I find the concept of consciousness applied here a bit disturbing. Shouldn't we treat the observation as information leaving the observed system and propagating causal effects to other systems? A data recorder is not conscious, but notes information that passes through it and, therefore, observes a single state instead of a superposition.
Yep, this is why quantum computers have to be kept at very low temperatures as opposed to asking everyone to look away from the computer while it operates
> I find the concept of consciousness applied here a bit disturbing
It's not just disturbing, it's going way, way beyond what the actual experiment tested. Articles like this (and even the underlying papers describing experiments like this) talk as if we are doing Wigner's friend type experiments involving actual humans. In fact the systems being experimented on are qubits, and the detectors are very simple detectors. The technical achievements involved in these experiments are certainly impressive, and experimenters should certainly continue pursuing these things, but the philosophical implications being claimed are IMO vastly overstated.
I agree, however you have to keep in mind that the Copenhagen interpretation [1] was invented before Vitalism [2] was debunked. So it is not too surprising that it also has this idea of a fundamental difference between the rules guiding living (including conscious) and dead matter at its core.
My usual argument against it is the mediocrity principle [3], we humans often fail to accept the insignificance of our existence. Thus, we tend to prefer theories which tell us that we are special. Just like the idea that we are able to trigger the collapse of the wave-function and the superposition, but a cat is not.
Does the copenhagen interpretation actually require the observer to be conscious rather than, say, a macroscopic measuring instrument?
Indeed the wiki article you link to says:
> Although the Copenhagen interpretation is often confused with the idea that consciousness causes collapse, it defines an "observer" merely as that which collapses the wave function.
Which seems like a very practical, albeit obviously incomplete, approach to take.
Consciousness comes indirectly into quantum mechanics through the measurement postulate. You have to assume something special about measurement which breaks unitarity. Not all physicist are convinced, but it is the traditional way quantum mechanics is taught (all of the "transition probabilities" are really measurement probabilities). The problem is that you can't tell if your apparatus caused the collapse or if it was you that measured the apparatus that then caused the collapse. And because you can only know that something was "observed" when a conscious physicist makes that final readout you end up with a solipsistic situation where the only thing you can be certain of is that it was that last conscious observer that could have for sure collapsed the wave function. It could be that the collapse happened in the apparatus itself but as far as I know you have no way of telling the difference between it projecting the wave function of the measured system or you projecting the wave function of the apparatus into a pointer state. Basically consciousness sneaks in through the fact that there is no definition of what constitutes a measurement - what makes one physical process a measurement as opposed to all others that are unitary and the only thing you are certain of is that final "conscious" readout should count as measurement.
This is misleading at best. A measurement has nothing to do with a conscious observer taking the measurement. An automated experiment that could note down the result of an experiment would note down the result without any conscious observer having to check in and the result would be the exact same.
How do you know it would be the same? That's an assumption. The measurement problem is a problem because quantum mechanics postulates two types of processes "normal" unitary evolution and "measurements" that project out the wave function into eigenstates of the measured operator. This is obviously inconsistent since there is no definition, formal or operational of what process is a measurement and what process isn't a measurement. How do you know that your measurement apparatus isn't just evolving unitarily (which is what you would expect if it was a "normal" system) until someone looks at it to read out the results. Consciousness enters only in the fact that for anyone to know what happened to the experiment someone has to do the readout (otherwise you're just writing equations). At the point of readout you can't tell who or what did the collapse.
Many worlds interpretation of QM is that everything continues evolving unitarily, even you when you hear about the result. It's just that "being in a superposition" doesn't feel like listening to the garbled sound of someone simultaneously telling you that the result was positive and that it was negative, or looking at a blurry instrument screen reporting two results at once. Each component of you in the superposition feels like it got a single clear definitive measurement result. It feels the same as not being in a superposition.
It appears as if one of the main reasons for almost a-priori rejection of the Many Worlds interpretation lies in the words "many worlds". More often this metaphor gets in the way instead of helping. It seems as if you have to accept something "more", yet, at its core, the Everettian interpretation is the simplest pure consequence of QM. We just have to grapple with the psychological consequences if being a cog in the machine and we devise further metaphors to help us talk about how it would "feel" to be in superposition.
Is there a better way to build intuitions how an information processing system would behave while being in superposition?
Things get complicated when we throw humans in the mix, perception and consciousness and all. But modelling even simpler machines and their "point of view" can be insightful.
In this day and age it shouldn't be hard to imagine the working of a computer that uses computer vision algorithms to perceive its world and take action as a result, acting as a "causality amplifier". For example image we feed it the output of some QM experiment and instruct it output a description of what it "sees" (as we routinely do with cat pictures classification).
I assume it would be far less controversial to think about the unitary evolution of the wave function if the system being described is a QM experiment plus a computer rather than the same QM experiment and a human.
Yet, there are many similarities. The output of this computer (the classification) would be in superposition, and when measured by us it would appear as if the wave function collapsed. But we could add another such computer in the mix and ask ourselves "does it also see the wave function collapsing"? Well can program it to take the measurement and record the answer and then convey the answer to us (or to another computer down the chain). These "answers" are the "point of view" of the computer. It will "observe" decoherence yet it won't be decohered itself, as its own statement about whether it observed decoherence is itself in superposition and thus can be used as a further input to other machines witnessing subjective decoherence.
> It's just that "being in a superposition" doesn't feel like listening to the garbled sound of someone simultaneously telling you that the result was positive and that it was negative, or looking at a blurry instrument screen reporting two results at once.
It seems like there's regularly articles saying stuff like "QM implies that both outcomes happen, but it's a longstanding mystery why we only see one outcome", as if they seriously expect your hypothetical to be the consequence of QM. I'm so frustrated at that, because as you say, seeing one outcome is exactly what you'd expect to see from inside of a superposition.
It's almost as frustrating to see as it would be to see an article saying "Newton's theory of gravity says mass attracts mass, but it's a longstanding mystery why we haven't all fallen into the sun". The theory already has an answer for that if you follow the chain of consequences from it.
I’m not sure why the measurement problem is so difficult to understand for some; your explanation of it is very clear. You don’t have to believe in anything supernatural or mysterious to recognize there is a clear inconsistency here with “quantum systems sometimes evolve unitarily”.
Traditional QM requires an outside observer but has nothing to do with consciousness. Consider the collision events recorded at the LHC: the vast majority have never been looked at by a human.
They have not been explicitly looked at, but they have been observed in the QM sense. That’s because decoherence has spread those records to conscious observers.
If the results had been kept completely isolated from all people, you could still say a measurement hasn’t occurred.
I guess I got confused at "If he thought he was in a superposition he would believe in many worlds."
Do you mean "he can either think he can be in superposition or he cannot be, if he thinks he can be then he would believe in many worlds, which is incompatible with 'traditional' QM, which thus implies 'Traditional QM does not allow Wigner’s friend to be in superposition.'" ?
I don't think the Traditional QM ever explicitly disallowed a person to be in superposition; being in superposition it's just not something "decent people" do, I suppose.
Thus we needed to find other ways to come to terms with what we observe. But the math is the same. The rest is only metaphors that help us reconcile what we observe with how we feel inside.
Thus, many worlds is not antithetic to traditional QM.
Do you think that many quantum physicists during the 21st century accepted the possibility of living in a splitting multiverse?
I don't think that's historically accurate. If you read about how Everett's ideas were treated it's pretty clear traditional QM is antithetic to many worlds.
That's just not what traditional QM says. Traditional QM separates quantum systems (microscopic) from measurements devices (macroscopic). A measurement occurs when the measurement device interacts with the quantum system. You do QM by predicting the results of these measurements. Consciousness does not enter into it.
Can it be considered as until it causally affects something else (the observer), its state is not defined? Isn't it a causal relation that collapses the superposition?
So why doesn't a measurement and/or decoherence occur when double slits or a half-silvered mirror interact with a passing particle? Are they not macroscopic objects which interact with the quantum information?
Perhaps it's only a "measurement" if the (alleged) particle has nowhere else to go after the interaction. But how does the (alleged) particle know which macroscopic interactions are terminal and should be counted as "measurements" and which are part of the rest of the experiment?
There is no answer to this in QM. You can calculate the probabilities and you will get predictable answers, but there are still >20 interpretations of what is really happening, and they all disagree with each other in important ways.
Wouldn't it be valid to consider the LHC and its unobserved data as a superposition of all their possible states?
Note that the recording of information has effects outside the system, even if no human looks at the data - recording a zero or a one will require different levels of power that, while they average to a mostly constant power consumption, are there nevertheless.
> Consider the collision events recorded at the LHC: the vast majority have never been looked at by a human
What tells you those records aren't in a superposition?
The only way to determine anything about the records is to observe them, and at that point you have a human (= consciousness) in the loop. Anything you put between yourself and the experiment might be in a superposition until you observe it.
> What tells you those records aren't in a superposition?
This! Now substitute the record taking device and record keeping substrate with a human brain and this stays true. To rephrase your question:
"What tells you the state of your brain isn't in superposition?"
A human in the loop is not the end of the story, it's just yet another interaction in the quantum system.
When we experience decoherence of a quantum system, we interpret it as if something happened to the thing we observe, yet what actually happens is that something happened to us (and in turn to every system that observes us, and so on).
This is all utterly unintuitive for us, who experience the world through those brains, who feel being there, conscious in the moment. That feeling is one of our strongest direct perceptions of the world, and yet it's affected by the mechanisms of the physical reality. It's hard to accept though; it runs counter to many deep intuitions we have about ourselves, our inner lives, our identity, our values, our belief systems.
> You can always consider your automated apparatus to be part of the system and hence governed by unitary evolution.
But then wouldn't an external measuring apparatus (or person) observe the system in a single state instead of a superposition? Isn't that the same as a series of nested systems, each measured and having the state recorded by an apparatus that's part of a system that encapsulates it?
> Consciousness comes indirectly into quantum mechanics through the measurement postulate. You have to assume something special about measurement which breaks unitarity. Not all physicist are convinced, but it is the traditional way quantum mechanics is taught (all of the "transition probabilities" are really measurement probabilities). The problem is that you can't tell if your apparatus caused the collapse or if it was you that measured the apparatus that then caused the collapse. And because you can only know that something was "observed" when a conscious physicist makes that final readout you end up with a solipsistic situation where the only thing you can be certain of is that it was that last conscious observer that could have for sure collapsed the wave function.
> Here we report matter wave interferometer experiments in which C70 molecules lose their quantum behaviour by thermal emission of radiation. We find good quantitative agreement between our experimental observations and microscopic decoherence theory. Decoherence by emission of thermal radiation is a general mechanism that should be relevant to all macroscopic bodies.
> We observe that at temperatures below 2,000 K the emission rate is negligible, whereas at higher temperatures the molecules may emit photons whose wavelengths are comparable to (or even smaller than) the maximum path separation of ,1 mm. They transmit (partial) which-path information to the environment, leading to a reduced observability of the fullerene wave nature. Around 3,000 K the molecules have a high probability to emit several visible photons
yielding sufficient which-path information to effect a complete loss of fringe visibility in our interferometer.
I don't think there is consensus that decoherence = measurement. This is a sophisticated experiment that controls the level of position information being "radiated" to the environment. But Born's rule and the measurement postulate are still baked into the fact that you are then measuring the interference pattern.
One thing I never quite understood about the decoherence argument for resolving the measurement problem is that it forces the "collapse" to happen with local interactions. This of course makes sense since you want to think of a measurement as a "normal", unitary process. This means the environment has to "enact" the projection operator through interactions. If you have the most basic EPR pair, how can this happen? How can you decohere the wave function of the spacelike separated counterpart (B) by locally measuring the other spin (A) and then letting the environment project out the state of B through local interactions? In this experiment https://arxiv.org/pdf/1511.03190.pdf they have two detectors 60 meters apart in a subbasement of a castle. What are the interactions in those hallways that are carrying the correlation from the decoherence/measurement on one side of the experiment to the other, faster than light to maintain the correlations. This seems too basic of a question so I don't know if I'm just missing something obvious, but to me it seems like a pretty straightforward argument that the collapse cannot happen through decoherence. I've only dug out one random paper that mentioned this argument with very little subsequent referencing of it.
Certain experiments have shown that quantum mechanics is incompatible with local realism, and this means either that locality is wrong, and the world has faster-than-light communication, or that realism is wrong, and that there are no single outcomes to events. Assuming that locality is true and realism is wrong, when you measure particle 1, it's not that the result is being instantly communicated to particle 2 and causing it to come out that way, but all possible outcomes for each particle measurement happen separately, and all versions of things affected by particle 1's measurement only interact with versions of the world where particle 2 has or will have a compatible measurement. No collapse ever happens, though from each of the perspectives of humans inside the superposition, they will see results that look like their own measurement caused a collapse, because they each will eventually see a single consistent outcome for the results of the particles' spins.
The results of nonlocal and nonrealism theories come out the same, though nonlocal theories imply a lot of mysteries around the contradiction between instant communication and relativity of simultaneity, while nonrealism just paints a picture of a MWI universe that's surprisingly larger than our expectations.
In quantum computing, I can implement important effects using the measurement operation. For example, I can reduce the number of Toffoli gates used during an addition by measuring at particular places in the circuit. From this we can easily see that measurement must matter a lot in quantum mechanics, since its presence allows you to reduce the cost of certain tasks.
No one has postulated a "conscious measurement operation" with additional useful effects beyond the measurement defined in textbooks. (Well, okay, Roger Penrose says things that kinda sound like that, but that's pretty fringe and beyond the scope of quantum mechanics.) In fact, there's literally no known experiment that a person or a machine could perform that would distinguish a "conscious measurement" from the usual mechanical measurements quantum computers use. That is the sense in which consciousness has nothing to do with measurement.
IMO quantum mechanics really doesn't have anything to say about consciousness that wasn't already present in classical mechanics. Instead of saying "but how can an assemblage of gears experience an outcome" we're saying "but how can an assemblage of superposed gears experience an outcome". It's just the same hard-problem-of-consciousness confusion dressed in new clothing.
It's a huge issue that observer is interpreted to be something like a person. An observation is a certain kind of interaction that causes the quantum state to collapse. It has nothing to do with consciousness.
In what sort of physical system does this interaction occur? Can you make an observer out of carefully arranged doped silicon? How does it manage to trigger a collapse rather than ending up in a superposition itself?
I'm not well informed on this, and I don't think consciousness is relevant here so by all means I hope someone corrects me. But I'm not sure it's that simple. As I understand it, you can have something set up to measure a property like momentum, then obfuscate it, and the eventual outcome will be an uncollapsed system.
Putting on my baseless assumptions hat, it seems to me the world just behaves as a weighted average of all possible pasts, that reality isn't "branching" into many worlds on measurements so much as just being a giant vector space of electron states that have some discrete gaps in an otherwise continuous space due to precise measurements being made, and that the phenomenon of particle-wave duality is just an artifact of some rounding the universe does under the hood when averaging together these possible pasts to render a given state*, and that any attempt to understand the casuality of the particles will fail because they don't arrive from a single cause, but a space of causes, which we can at best constraint to single points.
honestly, i felt the same way after a good 15-20 minute read - I think it's one of those where you have to read it a few times, leave space in between, and Google as soon as you start getting confused
I haven't read the new paper in enough detail to comment, but also mentioned is the Renner-Frauchiger paper, which Scott Aaronson heartily debunks here: https://www.scottaaronson.com/blog/?p=3975
The primary error there is one that people can't seem to stop making for some strange reason: if you don't measure a property X, and instead measure a (non-commuting) property Y, you are not free to say what X "would have been," and -- especially egregiously -- to use that to demonstrate a contradiction. There is no "would have been" in QM. Either you measure it or you don't, and even after you do, the value becomes invalid after it evolves or a complementary variable is measured.
Secondly, Wigner's friend was not a single particle. Asking what a single particle "experienced" only makes sense if you are a panpsychist (or "panpsychic" as amusingly misspoken in the article). In any case, it has no bearing on the essence of Wigner's question.
I think RF is a bit more subtle than that, and I'm not convinced that Scott Aaronson has really "debunked" it.
"But clearly, even if no one literally measures Charlie in the {|0〉,|1〉} basis, he’s still there, thinking either the thought corresponding to |0〉 or the thought corresponding to |1. And likewise Diane. Just as much as Alice and Bob, Charlie and Diane both exist even if no one measures them, and they can reason about what they know and what they know that others know. So then we’re free to chain together the “certainties” of Alice, Bob, Charlie, and Diane in order to produce our contradiction."
You can't use this argument without also explaining what "thinking" means - because in fact it makes no sense unless you assume consciousness exists as a quantum state which allows a phenomenon called "thinking."
And philosophically and practically, that's a very, very large can of wormns.
So this ends up in the usual place: it's not obvious how using an idea no one understands - like consciousness - to explain another idea no one understands - like QM - is supposed to solve any of these problems.
In order to implement the measurement described in the thought experiment, you basically have to uncompute Charlie's state back to the start of the experiment (to disentangle him from the qubit you want to measure), do the measurement on the single qubit, then recompute Charlie forward. This means that when Charlie is making deductions about the current state of affairs, like "if my qubit is |0> then the other qubit is |+>, that thinking needs to be valid during the computation and during the recomputation. But the conclusions Charlie is making are not correct during the recomputation, e.g. it is no longer the case that if his qubit is |0> then the other qubit is |+> at that time, and as a result the whole argument (based on Charlie's reasoning being valid so that an outside agent can use his conclusions) falls apart.
I'm going to be honest, and say that I have no idea if this comment on that page is relevant to the article in the OP. But if it is, it's hilarious:
> OK, point taken. But if one has any experience with experiments in the foundations of QM, one knows full well what’s going to happen next. Namely: some experimental group will do a slightly souped-up test of Hardy’s Paradox, of course getting just the results that QM predicts, and will then market it in Science or Nature as “the first experimental probe of the logical contradiction at the heart of QM … who could’ve imagined that the ‘impossible’ outcome would occur with probability 1/12?” And then the science journalists will wet themselves with excitement. It’s all nearly as predictable as QM itself!
>"Because the two photons appear to communicate faster than the speed of light — something prohibited by his theories of relativity—this phenomenon deeply troubled Albert Einstein, who dubbed it “spooky action at a distance.”
These concerns remained theoretical until the 1960s, when physicist John Bell devised a way to test if reality is truly spooky—or if there could be a more mundane explanation behind the correlations between entangled partners."
[...]
Such “Bell tests” have since been carried out, with a series of watertight versions performed in 2015, and they have confirmed reality’s spookiness."
Logic:
If you believe that the Bell Test is correct -- you must equal-and-oppositely believe that the speed of light is NOT the upper limit of speed in the universe, that is, that Faster-Than-Light (FTL) speeds are possible.
Conversely, if you believe that speed maxes out with the speed of light, and that nothing can travel faster than the speed of light -- then you equal-and-oppositely believe that the Bell Test is wrong.
But, you cannot believe that nothing can travel faster than the speed of light AND that the Bell Test is correct, simultaneously -- as this would result in a logical paradox...
The trick is to talk in terms of communicating information faster than the speed of light. Both relativity and quantum theories agree that there's no communication of information faster than the speed of light. Two wave-functions collapsing to opposite states lightyears away doesn't actually allow two people to communicate in an FTL way.
Exactly, so it becomes a question of the very inner working of the universe itself: How does the universe spontaneously assign the states consistently without communicating and also without assigning the states beforehand.
This is precisely why the experiment was proposed, to show this conflict in our understanding of the constraints.
If one bends a piece of paper such that two faces nearly meet then pokes a pin through the page and makes two holes in the page essentially simultaneously from the perspective of the two dimensional beings on the page it appears that the two events happened far apart and that "something must be moving information at faster than the speed of light" when the reality is more complex.
That's how I mentally think about entanglement. I doesn't need to operate through the universe as I understand (or operate within) it.
> If you believe that the Bell Test is correct -- you must equal-and-oppositely believe that the speed of light is NOT the upper limit of speed in the universe, that is, that Faster-Than-Light (FTL) speeds are possible.
That's not exactly correct, you just have to forgo locality, which some interpretations of quantum mechanics do, like the Bohmian one. Like the sibling comment points out, this still doesn't permit faster-than-light communication.
>"That's not exactly correct, you just have to forgo locality, which some interpretations of quantum mechanics do, like the Bohmian one. Like the sibling comment points out, this still doesn't permit faster-than-light communication."
Er, yes, but, forgoing locality -- changes the definition of speed -- because it changes the definition of distance:
Speed = Distance / Time
If you forgo locality, then you are now able to modify distance in this equation, that is, your equation now looks like this:
That's because, let's say you have a black hole/quantum entangled particle/stargate/thing A at one point in the universe that immediately influences thing B at some distance to it in the universe (call this "connected" concept by whatever name or metaphor you will), etc.
Well, if so, then the NON_LOCALITY_SCALING_FACTOR is there to adjust the shorter distance to the longer one.
Like let's say you had a stargate linking two points in the universe, and you walked 10 feet into it, but this 10 feet, after you had walked it, resulted in you appearing halfway across the universe.
OK, so that's 10 feet in local movement, in local distance, but billions of miles of actual (but non-local) distance.
That's why you now need NON_LOCALITY_SCALING_FACTOR in your equation, because you've redefined Speed in terms of it.
But, by redefining Speed (which must occur, by forgoing locality!), you now have agreed (because it is implied) that anything faster than light (communication, travel etc.) can now occur, that is -- all you need is a high enough NON_LOCALITY_SCALING_FACTOR in the equation for speed -- which you get by forgoing locality...
Thinking about it as a collapse used to be the most popular interpretation, but it really complicates more than it solves.
All the experiments become much clearer if you think of everything behaving primarily as a wave (all possible paths are taken). A particle is just a special (limited energy) case of this where all paths are still taken but since there is limited energy only one path can be 'registered', and this happens probabilistically.
If the cat is an observer, then is the bottle of poison also an observer? Are the cells of the cat? Are the atoms of those cells? What is it that gives the cat observer status?
The intent of the thought experiment was to show that the Kopenhagen Interpretation of quantum physics is ridiculous. In the many-worlds interpretation for example there is no paradox, as time goes on more and more worlds exist where the cat died at some point.
Going off the double slit experiment, it seems the definition of an observer is something that can take in light, interpret and measure it. The examples given are eyes and cameras. (I'm not claiming I understand this, just trying to)
"observation" isn't important, it's enough interaction to decohere the superposition, or in another sense, entangle it with the environment. A polarization filter could be considered an "observer" in some sense, but I think focusing on observation results in a less clear and truthful understanding of what a measurement is. A polarization filter rather interacts with the light in a strong enough way to force conditional behavior based on its polarization. This results in photos that have a superposition of different polarizations to probabilistically become one of the polarizations, wrt the respective amplitudes of each polarization for that photon.
The double slit experiment works with much more than just light. It works just as well with electrons, and even with large molecules (the current record seem to be molecules of 2000 atoms each) [1]. That gives you a pretty wide range of observers under your definition.
Looking for a solution of f(x)=1/x at x=0 is also ridiculous, because it is simply not defined there. But how does then a quantum cat in a box pass this test? Is a completely coherent box of cat and poison that could kill it before decoherence even possible? Quantum computers have to freeze everything to absolute zero+ to work without decoherence, but wouldn't that, applied to the box, actually save the cat (or kill it, depending on your viewpoint)? Poison couldn't do its job at zero kelvin. And you cannot have a normal temperature catbox that doesn't decohere immediately, unless it is a box of startrek quantum force shields. Yes, at zero temps we can claim anyway that process maybe has been started, but then when we look at it (by unfreezing), we actually begin to observe that one radioactive atom in our, unseparated reality, as if nothing happened before we opened a lid.
I'm likely wrong in details here, but isn't that thought experiment similar to "what happens at the origin when we draw a hyperbola" sort of question? (Nothing happens, as you can't draw a complete hyperbola.)
"Does the cat experience superposition?" Questions like these have always seemed to me like mistaking the map for the terrain. Quantum mechanics is a model, an abstraction of reality, trying to cope with uncertainties in reality that our observation & measurement technologies can't really settle. It's interesting but not to be taken quite so literally. Right? I dunno, clearly I'm not a physics expert. But I'm not a music expert either (actually I am but let's pretend), but I know that the sounds that please or impress advanced academic peers/mentors/funders rarely overlap with those that please the rest of the world. The Art and Architecture departments are like that also, and I suspect but can't prove that e.g. Economics, and more to the point, Physics, are too. I probably sound like I'm verging dangerously close to anti-intellectualism here, which I don't really want to do in the current cultural climate, but if anything it's the opposite: We should remember that believing someone implicitly just because they're an intellectual is also anti-intellectual. It's an appeal to authority which is one of the classic logical fallacies. Someone is not necessarily immune to institutional pressures (nor are they even necessarily an honest person) just because they're an intellectual. But you need to conduct your own research or at least a survey of the existing research before you can call yourself even a legitimate skeptic (as opposed to just a troll)! Which is why I'm stopping short of actually saying anything... so hey, my apologies for the time-wastage!
Here are a couple of things that are very rarely explained in QM pedagogy but which are very helpful in understanding Shroedinger's cat:
1. The wave function does not exist in physical space, it exists in configuration space. Configuration space is the same as physical space only in the one very special case of a single non-entangled particle. The intuitions of physical space do not generally carry over into the general case of configuration space.
2. In order to produce interference you have to split up the wave function into two pieces and then bring those pieces back together again with a phase change. But since the wave function exists in configuration space, "bringing it back together again" becomes increasingly difficult as the number of entangled degrees of freedom grows. As a practical matter, it is effectively impossible to do this once the number grows beyond a few hundred (this is why building quantum computers is hard). But an actual macroscopic observer consists of O(10^23) degrees of freedom, and is also entangled with their environment. This is the reason that large systems behave classically and don't exhibit interference.
3. Whether or not a system is in a superposition is not a physical fact about the system, it is a consequence of the measurement you choose to make. A vertically polarized photon (for example) is in a superposition relative to a measurement axis rotated away from the polarization axis. For a more complicated system, the measurement axis, like the wave function itself, is in configuration space. But we can only do measurements in physical space. This is the reason that in order to see interference in an entangled system it is necessary to do multiple physical measurements and look at the correlations between them. Doing those kinds of measurements on actual macroscopic objects is way beyond the current state of the art, and may well be impossible even in principle. But this is the sort of thing that is contemplated in the theory behind these experiments. This is the reason that single particles are used as stand-ins for Wigner's friend.
That is a really good question! The answer is: no one knows. There is at least one serious interpretation of QM that says that the 3-D physical space that we know is just one of many possibilities [1].
...which failed to measure Aether-Drift because of the fact that it was done in a basement, and which was later contradicted by Dayton Miller when he repeated the experiment on a mountain top:
This scientific tragedy, likely done by powerful oligarchs, set the progression of physics back 100 years. We now have fraudulent branch of particle physics, which has a terrible track record with pretty much every prediction it has made, and which would be laughed out of the scientific community if it weren't for the sustained push by the mainstream media to hammer it into the minds of intellectuals.
We've been told that light has a set speed of propagation, yet time after time there are articles posted about light exceeding the speed of light for example gamma ray jets from quasars, which is explained by positing that the space-time fabric is moving and causing the reference frame containing the light wave to increase it's speed above the limit.
The media has become so twisted that we now accept completely broken interpretations such as virtual particles (used to balance equations), string theory (completely made up and didn't work), multiple universes, time travel and a smorgasbord of hilarious interpretations from a fundamentally broken model of physics.
This article is just yet another desperate attempt to patch up this hilariously outdated physics branch. Einstein physics is on it's way out. And when it comes crashing down we will replace it with the work of Tesla and we will move faster in one decade the entire previous 150 years.
Wow, so much pseudo-science and just plain nonsense in one long comment. The wikipedia article on Orgone, https://en.wikipedia.org/wiki/Orgone is a good starting place in learning just how goofy these ideas are.
I good friend of me has an esoteric coworker with a strong focus on Orgone energy and the like. The stories I get to hear and the "research" we do to read up on that stuff are fun and quite a filler for good and entertaining evenings.
No worries man.
Around the end of the 19th century we start laying out long, I assume copper, metal wires stretching from shore to shore crossing the Atlantic.
They don't know much at this point, things are not working out, and some people even manage to total loss a wire by too much voltage.
At each end of shore, there sits the operator. Click clacking dots and lines. Mapping them to letters, words and sentences by Morse's mapping. One of the operators is Oliver Heaviside.
Heaviside develops the mathematical understanding of the action and scenario when a single pulse is generated at one shore. I would like to have a reference to the first occurrence of his telegraph equations.
Now, we have them in many forms.
The differential kind from the wiki article.
The operational calculus kind from Jan Mikusi?ski's .
The modeling of capacitor and/or coil as transmission lines by Ivor Catt.
The fourquadrant representation from Eric Dollard.
I will appreciate any elaboration on any of the subjects.
The modelling and experimentation in the area seems to be based on an opportunity to contract space.
Meaning that instead of having a 5,000 mile long wire, we are forced to be limited by our laboratory span.
How can we simulate in our lab a 5,000 mile long wire? Eric Dollard proposes the analog computer.
Four elements compromise a symbol. We lay the symbols in a serial line.
https://www.youtube.com/watch?v=nJ8drfI4j9o
Now, we got a 5,000 mile long wire in our lab. Space contraction.
It's a convenience to explain states within equations. When you say "the cat is both alive and dead at the same time", you're simply saying "in this equation, do not assume that this variable will be one or zero, calculate the results for both conditions", and the final outcome will possibly be one of these calculated results.
The more such variables you have, the more different results you may get.
88 comments
[ 3.5 ms ] story [ 147 ms ] threadIt's not just disturbing, it's going way, way beyond what the actual experiment tested. Articles like this (and even the underlying papers describing experiments like this) talk as if we are doing Wigner's friend type experiments involving actual humans. In fact the systems being experimented on are qubits, and the detectors are very simple detectors. The technical achievements involved in these experiments are certainly impressive, and experimenters should certainly continue pursuing these things, but the philosophical implications being claimed are IMO vastly overstated.
My usual argument against it is the mediocrity principle [3], we humans often fail to accept the insignificance of our existence. Thus, we tend to prefer theories which tell us that we are special. Just like the idea that we are able to trigger the collapse of the wave-function and the superposition, but a cat is not.
1: https://en.wikipedia.org/wiki/Copenhagen_interpretation 2: https://en.wikipedia.org/wiki/Vitalism 3: https://en.wikipedia.org/wiki/Mediocrity_principle
Indeed the wiki article you link to says:
> Although the Copenhagen interpretation is often confused with the idea that consciousness causes collapse, it defines an "observer" merely as that which collapses the wave function.
Which seems like a very practical, albeit obviously incomplete, approach to take.
Is there a better way to build intuitions how an information processing system would behave while being in superposition?
Things get complicated when we throw humans in the mix, perception and consciousness and all. But modelling even simpler machines and their "point of view" can be insightful.
In this day and age it shouldn't be hard to imagine the working of a computer that uses computer vision algorithms to perceive its world and take action as a result, acting as a "causality amplifier". For example image we feed it the output of some QM experiment and instruct it output a description of what it "sees" (as we routinely do with cat pictures classification).
I assume it would be far less controversial to think about the unitary evolution of the wave function if the system being described is a QM experiment plus a computer rather than the same QM experiment and a human.
Yet, there are many similarities. The output of this computer (the classification) would be in superposition, and when measured by us it would appear as if the wave function collapsed. But we could add another such computer in the mix and ask ourselves "does it also see the wave function collapsing"? Well can program it to take the measurement and record the answer and then convey the answer to us (or to another computer down the chain). These "answers" are the "point of view" of the computer. It will "observe" decoherence yet it won't be decohered itself, as its own statement about whether it observed decoherence is itself in superposition and thus can be used as a further input to other machines witnessing subjective decoherence.
It seems like there's regularly articles saying stuff like "QM implies that both outcomes happen, but it's a longstanding mystery why we only see one outcome", as if they seriously expect your hypothetical to be the consequence of QM. I'm so frustrated at that, because as you say, seeing one outcome is exactly what you'd expect to see from inside of a superposition.
It's almost as frustrating to see as it would be to see an article saying "Newton's theory of gravity says mass attracts mass, but it's a longstanding mystery why we haven't all fallen into the sun". The theory already has an answer for that if you follow the chain of consequences from it.
You can always consider your automated apparatus to be part of the system and hence governed by unitary evolution.
If the results had been kept completely isolated from all people, you could still say a measurement hasn’t occurred.
What if there is a person in an lab which is isolated from the rest of the people?
That’s something only conscious beings can do.
The friend only exists in many worlds from Winger's perspective, and only as long as their state has no causal effect on Wigner's own state.
Do you mean "he can either think he can be in superposition or he cannot be, if he thinks he can be then he would believe in many worlds, which is incompatible with 'traditional' QM, which thus implies 'Traditional QM does not allow Wigner’s friend to be in superposition.'" ?
I don't think the Traditional QM ever explicitly disallowed a person to be in superposition; being in superposition it's just not something "decent people" do, I suppose.
Thus we needed to find other ways to come to terms with what we observe. But the math is the same. The rest is only metaphors that help us reconcile what we observe with how we feel inside.
Thus, many worlds is not antithetic to traditional QM.
I don't think that's historically accurate. If you read about how Everett's ideas were treated it's pretty clear traditional QM is antithetic to many worlds.
But the QM theory is not its interpretation.
Perhaps it's only a "measurement" if the (alleged) particle has nowhere else to go after the interaction. But how does the (alleged) particle know which macroscopic interactions are terminal and should be counted as "measurements" and which are part of the rest of the experiment?
There is no answer to this in QM. You can calculate the probabilities and you will get predictable answers, but there are still >20 interpretations of what is really happening, and they all disagree with each other in important ways.
Note that the recording of information has effects outside the system, even if no human looks at the data - recording a zero or a one will require different levels of power that, while they average to a mostly constant power consumption, are there nevertheless.
What tells you those records aren't in a superposition?
The only way to determine anything about the records is to observe them, and at that point you have a human (= consciousness) in the loop. Anything you put between yourself and the experiment might be in a superposition until you observe it.
This! Now substitute the record taking device and record keeping substrate with a human brain and this stays true. To rephrase your question:
"What tells you the state of your brain isn't in superposition?"
A human in the loop is not the end of the story, it's just yet another interaction in the quantum system.
When we experience decoherence of a quantum system, we interpret it as if something happened to the thing we observe, yet what actually happens is that something happened to us (and in turn to every system that observes us, and so on).
This is all utterly unintuitive for us, who experience the world through those brains, who feel being there, conscious in the moment. That feeling is one of our strongest direct perceptions of the world, and yet it's affected by the mechanisms of the physical reality. It's hard to accept though; it runs counter to many deep intuitions we have about ourselves, our inner lives, our identity, our values, our belief systems.
But then wouldn't an external measuring apparatus (or person) observe the system in a single state instead of a superposition? Isn't that the same as a series of nested systems, each measured and having the state recorded by an apparatus that's part of a system that encapsulates it?
There are experiments that show decoherence in absence of conscious measurement. For example, https://www.physics.upenn.edu/~pcn/Course/250/Week.of.02.21/...
> Here we report matter wave interferometer experiments in which C70 molecules lose their quantum behaviour by thermal emission of radiation. We find good quantitative agreement between our experimental observations and microscopic decoherence theory. Decoherence by emission of thermal radiation is a general mechanism that should be relevant to all macroscopic bodies.
> We observe that at temperatures below 2,000 K the emission rate is negligible, whereas at higher temperatures the molecules may emit photons whose wavelengths are comparable to (or even smaller than) the maximum path separation of ,1 mm. They transmit (partial) which-path information to the environment, leading to a reduced observability of the fullerene wave nature. Around 3,000 K the molecules have a high probability to emit several visible photons yielding sufficient which-path information to effect a complete loss of fringe visibility in our interferometer.
One thing I never quite understood about the decoherence argument for resolving the measurement problem is that it forces the "collapse" to happen with local interactions. This of course makes sense since you want to think of a measurement as a "normal", unitary process. This means the environment has to "enact" the projection operator through interactions. If you have the most basic EPR pair, how can this happen? How can you decohere the wave function of the spacelike separated counterpart (B) by locally measuring the other spin (A) and then letting the environment project out the state of B through local interactions? In this experiment https://arxiv.org/pdf/1511.03190.pdf they have two detectors 60 meters apart in a subbasement of a castle. What are the interactions in those hallways that are carrying the correlation from the decoherence/measurement on one side of the experiment to the other, faster than light to maintain the correlations. This seems too basic of a question so I don't know if I'm just missing something obvious, but to me it seems like a pretty straightforward argument that the collapse cannot happen through decoherence. I've only dug out one random paper that mentioned this argument with very little subsequent referencing of it.
The results of nonlocal and nonrealism theories come out the same, though nonlocal theories imply a lot of mysteries around the contradiction between instant communication and relativity of simultaneity, while nonrealism just paints a picture of a MWI universe that's surprisingly larger than our expectations.
No one has postulated a "conscious measurement operation" with additional useful effects beyond the measurement defined in textbooks. (Well, okay, Roger Penrose says things that kinda sound like that, but that's pretty fringe and beyond the scope of quantum mechanics.) In fact, there's literally no known experiment that a person or a machine could perform that would distinguish a "conscious measurement" from the usual mechanical measurements quantum computers use. That is the sense in which consciousness has nothing to do with measurement.
IMO quantum mechanics really doesn't have anything to say about consciousness that wasn't already present in classical mechanics. Instead of saying "but how can an assemblage of gears experience an outcome" we're saying "but how can an assemblage of superposed gears experience an outcome". It's just the same hard-problem-of-consciousness confusion dressed in new clothing.
Putting on my baseless assumptions hat, it seems to me the world just behaves as a weighted average of all possible pasts, that reality isn't "branching" into many worlds on measurements so much as just being a giant vector space of electron states that have some discrete gaps in an otherwise continuous space due to precise measurements being made, and that the phenomenon of particle-wave duality is just an artifact of some rounding the universe does under the hood when averaging together these possible pasts to render a given state*, and that any attempt to understand the casuality of the particles will fail because they don't arrive from a single cause, but a space of causes, which we can at best constraint to single points.
https://en.wikipedia.org/wiki/Delayed-choice_quantum_eraser
The primary error there is one that people can't seem to stop making for some strange reason: if you don't measure a property X, and instead measure a (non-commuting) property Y, you are not free to say what X "would have been," and -- especially egregiously -- to use that to demonstrate a contradiction. There is no "would have been" in QM. Either you measure it or you don't, and even after you do, the value becomes invalid after it evolves or a complementary variable is measured.
Secondly, Wigner's friend was not a single particle. Asking what a single particle "experienced" only makes sense if you are a panpsychist (or "panpsychic" as amusingly misspoken in the article). In any case, it has no bearing on the essence of Wigner's question.
"But clearly, even if no one literally measures Charlie in the {|0〉,|1〉} basis, he’s still there, thinking either the thought corresponding to |0〉 or the thought corresponding to |1. And likewise Diane. Just as much as Alice and Bob, Charlie and Diane both exist even if no one measures them, and they can reason about what they know and what they know that others know. So then we’re free to chain together the “certainties” of Alice, Bob, Charlie, and Diane in order to produce our contradiction."
You can't use this argument without also explaining what "thinking" means - because in fact it makes no sense unless you assume consciousness exists as a quantum state which allows a phenomenon called "thinking."
And philosophically and practically, that's a very, very large can of wormns.
So this ends up in the usual place: it's not obvious how using an idea no one understands - like consciousness - to explain another idea no one understands - like QM - is supposed to solve any of these problems.
More details: https://algassert.com/post/1904
> OK, point taken. But if one has any experience with experiments in the foundations of QM, one knows full well what’s going to happen next. Namely: some experimental group will do a slightly souped-up test of Hardy’s Paradox, of course getting just the results that QM predicts, and will then market it in Science or Nature as “the first experimental probe of the logical contradiction at the heart of QM … who could’ve imagined that the ‘impossible’ outcome would occur with probability 1/12?” And then the science journalists will wet themselves with excitement. It’s all nearly as predictable as QM itself!
These concerns remained theoretical until the 1960s, when physicist John Bell devised a way to test if reality is truly spooky—or if there could be a more mundane explanation behind the correlations between entangled partners."
[...]
Such “Bell tests” have since been carried out, with a series of watertight versions performed in 2015, and they have confirmed reality’s spookiness."
Logic:
If you believe that the Bell Test is correct -- you must equal-and-oppositely believe that the speed of light is NOT the upper limit of speed in the universe, that is, that Faster-Than-Light (FTL) speeds are possible.
Conversely, if you believe that speed maxes out with the speed of light, and that nothing can travel faster than the speed of light -- then you equal-and-oppositely believe that the Bell Test is wrong.
But, you cannot believe that nothing can travel faster than the speed of light AND that the Bell Test is correct, simultaneously -- as this would result in a logical paradox...
(Choose one, but not both at the same time...)
This is precisely why the experiment was proposed, to show this conflict in our understanding of the constraints.
That's how I mentally think about entanglement. I doesn't need to operate through the universe as I understand (or operate within) it.
That's not exactly correct, you just have to forgo locality, which some interpretations of quantum mechanics do, like the Bohmian one. Like the sibling comment points out, this still doesn't permit faster-than-light communication.
Er, yes, but, forgoing locality -- changes the definition of speed -- because it changes the definition of distance:
Speed = Distance / Time
If you forgo locality, then you are now able to modify distance in this equation, that is, your equation now looks like this:
Speed = ((Distance * NON_LOCALITY_SCALING_FACTOR) / Time)
That's because, let's say you have a black hole/quantum entangled particle/stargate/thing A at one point in the universe that immediately influences thing B at some distance to it in the universe (call this "connected" concept by whatever name or metaphor you will), etc.
Well, if so, then the NON_LOCALITY_SCALING_FACTOR is there to adjust the shorter distance to the longer one.
Like let's say you had a stargate linking two points in the universe, and you walked 10 feet into it, but this 10 feet, after you had walked it, resulted in you appearing halfway across the universe.
OK, so that's 10 feet in local movement, in local distance, but billions of miles of actual (but non-local) distance.
That's why you now need NON_LOCALITY_SCALING_FACTOR in your equation, because you've redefined Speed in terms of it.
But, by redefining Speed (which must occur, by forgoing locality!), you now have agreed (because it is implied) that anything faster than light (communication, travel etc.) can now occur, that is -- all you need is a high enough NON_LOCALITY_SCALING_FACTOR in the equation for speed -- which you get by forgoing locality...
The collapse still propagates at the speed of light across the bridge.
[1]: https://www.youtube.com/watch?v=OBPpRqxY8Uw
All the experiments become much clearer if you think of everything behaving primarily as a wave (all possible paths are taken). A particle is just a special (limited energy) case of this where all paths are still taken but since there is limited energy only one path can be 'registered', and this happens probabilistically.
The intent of the thought experiment was to show that the Kopenhagen Interpretation of quantum physics is ridiculous. In the many-worlds interpretation for example there is no paradox, as time goes on more and more worlds exist where the cat died at some point.
1: https://en.wikipedia.org/wiki/Double-slit_experiment#Other_v...
I'm likely wrong in details here, but isn't that thought experiment similar to "what happens at the origin when we draw a hyperbola" sort of question? (Nothing happens, as you can't draw a complete hyperbola.)
1. The wave function does not exist in physical space, it exists in configuration space. Configuration space is the same as physical space only in the one very special case of a single non-entangled particle. The intuitions of physical space do not generally carry over into the general case of configuration space.
2. In order to produce interference you have to split up the wave function into two pieces and then bring those pieces back together again with a phase change. But since the wave function exists in configuration space, "bringing it back together again" becomes increasingly difficult as the number of entangled degrees of freedom grows. As a practical matter, it is effectively impossible to do this once the number grows beyond a few hundred (this is why building quantum computers is hard). But an actual macroscopic observer consists of O(10^23) degrees of freedom, and is also entangled with their environment. This is the reason that large systems behave classically and don't exhibit interference.
3. Whether or not a system is in a superposition is not a physical fact about the system, it is a consequence of the measurement you choose to make. A vertically polarized photon (for example) is in a superposition relative to a measurement axis rotated away from the polarization axis. For a more complicated system, the measurement axis, like the wave function itself, is in configuration space. But we can only do measurements in physical space. This is the reason that in order to see interference in an entangled system it is necessary to do multiple physical measurements and look at the correlations between them. Doing those kinds of measurements on actual macroscopic objects is way beyond the current state of the art, and may well be impossible even in principle. But this is the sort of thing that is contemplated in the theory behind these experiments. This is the reason that single particles are used as stand-ins for Wigner's friend.
[1] https://arxiv.org/abs/1812.06451
Tesla based his entire electric theory on the Aether.
This theory was wrongly suppressed by results of a Michelson Morley experiment...
https://en.wikipedia.org/wiki/Michelson%E2%80%93Morley_
...which failed to measure Aether-Drift because of the fact that it was done in a basement, and which was later contradicted by Dayton Miller when he repeated the experiment on a mountain top:
http://www.orgonelab.org/miller.htm
This scientific tragedy, likely done by powerful oligarchs, set the progression of physics back 100 years. We now have fraudulent branch of particle physics, which has a terrible track record with pretty much every prediction it has made, and which would be laughed out of the scientific community if it weren't for the sustained push by the mainstream media to hammer it into the minds of intellectuals.
We've been told that light has a set speed of propagation, yet time after time there are articles posted about light exceeding the speed of light for example gamma ray jets from quasars, which is explained by positing that the space-time fabric is moving and causing the reference frame containing the light wave to increase it's speed above the limit.
https://www.sciencealert.com/faster-than-light-speed-in-jets...
The media has become so twisted that we now accept completely broken interpretations such as virtual particles (used to balance equations), string theory (completely made up and didn't work), multiple universes, time travel and a smorgasbord of hilarious interpretations from a fundamentally broken model of physics.
This article is just yet another desperate attempt to patch up this hilariously outdated physics branch. Einstein physics is on it's way out. And when it comes crashing down we will replace it with the work of Tesla and we will move faster in one decade the entire previous 150 years.
Now, we have them in many forms. The differential kind from the wiki article. The operational calculus kind from Jan Mikusi?ski's . The modeling of capacitor and/or coil as transmission lines by Ivor Catt. The fourquadrant representation from Eric Dollard.
I will appreciate any elaboration on any of the subjects.
The modelling and experimentation in the area seems to be based on an opportunity to contract space. Meaning that instead of having a 5,000 mile long wire, we are forced to be limited by our laboratory span. How can we simulate in our lab a 5,000 mile long wire? Eric Dollard proposes the analog computer. Four elements compromise a symbol. We lay the symbols in a serial line. https://www.youtube.com/watch?v=nJ8drfI4j9o
Now, we got a 5,000 mile long wire in our lab. Space contraction.
This dude does great job http://www.am-innovations.com/transference-of-electric-power...
And please don't forget that Heavside basically single-handedly created the electrician's vocabulary of terms.
It's a convenience to explain states within equations. When you say "the cat is both alive and dead at the same time", you're simply saying "in this equation, do not assume that this variable will be one or zero, calculate the results for both conditions", and the final outcome will possibly be one of these calculated results.
The more such variables you have, the more different results you may get.
If that line would be in the beginning, I would not have bothered to read the article :/
At at any moment, there might be some n number of waves that are the highest. But not until the exact moment you measure the answer exists.
The superposition collapses way before the cat is affected, when more than a handful of atoms is concerned, way before the hammer even moves.