What makes the interpretations of Quantum Mechanics so important?

[timmdeeg]

TL;DR

Provided it's correct that the interpretations of Quantum Mechanics can be neither proved nor disproved why then do researchers invest so much time and talent in this field?

How valid is the statement "It means physics is ultimately concerned with descriptions of the real world" in the realm of QM? Heretic question, what is "real" besides the outcome of the measurement?

 

[DarMM]

It's difficult to know exactly what Quantum Mechanics is saying about the world and in the standard formulation it doesn't directly "represent" the systems you apply it to but only the statistics of their effects on systems you control and don't model with quantum theory.

People spend a lot of time on it because many think you should have an understanding of systems in and of themselves.

The answers to your other questions are more philosophical and differ between people, but preferences ultimately motivate why people tackle the issues I mentioned above.

 

[Vanadium 50]

why then do researchers invest so much time and talent in this field?

Before we can tell you why something is true, we need to know if it is true.

Judging by publications, <1% of the work discussing QM is about interpretations. Maybe even 0.1%, but I am unwilling to look at thousands of publications to count.

 

[vanhees71]

The interpretation of a physical theory is just telling you how to use the formal math to derive predictions for the outcome of real-world observations and quantitative measurements. Thus, the minimal interpretation (aka shutup and calculation or orthodox interpretation), which just takes Born's rule, i.e., the probabilitist/statistical interpretation of the quantum state seriously, is all you need.

Everything else is, in my opinion, just not belonging to physics but to philosophy and thus may be interesting to think about and maybe it may help to understand QT (and particularly why the minimal interpretation is indeed all you need), but I doubt this a bit: It's rather confusing the meaning of QT as a physical theory than anything else.

For me the most relevant publications of the physical part of the "interpretation issue" are experimental quantum-optics/AMO papers, where the foundations are tested by real-world experiments, showing with overwhelming significance that the implications of minimally interpreted QT, particularly entanglement and the corresponding long-ranged correlations between far-distant parts of quantum systems, are the (so far only) correct description of how nature behaves, while deterministic local hidden-variable theories a la Bell are disproven through the violation of Bell's inequality.

 

[jtbell]

Judging by publications, >1% of the work discussing QM is about interpretations.

Did you mean <1%?

 

[Vanadium 50]

Did you mean <1%?

Fixed. Thanks!

 

[Michael Price]

Summary: Provided it's correct that the interpretations of Quantum Mechanics can be neither proved nor disproved why then do researchers invest so much time and talent in this field?

The founders of QM invested a lot of effort in trying to "understand" QM. With their failure to agree or reach a consensus the subject has become something of an anathema in mainstream professional literature. Which is understandable - since giants such as Einstein, Schrödinger, Heisenberg, De Broglie etc could not agree, why waste effort ourselves?

But, of course, physicists do want to understand QM, but they mostly conduct this effort in private. I know the challenge of understanding QM was the main reason I choose to study physics. In my sophomore year I settled on many worlds because, it seemed to me then (and still does), the only intepretation to yield clear, realist, unambiguous answers about reality, independent of observers. I'm sure a pilot wave advocate would say the same thing. (Not sure what a Copenhagenist would say..)

Physics is about finding a model of the world, not just a scheme that calculates the results of observations. When someone declares that all they require of physics is a calculational tool, I don't believe them, in the sense that if such an acceptable model was found, they would adopt it. The problem is that the models on offer all have some deficiencies:
1) pilot wave - pushes around the Bohm particle at FTL velocities, which many find anathema
2) many worlds - existence of parallel time lines, which many find anathema
3) copenhagen - denies reality between observations, which many find anathema.

Take your pick.

 

[Aidyan]

I think most of those, even though not all, working on the interpretations of QM hope to recover a classical world or at least a hidden variable and/or local theory which 'saves the appearances' and can be recast back into an intuitive understanding of reality that meets our human macroscopic perception and understanding. That is, an ontology that deals with by point-like particles moving in space-time along definite paths as our mind and senses are accustomed to think of, but QM seems to challenge. The De Broglie-Bohm and Many World Interpretation were quite successful in this sense. No entanglements, superpositions and strange 'spooky action at a distance' are necessary here, furnishing an ontology that dispenses with something our human understanding would like to avoid at any cost. Indeed, these show that, in principle, it is possible. If that has anything to do with reality remains to be shown. I think it hasn't and these attempts are more the sign of a psychological need than a scientific one and that reflect our human resistance to change. Such as the resistance to accept the heliocentric theory replacing the geocentric appearances.

 

[Michael Price]

That is, an ontology that deals with by point-like particles moving in space-time along definite paths as our mind and senses are accustomed to think of, but QM seems to challenge. The De Broglie-Bohm and Many World Interpretation were quite successful in this sense.

Many worlds does not have particles moving along definite trajectories. The wavefunction alone is deemed to correspond to reality, with no collapse postulate, just the unitary dynamics of time evolution. Since the time evolution equations are local and deterministic it follows that many worlds is also a local, deterministic theory.
https://en.m.wikipedia.org/wiki/Many-worlds_interpretation

 

[Aidyan]

Many worlds does not have particles moving along definite trajectories.

As far as I understand it has many definite (in the sense of deterministic) trajectories,

 

[Michael Price]

The splitting in many cases worlds is normally taken to occur at measurements or at any entropic event. If we use this criterion then the path of a particle through the double slit apparatus, for example, remains indefinite. The timeline splits when the particle is detected by a classical or macroscopic detector.

 

[Aidyan]

But, at the time of detection, doesn't it split in many timelines, for each possible projection of the state vector?

 

[PeterDonis]

As far as I understand it has many definite (in the sense of deterministic) trajectories

The MWI has definite "trajectories" for the entire quantum system in its configuration space. But that is not at all the same thing as definite trajectories for each individual subsystem, or as a definite trajectory for the system in ordinary 3-space. So, for example, say we have a rock that undergoes an experiment in which quantum uncertainty is involved (for example, the radioactive decay of an atom determines whether the rock is diverted to the left or the right). Then, according to the MWI, there is a definite trajectory in the configuration space of the overall wave function of the system consisting of rock + radioactive atom, but there is no definite trajectory in ordinary 3-dimensional space for the rock itself.

 

[Michael Price]

But, at the time of detection, doesn't it split in many timelines, for each possible projection of the state vector?

No, it splits into many timelines, one for each possible state of the measuring device or some other macroscopic variable. So the double slit experiment splits according to the granularity of the photographic screen. But not according to all the possible paths (in the sum-over-histories sense) through the apparatus.

 

[timmdeeg]

Thanks for your responses.

The MW seem to require that the wave function is ontic. Would ontic versus non-ontic in principle make a tiny (not measurable) difference regarding the gravitational field of the particle before measurement?

 

[bhobba]

No, it splits into many timelines, one for each possible state of the measuring device or some other macroscopic variable. So the double slit experiment splits according to the granularity of the photographic screen. But not according to all the possible paths (in the sum-over-histories sense) through the apparatus.

What a world is in MW's is not the same for everyone:


It does not necessarily split into many worlds, timelines etc - simply possible histories are treated on equal footing except for probabilities. This lead to the consistent/decoderent histories approach. But the point I am making is some like Gell-Mann think its basically the same as MW. Its one of the things that make QM interpretations a minefield. I may get some push-back saying this, but it's what Gell-Mann thought and probably others like Feynman and De-Witt.

Thanks
Bill

 

[Michael Price]

What a world is in MW's is not the same for everyone:

I've always found Gell-Mann annoyingly vague about the existential issues in 'his' version of MW.

But I agree, 'world' and 'split' mean what you want them to mean. The definitions I use (entropy release causes splitting) works for me because it makes world splitting an irreversible process. However they are just words; the core of Everett's conception is quite simple. The wavefunction represents reality and there is no collapse.

 

[DarMM]

But the point I am making is some like Gell-Mann think its basically the same as MW.

I might not be understanding your meaning correctly, but he does say "only one of which occurs" when discussing entanglement. The other difference to me is that in decoherent histories the experimenter must choose the family of histories of which one element occurs.

 

[bhobba]

I might not be understanding your meaning correctly, but he does say "only one of which occurs" when discussing entanglement. The other difference to me is that in decoherent histories the experimenter must choose the family of histories of which one element occurs.

Only one history occurs but before it occurs they all have the potential to occur - we only know the probability.

Thanks
Bill

 

[DarMM]

Only one history occurs but before it occurs they all have the potential to occur - we only know the probability.

Thanks
Bill

Again this might be dumb, but don't we usually say that only one event or history (i.e. sequences of events) has the potential to occur prior to their occurrence. What makes MWI different is precisely that they all occur. To me this seems unlike MWI.

Also would it not be more accurate to say prior to their occurrence and given a complete context all the histories in that context/family have the potential to occur. Without a selected context I don't think one can discuss their potential to occur.

 

[Michael Price]

Again this might be dumb, but don't we usually say that only one event or history (i.e. sequences of events) has the potential to occur prior to their occurrence. What makes MWI different is precisely that they all occur. To me this seems unlike MWI.

Also would it not be more accurate to say prior to their occurrence and given a complete context all the histories in that context/family have the potential to occur. Without a selected context I don't think one can discuss their potential to occur.

Yes, in MWI all histories have occurred and all futures occur. This the essence of MWI. Gell-Mann, it seems, was never a MWorldist.

 

[ftr]

No entanglements, superpositions and strange 'spooky action at a distance' are necessary here, furnishing an ontology that dispenses with something our human understanding would like to avoid at any cost. Indeed, these show that, in principle, it is possible. If that has anything to do with reality remains to be shown. I think it hasn't and these attempts are more the sign of a psychological need than a scientific one and that reflect our human resistance to change. Such as the resistance to accept the heliocentric theory replacing the geocentric appearances.

I am not really quoting you to argue against it, I just want to comment differently. I think the goal of interpretation is very limited in the sense that science unlike religion (nothing against it) believes there is a natural reason for everything. Even if we assume as QM seems to say that the particles behave in a probabilistic way (no reason for its behavior) with undefined attributes before measurement, i.e. it just the way it is and don't ask why , but some don't like such stand and equate it with magic or religion faith type (that is don't ask why). OR at least if it is that way, they want to know why.

 

[Dale]

What makes the interpretations of Quantum Mechanics so important?

Nothing.

 

[bhobba]

Again this might be dumb, but don't we usually say that only one event or history (i.e. sequences of events) has the potential to occur prior to their occurrence. What makes MWI different is precisely that they all occur. To me this seems unlike MWI.

In the version of WWI most hold to, and Gell-Mann reefers to as what some very scholarly people hold to, MW is totally deterministic because its just governed by Schrödinger's Equation. All worlds occur simultaneously. But Gell-Mann does not like that, so has his variant. It's really decoherent/consistent histories but he thinks its basically the same as MW. The point is not which one is correct, I am these days heading toward the idea what interpretation you believe in etc doesn't really matter. Progress will be made and things that seem unclear now will be clearer. Just look at the difference between modern interpretations and Copenhagen. However like nowadays we have discussions about decoherence, we will have discussions about something else. As I jokingly said - turtles all the way down, I don't actually think it will be like that but what eventually comes to light who knows - to some extent at the moment we are groping in the dark. The minimum I think to make progress will be getting a better version of Quantum Gravity than the merely effective field theory one we now have. Of course, while its not generally talked about in pop-sci accounts, its likely all our theories are just effective. That needs to be rectified.

BTW I must add this is just my view - nobody really knows how things will pan out - understanding interpretations better may be the key - science is funny like that just like math is funny in the most seemingly useless looking mathematical result can turn out to be crucial in applications.

Thanks
Bill

 

[bhobba]

Nothing.

It seems like that doesn't it. But as you know science is funny in that what seems useless can turn out to be critical. My views about interpretations is slowly evolving and I am coming around to the idea we need to make progress in resolving that all our theories are likely just effective eg what does EM being trivial tell us? But who really knows.

Thanks
Bill

 

[Quanundrum]

Quantum Interpretations can be important in facilitating novel concepts, such as quantum computing through Deutsch's work on Everett, or the Bell Inequalities from Bell's contemplation on his preferred interpretation of Pilot Waves. I would say that the greatest value we can derive from interpretations is simply thinking outside of the box.

However, dogmatism of ones preferred interpretation lead to nothing but fragmentation of the field, putting people in camps. This is not conducive.
The main problem with interpretations as I see it is that they are inherently personal and ambiguous. Take MWI as a clear example, in the last 15 years it has seen a massive wave of support and some go as far as to proclaim its superiority and borderline imposing it on people and dismissing opponents. If you dig a little deeper you will be hard pressed to find two MWI adherents that actually agree on the fundamentals of their preferred interpretation. How to resolve the Born Rule within that picture is the epitome of this and there is nowhere close to a consensus among its leading promoters, but also topics like preferred basis, ontology (Beables vs pure wavefunction), whether the wavefunction is real and exist in hilbert space versus it not being real but representational for beables etc. etc. Even basic concepts such as whether there are new worlds born from worlds splitting/branching or whether they all exist from start and later on bifuricate/diverge is something they rarely agree on.

 

[Michael Price]

Take MWI as a clear example, in the last 15 years it has seen a massive wave of support and some go as far as to proclaim its superiority and borderline imposing it on people and dismissing opponents. If you dig a little deeper you will be hard pressed to find two MWI adherents that actually agree on the fundamentals of their preferred interpretation.

I don't think this is true. The disagreement between MWI-ists is mainly one of expression and language. They agree about the mathematics. For years DeWitt (worlds) and Everett (relative states) were proclaimed as different theories. But with the publication of more biographical data it now clear there were no differences at all, and they were fully in agreement with each other.

One exception is David Deutsch's idea of universes /worlds / minds already being "pre-split" - but he is explicit that he has augmented Everett's pared down QM with an extra assumption.

And yes, there is some disagreement about how to recover the Born rule, but that is hardly a fundamental disagreement - more like a work in progress.

 

[Michael Price]

Heretic question, what is "real" besides the outcome of the measurement?

The wavefunction is real.

 

[A. Neumaier]

The wavefunction is real.

The wave function in the Schrödinger picture, in the Heisenberg picture, or in the interaction picture? (These are different wave functions!)

 

[Michael Price]

The wave function in the Schrödinger picture, in the Heisenberg picture, or in the interaction picture? (These are different wave functions!)

Since they are isomorphic they are all real. Just as in classical physics, B and 2B are both real.