Where Our Views of Reality Go Wrong
December 5, 2018 2:48 PM   Subscribe

A thought experiment has shaken up the world of quantum foundations. That quantum mechanics is a successful theory is not in dispute. It makes astonishingly accurate predictions about the nature of the world at microscopic scales. What has been in dispute for nearly a century is just what it’s telling us about what exists, what is real. There are myriad interpretations that offer their own take on the question, each requiring us to buy into certain as-yet-unverified claims — hence assumptions — about the nature of reality.

Now, a new thought experiment is confronting these assumptions head-on and shaking the foundations of quantum physics. The experiment is decidedly strange. For example, it requires making measurements that can erase any memory of an event that was just observed. While this isn’t possible with humans, quantum computers could be used to carry out this weird experiment and potentially discriminate between the different interpretations of quantum physics.
posted by MovableBookLady (50 comments total) 48 users marked this as a favorite
 


The experiment has four agents: Alice, Alice’s friend, Bob, and Bob’s friend.

Oh just come out and say that it's Carol and Ted.
posted by Saxon Kane at 3:49 PM on December 5 [82 favorites]


Renato Renner, a physicist at the Swiss Federal Institute of Technology Zurich, devised the paradox along with Daniela Frauchiger, who left academia shortly thereafter.
In the ideal world, she left because of getting this result. "Fuck this weird quantum shit. I'm going to be an accountant."
posted by clawsoon at 4:10 PM on December 5 [28 favorites]


A few years back I tried to get into quantum theory, weighed down with my poor background in math, and it was a horrendous struggle and I think quantum theory won... but. My background in philosophy kicked in reading this.

We have an event in a lab with a human watching it. Outside the lab, another person is MODELING the event, lab, and person using quantum theory. There is another lab where a person watches an event. This person, lab, and event is then MODELED by a fourth person. Contradictions occur between the two results from the MODELED descriptions of what occurred. For example, in the first case, the modeled description puts the person, event, and lab into a superposition of both heads and tails. This description is in the mind of the person OUTSIDE the lab, as a quantum theoretical model. Can we assume that in the lab, the coin toss had an actual specific outcome? My simple mind says that there was a specific outcome. The model is not an accurate map of the person-event-lab. The model is not reality, though it is claimed to be an accurate model of reality. We are dealing with people making a description or model from the OUTSIDE. I can imagine that this would present problems. As my old friend Korzybski said, The map is not the territory. I feel that the “weirdness” in quantum theory is the result of an issue in modeling/mapping given that we are dealing with an area of reality that we can only view from the outside.
posted by njohnson23 at 4:16 PM on December 5 [10 favorites]


This is either huge, or inconsequential. Or both at the same time.
posted by Edgewise at 4:28 PM on December 5 [31 favorites]


new way to dismiss people's opinions: "that's just what -your- quantum measurement says".
posted by solarion at 4:31 PM on December 5 [2 favorites]


I'm still reading the article but an obvious point I think is that the three assumptions are just different kinds of consistencies; in that Assumption 1 is an assertion about consistency with respect to granularity or scaling the number of particles, Assumption 2 is consistency w.r.t. to different observers, and Assumption 3 is a physical analog of logical consistency such as law of excluded middle. So that's an interesting general conceptual pattern to organize all those candidate quantum theories around, which seems to be the gist of the article.
posted by polymodus at 4:37 PM on December 5 [2 favorites]


> This description is in the mind of the person OUTSIDE the lab, as a quantum theoretical model.

Quantum Superposition is not just an artifact of someone's knowledge of an event, because it affects the probabilities of events, and we are able to show by experiment that quantum systems behave according to quantum statistics and not classical statistics, which could only be the case if the system were actually in a state of superposition.

The tricky thing about quantum mechanics is that to properly understand it, you have to give up not only determinism, but also reductionism. For example, a good way of understanding entanglement is that it is a state where the entangled particles have no independent state of their own, and the state of the system consists of only the correlations between the parts. Such a state can not be analyzed by reducing it to the sum of its constituents, and this also leads to consequences that can be tested and have been confirmed by experiment.
posted by I-Write-Essays at 4:44 PM on December 5 [12 favorites]


In terms of theoretic assumptions, that's still interesting because while anti-reductionism is intended to mean rejecting the fallacy of composition (ok or something similar to it), the standard quantum narrative nevertheless is a reduction. It's a reduction to those correlations.
posted by polymodus at 4:50 PM on December 5


Oh, me too, notreally, me too. I just like wrestling with it and then reading the comments in hopes of enlightenment (sometimes it works, sometimes it doesn't).
posted by MovableBookLady at 4:54 PM on December 5


O thou who standest on the threshold between the pleasant world of men and the terrible domain of the dread lords of the outer spaces, hast thou the courage to really look quantum mechanics in the face?
posted by heatherlogan at 5:06 PM on December 5 [5 favorites]


This was a real weird thing to read in the middle of binge-watching Mr. Robot.
posted by lunasol at 5:34 PM on December 5 [2 favorites]


But sometimes it makes me feel totally inadequate.

Think of it like a Voight-Kampff test. If you aren't confused and unsettled by quantum mechanics, you're probably a replicant.

The map is not the territory.

The traditional interpretation of quantum mechanics implies that the map is the only thing that exists, up until the point that it is measured, whereupon it suddenly becomes the territory. It's comforting to think of this as a mathematical abstraction, but Bell's theorem (such as I understand it) tells us that, in order to be consistent with experimental results, we have to assume that at least some quantum variables are undefined prior to measurement, or else affected by nonlocal interactions. There's no mathematically consistent way to iron out all the spooky wrinkles.
posted by dephlogisticated at 5:35 PM on December 5 [12 favorites]


I've thought for a long while now that the passage of time is actually marked by information entering the universe. The reason it looks continuous is that by definition you'll never see anything between one bit and the next.

If you assume that newly created information is the heartbeat of the universe, Quantum Physics starts to feel a little more intuitive. Not understandable of course, but less paradoxical.
posted by Tell Me No Lies at 5:53 PM on December 5 [2 favorites]


If you aren't confused and unsettled by quantum mechanics, you're probably a replicant.

I might rephrase it thusly: if you think you can accurately convey anything about quantum mechanics without math, you either don't understand quantum mechanics or you're trying to sell something.
posted by tobascodagama at 5:54 PM on December 5 [12 favorites]


I really like Carlo Rovelli for his non-mathematical "explanation" of this stuff.
posted by Mei's lost sandal at 5:56 PM on December 5 [2 favorites]


> Nicolas Gisin of the University of Geneva favors spontaneous collapse theories as a way to resolve the contradiction in the Frauchiger-Renner experiment.

This is interesting, because my own (admittedly very non-reliable*) intuition would lead me in this same general direction. In the quantum domain, the quantum way of working very clearly holds sway. But on our everyday scale of dealing with reality, we just don't see it--or at best, some very very small hints of it.

The large discrepancy in behavior between those two domains is the main reason it is so hard for us to grasp how things really work at the quantum level.

But the obvious--or perhaps, very naive--conclusion given the discrepancy between those two domains, is that somewhere between the quantum domain and domain everyday life, something must give in a really big way. That would probably look something very much like spontaneous collapse, or perhaps the new theory about how different large-scale theories fit together, that is speculated in the last couple of paragraphs of the article.

Other theories about how discrepancies could be resolved, like many-universes, just seem kind of hand-wavy, vague, unproveable, and fantasy-like.

It's just interesting, because in many study of quantum physics, which now dates to many decades in the past, I didn't realize the ideas like quantum collapse had much currency among serious researchers in the field.

* Whenever discussing quantum theory and phenomena, it's always worth reminding yourself that your intuition is going to wrong in pretty much every possible way. So take the above rambling speculations with that in mind.
posted by flug at 6:01 PM on December 5


Other theories about how discrepancies could be resolved, like many-universes, just seem kind of hand-wavy, vague, unproveable, and fantasy-like.

This is, I think, a common misconception about the many worlds interpretation. It's actually quite rigorous, and so far on an equal footing with other interpretations as far as provability goes, which is to say, not.

MWI holds that every measurement which produces an apparent collapse of the wave function is in fact an entanglement. The wave function remains in uncollapsed superposition, but we, as the measurers, now have our own quantum state entangled with the state of what we've measured. We live on one branch of that wave function and, once entangled, have no access to any other branch of the wave function. On the large scale we see very few quantum effects because we are already entangled with most other large scale objects which we encounter, and so already locked onto a particular branch of the wave function with respect to each other.

Wave function collapse is very similar, but assumes that the other branches of the wave function no longer have any reality once this happens. We don't really know one way or the other, but MWI isn't as outlandish as a lot of people think.

I'm not following everything in the thought experiment, but it sounds like this is working towards an experiment which might help us distinguish between interpretations, which would be great.
posted by vibratory manner of working at 6:47 PM on December 5 [13 favorites]


Mouse: Do you know what it really reminds me of? Tasty Wheat. Did you ever eat Tasty Wheat?

Switch: No- but technically, neither did you.

Mouse: That's exactly my point! Exactly! Because you have to wonder now: how do the machines really know what Tasty Wheat tasted like, huh? Maybe they got it wrong. Maybe what I think Tasty Wheat tasted like actually tasted like uh.... oatmeal or uh.... or tuna fish. That makes you wonder about a lot of things. You take chicken for example. Maybe they couldn't figure out what to make chicken taste like, which is why chicken tastes like everything! And maybe they couldn't figure out-

Apoc: Shut up, Mouse.
posted by Cool Papa Bell at 6:52 PM on December 5 [7 favorites]


I've thought for a long while now that the passage of time is actually marked by information entering the universe. The reason it looks continuous is that by definition you'll never see anything between one bit and the next.

Rovelli shows how time having a direction is dependent on exchange of heat.
posted by Mei's lost sandal at 8:22 PM on December 5 [4 favorites]


I think I can safely say that nobody understands quantum mechanics. -- Feynman
posted by bukvich at 9:11 PM on December 5


new way to dismiss people's opinions: "that's just what -your- quantum measurement says".

"I reject your reality and substitute my own!"

So, is this where we have another unsettling discussion about probably living in some screwy simulation and a glitchy copy of "Roy: A Life Well Lived"?

Because that conversation keeps coming up in unsettling ways and at this point I don't really care unless someone has figured out a reliable way to poke a pointy stick into it and through to whomever is running this fucked up timeline and maybe have a vigorous nonverbal disagreement with the script writers.
posted by loquacious at 9:22 PM on December 5 [2 favorites]


In the ideal world, she left because of getting this result. "Fuck this weird quantum shit. I'm going to be an accountant."

Or maybe she just ceased to ever have existed in the first place. Because that's a thing that happens. It's why lenses have that purple-y sheen.
posted by sexyrobot at 11:19 PM on December 5 [1 favorite]


This thought experiment does not contain a cat and is therefore invalid.

Yours sincerely

Wigner’s Friend
posted by fallingbadgers at 11:52 PM on December 5


QM is so non-deterministic that I could have sworn I read a paper about a group successfully putting a couple of bacteria in superposition.
posted by wierdo at 1:05 AM on December 6 [1 favorite]


It's like a bunch of stoners lying around, high as hell, having an intense discussion about the nature of reality as stoners are wont to do.

But one stoner asks the wrong question _and reality itself breaks_.

There are some who believe that this happens on an annual basis.
posted by delfin at 3:53 AM on December 6 [3 favorites]


It's comforting to think of this as a mathematical abstraction, but Bell's theorem (such as I understand it) tells us that, in order to be consistent with experimental results, we have to assume that at least some quantum variables are undefined prior to measurement, or else affected by nonlocal interactions.

If I'm remembering correctly, via the "nonlocal interactions" route Bell's theorem demonstrates the possibility of superdeterminism; quantum phenomena don't actually necessitate determinism, it's just that any compatible form of determinism will be radically different from the sort of Rube Goldberg machine, things hitting things the same way every time, conception of determinism in classical physics.
posted by XMLicious at 4:22 AM on December 6 [2 favorites]


Scott Aaronson says meh.
posted by you at 4:29 AM on December 6 [6 favorites]


-N'T liste- to Bob or -Alic
unstuck agents
jus need m- coin back
-niverse Next -Oor has
shitty parking
posted by Lipstick Thespian at 4:37 AM on December 6


Scott Aaronson says meh.

Thank you you, that was an extremely clear blog post!
posted by heatherlogan at 5:05 AM on December 6


How come the characters' names in the experiment weren't Rosencrantz and Guildenstern?
posted by CheesesOfBrazil at 5:57 AM on December 6 [1 favorite]


Because they were all dead, instead of maybe-dead/maybe-not.
posted by Quindar Beep at 6:16 AM on December 6


Schrödinger's give-a-shit.

That's me right about now. I'm very conflicted on this and my ability to parse it.
posted by RolandOfEld at 7:33 AM on December 6 [1 favorite]


Umm...
posted by Naberius at 7:42 AM on December 6


I think there is a fourth assumption involved, the assumption that only "conscious" things, such as people, can make a measurement. Whereas, I think that if an electron is in a superposition of spins, and another electron comes nearby and "needs to know" the spin, such as whether it can enter the same orbital of a hydrogen atom, the first electron's spin gets measured and the second one either enters the orbital or is rebuffed. This explains why wave functions for macroscopic systems collapse almost immediately. The biggest entanglements are Bose-Einstein condensates of just a few hundred particles, and let the temperature rise a bit and the state collapses. So of course Schrodinger's cat is either alive or dead long before a human opens the box.

Turning this around, you could alternatively say that every particle is a tiny bit conscious and so makes a "measurement" when it gets information about another particle's state. The lesson learned is that, once again, there is nothing particularly unique about humans or, in this case, their measurements. The idea that we are conscious but not Schrodinger's cat is absurd. But I would say the same is true of Schrodinger's cat's molecules.

While this interpretation also has its problems, I think they are of a type and of the same magnitude as the other assumptions that may or may not get violated.
posted by M-x shell at 8:41 AM on December 6 [4 favorites]


Oh just come out and say that it's Carol and Ted.

Now we'll never know how fast they're going!
posted by chavenet at 8:46 AM on December 6 [4 favorites]


> I think there is a fourth assumption involved, the assumption that only "conscious" things, such as people, can make a measurement.

Yes, I think many people get confused by the anthropic-sounding terms "measurement" or "observation," but really we're talking about "interaction" and "entanglement." As I understand it, Quantum Mechanics is always predicated on dividing the world into one system being "observed" and another doing the "observing," and the act of observation boils down to creating entanglement between the two systems when they interact.
posted by I-Write-Essays at 9:29 AM on December 6 [5 favorites]


I-Write-Essays: the act of observation boils down to creating entanglement between the two systems when they interact.

That makes sense of the idea that this might be something we could someday test in a quantum computer.
posted by clawsoon at 9:56 AM on December 6


How does observation of a quantum system entangle the observer? (I do vaguely recall this from somewhere.) I thought the article article says that a measurement collapses the wavefunction of the particle. That classical regime sounds like the complete opposite of getting entangled.

Also, I'm annoyed that the article glosses over the polarized light example without giving citation. That whole paragraph buried in the middle of the article seems crucial.
posted by polymodus at 12:20 PM on December 6


"Collapse of the wave function" is an imprecise way of describing the situation from the point of view of the observing system. If I were home, I would link to some lectures, but perhaps this stackexchange question will provide a good discussion of the issue.

Relevant Snippet:

From an external point of view, when two physical systems interact, they become entangled. This apply even if one of the systems is large and semi-classical (say, a photon detector). Regardless of the scale of the systems, when there is an interaction, the total system keeps evolving under an unitary propagator, which respects time symmetry.

From the point of view of each system, the coupling does not seem unitary at all; it seems like the other system suddenly collapsed to a random eigenstate of the coupling perturbation. Part of the quantum information that existed in the other eigenstates disapeared and became physically inaccessible. This is how we perceive measurement.


The stuff about FTL communication is irrelevant: measuring the entanglement doesn't transmit information, because the piece you measured never had an independent state outside the combined system to begin with, and by interacting and becoming entangled with it, you broke the original entanglement.
posted by I-Write-Essays at 12:34 PM on December 6 [3 favorites]


I could be totally wrong, but my understanding has always been that to observe you have to bounce something (like light) off the electron, thus interacting/interfering with it, and collapsing the wave form.

This has kept my mind safe from the weirdness until I found out about that Delayed Choice Quantum Eraser and I've still not come to terms with it.
posted by CheapB at 2:02 PM on December 6


@CheapB:

The thing to keep in mind is that Collapsing The Wave Function is not a thing that actually happens, it is merely one way of interpreting the mixed state of the partial system that you have observed. After your system has interacted with the system you are observing, the two systems become entangled and now the state of either one can not be properly described on its own. Collapsing The Wave-function is a story told by the Copenhagen Interpretation of Quantum Mechanics, because a system is not capable of observing itself.

To observe the pure state of the combined system, a third system would need to be introduced, and it would not observe anything like a collapsed wave-function, but instead it would see that the two original systems can no longer be described independently of each other: the act of observation has caused them to become entangled. Such an observation would of course cause that third system to itself become entangled with the whole, from the point of view of a fourth system.
posted by I-Write-Essays at 2:43 PM on December 6 [2 favorites]


I felt like there was a crucial step in the thought experiment left out in the original article...
posted by Saxon Kane at 4:17 PM on December 6


Whether wavefunction collapse really physically happens isn't a settled matter: it's a central point of contention between different interpretations.

Personally I'm inclined to believe that the difficulties people have been having creating quantum computers (we're up to what, 50 whole qubits after half a century?) indicate that the universe is loathe to allow substantial entanglement, and that wavefunction collapse does objectively happen.
posted by Pyry at 4:30 PM on December 6 [2 favorites]


Pyry, supposing that the universe is loathe to allow substantial entanglement, what do you make of the proposition that the connectivity of space consists of the entanglement patterns in vacuum fluctuations, such as the ER=EPR principle advocated by Leonard Susskind?
posted by I-Write-Essays at 4:45 PM on December 6


Actually, I think like this version of the lectures better, given a year earlier. 1 2
posted by I-Write-Essays at 4:57 PM on December 6


Well thinking about the article, could you not argue that the wave function necessarily collapses if you accept Assumption 1? Under their framework, wavefunction collapse is an illusion (subjective to the measuring system) only because one rejects Assumption 1 i.e. on the consistency of quantum theories w.r.t. the size of the system. Particularly, if you keep glomming on entangled subsystems you eventually get a large system and that's why Assumption 1 needs to be reckoned with at some point. What their framework does is make this interpretive issue explicit.
posted by polymodus at 5:21 PM on December 6


If this is a simulation, then we have reached a crisis of ubiquitous and persistent fourth-wall breaking.
posted by tillermo at 1:43 AM on December 7


If this is a simulation, then we have reached a crisis of ubiquitous and persistent fourth-wall breaking.

This is why I like to make a lot of eye contact and wink at the cameras. I not only want them to know that I know that they know I know, but I also want them to know I'm coming for them.
posted by loquacious at 9:22 AM on December 8 [2 favorites]


Er, I suspect the difficulty with quantum computers is too much entanglement, not too little, in that it's hard to prevent entangement with environmental degrees of freedom.
I suspect that if quantum gravity comes along it will offer a better explanation for why we mostly don't see quantum superpositions in the everyday world. But by way of historical analogy, when general relativity came along it did offer a better explanation of why we don't notice the Earth hurdling through absolute space at 110,000 km/h than Newtonian physics--better but also far weirder and less intuitive. New physics is not going to save us from the weirdness of quantum mechanics.
posted by mscibing at 6:25 AM on December 11 [1 favorite]


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