Undergrad Nature Physics on quantum foundations

[vanhees71]

Yes, but this does not contradict my claim. Standard relativistic microcausal QFT is one possible model of reality consistent with existing experiments, but it's not the only possible model of reality consistent with existing experiments.

I only argue within standard physics, not about speculative future theories. Of course, one can never exclude any possibility that such a new theory is needed in the future.

 

[vanhees71]

No, entanglement + Born rule implies the corresponding correlations. The origin of Born rule is controversial, hence the origin of correlations is controversial.

For me the Born rule is simply a postulate included in standard QT in the minimal interpretation. In this sense there's no "origin" of it at all (as there are no "origins" of any other of the formal postulates) other than the fact that it is empirically very successful.

 

[Demystifier]

What kind of influence can there be if you cannot find any!

What number can violate the Goldbach conjecture if you cannot find any!

 

[Demystifier]

For me the Born rule is simply a postulate included in standard QT in the minimal interpretation.

Which, of course, is controversial.

 

[gentzen]

How can there be a non-local action at a distance and no faster than light signaling at the same time?

I guess that the confusion arises more from the unclear meaning of "action at a distance" than from the word "non-local". If you imagine the randomness itself as being nonlocal, then no contradictions to "no faster than light signaling" arises, and you even get an argument why there must be randomness:

Nicolas Gisin’s short book Quantum Chance nicely explains how the paradox arises that quantum mechanics is local and nonlocal at the same time: The randomness itself is nonlocal, and it must be really random, because otherwise this non-locality could be used for instantaneous signal transmission.

Even so there may be "interpretational dances" that can avoid even "nonlocal randomness", the notion itself is intuitive and unproblematic.

 

[Demystifier]

I only argue within standard physics

Except when you don't, as my 3 examples illustrate.

 

[vanhees71]

Which, of course, is controversial.

Within some philosophers' community may be, but not within the physics community, right?

 

[martinbn]

What number can violate the Goldbach conjecture if you cannot find any!

If there is such a number it can be found at least in principle. The action of Alice measurement cannot be found even in principle.

 

[vanhees71]

I guess that the confusion arises more from the unclear meaning of "action at a distance" than from the word "non-local". If you imagine the randomness itself as being nonlocal, then no contradictions to "no faster than light signaling" arises, and you even get an argument why there must be randomness:Even so there may be "interpretational dances" that can avoid even "nonlocal randomness", the notion itself is intuitive and unproblematic.

This is simply resolve by saying that relativistic microcausal QFT is local but it allows correlations between far-distant parts of a quantum system (described by entanglement).

 

[Demystifier]

Within some philosophers' community may be, but not within the physics community, right?

Wrong. Many physicists, including you, do philosophy.

 

[vanhees71]

Except when you don't, as my 3 examples illustrate.

So what's "standard physics" with regard to QT? Copenhagen with collapse? de Broglie-Bohm? Many worlds?

 

[Demystifier]

The action of Alice measurement cannot be found even in principle.

No, it's not a matter of principle. It's only a FAPP phenomenon, very much like the 2nd and 3rd laws of thermodynamics.

 

[Demystifier]

So what's "standard physics" with regard to QT? Copenhagen with collapse?

Yes. You and me both dislike it, but it's standard.

 

[gentzen]

This is simply resolve by saying that relativistic microcausal QFT is local but it allows correlations between far-distant parts of a quantum system (described by entanglement).

I guess I understand what is meant by "relativistic microcausal QFT is local". I don't get what is meant by "it allows correlations between far-distant parts of a quantum system (described by entanglement)", and how it would give me a notion as intuitive and unproblematic as "nonlocal randomness". So I get a feeling like "maybe those are nice words, but what do you want to tell me with those words". Somehow it feels "too abstract" to me.

 

[martinbn]

No, it's not a matter of principle. It's only a FAPP phenomenon, very much like the 2nd and 3rd laws of thermodynamics.

Are you saying that in principle Bob could figure out whether Alice has done or not something with her particle?

 

[martinbn]

I guess I understand what is meant by "relativistic microcausal QFT is local". I don't get what is meant by "it allows correlations between far-distant parts of a quantum system (described by entanglement)", and how it would give me a notion as intuitive and unproblematic as "nonlocal randomness". So I get a feeling like "maybe those are nice words, but what do you want to tell me with those words". Somehow it feels "too abstract" to me.

I don't want to be the translator from @vanhees71 to English, but I think that non-local randomness is something he would agree is part of QFT.

 

[Demystifier]

Are you saying that in principle Bob could figure out whether Alice has done or not something with her particle?

Yes, according to some interpretations/theories. Example is Bohmian mechanics out of quantum equilibrium.

 

[Demystifier]

I don't want to be the translator from @vanhees71 to English, but I think that non-local randomness is something he would agree is part of QFT.

I don't think that he would agree that randomness is non-local.

 

[vanhees71]

I guess I understand what is meant by "relativistic microcausal QFT is local". I don't get what is meant by "it allows correlations between far-distant parts of a quantum system (described by entanglement)", and how it would give me a notion as intuitive and unproblematic as "nonlocal randomness". So I get a feeling like "maybe those are nice words, but what do you want to tell me with those words". Somehow it feels "too abstract" to me.

The correlations between far-distant parts of a quantum system (e.g., an entangled two-photon state with the photons measured at far-distant places) described by entangled states of course a consistent with relativistic microcausal QFT, and for me this implies that these long-ranged correlations are described by a local theory (QFT) and thus that what's violated in Bell's local realistic HV theories is "realism" (i.e., the assumption that all observables take determined values).

 

[vanhees71]

I don't want to be the translator from @vanhees71 to English, but I think that non-local randomness is something he would agree is part of QFT.

I don't know, what "non-local randomness" means though.

 

[martinbn]

Yes, according to some interpretations/theories. Example is Bohmian mechanics out of quantum equilibrium.

But according to QM (and current experiments) it is not possible. Of course you can find theories that have all kinds of interactions. Newtonian gravity has action at a distance.

 

[martinbn]

I don't know, what "non-local randomness" means though.

It just means randomness.

 

[martinbn]

I don't think that he would agree that randomness is non-local.

Non-local here is superfluous.

 

[PeterDonis]

The momentum's direction cannot be inferred from position of the flash, if the two branches of the wave function are made parallel after their split by the magnet

But they aren't in the standard SG experiment, which is what we're discussing. Nothing is done to the wave function between exiting the SG magnet and hitting the detector.

What the position of the flash is really correlated with is the position of the particle at the time of detection, not its momentum.

If we want to get really, strictly technical, yes, the position of the flash is a measurement of the position of the particle, which gets entangled with its momentum (which output beam it is in) by the detector, and the momentum gets entangled with the spin by the SG magnet. So strictly speaking there are two stages of deduction required to get from the observed position of the flash on the detector to the "measured" value of spin.

 

[gentzen]

I don't know, what "non-local randomness" means though.

"Not predetermined" random outcomes at spacelike separated (spacetime) points/events which are not independent.

The "not predetermined" is the important part of that notion, and also the part where some vagueness enters. Just because the outcome was not yet "fully" predetermined at any point in the intersection of the past lightcones of the points/events doesn't mean that it got determined exactly at the moment where the random outcomes became known and recorded.

But in the simplest form of the notion, one could imagine it indeed as if the random outcomes only got determined at the moment where they got recorded. The notion is unproblematic even in this "simple but unrealistic" form.

 

[haushofer]

Wave-particle duality is no phenomenon but a theoretical concept that's outdated for about 100 years.

I see this claim more often on PF, but why exactly? To me, the duality states that quantum objects show both particle and wave behaviour, which is captured in a new ontological category for the quantum object we call "quantum particle". This is how I've understood this concept for a long time. What's outdated about that? I'd say the concept has evolved, not that it's outdated.

Didn't read all the replies, sorry if this has been asked before.

 

[Demystifier]

But according to QM (and current experiments) it is not possible. Of course you can find theories that have all kinds of interactions. Newtonian gravity has action at a distance.

Of course. Let me remind you that we discuss examples that illustrate the difference between superluminal signalling and superluminal action. Standard QM is not a good example, which is why we discuss other examples.

 

[Demystifier]

I see this claim more often on PF, but why exactly? To me, the duality states that quantum objects show both particle and wave behaviour, which is captured in a new ontological category for the quantum object we call "quantum particle". This is how I've understood this concept for a long time. What's outdated about that? I'd say the concept has evolved, not that it's outdated.

Nothing is outdated about notion of quantum particle. What is outdated is that sometimes it behaves like a classical wave and sometimes like a classical particle.

 

[haushofer]

Nothing is outdated about notion of quantum particle. What is outdated is that sometimes it behaves like a classical wave and sometimes like a classical particle.

But what's wrong with that? In certain cases the wavefunction is sharply peaked, which means that the quantum particle exhibits "classical behaviour". But that doesn't make it a "classic particle".

Just because a sheep can be fluffy it doesn't mean it's a pillow; we just perceive that in that case (unshaved) it shows "pillow-like behaviour".

Maybe it's nomenclature, but the wave-particle duality is not a statement about the ontology of quantum particles, afaik. That's why I'm surprised by VanHees' adament statement.

But maybe this is off-topic.

 

[Lynch101]

The correlations between far-distant parts of a quantum system (e.g., an entangled two-photon state with the photons measured at far-distant places) described by entangled states of course a consistent with relativistic microcausal QFT, and for me this implies that these long-ranged correlations are described by a local theory (QFT) and thus that what's violated in Bell's local realistic HV theories is "realism" (i.e., the assumption that all observables take determined values).

RQ: Measurements result in definite, singular values, don't they?

Q1: What does this interpretation say happens, such that the measurements made on far-distant parts of a quantum system result in definite, singular values?

Q1A: Do the [far-distant] parts have definite values from the moment they leave the preparation device and along their travel towards the measurement devices of Alice and Bob, with the correlations being explained by virtue of their shared preparation?

Q1B: Or, is each part in a superposition i.e. don't have definite values as they leave the preparation device and travel towards the measurement devices?

If the answer to Q1B is yes, then Q1 remains to be answered.