Many Worlds Interpretation existence

[martinbn]

I didn't thought that. The "now line" is also sometime pictured as a tiny wormhole between particle.

The way it was written you were talking about a now line that all observers agree on. This sounds like absolute simultaneity.

Now, why do you think QM could "hints that relativity might be wrong" ? Because despite the highly wrong terminology, entanglement is not about action nor distance.

I don't. I was relying to Dymistifiers post, where he said that some interpretations of QM have action at a distance.

 

[Boing3000]

The way it was written you were talking about a now line that all observers agree on. This sounds like absolute simultaneity.

It is. And as far as I know, it is compatible with relativity.

I don't. I was relying to Dymistifiers post, where he said that some interpretations of QM have action at a distance.

Indeed, but the way I understood those, some use relativity compatible objects (like tachyon), and other FTL pseudo particle (phonon like) emergent from a standard relativistic medium. I think none of them carried action (ie momentum) but would "trace out" correlation.

 

[Demystifier]

But the ontology is very strange, unsatisfying, probably even non sensible.

But in non-local BM, the ontology (namely particle positions) is local. What is strange about that?

 

[vanhees71]

To me this is not an interpretation, i think that Demystifier will agree. You may be happy working with a black box, but an interpretation is suppose to tell you what is inside the box. The unsettling thing is that Bohr may have been right and there is nothing inside.

What you call "black box" is the most successful and comprehensive model about the objectively observable part of Nature. Admittedly it's incomplete since we haven't been able to quantize the gravitational interaction, but you can't expect Nature to be as you like. She just behaves as she does, and as physicists we have to figure out how to describe this behavior as good as we can. You can't expect more from the natural sciences than the best possible description of phenomena that can be objectively quantified and observed.

 

[martinbn]

It is. And as far as I know, it is compatible with relativity.

How is absolute simultaneity compatible with relativity?!

 

[martinbn]

But in non-local BM, the ontology (namely particle positions) is local. What is strange about that?

That part is not strange, although it seems like a step back towards classical thinking. The strange bit is the action at a distance.

 

[martinbn]

What you call "black box" is the most successful and comprehensive model about the objectively observable part of Nature. Admittedly it's incomplete since we haven't been able to quantize the gravitational interaction, but you can't expect Nature to be as you like. She just behaves as she does, and as physicists we have to figure out how to describe this behavior as good as we can. You can't expect more from the natural sciences than the best possible description of phenomena that can be objectively quantified and observed.

But even without gravity it has problems. It has the measurement problem. QFT is mathematically not sound.

 

[Boing3000]

How is absolute simultaneity compatible with relativity?!

How is it not ?

 

[vanhees71]

But even without gravity it has problems. It has the measurement problem. QFT is mathematically not sound.

Hm, and I thought perturbative renormalized QFT makes a lot of sense (up to 12 digits or even more confirmation of predictions by experiment)!

 

[martinbn]

How is it not ?

Not sure how to respond to this. Given that relativity of simultaneity is one of the most famous things about relativity, if you claim the opposite you need to explain, not ask questions.

 

[martinbn]

Hm, and I thought perturbative renormalized QFT makes a lot of sense (up to 12 digits or even more confirmation of predictions by experiment)!

And yet there is no mathematically rigorous QFT in 3+1 dimensions.

 

[Boing3000]

Not sure how to respond to this. Given that relativity of simultaneity is one of the most famous things about relativity, if you claim the opposite you need to explain, not ask questions.

They are not opposite at all, one is the consequence of the other. I have explained what this "now line" is, and despite the ugly language, those absolute proper geodesics exist and define that "absolute proper simultaneity".

Experimenter are actually using them and try to measure it. I am pretty sure they use relativity quite extensively, especially to synchronize clocks.

Relativity allows us to define an absolute proper universe, whatever our particular relative perspective on it. And non-local entanglement is totally compatible with it. That's FTL that is not (hypothetical tachyon aside)

 

[Demystifier]

That part is not strange, although it seems like a step back towards classical thinking. The strange bit is the action at a distance.

I agree that action at a distance is strange. But to me, the known alternatives look even stranger. See Sec. 5.3 of my https://arxiv.org/abs/1703.08341

 

[Demystifier]

And yet there is no mathematically rigorous QFT in 3+1 dimensions.

I would say that QFT on 3+1 dimensional lattice is rigorous.

 

[name123]

Not sure how to respond to this. Given that relativity of simultaneity is one of the most famous things about relativity, if you claim the opposite you need to explain, not ask questions.

The concept of absolute simultaneity in reality is compatible with relativity (because Presentism is), but there would be no experimental means of establishing it (it would be a metaphysical concept). If simultaneity was given an experimental definition, it would be relative, and observers could disagree on what was simultaneous.

 

[PeterDonis]

How is it not ?

It's easy for people to think they disagree if they are using words to mean different things.

If "absolute simultaneity" means "there is a preferred notion of simultaneity built into the laws of physics", then that is incompatible with "relativity" if "relativity" means "the principle of relativity", i.e., "there is no preferred inertial frame built into the laws of physics".

But if "relativity" only means "we observe Lorentz invariance to the accuracy we can measure", then that is not necessarily incompatible with "absolute simultaneity", since one can always claim that the preferred notion of simultaneity is in principle unobservable. But doing that means you are talking about LET, which is a banned topic here on PF.

 

[LeandroMdO]

I would say that QFT on 3+1 dimensional lattice is rigorous.

It's worth pointing out that lattice theories are defined in Euclidean space, via the analogy between the path integral and the partition function from thermodynamics. For the most part, this means that such theories are not relativistic quantum lattice theories at all, but rather the classical statistical mechanics of discretized field theories. These theories are of course related to their quantum counterparts by a Wick rotation, which means a great deal of information about the continuum Minkowski theory is encoded in statistical properties of the classical ensembles, and this can be extracted if you know where to look for it. Whether or not this procedure is rigorous is a separate question, however.

That said, I don't see the relevance of the mathematical difficulties with rigorous quantum field theory for the present topic. Aside from the particular example you chose, the central message that QFT ought to make sense at least as an approximate effective theory for something more fundamental is essentially unimpeachable.

 

[vanhees71]

And yet there is no mathematically rigorous QFT in 3+1 dimensions.

There is no mathematically rigorous QFT beyond the level of an effective theory, and indeed it's pretty likely that at one energy scale the Standard Model fails, and one needs at least another effective theory which works there.

 

[Blue Scallop]

The preparation would determine the correlation between the two, but not the polarisation of B's photon.
Prior to measurement of A's photon, the polarisation of B's photon would be undetermined.
Upon measurement of the polarisation of A's photon the polarisation of B's photon would no longer be undetermined.

So how did the measurement at A's detector determine the polarisation of B's photon ?

vanhees71.. may I know what is your answer to the above question by name123?
I'm kinda confused about your position.. do you believe that in entanglement experiments like Clauser and Aspect experiments.. it's due to common causes like red and blue socks being determined from the origin? But in the experiments that violate Bell's Inequality, there were correlations. Or maybe you believed there were correlations but it was not "non-local" because there is no reality.. hence there is nothing to be local about (so spooky at distance is false)? (if so, then this is reasonable). Or do you indeed believe Bell's Inequality is violated due to some loophole that still proves it's like red and blue socks from the initial preparation (this was actually believed by Einstein (the EPR arguments) but disproven by Bell's Experiments that showed there were really correlations even if the entangled particles were light years apart).

 

[Boing3000]

If "absolute simultaneity" means "there is a preferred notion of simultaneity built into the laws of physics", then that is incompatible with "relativity" if "relativity" means "the principle of relativity", i.e., "there is no preferred inertial frame built into the laws of physics".

There is no "frame" involved here (and even less so a "preferred" one), and that maybe the reason of the confusion. "Absolute simultaneity" is defined by clocks once synchronized displaying the same time. Proper time is an absolute notion in relativity, and all frames agrees with that.

But if "relativity" only means "we observe Lorentz invariance to the accuracy we can measure", then that is not necessarily incompatible with "absolute simultaneity", since one can always claim that the preferred notion of simultaneity is in principle unobservable. But doing that means you are talking about LET, which is a banned topic here on PF.

I had to google LET to find out what you meant. And the "simultaneity" of entanglement correlation have nothing to do with LET (an obviously crackpot notion)

An more interesting case for experimentation would be if there are entangled properties that evolves with time. Spin orientation don't, but is there any that does ?

 

[martinbn]

To call this absolute simultaneity is very misleading. And what do you mean by clocks once synchronized? Once in what frame?

 

[Boing3000]

To call this absolute simultaneity is very misleading. And what do you mean by clocks once synchronized? Once in what frame?

Any "frame" of the one event called entanglement. The common start of the two world line.

 

[vanhees71]

vanhees71.. may I know what is your answer to the above question by name123?
I'm kinda confused about your position.. do you believe that in entanglement experiments like Clauser and Aspect experiments.. it's due to common causes like red and blue socks being determined from the origin? But in the experiments that violate Bell's Inequality, there were correlations. Or maybe you believed there were correlations but it was not "non-local" because there is no reality.. hence there is nothing to be local about (so spooky at distance is false)? (if so, then this is reasonable). Or do you indeed believe Bell's Inequality is violated due to some loophole that still proves it's like red and blue socks from the initial preparation (this was actually believed by Einstein (the EPR arguments) but disproven by Bell's Experiments that showed there were really correlations even if the entangled particles were light years apart).

This is a very good example for why I thik that quantum states are better interpreted in an epistemic sense and why the collapse hypothesis leads to problems with locality and causality.

My view is that the correlations, as all probabilistic relations of quantum systems, are described by the state of the system, and the state of the system is determined by preparation. The preparation in that case is when the entangled photon pair is created (e.g., by parametric downconversion by shooting a laser beam into a crystal). When A measures her photon's polarization state (her photon is defined by that it is registered by the detector at A's place), she immediately also knows the polarization state of B's photon (his photon is defined by that it is registered by the detector at B's place, which can be very far distant from A's detector). For B nothing has changed. He simply expects an unpolarized photon and gets with 50% probability the one or the other polarization when he measures it.

Let's now assume that A's detector is very close to the photon source, and B's very far, such that A measures her photon earlier than B. In other words, the measurement processes are assumed to happen as time-like separated events. Then "collapse" happens definitely at different times for A than for B: A changes the state of the photon pair due to her measurement result much earlier than B. Still, there is no contradiction by what's known to A and B concerning the outcome of their mesaurements. Both A's and B's photons are exactly unpolarized, i.e., the polarization state if maximally indetermined.

There's, of course, also no problem when the measurement processes are realized at space-like distances. Then you can always find a reference frame, where A and B register their result simultaneously or another reference frame, where A registers her result before B or again another frame, where B registers his result before A. Still there's no contradiction, because both, A and B always just find that their photons sent from the source of entangled photon pairs are precisely unpolarized.

When A and B compare their measurement protocols (always keeping detailed track about the time, when they registered their measurement outcome to be sure to relate always the pairs which where created together at the source), they find in any case the 100% correlations due to the preparation of the photon pairs in the polarization-entangled state.

Of course, here I made two assumptions: (a) the polarization measurements are local events as described by standard QED and thus the linked-cluster principle is valid, i.e., A's measurement cannot instantaneously affect B's photon and/or measurement apparatus (implied by microcausality) and (b) that all there is possible to be known about photons is what is described by quantum states, and since this is probabilistic knowledge (some may think only) it refers to ensembles of equally prepared quantum systems, i.e., the probabilistic information described by the prepared state can only be tested by collecting "enough statistics", i.e., using a sufficiently large ensemble.

The problems start, whenever you try to give more meaning to the quantum state then is implied by this minimal interpretation. Some think (in the past Einstein and Schrödinger were the most prominent physicists to do so) that this is not a complete description of nature since "in reality" (whatever "reality" means to them) all possible observables should have determined values always. It's not completely ruled out that maybe somebody one day finds some satisfactory theory, where this is the case, but Bell's work and the empirical precise findings with respect to it, imply that such a deterministic hidden-variable theory must be non-local, and so far there seems not to be a satisfactory such kind of theory in the relativistic realm.

 

[Blue Scallop]

This is a very good example for why I thik that quantum states are better interpreted in an epistemic sense and why the collapse hypothesis leads to problems with locality and causality.

My view is that the correlations, as all probabilistic relations of quantum systems, are described by the state of the system, and the state of the system is determined by preparation. The preparation in that case is when the entangled photon pair is created (e.g., by parametric downconversion by shooting a laser beam into a crystal). When A measures her photon's polarization state (her photon is defined by that it is registered by the detector at A's place), she immediately also knows the polarization state of B's photon (his photon is defined by that it is registered by the detector at B's place, which can be very far distant from A's detector). For B nothing has changed. He simply expects an unpolarized photon and gets with 50% probability the one or the other polarization when he measures it.

Let's now assume that A's detector is very close to the photon source, and B's very far, such that A measures her photon earlier than B. In other words, the measurement processes are assumed to happen as time-like separated events. Then "collapse" happens definitely at different times for A than for B: A changes the state of the photon pair due to her measurement result much earlier than B. Still, there is no contradiction by what's known to A and B concerning the outcome of their mesaurements. Both A's and B's photons are exactly unpolarized, i.e., the polarization state if maximally indetermined.

There's, of course, also no problem when the measurement processes are realized at space-like distances. Then you can always find a reference frame, where A and B register their result simultaneously or another reference frame, where A registers her result before B or again another frame, where B registers his result before A. Still there's no contradiction, because both, A and B always just find that their photons sent from the source of entangled photon pairs are precisely unpolarized.

When A and B compare their measurement protocols (always keeping detailed track about the time, when they registered their measurement outcome to be sure to relate always the pairs which where created together at the source), they find in any case the 100% correlations due to the preparation of the photon pairs in the polarization-entangled state.

Of course, here I made two assumptions: (a) the polarization measurements are local events as described by standard QED and thus the linked-cluster principle is valid, i.e., A's measurement cannot instantaneously affect B's photon and/or measurement apparatus (implied by microcausality) and (b) that all there is possible to be known about photons is what is described by quantum states, and since this is probabilistic knowledge (some may think only) it refers to ensembles of equally prepared quantum systems, i.e., the probabilistic information described by the prepared state can only be tested by collecting "enough statistics", i.e., using a sufficiently large ensemble.

The problems start, whenever you try to give more meaning to the quantum state then is implied by this minimal interpretation. Some think (in the past Einstein and Schrödinger were the most prominent physicists to do so) that this is not a complete description of nature since "in reality" (whatever "reality" means to them) all possible observables should have determined values always. It's not completely ruled out that maybe somebody one day finds some satisfactory theory, where this is the case, but Bell's work and the empirical precise findings with respect to it, imply that such a deterministic hidden-variable theory must be non-local, and so far there seems not to be a satisfactory such kind of theory in the relativistic realm.

Yes. Thanks for the details. Good to know at least you still believe in Bell's Theorem. I thought you were arguing in terms of Einstein EPR days where Einstein believed there was no correlations and there were hidden variables (akin to the red and blue socks being determined from beginning).

And yes. It's so difficult to visualize the wave function as ontic as this can violate relativity (except if one used the banned LET).

Whatever, the correlations in QM still work in QFT.. is it not.. what experiments in QFT (or QED) where there are long distant correlations like in the Aspect Experiments? Anyone knows?

Also remember the wave function in MWI lives not in physical space but in higher configuration space. And Demystifier calls it alocal...

 

[martinbn]

Any "frame" of the one event called entanglement. The common start of the two world line.

This is even less clear. What is a frame (in quotes) of one event?

 

[Boing3000]

This is even less clear. What is a frame (in quotes) of one event?

In any frame, there is only one event where entanglement start. I cannot be more clearer sorry.

 

[PeterDonis]

"Absolute simultaneity" is defined by clocks once synchronized displaying the same time.

When the clocks are spatially separated, how do you know they display "the same time"? Giving meaning to that statement requires a simultaneity convention, which is equivalent to choosing an inertial frame.

the "simultaneity" of entanglement correlation

What does this mean?

 

[PeterDonis]

In any frame, there is only one event where entanglement start

But then the entangled particles move apart, so they are spatially separated. See my previous post.

 

[Boing3000]

When the clocks are spatially separated, how do you know they display "the same time"?

Know ? I "know" it because I can compute any particle 4D path (and proper time along it), and pick the event pair whee both clock display the same time.

Giving meaning to that statement requires a simultaneity convention, which is equivalent to choosing an inertial frame.

No. The simultaneity of convention is when synchronized clock display the same time. There is no preferred frame involved here. Any frame will do.

What does this mean?

It means both measure can done at a perfectly valid absolute proper time. Simultaneity didn't disappear because different frame have a different perspective on some ordering. Events only happen once at one place, that's called causality.

 

[PeterDonis]

I "know" it because I can compute any particle 4D path (and proper time along it), and pick the event pair whee both clock display the same time.

Ah, ok. But then you have to deal with the twin paradox. You can separate your clocks and then bring them back together, and they won't read the same time--they will be sitting right next to each other, but reading different times. So which one is the "right" time--the time that is "absolute"?