I Quantum mechanics is not weird, unless presented as such

[ddd123]

So you have a pretty good determination of the protons' location with a pretty well determined momentum at the interaction point. Without that you'd not be able to get proton collisions in a collider with a sufficiently well defined collision (center of momentum) energy to be meaningful for particle physics. All this is, of course, fully consistent with quantum theory, and for sure it's nothing weird about it, although just remarkable and amazing to which precision one can construct accelerators and detectors testing the predictions of quantum theory (in this case the Standard Model of elementary particles, i.e., relativistic quantum field theory).

Are you referring to the HUP?

 

[vanhees71]

Of course science is embedded in social acitivity, but it is independent of a "worldview" since the only thing that counts is the success or failure to describe what's objectively and reproducibly observable. Of course, that limits its purpose to a subset of human experience but it cannot contradict any religion, and it's always possible that one day a reproducible observation invalidates the today "valid" theories and models. That happens astonishingly rarely but it happens. Famous examples are relativity which lead to abondaning the hitherto "valid" Newtonian theory about space and time. It also explained, why Newtonian mechanics is so successful in its realm of applicability. Even more extreme was the discovery of quantum theory, which lead to a total reconception of what "reality" itself means.

That's a great difference to religion, where you have to believe some basic principles without questioning them. This is contrary to any good practice in science. Although being better conservative and trying to understand any "new" phenomenon first with the so far considered valid models, one has to be open to the possibility that these models may be not always valid and observations and experiments may lead to a revision of the models. There's no "worldview" that supersedes this basic principle of how science works. You can argue as much as you like that, e.g., QT is incompatible with your worldview. From a scientific point of view this is fully irrelevant to the progress of science. Here only observable objective facts rule about the validity of models!

 

[vanhees71]

Are you referring to the HUP?

Among other things yes. I referred to the claim that due to quantum theory you don't know any property if you don't measure it. Of course, if you don't have any knowledge about whether there are protons or not, you don't know anything, but that's a tautology. I took as an extreme example to the contrary a modern accelerator, where one knows a lot about the protons accelerated by it, because it is obviously possible to prepare protons quite accurately, and all this is of course in accordance with quantum theory. If it were not, we'd have to give up quantum theory and look for a better model, but to the contrary QT is fully compatible with all observations so far, and it's simply not true that we don't know anything about particles only because quantum theory provides "only" probabilistic information about observables.

To put it in another, even more simple, way: Over all the mathematically sophisticated formalism, which is necessary because it's the only way to describe our observations and theoretical understanding adequately and unambiguously, one must not forget, what's really observed in the labs concerned with QT. Then the theory looses much if not all of its weirdness!

 

[ddd123]

Unless they're Bell tests or quantum erasers etc :)

 

[vanhees71]

Well, if you accept quantum theory (in the minimal interpretation), there's nothing weird anymore about Bell tests (to the contrary they confirm with high precision the predictions of quantum theory, violating the Bell inequality with a very high confidence level) or quantum erasers (you just choose different partial ensembles using a fixed measurement protocol). The very fact that such "postselection" works is also a strong confirmation for the principles of quantum theory.

Admittedly, from the point of view of our classically trained everyday experience these findings are quite weird, but not from the point of view of QT :-).

 

[vanhees71]

I don't know, how science should apply to single events like this story about G. W. Bush. It's a single event and most likely a coincidence that somebody could predict this. Is it clear that somebody really "predicted" this outcome of G. W. Bush's predidency and events concerning China or is this made up on some conspiracy web page?

I don't understand what you mean with this assertion concerning light. What's the context of this?

The objective observer of facts nowadays can be an electronic device providing measurement results at high accuracy (as used in all kinds of experiments in all kinds of labs across the world) and finally physicists evaluating these fixed facts about nature.

 

[A. Neumaier]

I think that flavors of Copenhagen interpretations that don't envoke the collapse postulate are the lest weird interpretations. Among them is the minimal interpretation

The minimal interpretation is significantly different from any version that deserves (in my view) to be called Copenhagen. In the Copenhagen interpetation (prevailing until the 1970es), each single object is in a well-defined (though possibly unknown) pure state, which collapses to a different state upon measurement. In contrast, in the (much later sensibly defined) minimal, statistical interpretation, the state is a property of the source (i.e., preparation procedure), not of the single quantum object. If you call the minimal interpretation a flavor of Copenhagen then the term ''Copenhagen interpretation'' loses its discriminating meaning.

At the collision point it's squeezed to μm size. At each bunch crossing up to 20 collisions occur. So you have a pretty good determination of the protons' location with a pretty well determined momentum at the interaction point.

I fully agree. My point is just that this is in flat contradiction to what one reads in the highly idealized presentations and discussions of axioms/postulates concerning the interpretation of quantum mechanics.

Both preparation and measurement are complex procedures with nontrivial qualifications of what it means to have prepared something and what counds as a measurement result, and to which accuracy something is prepared and measured. This is simply pushed aside by simplistic, strictly speaking invalid, statements given the status of postulates or axioms, and it is pretended that these are the ''foundations'' on which uantum mechanics rests. In reality, quantum mechanics rests on much stronger - and a bit more complicated - pillars that have almost nothing to do with complicated measurement processes (which are only used to verify the validity of the theory). The traditional foundations are but a caricature of the real thing.

 

[A. Neumaier]

Over all the mathematically sophisticated formalism, which is necessary because it's the only way to describe our observations and theoretical understanding adequately and unambiguously, one must not forget, what's really observed in the labs concerned with QT. Then the theory loses much if not all of its weirdness!

This is indeed the ostensible purpose of an interpretation. To relate shut-up-and-calculate to what's really observed in the labs. if it is done well, it proves the title of the present thread. But the textbook interpretations idealize far too much, so that if their postulates are taken too seriously, one ends up with lots of weirdness.

 

[ddd123]

Just to be sure, the minimal interpretation is the ensemble interpretation, right? As presented in Ballentine for example.

 

[A. Neumaier]

Just to be sure, the minimal interpretation is the ensemble interpretation, right? As presented in Ballentine for example.

Yes, minimal = ensemble = statistical interpretation, as in Ballentine and Peres. I prefer Peres, since he discusses it in a context that makes sense for real measurements.

 

[ddd123]

Okay I was wondering about this article on the HUP: http://plato.stanford.edu/entries/qt-uncertainty/ :

it is not straightforward to relate the spread in a statistical distribution of measurement results with the inaccuracy of this measurement, such as, e.g. the resolving power of a microscope. Moreover, the minimal interpretation does not address the question whether one can make simultaneous accurate measurements of position and momentum. As a matter of fact, one can show that the standard formalism of quantum mechanics does not allow such simultaneous measurements. But this is not a consequence of relation $\Delta_\psi p \Delta_\psi q \geq \hbar / 2$.

If one feels that statements about inaccuracy of measurement, or the possibility of simultaneous measurements, belong to any satisfactory formulation of the uncertainty principle, the minimal interpretation may thus be too minimal.

1) if it's not a consequence of that inequality, what is it a consequence of in orthodox quantum theory?
2) is the minimal interpretation too minimal as the article says?

 

[vanhees71]

The Heisenberg uncertainty relation, as proven in any modern textbook on QM, does not describe the disturbance of the system by measurement but a constraint on the accuracy with which position and momentum of a particle can be determined, i.e., it tells you that in any state of a particle, the standard deviations fulfill this inequality, and that's it.

I also disagree with the statement that the minimal interpretation is too minimal (at least in this context), since before you make statements about accuracy disturbance relations you have to define precisely what you mean by this. This is, by the way, an ongoing debate in the literature, but not a severe obstacle of quantum theory in my opinion, since this disturbance is defined by the kind of measurements you do on the particle and must be analyzed taking into account the mechanism behind the measurement apparatus for each experimental setup case by case.

 

[ddd123]

What about statements on the accuracy of simultaneous measurements of noncommuting observables on a single system? Could it be too minimal for that?

 

[vanhees71]

As I said, you have to define "simultaneous measurements of noncommuting observables on a single system" by giving a concrete description of the measurement apparatus. The usual Heisenberg uncertainty relation refers to measurements of each single observable on the single system with an accuracy much larger than the expected uncertainty of the single observables. The probabilistic nature of the physical meaning of the quantum state means that you have to measure each variable on an ensemble of independently but equally prepared systems (that's the definition of an ensemble).

There are of course much more general ideas on measuring procedures, i.e., you don't measure the observables accurately but minimize the influence on the system. This is quantified by defining accuracy-disturbance uncertainty relation, and it is still an open debate about them in the literature. Here are some (arbitrary) examples, I collected randomly when finding them on the web. Perhaps one of the other posters can provide a more systematic collection:

http://arxiv.org/abs/1201.1833
http://arxiv.org/abs/1504.04200
https://www.osapublishing.org/viewmedia.cfm?uri=QIM-2013-W6.10&seq=0
http://arxiv.org/abs/1007.3076
http://arxiv.org/abs/quant-ph/0307057
http://arxiv.org/abs/1306.1565

Here's an old posting of mine, nobody ever found interesting, but it summarizes the first citation above:

https://www.physicsforums.com/threa...elation-vs-noise-disturbance-measures.664972/

 

[naima]

The measuring device is a complex system in a metastable "neutral state", which then makes a transition into a stable pointer state through interaction with the microscopic quantity that is being measured. That's understandable. It's exactly what happens in classical mechanics, and is the reason that we can get discrete outcomes ("heads" or "tails") from continuous Newtonian dynamics.

But it's the pairing of distant measurement results in a correlated pair such as EPR that is mysterious. Alice's device is in a metastable state, and when it interacts with a spin-1/2 particle, it falls into a stable pointer state. Similarly for Bob's device. But to describe the transition using statistical mechanics seems to make the fact that Alice's and Bob's results are perfectly anti-correlated even more mysterious. If the measurement process is inherently statistical, then how does perfect anti-correlation come about?

I read this old question about weirdness.
When a system is prepared in a given state it is often in an eigenvector of an observable. if you re-measure the system to get this observable you get the same value. A measurement for something else give a random output.
There are devices which prepares pairs of particles with a global null spin along all directions. You can verify it even if they are separated. take any direction and ask Alice and Bob to locally measure the spin along it. Ask their results and add them. If you get 0 you have verified the preparation. Il the local directions are not the same you have a random result. It is not surprising because you have measured something else.
Weirdness is not absent but it is somewhere else.

 

[stevendaryl]

I read this old question about weirdness.
When a system is prepared in a given state it is often in an eigenvector of an observable. if you re-measure the system to get this observable you get the same value. A measurement for something else give a random output.
There are devices which prepares pairs of particles with a global null spin along all directions. You can verify it even if they are separated. take any direction and ask Alice and Bob to locally measure the spin along it. Ask their results and add them. If you get 0 you have verified the preparation. Il the local directions are not the same you have a random result. It is not surprising because you have measured something else.
Weirdness is not absent but it is somewhere else.

The weird thing is that (apparently) Alice's result is completely random, and so is Bob's, but they manage to always get the opposite result (when they measure using the same axis). That would not be surprising if their results were predetermined from the moment that the twin pair is created, but that isn't the case.

 

[naima]

Nature is only weird if you believe that it can abswer all YOUR questions.
Nature is a patient good teacher. It comes with datas, with answers. The problem is that the pupil does not understand what the teacher is talking about. It is like in Jeopardy. If you find the question the teacher will always give you as an answer the initial answer.
You know how to compute the spin density matrix as a linear combination of Pauli matrices and the identity matrix. the coefficient are the mean values of the yes/no "random" answers natures gives you. At the end although you never asked the good question your are able to win Jeopardy.
If the good question was about a number of particles (2 here) and a global property measuring a local thing of one of them is not rhe good question but nature does not refuse to help you.

 

[ddd123]

The weird thing is that (apparently) Alice's result is completely random, and so is Bob's, but they manage to always get the opposite result (when they measure using the same axis). That would not be surprising if their results were predetermined from the moment that the twin pair is created, but that isn't the case.

At the cost of sounding polemic, I think we should do either of the following:

1) admit it's weird and suspend judgement until a breakthrough comes, or at most say "I understand why it's weird for you but it doesn't bother me since I have the shut up and calculate framework, which is all I wanted";
2) explain what is missing in the intuitive picture that removes the weirdness in a clear straightforward manner.

I think so far we've seen either 1) or a moral statement that it shouldn't seem weird, then grasping at straws to justify that moral statement. 2) seems to be an unattainable goal at this moment.

 

[vanhees71]

The weird thing is that (apparently) Alice's result is completely random, and so is Bob's, but they manage to always get the opposite result (when they measure using the same axis). That would not be surprising if their results were predetermined from the moment that the twin pair is created, but that isn't the case.

The single results are not predetermined, but the correlation is. So what's surprising or even weird?

 

[vanhees71]

At the cost of sounding polemic, I think we should do either of the following:

1) admit it's weird and suspend judgement until a breakthrough comes, or at most say "I understand why it's weird for you but it doesn't bother me since I have the shut up and calculate framework, which is all I wanted";
2) explain what is missing in the intuitive picture that removes the weirdness in a clear straightforward manner.

I think so far we've seen either 1) or a moral statement that it shouldn't seem weird, then grasping at straws to justify that moral statement. 2) seems to be an unattainable goal at this moment.

My problem is indeed question 2). What's missing? Nothing (yet). We have quantum theory that works very well in describing everything we've observed so far. What else can you wish for and expect to get from the natural sciences?

 

[ddd123]

I take that as a 1)b) kind of answer. If nothing was missing we wouldn't have, to pick a random example, ER=EPR speculations which then get published on Scientific American.

 

[vanhees71]

What means "ER=EPR"?

 

[ddd123]

The Einstein-Rosen bridge speculated to be identical with (and the explanation of) EPR. There was a thread on that recently.

 

[naima]

I do not think that adding wormholes decreases weirdness!

 

[stevendaryl]

The single results are not predetermined, but the correlation is. So what's surprising or even weird?

The strange part is understanding how possibilities become actualities in QM. The wave function (or density matrix) gives probabilities for various outcomes. What we observe are definite outcomes. So the issue for me is: How does a single outcome picked out of a set of possible outcomes? There are various possibilities, but none of them really fit all the facts. One possibility is that outcomes are pre-determined, according to probabilities given by QM. Bell's theorem seems to rule out that possibility. Another possibility is that one outcome emerges through interaction between the system being measured and the system doing the measuring--that they both participate. But that being the case, then it would seem to require something nonlocal to insure that Alice and Bob always get opposite results when they measure along the same axis.

 

[stevendaryl]

The claim that QM only predicts correlations, not actual results, is in itself pretty weird, in my opinion. Here's an analogy. Suppose that, rather than a coin flip giving on the average an equal number of heads and tails, there was a law of nature stating that coin flips always alternated: heads, then tails, then heads, etc. If someone empirically discovered such a rule, he would suspect that there is some hidden state information that determined the result. I don't think most people would be satisfied by just saying: It's just a rule.

If we made it nonlocal, it would be even more remarkable. Suppose there were a pair of coins such that it's guaranteed that if the coins are flipped at the same time, they always give opposite results, no matter how far away they are when flipped. I think that most people would consider that pretty strange, and would want to find the mechanism that causes such correlations.

The fact that people accept similar correlations without wondering about them, in the case of quantum mechanics is itself weird.

 

[naima]

I do not know HOW possibilities become actualities, but i think this only occurs when details are erased or neglected. Take entangled photons they give no interference behind the slits just as if they were detected at the slits but they are not. The simple fact to consider one particle of the pair needs to trace out the degrees of freedom of the other and to neglect them.
To measure something you always need a barrier between the measured particle and a macroscopic apparatus whose details are unknown.
It seems that when all is known nothing occurs. Rovelli (who tells that time is an illusion) writes that "time is ignorance".

 

[stevendaryl]

I do not know HOW possibilities become actualities, but i think this only occurs when details are erased or neglected. Take entangled photons they give no interference behind the slits just as if they were detected at the slits but they are not. The simple fact to consider one particle of the pair needs to trace out the degrees of freedom of the other and to neglect them.
To measure something you always need a barrier between the measured particle and a macroscopic apparatus whose details are unknown.
It seems that when all is known nothing occurs. Rovelli (who tells that time is an illusion) writes that "time is ignorance".

In the case of EPR with an electron/positron pair, if Alice and Bob measure the spin of their respective particle along the same axis, they always get the opposite result. As I said in another post, it's as if there were a pair of coins such that if they are both flipped, they always give opposite results, no matter how far away they are when flipped. In the case of coins, people would strongly suspect that the results must be predetermined. But in the case of entangled twin pairs, such a way out is incompatible with Bell's theorem (or at least, it's very difficult to understand how it is consistent with Bell's theorem).

 

[fresh_42]

Suppose that, rather than a coin flip giving on the average an equal number of heads and tails, there was a law of nature stating that coin flips always alternated: heads, then tails, then heads, etc. If someone empirically discovered such a rule, he would suspect that there is some hidden state information that determined the result. I don't think most people would be satisfied by just saying: It's just a rule.

But there is a loophole in your analogy. Alternating outcomes are not predicted, only that it has to be either one or the other. And if 'head' is the outcome nobody wonders that the carpet on which the coin lands measures 'tail'.
What am I missing here?

 

[stevendaryl]

But there is a loophole in your analogy. Alternating outcomes are not predicted, only that it has to be either one or the other. And if 'head' is the outcome nobody wonders that the carpet on which the coin lands measures 'tail'.
What am I missing here?

But if there were a pair of coins such that whenever they are both flipped at the same time, they always gave opposite results (one heads and one tails), no matter how far apart they are flipped, I think people would consider it pretty weird. That seems analogous to the anti-correlated EPR type experiments.

 

[stevendaryl]

But there is a loophole in your analogy. Alternating outcomes are not predicted, only that it has to be either one or the other. And if 'head' is the outcome nobody wonders that the carpet on which the coin lands measures 'tail'.
What am I missing here?

I might have missed your point originally. You are making the analogy that anti-correlation in EPR is akin to the fact that if on one side of a coin you can see "heads", then on the other side, you can see "tails"?

As an explanation for anti-correlation, that's a hidden-variables theory. You have Bob looking at one side of the coin, and far, far, away, Alice is looking at the other side (presumably through a powerful telescope). But Bob's result is determined long before the light from the coin reaches him. In the EPR case, it is not consistent with Bell's theorem to believe that the results are predetermined. (I have to always make this caveat: It's very difficult to reconcile predetermination with Bell's theorem. It might be possible, but not in any straight-forward way.)

 

[ddd123]

I do not think that adding wormholes decreases weirdness!

Well it would restore a sense of locality. It's like a wormhole between steveandaryl's coins: if confirmed it'd explain what before was a mystery, so sure we might be amazed at the wormholes but it wouldn't be so weird as to make us feel there's a serious epistemological hole in our model.

 

[fresh_42]

As an explanation for anti-correlation, that's a hidden-variables theory.

Well, it was your analogy. And this only means that you cannot find an analogy in the classic macroworld that properly can be compared to entanglement. However, this fact might indicate that QFT is not a classical theory (comp. Bell) but it is not an indication of weirdness, only of the fact that we aren't trained (yet) to imagine it. There have been times people couldn't imagine non-Euclidean geometry.

 

[ddd123]

Non-euclidean geometry wasn't imagined because it wasn't discovered mathematically.

 

[A. Neumaier]

no matter how far away they are when flipped. I think that most people would consider that pretty strange, and would want to find the mechanism that causes such correlations.

The fact that people accept similar correlations without wondering about them, in the case of quantum mechanics is itself weird.

Once upon a time, even an intellectual giant such as Newton accepted action at a distance in case of gravitation. He had wondered about it but didn't find a mechanism that caused it. Nevertheless, he didn't find it weird.

In the mean time, we were spoilt by a brief period, ranging from 1915 (the birth of general relativity) to 1935 (the birth of the EPR paper and of Schrödinger's cat), where everything seemed to match our intellectual sense of naturality. Since 1935, we are partially back to the old times with regard to long range correlations, but for many, the subjective sense of weirdness born in 1935 hasn't subsided yet.

 

[fresh_42]

Non-euclidean geometry wasn't imagined because it wasn't discovered mathematically.

They knew the shape of Earth and that the axiom of parallels doesn't hold on a sphere. It has been simply ignored.
And my hope is that future ways of education will naturally provide a deeper understanding in mathematics and physics. At least in so far that the current curricula go beyond calculations and Newton mechanics. I prefer to hope that today's weirdness becomes tomorrow's understanding and intuition.

 

[stevendaryl]

Well, it was your analogy.

Yes, I know. In the case of coin flips, we certainly would look for a "hidden variables" explanation, and we would find it very weird if we were unable to discover one. You prove that point by immediately going to a hidden-variables explanation.

And this only means that you cannot find an analogy in the classic macroworld that properly can be compared to entanglement. However, this fact might indicate that QFT is not a classical theory (comp. Bell) but it is not an indication of weirdness, only of the fact that we aren't trained (yet) to imagine it. There have been times people couldn't imagine non-Euclidean geometry.

It certainly is not a classical theory. But as I have said before, what's weird about quantum mechanics is not any of the "rules", but the fact that there is no definitive answer to the question of whether the equations describe a physical property of the world, or describe our knowledge about the world.

In the EPR experiment, with anti-correlated spin-1/2 particles, suppose that Alice and Bob agree ahead of time on the axis that they will measure spin relative to. When Alice measures spin-up, she knows immediately that Bob will measure spin-down. That's pretty straight-forward. But then the question is: what is the nature of that knowledge? If Bob has not yet measured his particle's spin, then does Alice's result tell her something about Bob that she didn't know earlier? I think it clearly does. So that's a fact about the universe that she learns by making her measurement. Did that fact become true at the time Alice made her measurement, or was it true earlier, and Alice only discovered it? If it became true when Alice made her measurement, then it seems that Alice had an effect on Bob: He went from a state in which there were two possible future results to a state in which there is only one possible future result. The assumption that it was true beforehand, and Alice's measurement only revealed its truth is a hidden variables theory, which is ruled out by Bell's theorem.

You can argue that we're thinking classically when we assume that there is such a thing as "the state" of a subsystem such as Bob; maybe it makes no sense to talk about his state as something separate from Alice's state. I think that that's a possibility, but it's muy weird.

I've already been through this with different participants, so I will just be repeating myself if I go on, but I do not think it's true that the apparent weirdness of quantum mechanics is due to its being so far removed from our intuitions. Special and General Relativity were similarly far removed from our intuitions, but (in my experience) it only takes a few months of working with them to get to the point where they don't seem so weird any more.

 

[stevendaryl]

There have been times people couldn't imagine non-Euclidean geometry.

I don't think the analogy with non-Euclidean geometry is very apt. General Relativity may be contrary to our intuitions, but it can be presented in a realistic way: the universe is a 4-dimensional pseudo-Riemannian manifold, blah, blah, blah. The Hilbert space used to describe quantum mechanics is not particularly weirder, as a mathematical structure, than pseudo-Riemannian manifolds. But QM isn't making the claim that the universe is a hilbert space, or a point in Hilbert space. The whole apparatus of quantum mechanics is not about describing how the universe is, but is instead an elaborate way of formulating a recipe for making predictions about observations. That's what's essentially different about quantum mechanics. It gives us a way of making predictions, but it only very indirectly makes any claims about what the universe is like. (Although there are interpretations of QM that are sort-of realistic, such as Many-Worlds, which does claim that the universe has a state that is a point in some Hilbert space, and Bohmian mechanics, which claims that the world consists of positions of particles plus a pilot wave that influences the motion of those particles.)

 

[Jilang]

Seems to me that all correlations are local to the observer who puts the results together. Working backwards from there...

 

[ddd123]

Once upon a time, even an intellectual giant such as Newton accepted action at a distance in case of gravitation. He had wondered about it but didn't find a mechanism that caused it. Nevertheless, he didn't find it weird.

In the mean time, we were spoilt by a brief period, ranging from 1915 (the birth of general relativity) to 1935 (the birth of the EPR paper and of Schrödinger's cat), where everything seemed to match our intellectual sense of naturality. Since 1935, we are partially back to the old times with regard to long range correlations, but for many, the subjective sense of weirdness born in 1935 hasn't subsided yet.

Because influence across space is something Newton could handle, but not backwards in time depending on the observer. If we didn't observe time dilation and thus use a special relativity framework there would be much less weirdness. Since time and space got mixed up, nonlocality got a lot weirder, yes.

 

[Hornbein]

Once upon a time, even an intellectual giant such as Newton accepted action at a distance in case of gravitation. He had wondered about it but didn't find a mechanism that caused it. Nevertheless, he didn't find it weird.

Don't underestimate Izzy Junior.

That Gravity should be innate, inherent and essential to Matter, so that one body may act upon another at a distance thro' a Vacuum, without the Mediation of any thing else, by and through which their Action and Force may be conveyed from one to another, is to me so great an Absurdity that I believe no Man who has in philosophical Matters a competent Faculty of thinking can ever fall into it. [4]

— Isaac Newton, Letters to Bentley, 1692/3

 

[vanhees71]

The strange part is understanding how possibilities become actualities in QM. The wave function (or density matrix) gives probabilities for various outcomes. What we observe are definite outcomes. So the issue for me is: How does a single outcome picked out of a set of possible outcomes? There are various possibilities, but none of them really fit all the facts. One possibility is that outcomes are pre-determined, according to probabilities given by QM. Bell's theorem seems to rule out that possibility. Another possibility is that one outcome emerges through interaction between the system being measured and the system doing the measuring--that they both participate. But that being the case, then it would seem to require something nonlocal to insure that Alice and Bob always get opposite results when they measure along the same axis.

According to QT nothing is predetermined but the interaction of the particle with the measurement apparatus leads to the measurement of the observable the apparatus is constructed for, and the outcome is just random, because this observable was not prepared to have a determined value. There's no "explanation" in QT, why the apparatus shows the very result of a single measurement. It only tells you what to expect in terms of probabilities, i.e., if you prepare and ensemble of particles in this state, you'll get a frequency of finding a specific value which converges (in the weak sense) to the probability according to Born's rule (provided QT is correct, and up to know there's no hint that it is not).

On the other hand QT tells you also precisely that there can be correlations between observables of quantum systems, that can be measured at far distant places, although the single observables are random (even with maximum uncertainty in the sense of information theory, i.e., at maximum entropy for this observable) as is described by the entanglement in EPR like situations (like the famous polarization-entangled biphotons in Aspect-type experiments).

Of course, it is always possible that QT is not the theory of everything and that one day there will be another more refined theory be discovered which contains QT as an approximation, but as long as we don't have such a more comprehensive theory, it's all wild speculation what may be "behind the probabilities" of QT. In my opinion, there's no chance to find such a more comprehenseive theory by philosophical speculations and "reinterpretations" of QT but if it exists, it will be found from a clear observation of deviations of real-world phenomena from the predictions of QT. If you look at the history of about 400 years of physics, that's an always repeated pattern: There are sometimes people trying to figure out things from pure speculation, but even the best of them fail because they lack necessary empirical input. Even Einstein was caught in such a trap for about the last 30 years of his scientific live, and even he couldn't solve the problem of finding a "unified field theory" explaining quantum phenomena by a classical theory!

 

[A. Neumaier]

Don't underestimate Izzy Junior.

That Gravity should be innate, inherent and essential to Matter, so that one body may act upon another at a distance thro' a Vacuum, without the Mediation of any thing else, by and through which their Action and Force may be conveyed from one to another, is to me so great an Absurdity that I believe no Man who has in philosophical Matters a competent Faculty of thinking can ever fall into it. [4]

— Isaac Newton, Letters to Bentley, 1692/3

The more direct reference is here. Interesting. Did this make it into the later editions of the Principia Mathematica? It might have been just a temporary doubt.

 

[stevendaryl]

In my opinion, there's no chance to find such a more comprehenseive theory by philosophical speculations and "reinterpretations" of QT but if it exists, it will be found from a clear observation of deviations of real-world phenomena from the predictions of QT. If you look at the history of about 400 years of physics, that's an always repeated pattern: There are sometimes people trying to figure out things from pure speculation, but even the best of them fail because they lack necessary empirical input. Even Einstein was caught in such a trap for about the last 30 years of his scientific live, and even he couldn't solve the problem of finding a "unified field theory" explaining quantum phenomena by a classical theory!

It seems to me that a lot of the advances in physics were not from new observations but new ways of understanding observations that were already known. Newton, in developing his laws of motion, for instance, didn't have any observations that weren't already known to Galileo. He didn't use new planetary data to develop his law of gravity (Tycho Brahe's observations that led to Kepler's laws of motion were about 80 years old). Einstein in developing Special Relativity really was not using new data, or at least he wasn't driven by new data--the problem, reconciling Maxwell's equations and Newton's laws of mechanics, was 40 years old. In developing General Relativity, Einstein was concerned that his new theory be empirically testable, but he wasn't influenced by empirical data--he was driven the conceptual problem of how to reconcile gravity with relativity.

So I don't agree, as a general principle, that it is impossible to make theoretical breakthroughs unless guided by experimental results. I think that at least as important is the need to come up with a new way of understanding what we already know.

 

[A. Neumaier]

So I don't agree, as a general principle, that it is impossible to make theoretical breakthroughs unless guided by experimental results. I think that at least as important is the need to come up with a new way of understanding what we already know.

The two aspects don't contradict each other. The experimental results may be old ones. Fruitless is only speculation unchecked (or even uncheckable) by the known experimental constraints.

 

[naima]

In the case of EPR with an electron/positron pair, if Alice and Bob measure the spin of their respective particle along the same axis, they always get the opposite result. As I said in another post, it's as if there were a pair of coins such that if they are both flipped, they always give opposite results, no matter how far away they are when flipped. In the case of coins, people would strongly suspect that the results must be predetermined. But in the case of entangled twin pairs, such a way out is incompatible with Bell's theorem (or at least, it's very difficult to understand how it is consistent with Bell's theorem).

You quote what i said, but you speak of something else.
May be you are not interested in the "WHEN" that occurs.
Please read again post 427

 

[Hornbein]

The more direct reference is here. Interesting. Did this make it into the later editions of the Principia Mathematica? It might have been just a temporary doubt.

It seems clear to me that he fully understood that his model could not be correct. He continued to use it because it gave (almost) correct results.
Newton had remarkable intuition. There are a number of prescient speculations in the Principia. He opines that matter and energy are essentially the same thing. But I can't find a reference easily.

 

[stevendaryl]

You quote what i said, but you speak of something else.
May be you are not interested in the "WHEN" that occurs.
Please read again post 427

I guess I didn't understand it. I don't see how erasing or neglecting details leads to the EPR results.

 

[naima]

I guess I didn't understand it. I don't see how erasing or neglecting details leads to the EPR results.

The key point of my answer is that it does not answer to your HOW question.
I highlight the fact that probabilities only become realities when details are lost.
When you consider one particle of an entangled pair you have to trace out (neglect) the details of the other in a local measurement. then you get some result.

 

[stevendaryl]

I highlight the fact that probabilities only become realities when details are lost.
When you consider one particle of an entangled pair you have to trace out (neglect) the details of the other in a local measurement. then you get some result.

I don't get that. When Bob measures the spin of his particle, he's just looking at whether the particle goes left or right. He's not performing a trace.