Where does non-locality originate in dBB theory?

[LukeD]

Where does non-locality come from in dBB?

I've heard that when dealing with multiple particles, dBB is a non-local theory.
The standard knowledge from studying Bell's inequalities is that any hidden-variable theory must be either non-local or non-realist. I'm ok with non-realist theories, but non-local theories weird me out when I'm trying to describe physics that I think should be completely local.

So I'm wondering: What is dBB's description of a situation where non-locality shows up (I've heard that EPR is a good example)? In what sense is dBB "non-local"? Is there any way of interpreting non-locality in dBB as being due to local, but non-realist, effects?
Links to articles would be appreciated in lieu of or in addition to explanations.

--Sorry if this is answered clearly in another thread. I've been searching for the past few hours and haven't found a treatment of this.

 

[unusualname]

dBB type theories are an alternative to "non-realist" explanations. The whole point is that they are non-local by design.

The modern consensus is that some form of non-locality exists in nature at least at Planck-scale geometry, and for macroscopic non-local correlations in EPR etc the Holographic Principle is a possible mechanism.

But I don't think there's an accepted explanation for how the non-locality mechanism works in dBB theories, probably best for now to interpret the pilot wave as a descriptive idea rather than a "physical" non-local field.

 

[LukeD]

Ok, I did some more research.
It seems to me that the non-locality shows up when you try to make a 2nd order differential equation for the trajectories. The 1st order equation, however, seems to be completely local.
So the dynamics of dBB are completely local, but concepts like "force" do not seem to be.

 

[Galap]

Ok, I did some more research.
It seems to me that the non-locality shows up when you try to make a 2nd order differential equation for the trajectories. The 1st order equation, however, seems to be completely local.
So the dynamics of dBB are completely local, but concepts like "force" do not seem to be.

Yes. Exactly.

And as for nonlocality weirding you out, as humbling as it may seem, we must always remember that the way the universe is isn't based on what human beings do or to not find sensible or intuitive. in fact, it CAN'T be sensible or intuitive to us; our brains are extremely tiny compared to the entire system. If part of the universe could emulate perfectly the whole thing, information constraints would be violated and the rest of the universe would be redundant.

 

[Demystifier]

The 1st order equation, however, seems to be completely local.

That is wrong. In the 1st order form, the velocity of one particle depends on the instantaneous positions of all other particles. This is not local. And indeed, it must be nonlocal according to the Bell theorem, because it is a realist theory.

 

[Demystifier]

I'm ok with non-realist theories, but non-local theories weird me out when I'm trying to describe physics that I think should be completely local.

I never understood people who find non-realism less weird than non-locality.

After all, the good old high-school Newtonian gravity is nonlocal. When you learned about this theory the first time, did it really look so weird to you?

See also
http://xxx.lanl.gov/abs/quant-ph/0607057 [Foundations of Physics, Vol. 37 No. 3, 311-340 (March 2007)]

 

[LukeD]

Edit: Actually, I guess to summarize my question: Doesn't the velocity only depend on the local wavefunction? I thought non-localities only showed up when you tried to take the wave away. The wave propagates so you clearly can't take it away without non-locality. Is there another reason for non-locality though?
Below is a method of... i guess bringing the wave back into dBB? I don't know that it's local though.

-----

I never understood people who find non-realism less weird than non-locality.

Well.. standard Quantum Mechanics is already pretty "non-realistic", and it's local. To me adding more non-realistic elements is just like adding an unobservable field or wave to your theory, and these exist all over the place in Physics, so I have no problem with it.
Non-Locality, on the other hand, just doesn't seem useful. You have many more options for calculating a local law than a non-local law (for instance, it is easier to parallelize the calculations)

That is wrong. In the 1st order form, the velocity of one particle depends on the instantaneous positions of all other particles. This is not local. And indeed, it must be nonlocal according to the Bell theorem, because it is a realist theory.

Ok.. but now let's add a non-realistic element ala the Quantum Trajectory Method.
In QTM, we do our calculations with the whole set of Bohmian Trajectories at once. We use |\psi|^2 as a distribution of particle positions (rather than having just 1 point), and we have a velocity field. The velocity field is derived from an action field S that follows the usual differential equation from dBB:
\frac{\partial S(\mathbf{x},t)}{\partial t} = -\left[ V + \frac{1}{2m}(\nabla S(\mathbf{x},t))^2 -\frac{\hbar ^2}{2m} \frac{\nabla ^2R(\mathbf{x},t)}{R(\mathbf{x},t)} \right]
where R is |\psi| (square root of the distribution of positions)

The velocity field is then calculated as \nabla S /m, and the distribution of particle positions updates via the velocity field.
The wave function is then R*e^(iS/hbar)

--

This seems completely local. Am I wrong that QTM is a local, non-realistic formulation of dBB? I know that the velocity in dBB is supposedly non-local in the multi-particle case, but I just don't see where any non-locality would come into these formulas.

 

[LukeD]

Oh, I just remembered:

Newtonian Gravity is local. Poisson's Equation is local and it contains the full dynamics of gravity. The Newtonian field can even be quantized with spin-0 (Schroedinger) particles! I don't think that could be done if its dynamics weren't local (but the particles are massless and travel at infinite speed, so I might be wrong about that)

 

[Demystifier]

Doesn't the velocity only depend on the local wavefunction?

No!
If you know ONLY the wave function and the position of ONE particle, you CANNOT calculate the velocity of that particle. Instead, you must also know the positions of all other particles.

 

[Demystifier]

Newtonian Gravity is local. Poisson's Equation is local and it contains the full dynamics of gravity.

No!
Poisson equation is local, but acceleration of ONE particle CANNOT be calculated by knowing ONLY the solution of the of the Poisson equation and position of that particle. Instead, you must also know the positions of all other particles.

 

[Demystifier]

LukeD, perhaps you are confused by the difference between 1-particle case and many-particle case. The 1-particle Bohmian mechanics is indeed local. However, the many-particle Bohmian mechanics is not local, provided that the wave function is such that there is entanglement between the particles. Similarly, Newtonian gravity is local for the 1-particle case, but not for the many-particle case.

 

[unusualname]

If Newtonian Gravity was local it wouldn't even predict the stability of planetary orbits. Newtonian Gravity is (in)famous for being precisely non-local.

Gravity in General Relativity however is local, and propagates at c, you need quite difficult calculations to explain why this (hardly) effects the planetary orbits.

http://math.ucr.edu/home/baez/physics/Relativity/GR/grav_speed.html

(QFT is local by design, so a graviton inspired field theory of gravity will also be local)

 

[Demystifier]

The simplest answer to the question above is:
From the fact that the many-particle wave function depends on many particle positions at the same time.

This means that QM is, in a certain formal sense, intrinsically nonlocal even without dBB. See also
http://xxx.lanl.gov/abs/quant-ph/0703071
for an elaboration of that view.

 

[Demystifier]

(QFT is local by design, so a graviton inspired field theory of gravity will also be local)

Here you (like many others) are failing to distinguish field operators from quantum states. The field operators are indeed local in QFT, but nonlocality (more precisely, nonlocal correlations) is a property of quantum states. The nonlocality of many-particle states is related to the fact that QFT contains also NONLOCAL OPERATORS, which are certain PRODUCTS of many local field operators.

Or loosely speaking:
(LOCAL X LOCAL) + (LOCAL X LOCAL) = NONLOCAL
That's how nonlocal entanglement emerges from otherwise local quantum theory, such as QFT.

 

[unusualname]

Here you (like many others) are failing to distinguish field operators from quantum states. The field operators are indeed local in QFT, but nonlocality (more precisely, nonlocal correlations) is a property of quantum states. The nonlocality of many-particle states is related to the fact that QFT contains also NONLOCAL OPERATORS, which are certain PRODUCTS of many local field operators.

Or loosely speaking:
(LOCAL X LOCAL) + (LOCAL X LOCAL) = NONLOCAL
That's how nonlocal entanglement emerges from otherwise local quantum theory, such as QFT.

correlation functions in QFT are purely mathematical devices, they don't imply any physical non-locality. In a QFT of gravity, gravity will propagate at speed c.

 

[Demystifier]

correlation functions in QFT are purely mathematical devices, they don't imply any physical non-locality.

They DO imply (or are related to) nonlocal EPR correlations, which, by the way, are MEASURED. If it is not physical for you, then you have a very unusual definition of the word "physical".

But perhaps it should not be surprising that someone who calls himself unusualname uses unusualmeanings of the words. :-)

 

[unusualname]

They DO imply (or are related to) nonlocal EPR correlations, which, by the way, are MEASURED. If it is not physical for you, then you have a very unusual definition of the word "physical".

But perhaps it should not be surprising that someone who calls himself unusualname uses unusualmeanings of the words. :-)

Yes they do imply nonlocal correlations, (by "correlation function" I mean propagator which is a mathematical device to calculate probability amplitudes) but the correlations are purely probabilistic results. I don't think many people believe a physical non-local field mechanism is responsible for the correlations, a la dBB.

But obviously, no one really knows the physical basis behind the correlations, so it is possible that the mathematics are describing a physical non-local effect. (Personally I think it's more likely that the reality we are observing is a reconstruction or projection of the simple smooth mathematical space upon which the field theory is constructed, and the non-locality results from the reconstruction or projection)

 

[Demystifier]

I don't think many people believe a physical non-local field mechanism is responsible for the correlations, a la dBB.

That is true. However, the number of people who believe that reality does not exist before we measure it - is also small. Yet, the Bell theorem shows that at least one of these two weird options (neither of which is believed by many people) should be true. So the fact that not many people believe in any of these two options is merely a consequence of their ignorance. If they were aware of the meaning of the Bell theorem, the number of supporters of both of the two options would be much larger.

 

[inflector]

That is true. However, the number of people who believe that reality does not exist before we measure it - is also small. Yet, the Bell theorem shows that at least one of these two weird options (neither of which is believed by many people) should be true. So the fact that not many people believe in any of these two options is merely a consequence of their ignorance. If they were aware of the meaning of the Bell theorem, the number of supporters of both of the two options would be much larger.

One needs to make a choice in the matter, once one understands the implications of the state of the experiments.

I too, find non-locality to be more to my liking than non-reality. That's why I believe that dBB is the way forward.

 

[zonde]

That is true. However, the number of people who believe that reality does not exist before we measure it - is also small. Yet, the Bell theorem shows that at least one of these two weird options (neither of which is believed by many people) should be true. So the fact that not many people believe in any of these two options is merely a consequence of their ignorance. If they were aware of the meaning of the Bell theorem, the number of supporters of both of the two options would be much larger.

I wanted to ask if you consider this definition as belonging to option "reality does not exist before we measure it"?
If two observables are non-commuting then at least one of them can not be unambiguously defined as property of single particle.

 

[Demystifier]

I wanted to ask if you consider this definition as belonging to option "reality does not exist before we measure it"?
If two observables are non-commuting then at least one of them can not be unambiguously defined as property of single particle.

Yes, I agree with that. This is essentially the content of the Kochen-Specker theorem. That's why Bohmian mechanics has "preferred" observables (particle positions).

 

[PhilDSP]

Non-locality may be intrinsically present even in Maxwell's Equations - especially the original ones written and interpreted by Maxwell himself. Both Fitzpatrick and Heavyside objected to Maxwell's invocation of non-locality and recast the interpretation and usage of the equations to reflect what we see today. They were apparently the original relativists. Heavyside can be quoted as saying "Maxwell was only 50% Maxwellian"

If the Maxwell Equations, in either their original form or the "modern" relativistic form are intrinsically non-local then any quantum theory based on their usage would seem to require non-local observations or measurements also, wouldn't it?

PS. The non-locality occurs in the determination of the Vector Potential.

 

[DevilsAvocado]

After all, the good old high-school Newtonian gravity is nonlocal. When you learned about this theory the first time, did it really look so weird to you?

Newtonian Gravity is local. Poisson's Equation is local and it contains the full dynamics of gravity.

No!
Poisson equation is local, but acceleration of ONE particle CANNOT be calculated by knowing ONLY the solution of the of the Poisson equation and position of that particle. Instead, you must also know the positions of all other particles.

If Newtonian Gravity was local it wouldn't even predict the stability of planetary orbits. Newtonian Gravity is (in)famous for being precisely non-local.

Gravity in General Relativity however is local, and propagates at c, you need quite difficult calculations to explain why this (hardly) effects the planetary orbits.

??

What am I missing...? I always thought that anomalies in the theory of Newtonian gravity were discovered already in 1859?? That the ellipse of Mercury’s orbit was rotating slightly faster than predicted? And this problem was finally solved by General Relativity in 1915? And in 2002 http://en.wikipedia.org/wiki/Sergei_Kopeikin" ?


@Demystifier:

If we "must also know the positions of all other particles", does this include particles outside the http://en.wikipedia.org/wiki/Observable_universe" (47 billion light-years)?

If yes: If the universe is infinite, and the dBB "gravity interaction" is instantaneous without spatial limitations, wouldn’t that mean that every particle in the universe is influenced by infinite gravitation??

How does instantaneous dBB avoid "paradoxes" of http://en.wikipedia.org/wiki/Relativity_of_simultaneity" ?

I never understood people who find non-realism less weird than non-locality.

Agree! I have 'developed' my own "Stay-Sane-Little-Layman" methodology: If one assumes that the world is crazy and non-local, one should also assume that this crazy world does not exist, i.e. non-realism... to avoid all the "fuss", so to speak... also known as the "Don't Worry, Be Happy" method.

(Bell compatible)

 

[PhilDSP]

And in 2002 http://en.wikipedia.org/wiki/Sergei_Kopeikin" ?

While the popularizing sites and magazines certainly make wonderful science fiction reading in that they pander to the sensational and conveniently skip the caveats, they've probably done a great dis-service in radically distorting the issues and certainties involved with new and even very old experimental discoveries and theory.

This sounds like a more reasonable, level-headed assessment of the situation in the experiment:

http://www.aps.org/publications/apsnews/200306/gravity.cfm

 

[Demystifier]

What am I missing...?

You are missing the fact that we are discussing intrinsic properties of one particular THEORY (Newtonian gravity), not the properties of the actual world.

The motivation for discussing such a theory is the analogy with another theory (Bohmian mechanics) which MIGHT have something to do with the actual world.

 

[Demystifier]

If the universe is infinite, and the dBB "gravity interaction" is instantaneous without spatial limitations, wouldn’t that mean that every particle in the universe is influenced by infinite gravitation??

Yes. But due to quantum equilibrium and decoherence, it cannot be observed.

How does instantaneous dBB avoid "paradoxes" of http://en.wikipedia.org/wiki/Relativity_of_simultaneity" ?

See
https://www.physicsforums.com/showpost.php?p=2784237&postcount=131
and the posts preceding it.

 

[DevilsAvocado]

While the popularizing sites and magazines certainly make wonderful science fiction reading in that they pander to the sensational and conveniently skip the caveats, they've probably done a great dis-service in radically distorting the issues and certainties involved with new and even very old experimental discoveries and theory.

This sounds like a more reasonable, level-headed assessment of the situation in the experiment:

http://www.aps.org/publications/apsn...06/gravity.cfm

Hehe, I absolutely agree, we do not want to "conveniently skip the caveats".

http://en.wikipedia.org/wiki/Speed_of_gravity#Possible_experimental_measurements"

In September 2002, Sergei Kopeikin and Edward Fomalont announced that they had made an indirect measurement of the speed of gravity, using their data from VLBI measurement of the retarded position of Jupiter on its orbit during Jupiter's transit across the line-of-sight of the bright radio source quasar QSO J0842+1835. Kopeikin and Fomalont concluded that the speed of gravity is between 0.8 and 1.2 times the speed of light, which would be fully consistent with the theoretical prediction of general relativity that the speed of gravity is exactly the same as the speed of light.

Several physicists, including Clifford M. Will and Steve Carlip, have criticized these claims on the grounds that they have allegedly misinterpreted the results of their measurements. Notably, prior to the actual transit, Hideki Asada in a paper to the Astrophysical Journal Letters theorized that the proposed experiment was essentially a roundabout confirmation of the speed of light instead of the speed of gravity.[16] Further, there has been criticism of the means by which these results were presented, in that they were announced at a meeting of the http://en.wikipedia.org/wiki/American_Astronomical_Society" instead of being submitted for peer review.[17] However, Kopeikin and Fomalont continue to vigorously argue their case and the means of presenting their result at the press-conference of AAS that was offered after the peer review of the results of the Jovian experiment had been done by the experts of the AAS scientific organizing committee. Asada's claim was found theoretically unsound and disproved in later publication by Kopeikin and Fomalont [18], which operates with a bi-metric formalism that splits the space-time null cone in two - one for gravity and another one for light. The two null cones overlap in general relativity, which makes tracking the speed-of-gravity effects difficult and requires a special mathematical technique of gravitational retarded potentials, which was worked out by Kopeikin and co-authors [19][20] but was never properly employed by Asada and/or the other critics.

It is important to understand that none of the participants in this controversy are claiming that general relativity is "wrong". Rather, the debate concerns whether or not Kopeikin and Fomalont have really provided yet another verification of one of its fundamental predictions.

Ooh, I almost forgot, here are the peer reviewed papers on this "wonderful science fiction":

http://arxiv.org/abs/astro-ph/0302294"
E. B. Fomalont, S. M. Kopeikin
(Submitted on 14 Feb 2003 (v1), last revised 11 Jul 2003 (this version, v2))
Journal reference: Astrophys.J. 598 (2003) 704-711

http://arxiv.org/abs/astro-ph/0301145"
Clifford M. Will
(Submitted on 9 Jan 2003 (v1), last revised 6 Mar 2003 (this version, v2))
Journal reference: Astrophys.J. 590 (2003) 683-690

http://arxiv.org/abs/astro-ph/0308343"
Hideki Asada
(Submitted on 20 Aug 2003)

http://arxiv.org/abs/astro-ph/0311063"
Sergei M. Kopeikin (Univ. of Missouri-Columbia, USA), Edward B. Fomalont (NRAO, USA)
(Submitted on 4 Nov 2003 (v1), last revised 27 Mar 2006 (this version, v6))
Journal reference: Found.Phys. 36 (2006) 1244-1285

We describe our explicit Lorentz-invariant solution of the Einstein and null geodesic equations for the deflection experiment of 2002 September 8 when a massive moving body, Jupiter, passed within 3.7' of a line-of-sight to a distant quasar. We develop a general relativistic framework which shows that our measurement of the retarded position of a moving light-ray deflecting body (Jupiter) by making use of the gravitational time delay of quasar's radio wave is equivalent to comparison of the relativistic laws of the Lorentz transformation for gravity and light. Because, according to Einstein, the Lorentz transformation of gravity field variables must depend on a fundamental speed $c$, its measurement through the retarded position of Jupiter in the gravitational time delay allows us to study the causal nature of gravity and to set an upper limit on the speed of propagation of gravity in the near zone of the solar system as contrasted to the speed of the radio waves. We discuss the misconceptions which have inhibited the acceptance of this interpretation of the experiment. We also comment on other interpretations of this experiment by Asada, Will, Samuel, Pascual-Sanchez, and Carlip and show that their `speed of light' interpretations confuse the Lorentz transformation for gravity with that for light, and the fundamental speed of gravity with the physical speed of light from the quasar.​

 

[DevilsAvocado]

You are missing the fact that we are discussing intrinsic properties of one particular THEORY (Newtonian gravity), not the properties of the actual world.

The motivation for discussing such a theory is the analogy with another theory (Bohmian mechanics) which MIGHT have something to do with the actual world.

Okay, I understand. I just thought that stone dead "Newtonian nonlocality" was maybe a little "odd" in justifying "not so weird"...

 

[DevilsAvocado]

Yes. But due to quantum equilibrium and decoherence, it cannot be observed.

Okay, fair enough. But how do you "filter out" gravitation from the http://en.wikipedia.org/wiki/Local_Group" (10 million light-years in diameter), from gravitation from (hypothetical) galaxy group XYZ314, 1000 billion light-years from earth?

See
https://www.physicsforums.com/showpos...&postcount=131
and the posts preceding it.

Well... maybe "foliation-like structures" and "joint parametrization" are 'just' above my head...

But let’s take the classical and very simple example of a speeding train car. A is onboard and B is standing on the platform:

From the frame of reference of A, the light will reach the front and back of the train car at the same time.

From the frame of reference of B, the light will strike the back of the train car before it reaches the front.​

Is there a solution to this in dBB, that can be explained to a layman like me...??

 

[Demystifier]

Is there a solution to this in dBB, that can be explained to a layman like me...??

dBB is not very useful for this case because these are macroscopic objects on which all quantum effects (including nonlocal correlations) are negligible. Classical theory of relativity is sufficient for that purpose, but I have no intention to teach you this subject here.

 

[DevilsAvocado]

dBB is not very useful for this case because these are macroscopic objects on which all quantum effects (including nonlocal correlations) are negligible. Classical theory of relativity is sufficient for that purpose, but I have no intention to teach you this subject here.

Okay? How about replacing the "red spot" in t = 0 with a laser and a BBO crystal for spontaneous parametric down-conversion, and place measuring polarizers in the front & back of the train car, to implement a "Speeding EPR-Bell experiment"? Would that also be considered "negligible"?

Honestly, isn’t the incompatibility between non-relativistic QM and Special Relativity a BIG problem? According to John Bell:

"Those paradoxes are simply disposed of by the 1952 theory of Bohm, leaving as the question, the question of Lorentz invariance. So one of my missions in life is to get people to see that if they want to talk about the problems of quantum mechanics — the real problems of quantum mechanics — they must be talking about Lorentz invariance."​

When I looked around, I found this ("toy model" according to SEP):

http://arxiv.org/abs/quant-ph/9801070"

Hypersurface Bohm-Dirac models
Authors: D. Duerr, S. Goldstein, K. Muench-Berndl, N. Zanghi
Journal reference: Phys.Rev. A60 (1999) 2729-2736

Abstract: We define a class of Lorentz invariant Bohmian quantum models for N entangled but noninteracting Dirac particles. Lorentz invariance is achieved for these models through the incorporation of an additional dynamical space-time structure provided by a foliation of space-time. These models can be regarded as the extension of Bohm's model for N Dirac particles, corresponding to the foliation into the equal-time hyperplanes for a distinguished Lorentz frame, to more general foliations. As with Bohm's model, there exists for these models an equivariant measure on the leaves of the foliation. This makes possible a simple statistical analysis of position correlations analogous to the equilibrium analysis for (the nonrelativistic) Bohmian mechanics.

Is this problem really solved in dBB??

 

[Demystifier]

Honestly, isn’t the incompatibility between non-relativistic QM and Special Relativity a BIG problem?

Is this problem really solved in dBB??

In my opinion, it is solved:
http://xxx.lanl.gov/abs/1002.3226 [to appear in Int. J. Quantum Inf.]
If you have any objections or questions on THAT solution, I would be happy to respond on them.

 

[Demystifier]

How about replacing the "red spot" in t = 0 with a laser and a BBO crystal for spontaneous parametric down-conversion, and place measuring polarizers in the front & back of the train car, to implement a "Speeding EPR-Bell experiment"?

Do you actually ask "What is the Lorentz frame with respect to which the Bohmian force is instantaneous?". If THAT is your question, then the answer in the relativistic-covariant version of dBB is simple: If you repeat the experiment many times, then each time it will probably be ANOTHER Lorentz frame, because it (the Lorentz frame) is determined by initial conditions on Bohmian spacetime positions, which are usually different in each repeat of the experiment.

If you wanted to ask something else, then it would help to spell the question more precisely.

 

[DevilsAvocado]

In my opinion, it is solved:
http://xxx.lanl.gov/abs/1002.3226 [to appear in Int. J. Quantum Inf.]
If you have any objections or questions on THAT solution, I would be happy to respond on them.

THANKS! This is very interesting. I just love that Nikolic has a large conceptual section, explaining the main ideas. This is what I’ve been looking for! :!)

I must read carefully and get back ASAP. Thanks again.

 

[DevilsAvocado]

In my opinion, it is solved:
http://xxx.lanl.gov/abs/1002.3226 [to appear in Int. J. Quantum Inf.]
If you have any objections or questions on THAT solution, I would be happy to respond on them.

Hrvoje Nikolic is a really smart guy, and I like what he has written, for example http://arxiv.org/abs/quant-ph/0609163" ?

Please correct me if I’m wrong, but I find this a little 'peculiar':

1) Take the laws of physics seriously!
...
By 1) I mean that everything, including the human brain, obeys the physical laws. They
will turn out to be deterministic laws, which excludes the existence of free will.

To me this is the same as http://en.wikipedia.org/wiki/Hard_determinism" .

And afaict Hard determinism is equal to Superdeterminism, and this is a well known loophole and theoretical escape route from Bell's theorem. Bell discussed superdeterminism in a BBC interview:​

There is a way to escape the inference of superluminal speeds and spooky action at a distance. But it involves absolute determinism in the universe, the complete absence of free will. Suppose the world is super-deterministic, with not just inanimate nature running on behind-the-scenes clockwork, but with our behavior, including our belief that we are free to choose to do one experiment rather than another, absolutely predetermined, including the "decision" by the experimenter to carry out one set of measurements rather than another, the difficulty disappears. There is no need for a faster than light signal to tell particle A what measurement has been carried out on particle B, because the universe, including particle A, already "knows" what that measurement, and its outcome, will be.

The only alternative to quantum probabilities, superpositions of states, collapse of the wave function, and spooky action at a distance, is that everything is superdetermined. For me it is a dilemma. I think it is a deep dilemma, and the resolution of it will not be trivial; it will require a substantial change in the way we look at things.

Afaict, all of Nikolic’s reasoning becomes redundant if we accept Superdeterminism?

This is also somewhat strange:

R: Actually, it is quite natural. Humans are macroscopic beings who perceive the
world in terms of classical phenomena. There is a lot of evidence, especially from the
theory of decoherence [7, 8], that macroscopic classical physics emerges from the funda-
mental microscopic quantum physics. By assumption, superluminal signals are inherently
quantum phenomena responsible for nonlocal correlations between entangled particles, so
it is quite natural to expect that their effect cannot be seen at the classical macroscopic
level at which decoherence effectively destroys the quantum correlations.

What is Nikolic saying? Is Alice incapable of reading the measurement of her polarizer? I don’t understand?? Or does he mean that that entangled photons exchange superluminal signals, but this is somehow "delayed" to non-superluminal before Alice can actually read the measurement...??​

I’ll get back to you on post #33.

 

[inflector]

I too have found Demystifier (Hrvoje Nikolic) to be a really smart guy. His posts on dBB have been instrumental in my personal belief that dBB is the most logical interpretation of QM.

What is Nikolic saying? Is Alice incapable of reading the measurement of her polarizer? I don’t understand?? Or does he mean that that entangled photons exchange superluminal signals, but this is somehow "delayed" to non-superluminal before Alice can actually read the measurement...??

I always envision it being something like a mini wormhole that develops around entangled photons so that they maintain a superluminal connection and that a change in one results in a superluminal change in the other because of this link. I imagine the process of creation of entangled pairs creates a superluminal bridge between the entangled photons and that the process of decoherence destroys this bridge (i.e. there is a mini wormhole collapse).

NOTE: This is just an analogy I use to try to understand what might possibly be going on, NOT a specific proposal or theory.

If this analogy is off base, please let me know as I'm trying to understand dBB at a deeper level over time.

 

[DevilsAvocado]

I too have found Demystifier (Hrvoje Nikolic) to be a really smart guy.

Jeez! Demystifier = Hrvoje Nikolic !?!?
PF never stops to surprise me. What a wonderful place for discussions and learning! (Time to pay some contributions... )

His posts on dBB have been instrumental in my personal belief that dBB is the most logical interpretation of QM.

I haven’t decided what to "believe in". I’m open for anything that "works".

I always envision it being something like a mini wormhole that develops around entangled photons so that they maintain a superluminal connection and that a change in one results in a superluminal change in the other because of this link. I imagine the process of creation of entangled pairs creates a superluminal bridge between the entangled photons and that the process of decoherence destroys this bridge (i.e. there is a mini wormhole collapse).

NOTE: This is just an analogy I use to try to understand what might possibly be going on, NOT a specific proposal or theory.

It this analogy is off base, please let me know as I'm trying to understand dBB at a deeper level over time.

I think I understand what you’re saying. You are making an analogy for the mechanism.

The thing that "bothers" me is maybe not the "mechanism", but the contradictions between QM/SR, microscopic/macroscopic, causality/FTL, etc. Or to put it in short – If we try to talk about these things, it just doesn’t make sense.

If we can trust our senses, this is what happens:

• A laser source produces entangled pairs of photons through a BBO crystal. This is a very ineffective process and only one out of 10⁶ photons converts into two entangled photons, one in a million.
  
  
• There measuring polarizers are separated by 20 km, and it takes light 66 microseconds (10^-6) to travel 20 km (in vacuum) from Alice to Bob.
  
  
• The total time for electronic and optical processes in the path of each photon at the detector is calculated to be approximately 100 nanoseconds (10^-9).
  
  
• The settings of the polarizers at Alice & Bob are independently and randomly chosen every 100 nanosecond (10^-9).

As you can see there is no way for Alice & Bob to exchange information about their random polarizer settings, at the speed of light, as they are outside each other light-cone and there is no time for communication.

Yes, the photon is a microscopic object, and entanglement is only possible in the QM world. But, these "phenomena" create a macroscopic manifestation in the polarizer measurement.

According to Special Relativity, depending on which frame of reference you are, you will see Alice first performing her measurement and thereby decohere the shared wavefunction, and decide what Bob will measure. In another frame of reference you will see the opposite, Bob will decide what Alice will measure. And in a third frame of reference, all will be simultaneous.

This doesn’t make sense, it doesn’t work...

 

[Demystifier]

Afaict, all of Nikolic’s reasoning becomes redundant if we accept Superdeterminism?

If I understand you correctly, you reason in the following way. With superdeterminism, we can avoid nonlocality. So why do we deal with a nonlocal superdeterministic theory, such as Bohmian mechanics?
The answer is simple: Because it is MUCH EASIER to construct a nonlocal (Bohmian) superdeterministic theory that agrees with predictions of QM, than a local one. There are people who struggle with construction of an explicit local superdeterministic theory (like 't Hooft), but it is much more difficult to do it, even if possible in principle. Or to quote from (my) paper:
"R: I'm glad that you asked it, because the most remarkable part of the theory is the fact that it follows from some rather simple and natural principles."

What is Nikolic saying? Is Alice incapable of reading the measurement of her polarizer? I don’t understand?? Or does he mean that that entangled photons exchange superluminal signals, but this is somehow "delayed" to non-superluminal before Alice can actually read the measurement...??

Alice, of course, is capable of reading the measurement of her polarizer, but here the point is that she (as well as Bob) cannot CONTROL the reading of her/his measurement apparatus, in the sense that they cannot make the apparatus to be in the state they WANT. For that reason, they do not interpret nonlocal correlations as true exchange of information.

See, however, a way to (apparently) avoid this problem as well, leading to a possibility to use entanglement for an (apparent) superluminal communication:
http://xxx.lanl.gov/abs/1006.0338

 

[Demystifier]

As you can see there is no way for Alice & Bob to exchange information about their random polarizer settings, at the speed of light, as they are outside each other light-cone and there is no time for communication.

Actually, as I mentioned in the previous post, there is a way to do it:
http://xxx.lanl.gov/abs/1006.0338

 

[Demystifier]

J
According to Special Relativity, depending on which frame of reference you are, you will see Alice first performing her measurement and thereby decohere the shared wavefunction, and decide what Bob will measure. In another frame of reference you will see the opposite, Bob will decide what Alice will measure. And in a third frame of reference, all will be simultaneous.

This doesn’t make sense, it doesn’t work...

See #33, and a quote from the paper:
R: "... However, due to the superluminal influences, 'prior' does not always need to mean 'at an earlier time'."

The following quotes may also help:

On nonrelativistic BM:
No one can understand this theory until he is willing to think of psi as a real objective field rather than just a 'probability amplitude'.
John S. Bell

On relativistic BM:
No one can understand this theory until he is willing to think of x as a position in a 4-dimensional space, rather than just a collection of two conceptually different entities: 3-space position and 'time'.
H.N.

 

[ThomasT]

Because it is MUCH EASIER to construct a nonlocal (Bohmian) superdeterministic theory that agrees with predictions of QM, than a local one.

Well of course it is. Because there's no physical mechanism involved. But do you really want to call this physics??

Nikolic, while you have definitely clarified (demystified) some things for me, I still don't understand your apparent adherence to the notion of physical nonlocality. I mean, it makes no sense to me. Everything we know suggests that our universe is evolving deterministically in accordance with local causal relativistic principles, and yet you cling to this strange, fringe, interpretation of reality. Why?

 

[Demystifier]

Everything we know suggests that our universe is evolving deterministically in accordance with local causal relativistic principles, ...

Except nonlocal EPR correlations, of course.

...and yet you cling to this strange, fringe, interpretation of reality. Why?

Because I don't know any local interpretation of reality compatible with QM. Do you?

 

[ThomasT]

Except nonlocal EPR correlations, of course.

EPR correlations don't suggest nonlocality. They suggest a common cause. In fact, EPR states are quite amenable to a local explanation. If you're talking about the spectrum of results in Bell tests regarding non-EPR states then I suggest that you look at the experiments more closely. The discrepancies between certain LR models and results are minimal. The results suggest an explanation via standard, local, optics.

Because I don't know any local interpretation of reality compatible with QM. Do you?

If you want to call certain formal aspects of qm nonlocal, fine. So do I. But what does it have to do with reality?

Why not just accept the qm formalism as a probabilistic accounting of experimental preparations and leave it at that?

 

[Demystifier]

Why not just accept the qm formalism as a probabilistic accounting of experimental preparations and leave it at that?

Because such interpretation says nothing about REALITY (where, by reality, I mean properties of the system existing even when they are NOT measured).

In other words, I think in the following way.
IF I assume:
1. QM
2. reality (in the sense above)
3. simplicity
THEN I obtain Bohmian mechanics, which implies nonlocality.

 

[Demystifier]

EPR correlations don't suggest nonlocality. They suggest a common cause.

The same facts may suggest (but not prove, of course) different, even mutually exclusive, possibilities. So I would say that EPR correlations suggest (but not prove) BOTH nonlocality and common cause.

 

[Demystifier]

In other words, I think in the following way.
IF I assume:
1. QM
2. reality (in the sense above)
3. simplicity
THEN I obtain Bohmian mechanics, which implies nonlocality.

Let me also briefly explain how the reasoning above refutes some alternatives:

Copenhagen/instrumental QM:
Satisfies 1. and 3., but not 2.

Objective collapse theories:
Satisfies 1. and 2., but not 3.

Many worlds:
Definitely satisfies 2. However, as long as it satisfies 3., it does not satisfy 1. (cannot explain the QM Born rule). Alternatively, if it is modified such that it satisfies 1., it does not longer satisfy 3.

 

[ThomasT]

Why not just accept the qm formalism as a probabilistic accounting of experimental preparations and leave it at that?

Because such interpretation says nothing about REALITY (where, by reality, I mean properties of the system existing even when they are NOT measured).

In other words, I think in the following way.
IF I assume:
1. QM
2. reality (in the sense above)
3. simplicity
THEN I obtain Bohmian mechanics, which implies nonlocality.

First, Demystifier, let me say that I have of course known for quite some time that you are Nikolic the theoretical physicist. I have read your blogs and many of your posts and been enlightened by them. So, I thank you for that. Now, as to my current apparent confusion. Don't take it too seriously. I certainly don't. After all, if the world really is nonlocal and Bohmian mechanics really is correct, it hasn't seemed to matter too much.

Now to your points. Yes of course standard qm formalism says nothing, definitively for certain, about reality. That's the point. Anything you infer from it about an underlying reality is metaphysical speculation. Yes I agree that Bohmian mechanics is a simplistic representation of reality. And, I don't consider it realistic (in the sense above). It's just an easy way to 'account' for entanglement correlations.

On the other hand, certain aspects of Bohm's conceptualization do appeal to me. For example, the idea of particle and guiding wave is more than interesting. It makes conceptual sense to me. And, apparently, it makes conceptual sense to some rather significant physicists as well.

EPR correlations don't suggest nonlocality. They suggest a common cause.

The same facts may suggest (but not prove, of course) different, even mutually exclusive, possibilities. So I would say that EPR correlations suggest (but not prove) BOTH nonlocality and common cause.

I disagree. EPR states suggest, and are easily explainable via, a common cause. Non EPR states, but Bell-states producing entanglement stats, still suggest a common cause. Nonlocality is an ad hoc 'explanation' for what seems to be a local, but not so easily describable, phenomenon.

And, I don't think you've refuted the Copenhagen Interpretation -- at least my understanding of it as the minimalist instrumental/probabilistic interpretation that is the de facto standard of modern physics.

 

[Demystifier]

And, I don't think you've refuted the Copenhagen Interpretation

Of course I didn't, nor it was my intention to do so. I have only explained how the ASSUMPTION of 1. 2. and 3. refutes Copenhagen interpretation (CI) . To really refute CI, I would need to prove that 1. 2. and 3. are NECESSARY (not merely desirable), which of course I can't.

 

[ThomasT]

Ok, thanks Demystifier. I'm not going to intrude on your thread any more. At least not until I have something specific to say about the formalism of dBB. I'm sure you'd rather be talking to physicists and post-grads or whatever. Anyway, thanks for taking the time to respond. It's always a thrill for laymen like me to get to actually talk to physicists.

I'm eating breakfast now. Where are you? Italy? Romania? You're somewhere over there, right? What do you eat for lunch? Sorry for the off topic, but all I can think about now is food.

 

[Demystifier]

EPR states suggest, and are easily explainable via, a common cause.

I strongly disagree that they are EASILY explainable via a common cause (without nonlocality). Namely, to EXPLAIN it, it is not sufficient to say that there is SOME common cause. Instead, an explanation requires a detailed theory which quantitatively predicts the results of all experiments. No such local realistic theory is known, so even if such theory can be constructed, it is NOT EASY to do it.