Gravity is local and apparent collapse is non-local; it doesn’t even pass the smell test.
It’s one of the oddest tenets of quantum theory: a particle can be in two places at once—yet we only ever see it here or there. Textbooks state that the act of observing the particle “collapses” it, such that it appears at random in only one of its two locations. But physicists quarrel over why that would happen, if indeed it does. Now, one of the most plausible mechanisms for quantum collapse—gravity—has suffered a setback.
Our higher level concept of gravity (at least in the classical sense) has been known to break down to the point that it doesn't apply to quantum mechanics for quite a long time now. The writer should just google "quantum gravity" to discover what a complicated subject that becomes.
Basically all the work in getting large scale quantum computers to work lies in these fields. They will continue to grow.
The measurement problem, which deals with collapse is still unsolved. It is also the reason why there are so many weird interpretations of quantum mechanics and that even top physicists can't agree on one. In fact the same physicists tend to sweep the problem under the rug and instead focus on the equations that, to be fair, did a lot more to science and technology than trying to solve the measurement problem.
Oh yeah? Then what is the physical explanation for collapse?
Fox example you can split a ray of neutrons, direct each beam through a different path with different height and then make them collide and see the interference pattern. The idea is that the split creates a superposition and each half has a different gravity potential, changing the orientation of the experiment produce different interference patterns. (The details are in the book of Sakurai "Modern Quantum Mechanics" pp127-129, with data from an experiment of Colella, Overhauser, Werner (1975).)
I don't understand why the old experiment was not enough to falsify this theory.
Why not? Gravity is (probably) mediated by a particle, and therefore all matter will interact with all other matter ... so why shouldn't gravity therefore cause a collapse?
Electromagnetism is also mediated by particles (photons) and the quantum states can survive a lot of electromagnetic interactions without collapsing. One of my favorites https://en.wikipedia.org/wiki/Stern%E2%80%93Gerlach_experime...
I'm not a physicist, but from the few years of QM I took in college my take is that there is nothing special about "measurement", it's just a label we apply to certain states becoming entnagled. As long as you don't believe there is anything magical about humans or other "conscious" observers, then there doesn't seem to be anything to figure out about collapse.
Then why is quantum state evolution seemingly continuous and unitary some of the time, and sharply discontinuous at other times?
If you treat a measurement as entanglement then by the Kochen-Specker theorem you can't condition on the measurement outcome. However we seem to be able to do this in actual experiments.
Thus measurement is not entanglement alone, but also the elimination of other bases.
A simple example: As long as one doesn't look at a coin, the probability that it shows head or tail is 50%. After the measurement it "collapses" to 100% for one of the options.
But the "collapse" is only a mathematical "collapse", not a physical one.
Physics only limits how precise and fast your measurement apparatus can be.
After the experiment, the experimenter did not "learn" or "reveal" some objective fact about reality, ie that the true state was UP rather than DOWN. Instead, after the experiment the experimenter becomes entangled with the UP/DOWN system in such a way that the experimenter measured both UP and DOWN, but all observables relating to the experimenter are either wholly consistent with UP or wholly consistent with DOWN.
If the universe was at all optimized for simulation costs for human experience, we probably wouldn't expect there to be be trillions of galaxies with hundreds of millions of stars each, for the smallest particles to be on the scale of billion-billionths compared to humans, or for QM to work anything like it does.
I mean, quantum fluctuations during inflationary expansion are (in my understanding) what's responsible for the tiny differences in mass distribution that led to the eventual formation of gas clouds/stars/galaxies. The negative gravity during expansion worked to keep the matter in an extremely low-entropy (highly ordered) state, but those unfathomably small quantum fluctuations were blown up by the same unfathomably massive proportions as everything else.
Universe needed that RNG.
So the Kochen-Specker theorem says there was no pre-existent noncontextual state for the particle. However that doesn't in any way imply the particle was both UP and DOWN or that the device measured both UP and DOWN. Especially for the device as the Kochen-Specker theorem is proved in a context where observable outcomes are assumed to be single-valued.
However in real practice we can condition on the states of our devices following measurement, hence they don't seem to be susceptible to a Kochen-Specker result when viewed as the system for some "super"-observational device. Which they would be if they simply entered an entangled state. Thus the assumption of measurement as simple entanglement does not match actual observed reality. This is a point made in many texts such as those of Schlosshauer, Omnès, Peres and at a very rigorous level in the theory of C* algebras and Category theory by Fröhlich and Landsman respectively.
The weird thing is that the writer of that article has apparently written a popular science book about quantum mechanics too: https://us.macmillan.com/books/9780374536619
Flash news. Nobody has ever produced any.
To the contrast we have lots of lines of evidence that an observer described by quantum mechanics should, upon observing a quantum experiment, be thrown into a superposition of observers. Each of which appears to have observed collapse. The notion is utterly repugnant to our biases so many reject the idea out of hand.
But as we create ever more complex but controlled systems, we can perform ever more elaborate experiments verifying that quantum mechanics works exactly as predicted. At some point if we take seriously the idea that the most successful scientific theory of all time is an accurate description of ourselves, then we have to accept that perhaps there is no collapse after all.
What's the alternative? Assuming unitary evolution and some fairly common-sense axioms about how we'd expect subjective experience to behave (things like: we never experience being in a branch that has amplitude zero; if we experience being in a given branch then we continue to be in that branch), the Born probabilities are the only model anyone's ever come up with for how our subjective experience should go. So what's there to explain?
This is were we have to invoke philosophy. Specifically how does consciousness interact with time? The common-sense thinking is that our soul is tied to our body and is traveling forward through time with it. Another way of thinking is that the soul is tied to a given position of the space-time-probability. It does not travel. You today is not the same as you tomorrow or yesterday. The you that observes spin up is not the same you as the one that observes spin down. Your soul is perceiving reality from a randomly chosen vantage point among all the possibilities with which have a compatible body. If we condition on those bodies belonging to experimenters who have observed frequencies, then we get the distribution.
This is one possibility anyway.
So under the assumption that the state encodes probabilities, state space assumptions and consistency with unitary evolution you get the Born rule. However this is not the same as the Born rule arising dynamically from unitary evolution alone.
It's true that from the perspective of an external superobserver the quantum state evolves to contain terms for each observer observation state. However since all interference observables turn out to be non-physical for macroscopic systems we get a superselection rule and so the probabilities for different macrostates are classical probabilities and thus reflect simple ignorance of the observer's post measurement state.
There's very little motivation for reading the quantum state "ontically" in the way you are doing.
This is what I never understood about MWI, in what physical sense can the many worlds be said to exist? Where are they in our universe? What direction would we have to travel to find them? Do they exert gravity on us? If not, then how can we claim that they exist in a physical sense?
So the universes all exist in the same place, since they are the same universe. Your idea of what an observation is, is just an eigenvalue of that corresponding operator.
But anyway, the other worlds do effect our world - that's why we get interference patterns in double slit experiments.
I think there’s also an experimental setup, whose name I forget, but which is essentially nested Schrödinger's cat setups: Alice is in a box, Bob is in a box which contains Alice’s box, Carol is outside; Alice goes into superposition of |Alice+> and |Alice->, Bob opens the box and Carol can now demonstrate that Bob is in a superposition of |observing Alice+> and |observing Alice-> instead of the combination of 100%|observing> and a superposition of |Alice+> and |Alice->.
Apparent wave function collapse can be explained using quantum decoherence.
If the measurement apparatus is coherent, measurements can be performed without collapse. This was well thought out even within wigner and einstein's lifetimes. c.f. the vaidman bomb detector.
This solves the "consistency/small problem", i.e. treating the macroscopic apparatus as boolean is justified.
It doesn't resolve the "outcome problem", i.e. which outcome is selected. Of course if you accept the world is not deterministic this isn't really a problem.
Honest question.
You need something stronger, namely superselection or irreversibility.
I'd love to read more but my google results aren't turning up a good definitive introduction.
Then there are a lot of Nobel prize winning physicists who would love to be enlightened about how simple the mystery actually is.
The issue is linear evolution means the measurement of a superposition leads to a superposition of measurement devices. If the quantum state is real that gives you many worlds.
If you are suggesting there is nonlinear evolution, well a) it must be non-local and b) the theoretical research suggests it would be inconsistent. QM is a very rigid theory - “an island in theory space”. It isn’t easy to slightly modify.
And that's problematic because? Because it explains away the whole measurement problem, there is nothing to explain, it's an artifact of a macroscopic observer's point of view?
It's like saying that SR/GR with their space-time continuum being real is problematic, so let's keep to the (post)-Newtonian point of view, but oh no, it now has all this weird amendments and additional terms, and when you try to extend it to the whole of the Universe, it breaks down/gives really weird stuff. Well, duh, of course it does, if one tries to pull an owl on a globe, it simply won't fit.
The QM is "an island in theoryspace" idea isn't strictly true either. QM is one among an entire family of probability theories. It's only rigid when considered purely from the point of view of Probability theories based around vectors in Hilbert space. However considered as part of OPTs in general there's nothing that makes it difficult to modify.
This is very unfair. This is a niche field with contested interpretations, don't make people feel stupid for asking fair questions.
It's obvious what the other person meant: what does 'our world' and 'other worlds' mean, and how do you know it's not just a figment of your imagination, as a scientific theory must be falsifiable -> i.e. measurable and provable / disprovable somehow.
You should at least point people to reading material before making fun of them.
I’m pretty sure he suggested that something only exists physically if it has some measurable effect on us.
I suspect most people would say their friend continues to exist. This is very analogous to the many worlds situation.
I guess the best answers about MWI is that the other versions of these particles continue to exist at different coordinates in Hilbert space, and that they do interact with each other in observable ways, such as the interference patterns in double-slit experiments.
Chaotic systems don't have a Kochen-Specker or PBR theorem.
Regarding what's to explain, it's quantum randomness (which distills the Born rule objection). Our subjective experience is that we see spin-down 1/3rd of the time, and our theories say the result is otherwise impossible to predict, even in principle. But a deterministic theory cannot produce a random outcome, even a subjective one.
Whyever not? What else would you expect the subjective experience of being in a state like 1/sqrt(3)|x> + 2/sqrt(3)|y> to be like?
I've never seen the Born rule derived from unitary evolution and axioms for how subjective experience should work, so I don't even see this as one of the ways.
> the structure of the wavefunction is that it divides cleanly into those two branches, and that's true in any basis.
It's not. It only has this Schmidt decomposition in one basis. In other bases there will be cross terms among the basis elements. What you're doing is privileging Schmidt bases as ones that give experiences. In another basis with states w,z say the state will be: |w>|w> + |z>|z> + |w>|z> + |z>|w>
So you won't be able to give this clean "experience" reading unless you posit we can't experience in things like the w,z basis here and only in Schmidt bases, but then you run into the problem that for real macroscopic systems they won't admit a Schmidt basis.
This seems like the kind of a "vague" Many Worlds where one doesn't look any deeper than pretending a macro-device is a qubit (e.g. no thermal states etc) and looking at one basis. There's a reason properly developed MWI is nothing like this such as the Spacetime State realism of Wallace and Timpson.
Why one would believe in quantum state realism at all is a separate question.
>Of course you can
No you can't, it's a direct consequence of the Kochen-Specker theorem. If the device is treated quantum mechanically and it enters an entangled state of the form you gave then you cannot perform conditioning as the Kochen-Specker theorem, via the non-uniqueness of Hilbert space orthogonal decompositions, prevents an unambiguous formulation of Bayes's law. I can link to papers proving this if you wish.
The fact that we do experiments where we can condition is, in light of this theorem, a demonstration that our measurement devices do not enter into the kind of CHSH states you're giving.
The state's evolution will be completely equivalent to (a linear superposition of) the evolution of |x>|x> and |y>|y>. That's a physically observable fact that's independent of your choice of basis (it's less obvious in the |z>/|w> basis, but it's still true).
Any physically valid concept of "experience" would have to behave the same way. If your state is equivalent to a linear superposition of "experiencing x" and "experiencing y" then it can be characterised completely in terms of "experiencing x" and "experiencing y", and that's not dependent on your choice of basis (though it may be easier to see in one basis or another).
> No you can't, it's a direct consequence of the Kochen-Specker theorem. If the device is treated quantum mechanically and it enters an entangled state of the form you gave then you cannot perform conditioning as the Kochen-Specker theorem, via the non-uniqueness of Hilbert space orthogonal decompositions, prevents an unambiguous formulation of Bayes's law. I can link to papers proving this if you wish.
> The fact that we do experiments where we can condition is, in light of this theorem, a demonstration that our measurement devices do not enter into the kind of CHSH states you're giving.
I don't know what you're trying to claim here. All the available evidence is that measurement devices, being ordinary physical objects, follow the laws of quantum mechanics, and that includes conditioning behaving as entanglement; if you've got evidence that that's not the case then a Nobel prize awaits. Non-uniqueness is a red herring, because choice of basis does not and cannot change experimental predictions; the basis exists only in the map, not the territory.
The point is that there is no reason to select out any particular basis over another. You can't just retreat into "well this is the only basis I can experience" because the human sensory apparatus would be able to select out a range of bases in a full unitary account and also the ambiguity of basis decomposition means you can't perform conditioning which we do all the time in experiments.
I mean that if you decompose along a different basis than experiencing x/experiencing y, you just get an ensemble of states each of which is a superposition of experiencing x and experiencing y. So you end up with the same thing.
It's like looking at an entangled state (because that's exactly what it is) - if we have a two-particle state like 1/sqrt(2)(|x>|x> + |y>|y>), that behaves like the first particle being in |x> and experiencing the other particle being in |x>, or being in |y> and experiencing the other particle being in |y>, and it might look like that's an artifact of this particular basis decomposition, but it actually isn't - the structure of the wavefunction is that it divides cleanly into those two branches, and that's true in any basis.
> You can't just retreat into "well this is the only basis I can experience" because the human sensory apparatus would be able to select out a range of bases in a full unitary account
A system that's freely interacting will become entangled; whatever we consider ourself is constantly interacting with the rest of ourself, almost by definition.
> also the ambiguity of basis decomposition means you can't perform conditioning which we do all the time in experiments.
Of course you can, and it works exactly the way you'd expect - we already do experiments where some isolated apparatus inside the experiment does something if it detects one thing and something else if it detects something else. Choice of basis is a tool for understanding the wavefunction, not a physically real thing.
The device has to have its contextual observable algebra develop a non-trivial center, not just be entangled as is mentioned in section 5 of the paper I linked. It's well known entanglement alone isn't enough which again is why entanglement alone has been called "pre-measurement" since the 1980s.
Note how this involves hard mathematics, not vague talk about "obvious features of subjective experience". I'll also note that this is a general feature of discussions about this stuff among non-physicists online, especially programming communities like this one, the knowledge is stuck in the late 1970s.
Of course the state can be written in the form |xx> + |yy>. I never denied that. The point is that it can be written in other forms. So it's equally correct to say we'd "experience" |zz> + |ww> + |zw> + |wz> as to say we'd experience |xx> + |yy> so there's no reason to say we'd "obviously" experience only the latter. Your argument is just "that expansion is always available", but since other expansions are also always available I don't see what the force of this argument is.
Even worse in QFT there isn't an expansion of the form |xx> + |yy> available due to the Reeh-Schleider theorem so your whole construction is moot anyway. Again where is this paper deriving the Born rule from unitarity and basic facts about subjective experience.
> I don't know what you're trying to claim here. All the available evidence is that measurement devices, being ordinary physical objects, follow the laws of quantum mechanics, and that includes conditioning behaving as entanglement.
I'm claiming a consequence of a well known theorem from Quantum Probability. See section 4.2 of this paper https://arxiv.org/abs/1310.1484
Quantum states without superselection (e.g. the entangled states of the form you are considering) leave Bayesian conditioning undefined. As the paper mentions this is a direct consequence of the Kochen-Specker theorem via non-unique orthogonal expansion. It's not a red herring but a rigorously proved theorem.
I don't know what the "Nobel prize" remark is about as it is well known that entanglement doesn't give well-defined conditioning. That's why entanglement with the device alone is called "pre-measurement" in most papers in measurement theory following terminology introduced by Zurek in the early 80s. A good example of the issues with pre-measurement alone is here https://arxiv.org/abs/2003.07464. You can't just treat the device as simply entering some CHSH or GHZ style entangled state and think that solves everything about measurement. It doesn't via the theorem I gave in the paper above (and other issues).
If there's a simple description of the wavefunction that's valid then there should be a correspondingly simple description of our experiences that's valid. The fact that there's also a more complicated valid description of the wavefunction is neither here nor there. It's like looking at a basket of 4 apples and asking why your experience doesn't correspond to there being 6 - 2 apples.
> Quantum states without superselection (e.g. the entangled states of the form you are considering) leave Bayesian conditioning undefined. As the paper mentions this is a direct consequence of the Kochen-Specker theorem via non-unique orthogonal expansion. It's not a red herring but a rigorously proved theorem.
Ok, I take your point, saying that we can just condition is overly flippant: if there are cross terms (i.e. entanglement) then classical conditional probability doesn't always accurately describe the behaviour of a system, and of course that's true for a system that includes experimenters inside it. But if we treat an experimenter's conditioning as creating entanglement, like any other QM interaction, and treat the subsequent evolution of the system quantum-mechanically, then there's no problem.
> A good example of the issues with pre-measurement alone is here https://arxiv.org/abs/2003.07464. You can't just treat the device as simply entering some CHSH or GHZ style entangled state and think that solves everything about measurement. It doesn't via the theorem I gave in the paper above (and other issues).
That paper amounts to nothing more than redefining "outcome" as something that cannot be in a superposition, and then using this to argue that it makes their unfounded notion of decoherence physically meaningful. If we assume that experimenters are physical systems that can undergo superpositions like any other, then of course Bell-style "no hidden variables" results apply when those variables are the outcomes of experiments. Big whoop. (Would you find the following argument convincing: "Pre-measuring the polarisation of the photon might have one of two possible results, so it doesn't have an outcome according to any reasonable notion of "outcome". Therefore if any observer has measured a photon's polarisation, a physically meaningful process of decoherence must have occurred"? Put like that it's hopefully obvious that this is nothing more than asserting the primacy of the Copenhagen interpretation).
> Note how this involves hard mathematics, not vague talk about "obvious features of subjective experience". I'll also note that this is a general feature of discussions about this stuff among non-physicists online, especially programming communities like this one, the knowledge is stuck in the late 1970s.
Look, I'm not a big fan of credentialism, but I do have a master's in this from a reputable institution. If working physics has found a compelling argument that there's something mysterious about measurement or experience, then that knowledge hasn't made its way as far as even taught postgrad courses, yet alone the wider public, and the blame for that has to rest with the physicists. (I rather suspect that there's no such argument that has reached any significant consensus among working physicists, and that that the "late 1970s" view in the public sphere reflects that).