producing just silence. The record is still being played but the effect is all the same.. Until a new record is placed on the player. Do you identify yourself with the record or the needle? ;-)

I'm right between the record and the needle!

When discussing this exact topic with classmates this semester (philsophy undergrad in a medieval philosophy class that is doing a lot with the interplay with theology) I've used the following metaphor to explain how I've come to understand the idea of the supposed symbiosis between God's omniscience and our free will:

Think of God as an observer at the top of the Grand Canyon, looking down into the winding path of the river below. Humans are afloat on the river down in the canyon, unable to see what lies ahead. They can choose how to react to obstacles, for example, yet do no see them coming. God, however, sees our entire path on this metaphorical river laid out for us since birth; each of our individual choices are "perceived"[0] by him simultaneously.

[0]: Note, however, that many theologians would protest anthropomorphizing God in this way. Maimonides, as an example, says that we can speak of God as exhibiting mercy but not being merciful. He simply acts in a way that is analogous to human mercy.

I don't understand this metaphor. It sounds like if the river represents the events that will happen to us throughout our lives, then our free will is how we navigate through it. So does God know that ahead of time? Can He see me approaching a rock and know I will go around it on the left or right? Does He know I'll get smashed on it? If he doesn't, then that means there is a limit to His omniscience. If He does, then my "free" choices are predestined.

Furthermore, in this metaphor God doesn't just see the river, He created the river, and all the people on it. He laid out its twists and obstacles, with a perfect understanding of how it would affect the humans afloat on it. Moreover, at the moment of creation, He had an unbounded freedom to create the river in any other way He saw fit. If I get smashed on a rock half way down, it's because He chose to create the river in such a way that I would get smashed on it. If He didn't know this, or didn't have a choice, then He is either not all knowing, or not all powerful.

This, for me is the fundamental problem with any theist philosophy that holds that God is both omniscient and the prime mover of creation, but still suggests concepts like free will and morality have any meaning. Not only are "our" decisions predestined, they're a direct result of another being's free will, and the concept of us then being judged on them by that same being is farcical. It's like a puppeteer putting one their puppets on trial.

You can judge your programs as good or bad despite being the programmer. God doesn't judge morality how we judge. I think God's judgement might be more akin to how a programmer might judge which program to enter in a competition, after generating billions of programs through some generative process.

We have free will to the extent that we can seperate internal and external forces, and to the degree that internal forces play the causal role in any given event. Of course, in reality, this is just an abstraction, and there's no true seperation between internal and external just as there's no real seperation between bodies of water, but the categories are useful, so we use them.

> The argument isn’t that “everything is predestined” but there is still free will. That’s illogical by definition

It's actually not. The assumption that determinism precludes moral responsibility has turned out to be false.

>The assumption that determinism precludes moral responsibility has turned out to be false.

When did this happen? I missed that memo.

I suppose it depends how deep you want to go on HN. If you're busy and you'll accept an argument from expertise, then the majority of philosophers are Compatibilists, with a tiny minority holding out for incompatibilism (something like 10%).

If you want a review of the failures of every challenge meant to demonstrate how moral responsibility and determinism are incompatible, and that Compatibilism empirically matches our use of moral language and so is what people actually mean by free will, that will take more time.

For starters, majority of philosophers never make a stable ground for absolutist statements ("turned out to be false“).

Compatibilism is a kind of comfortable opinion, because it simply avoids thorny questions regarding logic, and morality. E.g. C-sts belive that a criminal is guilty, when he allegedly acts in accordance with his will, because, as you noted, that's what people mean when they say "free will". However, it still may sound unconvincing when one believes that the criminal's mind contents was predetermined aeons before he was born.

> For starters, majority of philosophers never make a stable ground for absolutist statements ("turned out to be false“).

Standards of evidence for HN are lower. Calling something false needs a preponderance of evidence, not a formal proof. Centuries of challenges attempting to show an inconsistency have all failed.

> However, it still may sound unconvincing when one believes that the criminal's mind contents was predetermined aeons before he was born.

Anything can be made to sound unconvincing when it's completely divorced from its context. The point in context, is that bad behaviour felt justified in the moment, and holding a learning agent responsible is the moral feedback needed to correct that error in judgement.

holding a learning agent responsible is the moral feedback needed to correct that error in judgement.

That's exactly where the trouble is. In case of strict logical determinism we correct nothing because everything is predestined. However, choosing limited definitions, and restricting ourselves in comfortable contexts we can certainly avoid discomfort of the thought. Not that I have better idea, but I see clearly why non-compatibilists say "word juggling" towards compatibilists.

> That's exactly where the trouble is. In case of strict logical determinism we correct nothing because everything is predestined.

A predestined correction is still a correction though. Unless you assume "correction" to have some bizarre and probably incoherent non-deterministic meaning. In context, "correction" means "learning to choosing correctly as opposed to incorrectly", which is precisely what happens in this scenario.

> However, choosing limited definitions, and restricting ourselves in comfortable contexts we can certainly avoid discomfort of the thought.

I dispute this characterization. It's not "limited definitions" and "comfortable contexts" (presumably pejoratively), it's "appropriate definitions" and "relevant contexts".

> but I see clearly why non-compatibilists say "word juggling" towards compatibilists.

I think you'll find that the incompatibilists are just as guilty of this. They've spent centuries arguing for a definition of free will that no one actually holds. Philosophy is sometimes about drawing fine but relevant distinctions, and that's exactly what Compatibilism does: it carves out a fine line behind which we can make sense of holding intelligent learning agents responsible for their choices.

They've spent centuries arguing for a definition of free will that no one actually holds

This statement is incorrect, and actually prove the very thing you intended to deny. It's not like adherents of C. held a plebiscite, isn't it? (And if they would, I dare to say they would be in danger of huge disappointment.) They made a choice to call an action free when it's inescapable consequence of a butterfly flapping wings millenia ago on another continent, but not free if it's e.g. equally inescapable consequence of a another human doing something recently, and closely (even if this forcing person is equally inescapably forced by a butterfly wings before). It is not, however, some sort of obvious conclusion, accepted by everybody. It's something C. philosophers prefer to call a common sense, because... well because this way we can make sense of things we do. So it feels much better.

As a side-note, your explanation of 'correction' makes perfect sense if you took the C. pill already, but doesn't make any useful proof of C. One have to accept that the only available variant of future (if we are in a purely determinist universe) still presents a free choice between correct/incorrect. Which is a circular reference to basic presumption of C.

Anyway, thank you for taking time to write the long response!

> That’s illogical by definition

What’s the definition of “free will”?

You're in good company:

"There's a divinity that shapes our ends/ Rough hew them how we will."

Hamlet, to Horatio, in Hamlet Act 5 Scene 2.

I've never quite understood those kind of arguments. It seems that if that were true, then either the logic that the physical or metaphysical world runs on would be inconsistent. I suppose that isn't necessarily a deal-breaker, though.

If everything was predetermined, you'd simply be predetermined to feel justified in the belief that free will exists. So we would all agree to it, even if it were false.

Personally, I don't see any reason it isn't a figment of the mind. Nobody is debating whether love exists. Our ability to model counterfactuals in our mind doesn't have any broader philosophical implications, so I don't see why free will would, when that may as well be all that it is.

As we gather more information, the free will argument seems to be looking less viable, but it'll be interesting to see how things pan out as ANNs start to guess our next moves. It might be something like picking the next word using a string of past words.

Perhaps such a system could predict our actions and intervene when some poor outcome is predicted and direct us toward more positive outcomes?

I like to explore the idea that there is premeditation in asking a question, but the experiments you run to get there may contain free will.

> It seems that if that were true, then either the logic that the physical or metaphysical world runs on would be inconsistent.

This statement itself depends on the logic which are you supposing might be deprecated.

How does that work exactly?


Freedom is choice is important because you might decide to base your choice on a property of a photon that originated in a galaxy far away.

If you have no choice, than that property must have been predetermined to have a specific conspiratorial outcome, essentially since the beginning of the universe. Just to make sure you and your friend make compatible measurements.

Without that assumption, every outcome can be explained as due to a highly unlikely conspiratorial setup.

> It begins with a single polarized photon that, when measured, can have either a horizontal polarization or a vertical polarization. But before the measurement, according to the laws of quantum mechanics, the photon exists in both polarization states at the same time—a so-called superposition.

Pardon my basic understanding, but doesn't it meant that it's both simply because it hasn't been measured - and therefor we don't know? Meaning it's either one or the other; at a very fast rate, it's both.

"Exists in both states" is a very misleading description of how quantum states work. I wish introductory texts didn't use that phrasing (or "superposition", for that matter).

A photon can have one of an infinity of polarizations. These states are describable as a mixture of any two orthogonal polarizations (a "basis"), such as "horizontal" and "vertical". "Left circular" and "right circular" are another choice.

The funny thing happens when you measure "how much" a photon is polarized into any given orientation. It appears as being polarized either in that orientation, or in the polarization complementary to what you measured. E.g., horizontal or vertical. The outcome is determined with a probability based on the actual polarization of the photon prior to your measurement. Then – the funny thing – the photon takes on that polarization. Repeated measurement using the same basis gives the same results. Measure again with a different basis, and the game starts over again.

You can see this in the macro world with a laptop screen and two pairs of polarizing sunglasses. Using one pair of sunglasses, you should be able to block the light from the screen entirely – you are holding them perpendicular to the polarization of the screen. Place the second pair in between the first and the screen, and rotate it. At certain orientations, you will be able to see the screen! The second pair (nearest the screen) is diagonally polarized, and will "convert" the light from the screen into that polarization, before it reaches the first pair (furthest from the screen), which is orthogonal to the screen only.

(Note: Things change completely once a second photon enters the picture. It can be entangled with the first, and their states are no longer independently describable. Measurement of one affects the pair.)

> it's both simply because it hasn't been measured ... Meaning it's either one or the other

A common misconception about quantum superposition is that the particle "has" some state, and we only use superposition to model that we "dont know". This is called the hidden-variable theory (https://en.wikipedia.org/wiki/Hidden-variable_theory). It's been proven that this hidden variable theory can't be true because of https://en.wikipedia.org/wiki/Bell%27s_theorem.

This is completely non-intuitive. See this video for a laymen's explanation: https://www.youtube.com/watch?v=z-s3q9wlLag (you may also need to watch the prev. video about the EPR paradox https://www.youtube.com/watch?v=5HJK5tQIT4A).

Edit: actually, this is a much better video https://www.youtube.com/watch?v=ZuvK-od647c - less technical and more visual.

Brilliant, thank you!

What you're referring to is known as a "hidden variable theory," (the photon does have some fixed polarization, but the value is hidden from us). This was argued about for decades, until tests of Bell's inequality (https://en.wikipedia.org/wiki/Bell%27s_theorem) essentially disproved hidden variables. Particles really can exist in a superposition of multiple states simultaneously, and it's not because of our lack of knowledge or our frame of reference. It's a delightfully strange result.

Yes. Although maybe it's worth adding that what was disproved is local hidden variables. The not-disproved hidden variable ideas must have explicit instantaneous action at large distances, i.e. faster-than-light communication among the hidden variables. And that is a pretty ugly thing to have; before Bell (and Aspect, maybe) one could hope that this would not be necessary.

Piggybacking on my similarly basic understanding of wave-particle duality: it is literally both, and the mode of measurement is what causes the superposition to collapse into one state or another. In other words, you can “force” a superpositioned particle into the desired state simply by measuring it appropriately. We infer from this odd arrangement that the particle contains both states at once.

Pardon any incorrectly used terminology above, but I believe my explanation is conceptually sound.

That is the sane reaction after learning some quantum physics, but the answer is that no, it doesn't exist on both states.

It is easier to understand if you look at photons position. Try to calculate the interference between any two waves, and compare to the odds of finding a photon somewhere it they were dispersed proportional to the waves amplitudes at the start.

It's essentially the schrödinger's cat - the first observer is the observer, while his friend is the cat. But the time won't stop going for the cat, it does experience its death and not, but it's vawefunction only collapses for the observer once the box is opened.

> Especially given how our understanding of the world changed during the last 200 years, I would say we are still very much clueless. I find that exciting.

Exactly. The universe would be rather boring if we learned everything about it during my lifetime.

What's really interesting to me is the philosophical interpretation of this... as if something wants the experience of things within the universe to be somehow consistent. That implies intelligence and not just randomness.

Alternately, maybe you're looking at progressive 3D slices of a 4D spacetime blob, and consistency is tautological in the same way as fitting together 2D slides of a 3D object.

From what I understand, this basic idea of 4D geometry also provides some of the more elegant explanations for the effects of relativity when approaching the speed of light, because time dilation effects reduce down to (relatively) simple 3D trigonometry in a 4D graph.

Of course, that raises predestination questions, because you're treating all of time and space as a single "object".

That's not just a dumbed down version, that's a version with no useful information about this particular experiment.

Maybe we can just say "true randomness" instead of "freedom of choice"?

Just going to repeat from the HN thread a few days ago[0]


Here's Scott Aaronson arguing that the theoretical work underpinning this experiment is flawed.

[0] https://news.ycombinator.com/item?id=19369655

I don't think that blog post is referring to the same paper. This new paper is about an experimental demonstration of Wigner's Friend: https://en.wikipedia.org/wiki/Wigner's_friend. Scott's blog post is talking about a more controversial extension to it, which combines Wigner's Friends with Hardy's Paradox to attempt to construct a reflective or meta limitation on the applicability of QM. The wiki link also discusses: https://en.wikipedia.org/wiki/Wigner's_friend#An_extension_o...

This paper cites the one Scott argues against.

Yes it does and yet you are still wrong AFAIK. Citing a paper does not necessarily mean that said paper would form the theoretical basis of the citing paper.

This paper about the experiment is trivially true, there were no surprises regarding to what QM predicts.

Here is a direct quote from Scott: >My comments about the Frauchiger-Renner paper never doubted for a second the result predicted by QM. So an “experimental demonstration” of their setup—by which one really means an experimental demonstration of Hardy’s experiment, since the Frauchiger-Renner one would require superposed conscious observers (!)—has zero effect on any of those comments.

And here is a prescient direct quote from Craig Gidney: >I wonder how long it will be until someone 1) executes the circuit from Hardy’s paradox (which has very few qubits and very few gates, so you could run it on a NISQ machine), 2) confirms that of course the results match the predictions of quantum mechanics, 3) frames the result in terms of Frauchiger et al’s variant, and 4) journalists telephone-game it into something like “quantum researchers prove decisions aren’t real”.

Please see also the relevant discussion in comments here: https://www.scottaaronson.com/blog/?p=4147

The experiment is uninteresting, as far as I can tell, without the interpretation from Frauchiger-Renner.

My point in linking the original article was that the media discussion of this (in part driven by the paper's own framing) as disproving objective reality is wrong, something Scott clearly agrees with.

Ah, I knew this rang a bell.

Reflecting on this a second time, I wonder whether it is possible to interpret this as the universe not caring until the information interacting actually matters. For example, if someone tells me that they have measured a value, they could very well be lying, and until I say "prove it, what value did you measure?" the underlying implementation of the universe is in a superposition where they both did and didn't measure the value (where the friend is telling the truth vs lying).

The theoretical possibility of someone lying to you isn't quantum-based and doesn't work as a superposition.

If they generated a random bit and lied depending on that value, then it would be.

I am even more of a layman than most here, but — why is it different? Isn't that basically Schrödinger's cat?

It just doesn't bring anything new. It was obvious it has to work this way, since non-elementar particles can be in superpostion. If we assume that the time is still going for systems that appear to be in superposition for the outside observer, there doesn't seem to be any other explanation other than superposition being relative - the observers in the system still experience time as usual, and the superpostion only exists for the outside observer.

I think this needs many up-votes.

It reminds me how sometimes converse is confused with contrapostive.

It is quite an exaggeration to talk about this experiment as actually realizing a "Wigner's Friend" type scenario. In the Wigner's Friend thought experiment, the friend--an actual conscious macroscopic human being--is put into a superposition of different, mutually exclusive conscious experiences that Wigner, from the outside, is able to measure as such a superposition. Nothing like that is being done in this actual experiment: the only things being placed into superpositions are microscopic systems that have no experiences.

This experiment is certainly interesting, and it realizes the basic structure of the multiple Wigner's Friend scenario that a number of theoretical papers have been written about recently, but I don't think it comes anywhere close to probing the actual issues raised by those theoretical papers. The kinds of real measurements that would have to be realized in order to probe those theoretical issues are many orders of magnitude more complex than those in this experiment.

There’s an interpretation that keeps observer as the central element. It is not as popular as many worlds or Copenhagen but I find it very intriguing. Check out QBism: https://en.m.wikipedia.org/wiki/Quantum_Bayesianism?wprov=sf...

In QBism, the Born rule is normative in that it tells the observer which observations to bet on, and observations are data points to update the observer’s subjective assessment of outcomes. Taking this perspective, a lot of paradoxes in QM seem to get resolved trivially. It’s controversial for sure but mathematically it’s rigorous.

> Wigner can even perform an experiment to determine whether this superposition exists or not. This is a kind of interference experiment showing that the photon and the measurement are indeed in a superposition. From Wigner’s point of view, this is a “fact”—the superposition exists. And this fact suggests that a measurement cannot have taken place.

> But this is in stark contrast to the point of view of the friend, who has indeed measured the photon’s polarization and recorded it. The friend can even call Wigner and say the measurement has been done (provided the outcome is not revealed).

A bit of a stretch saying that protons lose their superposition based on the content of a conversation.

From a Many Worlds point of view at least, they absolutely could, because the superposition represents ignorance in the mind of infinitely many Wigners, but once you remove the ambiguity you are just dealing with one specific Wigner, who would then know which Everett branch he is actually on (at least ip to the ambiguity of this particular experiment).

In this experiment, in both branches he receives the same information "A measure has been done.", so it doesn't remove the ambiguity, and it doesn't destroy the superposition.

I was talking about the case when the friend is told the outcome of the measurement, that’s the only part relevant to what the parent comment said.


Because it implies that receiving information affects the material world. And communication isn't infallible.

What if you kind of hear me and are pretty sure I said 'positive' but not 100% sure? What if you're 50% sure? What two people are listening and one heard me say 'positive' and one heard 'negative'?

It's hard to imagine a physical process being dependant on something external and near impossible to quantify or measure.

The general idea is that as long as your particles are not entangled with the system’s particles then from your point of view they remain in a superposition.

The conversation simply represents the idea that the causal cone of the observed system is now interacting with the particles of the observer, making them entangled and so ‘collapsing’ the wave function or whatever you want to calm it.

> Because it implies that receiving information affects the material world.

Information is physical, although I agree that the conversation isn't the important part, it's just a useful metaphor.

Well, you have Lucretius' clinamen.

The measurement is itself still in superpostion though. You know it's taken place, but not with what result. It's kind of like quantum eraser.

Ron Garrett gave a presentation at Google years ago on Quantum Information Theory that kind of demystified a lot of spooky stuff (like quantum eraser), and I'm wondering how much of this is merely an artifact of the way we're interpreting the math.


As they say, the only mystery in QM is the Born rule.

If you're reading this, congratulations for observing a universe that contains this experiment.

And condolences to our selves in parallel timelines where this experiment was never conducted.


Just don’t expect to measure the paper’s impact factor.

I am very far from even understanding quantum physics theories but for all this experiment to be valid to begin with,the idea that superpositions exist must be true, correct?

What if what they're proving is really the abscence of a superposition state? I never understood how superposition can be verified independent of assumptions. In this case,how can Wigner measure the photon has veritcal and horizontal polarity at the same time?

Lastly,could superposition be explained by human's limit of sampling reality? For example audio recorded at 10Hz means 10/samples of audio a second,a sort of audio-resolution. If our reality-resolution is low,some things might require ability to measure their state at a better time-space resolution than what humans can currently sample (or build equipment that can measure this)? But with limited time-space resolution it can appear as if the object is in two states at the same time. Sort of like watching a movie at a very slow frame rate and you capture a moment where half the screen is of one frame but the rest is of a different frame since the display is only half finished rendering the raster/rows.

Apologies if I am being absolutley clueless.

> for all this experiment to be valid to begin with,the idea that superpositions exist must be true, correct?

Depends what you mean by "exist". All that needs to be true - and is true - is that QM provides an accurate model of the experiment. For many of us, QM is just probabilistic mechanics; i.e. probability theory applied to mechanics[1], and so long as you maintain the distinction between (epistemic) probability space states and (ontic) configuration space states (something the article under discussion here unfortunately fails to do), no ontic existence of [superposition] states is ever assumed in the first place.

[1] More precisely, a natural and necessary generalisation of the classical, Kolmogorovian probability theory you might be familiar with (necessary because of the noncommuting random variables):


See also: http://blogs.discovermagazine.com/cosmicvariance/files/2011/...

The classic demonstration of superposition is the double-slit experiment [0], where an experimenter shines a (coherent) light through a pair of slits at a screen (or fires a stream of electrons through a pair of slits, or small particles- it works with anything.) Because light is waves [1], light coming through one slit will interfere with light coming through the other slit, producing an interference pattern. But weirdly, this still happens if you turn the light source down so low that it emits only one photon at a time. In that case, the photon is in a superposition that travels through both slits, and the interference is between "the part of the superposition going through the right slit" and "the part of the superposition going through the left slit."

So it's not a matter of our being unable to make detailed enough measurements; we can actually observe phenomena that directly indicate superposition happens.

[0] https://en.wikipedia.org/wiki/Double-slit_experiment [1] And so are electrons, particles, humans, etc.

If I were to review this paper, my critique would focus on this little paragraph:

> We note that, although Bell’s mathematical machinery [9] is used to show the result, the set of assumptions considered here — and therefore the conclusions that can be drawn from a violation of inequality (2) — are different from those in standard Bell tests.

That statement contains a lot of handwavium and in fact the interpretation of the results depends a lot on, if this experiment may be interpreted as another test on locality. Which I strongly presume, it does.

"they seem to assume that all the photons' degrees of freedom are described by pure states according to both observers. This seems wrong to me. The observation of the photons by Wigner's friend creates an entanglement with the degrees of freedom in the friend's body, so the state of the photons themselves become mixed according to Wigner (I mean the external observer), not pure. So I think that when these things are done correctly, it's not possible to find this kind of a "contradiction" in practice."


Objective truth!? A question for epistemologists to decide.

How could they record their high entropy (?) solipsistic observations in an immutable datastore in such as way as to have probably zero knowledge of the other party's observations?

Anyways, that's why I only read the title and the abstract.

Wigner's friend experiment: https://en.wikipedia.org/wiki/Wigner%27s_friend

This is an ARXIV pre-release. Has anyone peer-reviewed this?

If this turned out to be wrong it would be bigger news than if it turns out to be right. This result is 100% expected.

Not a physicist but I was under the impression we already knew this. For example if someone is falling into a black hole, to an outside observer they never cross the event horizon, just fall asymptotically slower towards it. But to the person falling in, he gets ripped to shreds. But I guess we haven’t tested that out.

That's the general relativity world, not the quantum world. They are different models, which is where these things are being proven.

An interesting thing about deterministic systems. If the deterministic system ever enters into a state exactly like a prior state, then from then on, that system will be in an inescapble loop destined to repeat forever.

If there ever was a repeatable state, I'd bet it would be at thr big bang, though i am no physicist.

It actually doesn't reject observer-independence unless you agree to certain axioms that an observer-independent interpretation of QM wouldn't feature. Bohmian mechanics is still perfectly viable, for instance.

Does this imply Leggett's inequalities have been proven?

Did the results of this experiment differ from what quantum mechanics predicts? No? Then what is the point of publishing it? It's not as if different interpretations make different testable claims, or else they would be theories rather than mere interpretations.

The ironic part of your comment is that the entire scientific method demands independent verification; and this thought experiment, now tested experimentally, calls into question the idea of objective observation underpinning that method. New discoveries are nice and all, but the rigorous work of science comes from making sure the guy before you did his homework.

> Did the results of this experiment differ from what quantum mechanics predicts? No? Then what is the point of publishing it?

Experimental confirmations of theoretical predictions are very valuable.

You don't want people to publish experimental results that confirm predictions?

Quantum mechanics has already been experimentally confirmed ad infinitum. If I experimentally verify predictions of Newtonian mechanics with some new configuration of billiard balls, would you consider that worthy of publication?

If it’s a brand new configuration of billiard balls that nobody has ever seen before, sure.

And even if not, it never hurts to double check.

Certainly not hurting anyone to publish a "repetitive" paper. Worst thing that happens is more people have a chance to hear about it.

> Did the results of this experiment differ from what quantum mechanics predicts? No? Then what is the point of publishing it?

Quick counterexample: By that logic, do you think results showing agreement with Einstein's GR should have never been released? I'm taking a wild guess that your answer is "no". This case is not any different.

Meanwhile, the world could sure use more kinds of biodegradable containers. If these philosophers could spare a minute.

Meanwhile, people complain about quantum theory research on a device that exists thanks to years of quantum theory research.

You wouldn't even care about biodegradability had it not been for previous "philosophers" so there's no need to be glib. And who's to say the downstream effects of this research don't lead to advances in areas you care about?

Theoretical research leads to applied science which ends up in products. Remember that GPS is only possible thanks to relativity, and e-commerce only works thanks for number theory.

Please tell me you work for a FAANG because that would make my day!


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