Quantum mechanics is not weird, unless presented as such

[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.

 

[rubi]

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.

That's not true. Bell's theorem rules out non-contextual hidden variables. This is a critical assumption in the derivation of the inequality.

 

[stevendaryl]

That's not true. Bell's theorem rules out non-contextual hidden variables. This is a critical assumption in the derivation of the inequality.

Well, I'm not sure what the "non-contextual" adjective implies here. What would be an example of a contextual hidden-variables theory?

 

[rubi]

Well, I'm not sure what the "non-contextual" adjective implies here. What would be an example of a contextual hidden-variables theory?

Non-contextual means that the hidden variables can be modeled on a single joint probability space. One could call QM itself a contextual hidden variable theory.
This is a nice introduction: http://www.mdpi.com/1099-4300/10/2/19/pdf

 

[stevendaryl]

Non-contextual means that the hidden variables can be modeled on a single joint probability space. One could call QM itself a contextual hidden variable theory.
This is a nice introduction: http://www.mdpi.com/1099-4300/10/2/19/pdf

I've read such papers before (maybe that very paper), and it doesn't do a thing for me. I don't see how it contributes anything to the discussion of Bell's theorem. If Bell made an unwarranted assumption about the existence of a single joint probability space, so his proof of the nonexistence of hidden variables is incorrect, then I would like to see that loophole exploited by seeing an explicit hidden-variables model that reproduces the statistics of EPR.

 

[stevendaryl]

I've read such papers before (maybe that very paper), and it doesn't do a thing for me. I don't see how it contributes anything to the discussion of Bell's theorem. If Bell made an unwarranted assumption about the existence of a single joint probability space, so his proof of the nonexistence of hidden variables is incorrect, then I would like to see that loophole exploited by seeing an explicit hidden-variables model that reproduces the statistics of EPR.

I think I understand the idea behind "contextuality". Suppose that you have a source of coins that sends them spinning on edge toward you. When a coin reaches you, you slap it to the floor, and check whether it's "heads" or "tails". It might be a mistake to assume that there is a "hidden variable" in the coin that determines whether it ends up heads or tails. The act of "measurement" in this case creates the measurement result. If the slapping action were slightly different, you may have ended up with a different result.

On the other hand, if we had a pair of coins sent spinning in opposite directions, such that the measurement of coin always produced the opposite of the measurement of the other coin, then we would suspect that the details of the measurement act were irrelevant. So we would suspect that this anti-correlation was due to noncontextual hidden variables (to use the physics terminology). That's the case with EPR measurements (in the case of anti-correlated spin-1/2 particles), when Alice and Bob both measure spin relative to the same axis. The details of the entire measurement setup seem irrelevant, because if Alice gets spin-up, then regardless of the details of Bob's apparatus, he will get spin-down.