Physicists Observe ‘Unobservable’ Quantum Phase Transition
quantamagazine.org
quantamagazine.org
Very false. Many many physicists still accept non-locality
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.
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.
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
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?
There are none
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
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.
See Everett or many-worls interpretation
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
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
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.
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.
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.
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.
QFT is no more non-local than QM in general
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).
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.
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.
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.
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.
> 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.
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.
> 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.
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?
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?
Well, because that's the postulate
I don't think many physicists will argue they are wrong.
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.
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.
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.
“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.
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.)