B Entanglement: How Does it Work and Its Implications in Everyday Life?

[Ken G]

Aren't you puzzled by it, or do you think that there's no need to bother trying to comprehend, because it will always remain out of our reach?

I'm very puzzled by it, and the resolution that satisfies me is that the same environment that answers a question is also what poses that question. The important ramification of this is that if the environment leaves an answer indeterminate, it simply means the question is never posed in the first place! It's a bit like when you take an exam in school and go over the answers afterwards, you look at questions that were asked that you either knew or didn't know the answers, and you might also wish certain questions were asked that you knew the answer to, but I'll bet you spend zero time thinking about questions that weren't asked that you wouldn't have known the answer to! Apparently nature is a bit like that too, in regard to the two-slit experiment.

"Nature does not establish which slit", that does sound like the particle did go through both slits, to me.

I can't agree, not saying which is not saying both. It's like if you have neither a like for beets nor a dislike for them, it doesn't mean you both like and dislike them, it means you have no opinion on them. The mistake is in thinking the particle has to either go through one slit or the other, or both-- that leaves out the possibility that the issue is simply indeterminate.

Or at least, it doesn't exclude it, whether it's in one or multiple universes, or whatever.

Oh sure, there are plenty of other interpretations, I'm just saying it already invokes an interpretation to say "both," and in my opinion, not a terribly useful interpretation.

Or yeah, perhaps the particle itself isn't everywhere, but follows a "guide" that's the result of all possibilities - but that wouldn't be much different, and equally weird & interesting.

Yes, I think as long as you regard it as weird and interesting, there's not much better you can do.

You say it's not answered by nature, but isn't that itself an interpretation? What if it really is reality that the particle passed through both slits? Would experiments spit out different results if that was the case?

Yes, my approach is indeed an interpretation, but I like to think it is a kind of "minimal" interpretation that adds the least to what we are actually being given. It does not appear that experiments can distinguish these interpretations, as any experiments that agree with quantum mechanics predictions can be interpreted in multiple ways.

Ok, but that's what I'm interested in, every interpretation that still sticks with the maths & experiments, considering I will never get deep into the maths or experiments.

Which leaves you to pick your own favorite interpretation, or even to accept a little dose of them all.

Plus, isn't what physics is all about, trying to find models that explain experimental results?

This is already an interesting question in the philosophy of science. Are we only trying to get power over our environment via successful predictions, or is there also an aesthetic goal to feel like we understand something, that we are learning some kind of lesson? I think almost all scientists have a significant portion of that latter perspective, it's usually what draws them to science in the first place. Even those who claim they only "shut up and calculate" rarely really do restrict themselves to that.

Is it certain that the spins of entangled particles are constantly anti-aligned, or only at the time of measurement?

That's interpretation dependent. Personally, I don't even regard the spin as an attribute that the particle possesses at all, neither all the time nor during measurement. I see it more like information that we have about the particle, which reflects simultaneously (another type of superposition, perhaps) some truth about the reality and some truth that our thought processes interpret into the reality. In other words, all these "attributes" reflect a kind of dialog between us and nature (and that dichotomy is already an idealization), where both parties play a crucial role and could not be the same without either one.

Ok, the question hasn't been posed at all, yet the result of what we observe is the result of all of the possibilities (the particle/wave interacting with itself), not just one. Doesn't that sound like it is all possibilities, until observed, if the result is the combination of them all?

And that's why many people like to say it goes through "both." But I prefer to say it arrives at the detector, because that question was posed, and how it got there is simply a question that is not posed, so there is no truth to saying the particle actually went through both-- however, the mathematical waves we use to predict the answer to the question that was posed (where it arrived) does involve amplitudes that go through both slits. But remember that amplitudes aren't "things" so don't really "go through" anywhere, they are mathematical constructs.

I mean: whether it's really the particle that was everywhere, or some weird guide in space itself & not the particle, the fact that 1 single particle at a time will produce an interference pattern, should mean that something, whether it's the guide or the particle, was the product of all possible states, thus "all at once", no?

I don't mind saying the "guide" involves hypotheticals that, by themselves, would look like a particle going through one slit or the other, so the combination of them kind of looks like going through both slits, but there's still no need to say the particle itself goes through both, when the slit it goes through seems more like it is fundamentally indeterminate.

Consider this analogy: a photon that is polarized at a 45 degree angle has an indeterminate polarization in regard to being either vertical or horizontal. Should we then say that the photon is polarized both vertically and horizontally? That sounds a bit incoherent, so we instead say it is polarized at a 45 degree angle, which sounds like something quite different but which can be regarded as a superposition of vertical and horizontal, and hence is indeterminate in regard to those directions. A superposition of two slits is not as clearly a "thing" as the polarization at 45 degrees, but that's just because we haven't figured out a measurement that gives a definite result if it's a superposition of two slits, whereas we can tilt a polarizer 45 degrees. Does that represent a fundamental difference in those types of superpositions? I couldn't say, but I won't regard them as fundamentally different without a good reason to.

 

[bhobba]

Regarding the double slit which seems to be mentioned a fair bit for me its utterly boring - see the paper I posted.

Its a good illustration, for beginners, how to apply in practice, the Principe of Superposition and the uncertainty principle - the wave particle duality explanation is a crock. Feynman's path integral approach is good, but unfortunately some take it too literally - its not really going down both paths. The path integral approach is simply suggestive of that view. If you take it literally you end up with a hidden variable interpretation but of a very novel and unusual type - if it attracts you - that's OK - but I personally would rather face QM head on - I find that more instructive. What then is QM - formally its simply the most reasonable extension to probability theory to continuous changes in states, in the general sense of probability models. But maybe I am strange.

Thanks
Bill

 

[Ken G]

Regarding the double slit which seems to be mentioned a fair bit for me its utterly boring - see the paper I posted.

Yes, though entanglement gets interesting for just about anyone-- so that's just the extension of the superposition principle to multiple particle systems.

What then is QM - formally its simply the most reasonable extension to probability theory to continuous changes in states, in the general sense of probability models.

This is very insightful, I think. For those not following the crux of this remark, it encapsulates the concept of indeterminism when dealing with discrete states (which are all the states in any measurement system we could ever employ), because if there would be continuous changes in states, where the observables are discrete, then indeterminism in the observables is required. So then, one can regard the issue of superposition as a kind of tension between the need to have states change continuously, and the need for the observables of any obtainable experiment to be discrete.

The fact that this tension does not appear classically is often interpreted as saying that classical systems are "normal," whereas quantum systems are "weird." But what I was pointing out above, by quoting ancient philosophers using pure logic rather than classical instruments, is that it seems the only reason classical systems don't encounter this rather fundamental and logically required tension is that the extreme complexity of classical systems allows us to pretend that continuous outcomes of observations are actually possible, even though none of our instruments are actually capable of it. The simplicity of quantum systems often force us to abandon that pretense, and hence the tension appears.

Incidentally, if you'd like a quote attributed to ancient philosophers that points straight to the logical tension between continuously varying states and discretely accessible observables (which you can also interpret as a tension between how we can mathematically rationalize systems as continuous superpositions, yet how we can only measure systems in terms of definite discrete attributes we call "collapsed states"), consider Zeno's statement of his paradox of denseness:
"If there are many, they must be as many as they are and neither more nor less than that. But if they are as many as they are, they would be limited. If there are many, things that are are unlimited. For there are always others between the things that are, and again others between those, and so the things that are are unlimited."
Ponder those words from over 2000 years before quantum mechanics, and see if you don't hear in them the tension between the continuousness of the possible "superposition states," and the necessity of finiteness in what we can actually observe or know, the discrete "eigenstates" of a quantum system. Yet quantum mechanics is regarded as weird, even though Zeno was hinting at its logical requirements thousands of years before Newton's classical dynamics!

 

[bhobba]

This is very insightful, I think.

I have been saying it for a long time - but answering the above ie how insightful is it - the answer IMHO is yes and no.

From a mathematical modelling point of view it's very very insightful and easily explains the formalism of QM elegantly from very intuitive assumptions. Basically the why of the formalism is solved - and beautifully solved at that.

The no bit is - like all mathematical models - what does it mean. That is very hard and leads to all sorts of long debates with great subtlety on all sides.

My personal view, and its just my view, is the model is the physics - the rest is just endless debating - which is why I subscribe to the ignorance ensemble interpretation. But like with Bell it occasionally leads to profound and valuable insights - but unfortunately only occasionally. I do like understanding other interpretations though because they all shed light on the formalism.

So what is the central mystery of QM? Its simply we have so many different interpretations - pick something you do like - and you will find an interpretation that has it - but you can bet your bottom dollar it will have something others don't like. No other theory of physics is like that.

Thanks
Bill

 

[Lord Jestocost]

I don't have much of a background in quantum physics so be patient with my questions please. Basically I want to know how does entanglement actually work? Is information being transferred faster than we can detect it or is there some invisible link between particles that causes the phenomenon we call entanglement?
Also just an extra question how does all of quantum mechanics translate to every day life? Does my bed disappear when I am not looking at it?

Entanglement, what is it?

Quote from E. Schrödinger, "Discussion of probability relations between separate systems", Proceedings of the Cambridge Philosophical Society, 31, 1935.

“When two systems, of which we know the states by their respective representatives, enter into temporary physical interaction due to known forces between them, and when after a time of mutual influence the systems separate again, then they can no longer be described in the same way as before, viz. by endowing each of them with a representative of its own. I would not call that one but rather the characteristic trait of quantum mechanics, the one that enforces its entire departure from classical lines of thought. By the interaction the two representatives (or ψ-functions) have become entangled. To disentangle them we must gather further information by experiment, although we knew as much as anybody could possibly know about all that happened. Of either system, taken separately, all previous knowledge may be entirely lost, leaving us but one privilege: to restrict the experiments to one only of the two systems. After re-establishing one representative by observation, the other one can be inferred simultaneously. In what follows the whole of this procedure will be called the disentanglement. Its sinister importance is due to its being involved in every measuring process and therefore forming the basis of the quantum theory of measurement, threatening us thereby with at least a regressus in infinitum, since it will be noticed that the procedure itself involves measurement.

Another way of expressing the peculiar situation is: the best possible knowledge of a whole does not necessarily include the best possible knowledge of all its parts, even though they may be entirely separated and therefore virtually capable of being "best possibly known", i.e. of possessing, each of them, a representative of its own. The lack of knowledge is by no means due to the interaction being insufficiently known - at least not in the way that it could possibly be known more completely - it is due to the interaction itself.”

 

[anothergol]

To me, coming from programming, the "quantum" in QM is actually -more- intuitive. That is, things like a definite speed of causality, the Planck length/time, that's more logical to me.
In the musical/audio world, logarithmic scales are more often used than linear, and that seems to be the case in nature in general. But what's a log scale without a base? That's why I find it more intuitive that there would be a smallest length & time (with everything a multiple or exponent of that).

It's the superposition that I don't find intuitive at all. Which is why I don't understand why Everett's theory is one of the least liked, because to me, it's what makes the most "sense". Where the superposition wouldn't be just maths, the particle would really be everywhere, in an infinite branches of the universe, "close enough" so that there is an interaction between them, and once there is interaction with another particle, there is no "collapse" at all, because there doesn't need to be any. I mean, we asked the particle where it is, and we didn't get one result, we got all of the results. But our brains being made of particles, we can only feel being in one universe, even though we are in all of them, and we did observe the infinity of states the particle could have.
It makes the most sense with entanglement as well, here entanglement is elegant, entangled particles simply shared branchings of the universe, they wouldn't need any other link than that.
Is it this infinity of branching that's so much disliked?

But is it really just debating? Aren't some interpretations fragile enough that specific not possible yet experiments, or things not found yet, will discard them?

 

[Ken G]

Which is why I don't understand why Everett's theory is one of the least liked, because to me, it's what makes the most "sense".

A lot of people do like Everett's approach, which is essentially the bent of the "rationalist"-- someone who thinks the mathematics is the reality, and observations are only there to check which is the right mathematics. (As opposed to the empiricist, who thinks the observations are the reality, and the mathematics is just our best stab at making sense of it.) I'm not a fan of taking the mathematics as literally as a rationalist, because I regard it as a "maximal" interpretation (one that adds a great deal to what we can know and test), whereas I feel interpretations should be "minimal" (add as little as necessary).

In particular, it requires taking the theory quite literally, even though mathematical theories have a way of getting replaced later. We saw this with Newton's laws, which if you take literally seem to imply that the conditions in the past completely determine the conditions we will experience in the future. Then along comes quantum mechanics, which says that what we will actually experience in the future is very far from determined. So which was the mathematics we were supposed to take literally as how things actually work?

But our brains being made of particles, we can only feel being in one universe, even though we are in all of them, and we did observe the infinity of states the particle could have.

I think it's hard to argue that "we" are in all the universes, given how different the people could be from us in those other universes-- especially if the outcomes in question have a significant impact that could even kill us in some universes. Also, if you think that "you" in some sense are present in all the branches in which you survive, you can run into the "quantum suicide" perspective, which would doom all of us to horrendously extended and infirm old ages, and which has a logical basis that I regard as highly strained.

It makes the most sense with entanglement as well, here entanglement is elegant, entangled particles simply shared branchings of the universe, they wouldn't need any other link than that.

Yet treating entanglement as a type of information denies that some kind of physical link is maintained, we are only culling possibilities that are constrained in a way we are not used to. If one does not take the existence of particles too literally either (another example of minimal interpretation), then there is no particular difficulty, it's all about culling possibilities according to unfamiliar constraints as new information comes in.

Aren't some interpretations fragile enough that specific not possible yet experiments, or things not found yet, will discard them?

I think what will happen is eventually quantum mechanics will need to be replaced, and the new theory might ascribe more obviously to one of the current interpretations, even if it also introduces some new ones. If so, then it will be useful to be versed in all the interpretations, because we never know which one will be the most conducive to the development of the new theory. We saw this with classical mechanics, where the Hamiltonian formulation is more conducive to quantum mechanics and the Lagrangian formulation is more conducive to quantum field theory. Ironically, often overlooked is the fact that the interpretation of the existence of "forces" is not particularly conducive to either! Yet we prefer that interpretation so much that we still teach it in high schools, which goes to show you that perhaps we should not be interpreting our interpretations as "what is really happening" anyway!

 

[anothergol]

I think it's hard to argue that "we" are in all the universes

no, I meant "we are in every branching after our existence". I meant, after measuring the state of a particle, we are in every branch for every state of that particle, but obvisouly we can only see ourselves in one, even though in each branch we are there & concluded that we measured something different.
You say this is a maximal interpretation, but to me it looks like the opposite, the one that seems to add the least. The need for a collapse function, the fact that randomness is introduced, entanglement not being explained, yeah perhaps the Copenhagen version limits itself to what we can safely conclude from observations, but.. perhaps the observation of the ant seeing the shadow of things with an extra dimension, to get back to that analogy.

 

[Ken G]

You say this is a maximal interpretation, but to me it looks like the opposite, the one that seems to add the least.

You need to take the equations as the literal truth, rather than some kind of effective approximation. That's adding far more than is ever necessary for science.

 

[anothergol]

But the concept of collapse doesn't come from equations but experiments, if I'm right. And entanglement has to be explained as something else than what seriously conflicts with with established theories (FTL communication). I mean, stating things as they are observed is hardly "interpreting", so the Copenhagen interpretation is simply not interpreting entanglement (or is it?).

Interesting new video of PBS space time btw, on that subject of whether particles are everywhere or not, questionning if those virtual particles are purely mathematical or could be real .

Now I thought that it was already stated that particles pop in & out all the time in vacuum, but now I'm reading that we've ever only observed the results of those (like, Casimir effect), not the particles directly (I assume, because they have to pop in/out in times short enough that they cannot be observed?).

That's interesting because this doesn't really fit in Everett's theory.. unless those would be particles traversing parallel universes perpenticularly.. thus appearing in a vacuum because coming from some universe in which it wasn't a vacuum.

But how do "virtual particles out of nowhere" fit in the Copenhagen interpretation anyway? Say those virtual particles are mathematical helps to compute where a real particle can be, they are all related to one real particle. But then why do you find those in a vacuum?

 

[zonde]

And entanglement has to be explained as something else than what seriously conflicts with with established theories (FTL communication).

Where do you see serious conflict with with established theory? If entanglement is explained using FTL coordinated changes of quantum phase it does not lead to FTL communication. Absolute phase is unobservable but to observe relative phase you have to compare both ends. There is no way how you can use quantum phase to communicate FTL. And quantum phases are out of scope of relativity so it says nothing about them.

 

[anothergol]

Where do you see serious conflict with with established theory? If entanglement is explained using FTL coordinated changes of quantum phase it does not lead to FTL communication. Absolute phase is unobservable but to observe relative phase you have to compare both ends. There is no way how you can use quantum phase to communicate FTL. And quantum phases are out of scope of relativity so it says nothing about them.

No I'm not talking about "third party message communication" FTL, but about that "coordination" itself. Simply stating that the changes of phase are coordinated is stating the observation, it's not interpreting it. Such a coordination, if we accept that it exists (because it doesn't seem to have to in Everett's theory), requires FTL communication, or something else like a shared property in another dimention, but in any case it requires something. If entangled particles are -really- linked, and that link can't be made through space because it would imply it's FTL, then it's made from something/somewhere else. Well sure you can say "it's the way it is, just accept it" but that sounds a little religious to me.

 

[zonde]

No I'm not talking about "third party message communication" FTL, but about that "coordination" itself. Simply stating that the changes of phase are coordinated is stating the observation, it's not interpreting it. Such a coordination, if we accept that it exists (because it doesn't seem to have to in Everett's theory), requires FTL communication, or something else like a shared property in another dimention, but in any case it requires something. If entangled particles are -really- linked, and that link can't be made through space because it would imply it's FTL, then it's made from something/somewhere else.

Yes, this "coordination" requires FTL physical process. But where is this "serious conflict" with established theory (not interpretation or arbitrary extension of that theory)?

 

[anothergol]

Well define "process". Speed of light is supposed to be the speed of causality, and here you're describing a causality.. happening instantly.
Perhaps speed of causality is an interpretation, but it seems to be the way it is, what else violates it?
Seems simpler & safer to reword entanglement as not being a coordination, since it doesn't need to be. It has been established that it wasn't a local hidden variable, but that doesn't go against the idea of multiple universes.

 

[zonde]

Well define "process". Speed of light is supposed to be the speed of causality, and here you're describing a causality.. happening instantly.
Perhaps speed of causality is an interpretation, but it seems to be the way it is, what else violates it?

Yes, I agree that it is in conflict with our experience. But I do not agree that it is conflict with established theory.

Seems simpler & safer to reword entanglement as not being a coordination, since it doesn't need to be.

Go ahead, try to do that. This is the topic of this thread after all.

 

[Ken G]

Entanglement isn't any less mysterious is Everett's interpretation. Remember, the guts of entanglement is that two particles can share a property like "same polarization" even when the polarization of each particle is completely indeterminate. It doesn't matter if your interpretation includes collapse or not, the bizarreness of that fact is still there. Indeterminacy doesn't go away in many worlds-- you just have more versions of scientists who cannot predict what they will see. If you have many worlds, you escape the need for the particles to yield a single seemingly random result in just one world, but you still have to explain why in every world where one particle passed a polarizer at some arbitrary angle, the other one did too. Tack on as many worlds as you like, that still requires interpreting the outcome of the observations. (As for virtual particles, that's a whole other can of worms you don't want to get into, so let's stick to entanglement.)

 

[anothergol]

Entanglement isn't any less mysterious is Everett's interpretation. Remember, the guts of entanglement is that two particles can share a property like "same polarization" even when the polarization of each particle is completely indeterminate. It doesn't matter if your interpretation includes collapse or not, the bizarreness of that fact is still there. Indeterminacy doesn't go away in many worlds-- you just have more versions of scientists who cannot predict what they will see. If you have many worlds, you escape the need for the particles to yield a single seemingly random result in just one world, but you still have to explain why in every world where one particle passed a polarizer at some arbitrary angle, the other one did too. Tack on as many worlds as you like, that still requires interpreting the outcome of the observations. (As for virtual particles, that's a whole other can of worms you don't want to get into, so let's stick to entanglement.)

But isn't Everett's theory more deterministic?

but you still have to explain why in every world where one particle passed a polarizer at some arbitrary angle, the other one did too

ah, so that really implies that there is a link. A problem indeed

 

[zonde]

ah, so that really implies that there is a link. A problem indeed

Before attempting to solve the problem yourself you can try to look how others tried to do that. There are some references at the end of this article. In particular you can take a look at this reference (chapter 6.3).
It is attempting to make MWI local by introducing additional splits where future light cones meet. Well, because intersections of lightcones initially would be spacelike I would say you get ever increasing microsplits along this intersection and subsequent intersections. At the end you get very contrived mechanism that at single world level (if is conceivable at all in such a model) looks just like FTL coordination of outcomes.

 

[Ken G]

But isn't Everett's theory more deterministic?

Determinism isn't the issue with entanglement-- correlation is. All many worlds do is allow you to escape the question "which definite outcome occurs when you have an indeterminate state?", but the question with entanglement is "what maintains the tight correlation between polariation of two particles that can be widely separated even before the polarization angle is decided?" It's the nonlocality of the correlation that is the puzzle.

By the way, there are two very different flavors to "collapse" that often get confused. Let's take the case of two photons entangled to be in the same polarization state, but that state is indeterminate. You choose a random angle for your polarizer, and perceive both photons either going through, or not going through. The two different flavors of collapse happen in two steps-- first there is the "decoherence", which happens in any interpretation, it means that when you pass the photons through, the only realities that remain "coherent" are the ones where both photons pass through, and we perceive they both pass through, and where neither photon passes through, and we perceive neither photon passing through. What experiences destructive interference, so doesn't happen, is that the photons pass through, and we perceive them not passing through, and the inverse. The interpretations only kick in after that stage, when we wrestle with the issue of which of those allowed possibilities will happen, or will all of them happen. But the entanglement puzzle already appeared, which is how did the behaviors get bundled together in the first place.

ah, so that really implies that there is a link. A problem indeed

Exactly.

 

[bhobba]

Say those virtual particles are mathematical helps

That's exactly what they are. Specifically they are lines on a Feynman diagram which is just a pictorial representation of what's called a Dyson Series:
https://en.wikipedia.org/wiki/Dyson_series

They don't exist in the same sense as real particles.

Thanks
Bill