> Physicists have since largely shed that discomfort. They now understand what Einstein, and perhaps Schrödinger himself, had overlooked — that entanglement has no remote influence.
Very false. Many many physicists still accept non-locality
I took it to refer to the various quantum no-go theorems that prohibit using entaglement as a means to transmit information in a spacelike manner without an accompanying timelike counterpart.
I don't think many physicists will argue they are wrong.
To be perfectly frank, quantum entanglement and wormholes are just the manners in which QFT and GR respectively manifest the concepts of nonlocality. That’s my understanding of the ER=EPR dictum, anyway: physics is fundamentally nonlocal, and these are the means by which the two leading theories express that nonlocality within their own lexicon of information/observation and curvature/trajectories (respectively).
I think ER=EPR still implies physics is local, but locality is dynamic rather than static. It is a mechanism by which spacetime can be constructed from quantum physics, but all the physics still takes place locally on it.
It doesn't rule out non-local interactions as in general we can rule out local non-hidden variables but not global ones.
What these party-pooper theorems do is largely rule out being able to differentiate non-local interactions from local interactions... which doesn't mean they don't exist, but if they do, there's large pieces of the puzzle missing before we can show they do.
If there is a non-rude way of conveying the message “when I said that it depends on the definition of interaction you gave me one and then you told me to ignore it” please assume that I had written that instead.
Most of the times that's because we find it hard to imagine how it would "feel" being ourselves entangled with the thing we observe. Accepting that there is effectively a copy of you that is effectively fully casually disconnected from you goes strongly against our instinct that wants us to be "us" unique and objectively existing
It has nothing to do with instincts or what is easy or hard to imagine, it's almost always because of special relativity and that all of our theories of the different forces are all local.
If entanglement is the only non-local interaction, then why?
I don't think it has anything to do with the human's idea of themselves
EDIT: If you meant why people accept it, well that's for the same reason that the EPR paradox was formulated in the first place: the basic formulation of quantum mechanics and the most obvious interpretation of it basically straight up says there is non-local action
Entanglement is not a non-local interaction, it means that state of the system can not be separated into independent states of its constituents. That altering the state of one particle could non-locally influence the state of another particle far away is not really a mind-bending idea, pressing the power button on a remote control turning on the TV across the room is pretty much the same thing, at least to our human perception as we can not see the IR light and the speed of light is essentially infinite on a human scale.
But that is not how to think about entanglement properly, that is much more mind-bending. The remote control and the TV become a unit and only this unit has a state, neither the TV nor the remote control have an independent state of their own. Note that entanglement requires superposition, without it you can separate the state. The state power button not pressed and TV on can obviously be separated into independent states for both devices, but power button not pressed and TV off or power button pressed and TV on can not.
You could try to say the power button is pressed or not pressed and the TV is on or off, but that description would not account for the correlation that power button pressed implies TV off (and vice versa) and power button not pressed implies TV on (and vice versa).
> Entanglement is not a non-local interaction, it means that state of the system can not be separated into independent states of its constituents.
Yes, and then what happens when you decide to measure the state of the part of the system that lies in one region of space?
> That altering the state of one particle could non-locally influence the state of another particle far away is not really a mind-bending idea, pressing the power button on a remote control turning on the TV across the room is pretty much the same thing, at least to our human perception as we can not see the IR light and the speed of light is essentially infinite on a human scale.
That's irrelevant, because EM is local in physics.
Yes, and then what happens when you decide to measure the state of the part of the system that lies in one region of space?
If I had any good idea, I would go pick up my Nobel Prize. The only thing I can really say is that I do not think that the wave function instantly collapses as this would be a non-unitarity evolution of the wave function.
That's irrelevant, because EM is local in physics.
You missed my point, I of course did not want to say that turning on the TV is a non-local interaction, just that it looks like one to humans. So if non-local interaction was all that is going on in quantum mechanics, than it would not be that hard to make sense of it, as we have an intuition for non-local interactions, even though only from things that look like non-local interactions to us but are not actually non-local interactions.
> If I had any good idea, I would go pick up my Nobel Prize.
Well, we already have many ideas, as summarised by the various interpretations of quantum mechanics, some of which have instantaneous non-local interaction.
> The only thing I can really say is that I do not think that the wave function instantly collapses as this would be a non-unitarity evolution of the wave function.
Sure, but many people do think it does, and such an interpretation is totally compatible with QM.
> You missed my point, I of course did not want to say that turning on the TV is a non-local interaction, just that it looks like one to humans. So if non-local interaction was all that is going on in quantum mechanics, than it would not be that hard to make sense of it, as we have an intuition for non-local interactions, even though only from things that look like non-local interactions to us but are not actually non-local interactions.
But it's irrelevant because physics isn't about what things "seem like to humans", it's about what we actually measure and how to create theories that explain and predict it.
Sure, but many people do think it does, and such an interpretation is totally compatible with QM.
The Schrödinger equation and wave function collapse are not compatible. Maybe they could be integrated in some theory, but that will need a lot of additional explaining when and how the evolution changes between being unitary and non-unitary.
But it's irrelevant because physics isn't about what things "seem like to humans", it's about what we actually measure and how to create theories that explain and predict it.
You are still missing my point. Quantum mechanics in general and entanglement in particular are arguably hard to understand because they are unintuitive. If all that was going on were some non-local interactions, than entanglement would not be hard to understand because non-local interactions are intuitive to us because we are familiar with things that look like non-local interactions to us.
Now turn this argument around. We are still having a lot of trouble making sense of quantum mechanics, therefore it seems unlikely that non-locality is at the core of quantum mechanics, because if it was and would explain everything, than it would not be so difficult to grasp quantum mechanics because we have an intuition for non-local behavior. This is obviously no rigoros argument but it was also never was meant to be one.
> The Schrödinger equation and wave function collapse are not compatible.
They don't need to be because they describe totally different phenomena.
> Maybe they could be integrated in some theory,
They already are integrated into the theory of quantum mechanics. Take a look at the postulates of quantum mechanics again, one of the postulates is unitary evolution through time, another is non-unitary collapse with measurement. All the experimental results of quantum mechanics are compatible with those postulates.
> Quantum mechanics in general and entanglement in particular are arguably hard to understand because they are unintuitive. If all that was going on were some non-local interactions, than entanglement would not be hard to understand because non-local interactions are intuitive to us because we are familiar with things that look like non-local interactions to us.
Well, entanglement _is_ intuitive in that sense, which is one of the reasons why seeing entanglement as linked with non-local collapse is so popular.
> We are still having a lot of trouble making sense of quantum mechanics, therefore it seems unlikely that non-locality is at the core of quantum mechanics, because if it was and would explain everything, than it would not be so difficult to grasp quantum mechanics because we have an intuition for non-local behavior.
I don't really buy this, since, quantum mechanics is intuitive and simple if we accept non-local collapse and wavefunction primacy, i.e., if you actually accept the five postulates on face value, it is not difficult to accept quantum mechanics.
As a physicist myself, I don't feel bewildered or confused by quantum mechanics. I have quite a strong intuition for it, in fact.
They don't need to be because they describe totally different phenomena.
In which way are they describing different phenomena? They both describe the evolution of the wave function, the Schrödinger equation while everyone is leaving the system alone, wave function collapse at those special moments when somebody is looking at the system, performing one of those mysterious measurements.
Without explaining more precisely when I should use unitary evolution and when I should collapse the wave function, the Schrödinger equation together with wave function collapse are at the very least an incomplete and maybe even an inconsistent theory.
Again, just take a look at the postulates: Schrödinger equation describes the unitary time evolution of the system under a given Hamiltonian, wavefunction collapse describes the outcome of the non-unitary measurement process.
> Without explaining more precisely when I should use unitary evolution and when I should collapse the wave function, the Schrödinger equation together with wave function collapse are at the very least an incomplete and maybe even an inconsistent theory.
I don't think that QM postulates are incomplete, you just take the set of things that constitute a measurement as a premise.
Let's put it this way, we have never designed an experiment where this has been an issue. So long as we define which part is the measurement beforehand, QM always predicts the results we measure. Usually it will be a photodiode or something of the sort. If QM were incomplete or inconsistent, surely we would be able to find contradictory experimental results?
To summarise, the 5 basic postulates of quantum mechanics, which contains unitary evolution and non-unity measurement collapse, form a microscopic theory which no experiment has ever been able to contradict.
Again, just take a look at the postulates: Schrödinger equation describes the unitary time evolution of the system under a given Hamiltonian, wavefunction collapse describes the outcome of the non-unitary measurement process.
The outcome distribution of a measurement is described by the Born rule, the collapse postulate on the other hand tells us that the wave function will change to the actually measured state. Related but not the same.
For the state |0> + |1> (unnormalized) the Born rule tells us that a measurement will yield |0> or |1> with 50 % probability each. The collapse postulate tells us that the state will change from |0> + |1> to either |0> or |1> depending on the measurement outcome.
This change is non-unitary and therefore incompatible with the Schrödinger equation which tells us that quantum systems evolve unitarily. If you want, consider the combined quantum system of the system under investigation and the measurement device, why should that system not evolve unitarily?
> The outcome distribution of a measurement is described by the Born rule, the collapse postulate on the other hand tells us that the wave function will change to the actually measured state. Related but not the same.
Born rule is also a postulate.
> This change is non-unitary and therefore incompatible with the Schrödinger equation
Yes that's why unitary evolution has a separate postulate
> If you want, consider the combined quantum system of the system under investigation and the measurement device, why should that system not evolve unitarily?
The postulates are roughly quantum system evolve unitarily except when they are measured, then a non-unitary wave function collapse happens. That is inconsistent. For the system under investigation this works, it evolves unitarily until measured, then the wave function collapses non-unitarily. The combined system of system under investigation and measurement device however is never measured, therefore must evolve unitarily. This is a conflict, the combined system can not evolve unitarily and also have some subsystem, the system under investigation, evolve non-unitarily, i.e. undergo a wave function collapse. This problem has been recognized for the better part of a century, that is nothing I am pulling out of thin air.
It is objectively inconsistent, the postulates contradict each other. But for almost a century we did not make any substantial progress, neither resolving the issue theoretically or philosophically, nor experimentally demonstrating any problems. Or at least nothing reached broad consensus.
This seems debatable, but let us assume they are not obviously inconsistent. Those postulates only really mean something if you apply them in some situation, and if you apply them as laid out a few comments before, they make inconsistent claims. If applied to the subsystems, they postulate non-unitary evolution of the combined system, if applied to the combined system, they postulate unitary evolution. If inconsistent is to strong of a claim, I guess this could be changed to incomplete, maybe there are missing postulates resolving the issue.
General Relativity admits the concepts of wormholes (Einstein-Rosen bridges) and these are a nonlocal phenomenon. So it is incorrect to assert that quantum entanglement is the only nonlocal interaction or phenomenon. Both General Relativity and Quantum Field Theory seem to manifest a need for nonlocality and each theory expresses it with its own vocabulary (in terms of information and observation in the case of QFT, and with wormholes and multiple connectedness in the case of GR). That’s the kernel of truth in the famously hermetic pronouncement that “ER=EPR”.
A wormhole is not really non-local, I would argue, you just get another path between two points that might be much shorter than the regular shortest path avoiding the wormhole. But even travelling through a wormhole would not make you disappear in one place and reappear in another without following a continuous world line.
Wormholes are not non-local though. They continuously connect two regions of spacetime. It's not different than saying you can go from the south to north pole by passing through Paris or by passing through Beijing.
I’m sure there’s a vocabulary in terms of local domains and being affected beyond the constraints of the light-cone as expressed on space time absent the wormhole so the absolute definition you offer can probably be tempered.
You don't have to give up locality it you just accept that we as observers get entangled with what we observe and the wave function doesn't really collapse. This is compatible with bell inequality and with locality.
There are interpretations of quantum mechanics covering all possible situations, but we don't have a way of choosing between them. You are suggesting religion, not science, not even philosophy.
The Everett interpretation is just taking the Schrödinger equations and not adding anything else to it.
The "interpretation" part is just about explaining how to make sense of what _we_ see and how to make that compatible with the Schrödinger equation. There is nothing mystic about it, quite the contrary I posit, the rejection of it requires the introduction of something anti-scientific, namely that we humans are somehow special and detached from the physical world
> The Everett interpretation is just taking the Schrödinger equations and not adding anything else to it.
That's literally what people claim Copenhagen to be
Everett's interpretation requires positing a universal wavefunction and some kind of branching mechanism
It's not some basic, straightforward interpretation of QM without any additional postulates, and even if it was, that would not be evidence for it to be true over others
> [..] and even if it was, that would not be evidence for it to be true over others
sure, it's not possible to distinguish it from some of the others.
This thread started with comparing it to some theories that instead invoked non-local effects, which would violate general relativity, so there's that.
I don't agree with your absolutism here. Not knowing things doesn't mean everything is on equal footing.
All I'm saying in this thread is that, when comparing possible theories, the ones that do not violate general relativity while explaining other observations IMO deserve not to be dismissed just because people cannot wrap their heads around non-physical consequences of the theory.
no, copenhagen interpretation is wave function + born rule.
the "branching mechanism" is not part of the fundamental mechanism of the everett interpretation. The "branching" is an emergent phenomenon similar to what happens when you mix to liquids of two different colors. At "some point" the the fluids intermix in such a way that you no longer distinguish them and finding the right boundary when that exactly happen may seem a bit "fuzzy", but the physics of what's going on is perfectly clear to anybody; the two substances are still there, but just mixed up in way that is completely intractable to reverse in practice.
It still posits that we are in one of (probably) infinite universes corresponding to each possible outcome (unless you are getting confused with the "Consistent histories" interpretation). That is a part of the fundamental mechanism of the Everett interpretation, it's literally why it's called the Many World Interpretation. Just because you can make a physical analogy for that branching doesn't mean it isn't an extra element in your ontology
yeah, I'd argue though that it's not adding those universes. They're there all along. Branching is an epiphenomenon of there being information processing machines that are embedded in the real universe, and those machines interpret their interactions with the surrounding as if they were only part of a subset of that universe, and upon reasoning about that they call the real universe the "multi-verse"
well, whether the postulate is crazy or not is a matter of perspective.
Postulating that nature just does the Born rule is postulating an additional thing that nature does.
Postulating that hey perhaps we ain't that special and perhaps we should just think through the consequences of our own brains being embedded in the very nature we're trying to describe.
On one hand, it feels like we're adding a lot by buying into the Everrettian model, because it does require us to think and throw away some intuition about ourselves. But OTOH, it actually requires adding much much less to what nature actually does.
We have to assume that nature “does” those consequences of our own brains being embedded in the very nature we're trying to describe though. Even if we can’t even articulate precisely what they are the assumption is not so mild.
Can you dive into this a little more? I think I understand what you're saying but it's a very interesting idea and I'd love to understand it more if you don't mind. :)
I, personally (and a bunch of other physicists), think that the issue is not exactly the entanglement. If you take a pair of gloves and send each piece to two separate people, the moment either of them open the box they will know instantly which piece (left or right) the other got, because both of them knows that a pair of gloves comes with a right piece and a left piece. This is why entanglement doesn't violate general relativity, each person updates their knowledge instantaneously but there is no flow of information from one person to the other.
The true issue, according to some people (me, an experimental materials science-focused physicist, included), is the measurement problem. It updates the state of entangled systems instantaneously, including very space-separated systems. In this way the measurement is provokes the collapse of the quantum state in a non-local way. The problem is that the effects of measurement in quantum systems described by a postulate in quantum theory. So, in my opinion, the non-locality remains not explained and is generally neglected as a non-issue by the community at large.
> I, personally (and a bunch of other physicists), think that the issue is not exactly the entanglement. If you take a pair of gloves and send each piece to two separate people, the moment either of them open the box they will know instantly which piece (left or right) the other got, because both of them knows that a pair of gloves comes with a right piece and a left piece. This is why entanglement doesn't violate general relativity, each person updates their knowledge instantaneously but there is no flow of information from one person to the other.
Quantum entanglement is different though than the classical case you describe. There is no classical correlation that is as strong as the quantum correlations. That's what Bell proved in his theorem: if you only have local interactions and assume that the two systems are correlated and actually already have the states really and we are simply ignorant of them, then the amount of correlation you get between the two systems is smaller than the amount of correlation that quantum mechanics predicts, and the amount verified by quantum experiments.
Therefore, the explanation you describe is not adequate to explain quantum mechanics. It explains how there can be correlations without the need to transfer information, however the amount of correlation this predicts is smaller than the quantum correlation.
(Aside: that is, unless, you drop the condition for statistical independence of your measurements, i.e., you assume you are free to choose which measurement to do. If you drop this assumption, this interpretation is called superdeterminism)
> The true issue, according to some people (me, an experimental materials science-focused physicist, included), is the measurement problem. It updates the state of entangled systems instantaneously, including very space-separated systems. In this way the measurement is provokes the collapse of the quantum state in a non-local way. The problem is that the effects of measurement in quantum systems described by a postulate in quantum theory. So, in my opinion, the non-locality remains not explained and is generally neglected as a non-issue by the community at large.
I don't see what the measurement problem has to do with locality. If the interpretation of quantum entanglement that you described above was correct, this would also explain the measurement problem. The problem with measurement is more that it is non-unitary. Most likely imo, measurement is an unrealistic abstraction of a complex interaction, which only appears to be non-unitary on the face of it.
>Quantum entanglement is different though than the classical case you describe. There is no classical correlation that is as strong as the quantum correlations. That's what Bell proved in his theorem: if you only have local interactions and assume that the two systems are correlated and actually already have the states really and we are simply ignorant of them, then the amount of correlation you get between the two systems is smaller than the amount of correlation that quantum mechanics predicts, and the amount verified by quantum experiments.
First of all, I kind agree with you. After all, the experimental observation of the violation of the Bell inequality is a landmark win for the probabilistic nature of quantum mechanics and it is undeniable. Also, the gloves example is just a particular case of "Bell's theorem experiment" with sensors aligned along the quantization axis of the conserved observable being measured (Measuring Sz in superposition of |+>z and |->z, for example).
Second, I used this example to simply say that "there can be correlations without the need to transfer information" at superluminal speeds. That is, entanglement either quantum or classical is not incompatible with general relativity nor non-local.
>I don't see what the measurement problem has to do with locality.
In the way I understand Bell's theorem (keeping statistical independence), it mainly tells us that i) if there is a hidden variables theory that explains QM results, it needs to be non-local. And, ii) if there is not a hidden variables theory, the state prior to the measurement is truly indetermined in the sense that we are not simply ignorant of the true state.
For me, ii) introduces non-locality in the sense that measuring one entangled particle will change the state of particle 2 irrespectively of time-space separation. You can say that there is no such a thing "state of particle 2" there is just the "entangled state". But this, in turn, invites the discussions of "physicness", "realness", etc of "states" and "wavefunctions". Which seem to be sidestepped due to the overwhelming success of the theory.
>The problem with measurement is more that it is non-unitary.
I am not aware of the consequences of this. If not too bothersome, could you discuss possible (EDIT: possible consequences that impact) experimental results of this? Thanks
> You can say that there is no such a thing "state of particle 2" there is just the "entangled state". But this, in turn, invites the discussions of "physicness", "realness", etc of "states" and "wavefunctions". Which seem to be sidestepped due to the overwhelming success of the theory.
I mean, we could sidestep it, but it's a fundamental part of the formalism of quantum mechanics. It's even worse if we consider quantum gravity: is the spacetime also in a non-separable state?
I'm not sure about local non-hidden variable theories though, to be honest
> If not too bothersome, could you discuss possible (EDIT: possible consequences that impact) experimental results of this?
It’s interesting to note that by this point “Quantum Supremacy” is well and truly established and is being used for something that’s actually useful. What we still lack, however, is any demonstrated supremacy outside the realm of “model quantum mechanics”.
Can you provide some evidence for your first sentence?
As far as I can see, this article is about a prediction made by theorists about the evolution of certain quantum devices when you frequently measure part of it. This was a nice theoretical idea which has now been tested experimentally.
The article itself does not seem to contain any claim of quantum supremacy, even for this specific subject. Also, while I think the research is really cute, I do not see it being 'actually useful' in any sense - and the article also does not seem to make such a claim.
I’d claim this highlights just quite how established quantum supremacy is. Specifically, they just did the work using quantum computers because they were the best tool for the job. We’ve moved on from the question of “are they good for anything”? In the field of quantum simulation they’re manifestly useful.
“Useful”, however, doesn’t need to be transitive. It’s perfectly possible for it to be useful to perform research without the research itself being useful.
Preface: I know nothing. Is it really a problem that we can’t apply special relativity to subatomic particles? Doesn’t the very nature of our measuring systems being made of atoms and subatomic particles mean they will always destructively interfere with any quantum effect we wish to measure?
But we do apply special relativity to subatomic particles, using a framework called relativistic quantum field theory. It is important to consider relativistic effects when the subatomic particles move close to the speed of light, which they do in particle accelerators. So the outcome of accelerator experiments is described extremely well by relativistic quantum field theory.
The hard part is combining the principles of relativistic quantum field theory with general relativity, or gravity, which describes how matter propagates in a space curved by its very presence. The effects of gravity are indeed completely negligible in particle accelerator experiments, which is why we can describe their outcome in a theory that does not include gravity. But these same effects dominate in other scenarios, for example in early universe cosmology and black hole physics.
Understanding the physics of these other scenarios is one motivation, but on top of that physicists just find it really irksome to not have a single theory that (at least in principle) describes all physics at once, because it indicates that they are missing something fundamental. That is why they are working so hard on topics like string theory.
All of this is is no way related to the article, by the way. First, everything related to the measurement 'paradox' is a big deal in popular science but firmly in the rear view mirror for almost all practicing physicists. Second, the experiments discussed in the article are described perfectly with ordinary quantum mechanics, which in the above context is just a limiting case of relativistic quantum field theory. (In particular, of course doing a measurement means interfering with the system, but that is all understood perfectly well and does not mean that we cannot observe quantum effects.)
Even if your measuring system interferes with the measurement, if the interference is small enough or diverse enough based on the conditions, you can still learn useful things.
Very false. Many many physicists still accept non-locality