I What is the true nature of reality according to quantum mechanics?

[deepalakshmi]

TL;DR Summary

What is scientific realism?

I read in an article that "Quantum physics is a highly mathematical theory that describes the nature of reality at the atomic and subatomic level". I also read another article that says quantum physics does not tell anything about reality. Can you give me some context about it in a way that is easy to understand?

 

[Dale]

Reality and realism are primarily philosophical terms, not scientific terms. Many different philosophies are consistent with the observed facts, often with rather divergent views of what is real.

 

[Hornbein]

Summary:: What is scientific realism?

I read in an article that "Quantum physics is a highly mathematical theory that describes the nature of reality at the atomic and subatomic level". I also read another article that says quantum physics does not tell anything about reality. Can you give me some context about it in a way that is easy to understand?

Quantum physical theories are mathematics for predicting what minuscule elementary particles will do. It has nothing to do with everyday social "reality" and is related but distantly to what everyday objects will do.

Some, including at least one Nobel Prize winner, think QM is closely related to consciousness. I don't think so, but neither hold I a Nobel Prize. Some think that QM shows that everything is created by human will. I guess someone who disagreed with that wrote "does not tell anything about reality." Also, ultimately QM doesn't tell you why it's predictions are true. It's just math that matches with observations.

 

[PeroK]

Summary:: What is scientific realism?

I read in an article that "Quantum physics is a highly mathematical theory that describes the nature of reality at the atomic and subatomic level". I also read another article that says quantum physics does not tell anything about reality. Can you give me some context about it in a way that is easy to understand?

If the universe was very simple, then we might not have much doubt about what is meant by "real". That said, if the universe was very simple, then there would not be complex organisms like humans to study it. But, the universe is not simple.

Likewise, when we talk about most everyday, macroscopic events we have a concept of "reality". If you use physics to study aeronautics, say, then you may have the "reality" that a component failed, an aircraft crashed and people died.

However, and I think it was Heisenberg who first realized this, atomic and subatomic processes may only be describable in mathematical terms. The interaction of a proton and an electron to form hydrogen cannot be adequately described in terms that reflect our everyday understanding of reality.

I think this leads to two seemingly opposite statements:

1) QM does not say anything about reality. It is only a mathematical description of interactions.

2) What QM says is reality.

The only real difference I can see is how you define reality. Personally, as @Dale suggests, I would leave "reality" as an undefined term in physics and leave it to philosophy. And I would focus on physics as providing a mathematical description of physical phenomena - whether that's galaxy formation or atomic spectroscopy.

 

[fresh_42]

Summary:: What is scientific realism?

I read in an article that "Quantum physics is a highly mathematical theory that describes the nature of reality at the atomic and subatomic level".

This sounds to me like an artificial distinction. Classical physics is also described by mathematics. The only difference is that Newton's law of gravity is easier to grasp than a sum of irreducible representations of certain tangent spaces of certain continuous symmetry groups of certain differential equations. To call the latter highly mathematical and the former real is rather arbitrary. 20,000 years ago we would have both called highly mathematical. Maybe we only have to wait another 20,000 years.

 

[DennisN]

I also read another article that says quantum physics does not tell anything about reality.

"Reality" can be a tricky word, as others have noted above, but I would argue that the sentence above is too harsh.

Quantum physics tells us that nature/reality obeys the laws of quantum mechanics. As an example there most definitely are distinct lines in atomic spectra (see e.g. 1 and 2).

 

[Lord Jestocost]

"Reality" can be a tricky word, as others have noted above, ...

A viable definition might be the following:

“By ‘reality’ I refer to that which exists or is assumed to exist, without making any claims concerning the character of this existence. I understand existence as the capacity to have effects upon the world with which we interact.”

Arkady Plotnitsky in “Niels Bohr and the Philosophy of Physics: Twenty-First-Century Perspectives” (edited by Jan Faye and Henry J. Folse):

 

[vanhees71]

This sounds to me like an artificial distinction. Classical physics is also described by mathematics. The only difference is that Newton's law of gravity is easier to grasp than a sum of irreducible representations of certain tangent spaces of certain continuous symmetry groups of certain differential equations. To call the latter highly mathematical and the former real is rather arbitrary. 20,000 years ago we would have both called highly mathematical. Maybe we only have to wait another 20,000 years.

Well, if you look at Newtonian physics from the (imho simplifying and clarifying) point of view of symmetry principles, it's not so much different.

 

[fresh_42]

A viable definition might be the following:

“By ‘reality’ I refer to that which exists or is assumed to exist, without making any claims concerning the character of this existence. I understand existence as the capacity to have effects upon the world with which we interact.”

Arkady Plotnitsky in “Niels Bohr and the Philosophy of Physics: Twenty-First-Century Perspectives” (edited by Jan Faye and Henry J. Folse):

Way too religious! I think scientists are better off with Plato. It basically means the same, but hasn't this religious touch of or is assumed to exist.

 

[PeroK]

Way too religious!

Religious?

 

[fresh_42]

Religious?

Yes, because it simply includes what Seneca, Pascal, Descartes, and Leibniz tried to reason without a holistic definition.

 

[Lord Jestocost]

Way too religious!

My god, where is now your problem?
In the case of physics, it is matter, which is usually assumed to exist independently of our interactions with it.

 

[PeroK]

Yes, because it simply includes what Seneca, Pascal, Descartes, and Leibniz tried to reason without a holistic definition.

Your religious education must have been significantly different from mine. Seneca, Pascal, Descartes and Leibnitz are not quite the New Testament gospels.

 

[Dale]

Please let’s drop the religion topic here and now so that the thread may remain open

 

[CelHolo]

The whole "realism" stuff is basically a shorthand for the fact that Quantum Theory doesn't seem to describe systems outside of preparation and measurement unlike classical physics. You get into difficulty in Quantum Theory if you try to claim that the observables we measure have values independently of measurements. See results like the Kochen-Specker theorem or the Hardy paradox for basic examples.

 

[votingmachine]

Summary:: What is scientific realism?

I read in an article that "Quantum physics is a highly mathematical theory that describes the nature of reality at the atomic and subatomic level". I also read another article that says quantum physics does not tell anything about reality. Can you give me some context about it in a way that is easy to understand?

We intuitively think in terms of analogies to the physical world we observe. We see objects, like rocks and balls. We see waves on the ocean.

QM deals with a lot of things that are very small, and are not analogous to the macroscopic things we have personal experience with. And are kind of weird. That means that we can't just use our normal tool of understanding, similarities to prior things.

JJ Thomson proposed a model of the atom which he likened to plum pudding. The negative electrons represented the raisins in the pudding and the dough contained the positive charge.

Ernest Rutherford proposed a system consisting of a small, dense nucleus surrounded by orbiting electrons—similar to the structure of the Solar System.

It is quite nice to have a mental image that reflects ordinary things we already understand. QM really misses the boat on a LOT of that. Things are just incredibly weird, when compared to ordinary macroscopic things. And some things that attempt to be analogous, such as "spin" really don't quite match.

https://www.scientificamerican.com/article/what-exactly-is-the-spin/

So in one sense, the math doesn't tell you about "reality". But in another sense, it tells you about reality, it just doesn't make ordinary sense.

 

[EPR]

It just tells you that everything is not made of particles, but of fields. Lay people got fooled because appearances can often be deceptive and their "making sense" of everything couldn't be further from the truth.

 

[Ian J Miller]

I think it is too easy to dismiss "realism" and say QM is merely mathematical. First, surely we cannot dismiss there is a cause for various effects? If there is a cause, there must be something we could consider as real, even if we do not understand it, and our failure to understand surely cannot be a guiding rule for physics. Further, if we take expectation values, following Ehrenfest's theorem, it is surely as real as classical physics. As for QM being solely mathematical, if so, it would be "merely conveying information" and if that is the case, it would violate Pusey's "no go theorem" which argues if the state is merely information it must contradict QM (See Pusey et al. (2012) Nature Phys. 8: 475 ).

 

[PeterDonis]

Moderator's note: Moved thread to QM interpretations forum and adjusted title to better reflect the thread topic.

 

[AlexCaledin]

Perhaps it's good to consider (according to Feynman) the QM to be a merely extension of the classical Least Action principle, showing how the small particles action functions are participating in the classical trajectory calculation? Then, the QM applicability is a property of the undoubtedly real classical reality.

 

[vanhees71]

We intuitively think in terms of analogies to the physical world we observe. We see objects, like rocks and balls. We see waves on the ocean.

QM deals with a lot of things that are very small, and are not analogous to the macroscopic things we have personal experience with. And are kind of weird. That means that we can't just use our normal tool of understanding, similarities to prior things.

JJ Thomson proposed a model of the atom which he likened to plum pudding. The negative electrons represented the raisins in the pudding and the dough contained the positive charge.

Ernest Rutherford proposed a system consisting of a small, dense nucleus surrounded by orbiting electrons—similar to the structure of the Solar System.

It is quite nice to have a mental image that reflects ordinary things we already understand. QM really misses the boat on a LOT of that. Things are just incredibly weird, when compared to ordinary macroscopic things. And some things that attempt to be analogous, such as "spin" really don't quite match.

https://www.scientificamerican.com/article/what-exactly-is-the-spin/

So in one sense, the math doesn't tell you about "reality". But in another sense, it tells you about reality, it just doesn't make ordinary sense.

It's the opposite! Classical physics doen't tell us about reality in many aspects. E.g., with Rutherford's model of the atom, based on his famous gold-foil experiment, the conclusion was that most of the mass of an atom must be concentrated in a very small positively charged nucleus around which the electrons move like the planets around the Sun, but this cannot be, because charged accelerated particles radiate off electromagnetic radiation, and according to Larmor's formula the electrons would crash into the nucleus in a very short time, which is in complete contradiction to the stability of matter around us. Only modern quantum theory explains why there are static configurations of the electrons around the nucleus. In addition this theory also explains with high accuracy the discrete line spectra of the atoms, another fact that cannot be explained in any way by classical physics.

 

[votingmachine]

It's the opposite! Classical physics doen't tell us about reality in many aspects. E.g., with Rutherford's model of the atom, based on his famous gold-foil experiment, the conclusion was that most of the mass of an atom must be concentrated in a very small positively charged nucleus around which the electrons move like the planets around the Sun, but this cannot be, because charged accelerated particles radiate off electromagnetic radiation, and according to Larmor's formula the electrons would crash into the nucleus in a very short time, which is in complete contradiction to the stability of matter around us. Only modern quantum theory explains why there are static configurations of the electrons around the nucleus. In addition this theory also explains with high accuracy the discrete line spectra of the atoms, another fact that cannot be explained in any way by classical physics.

Yes ... the point I was making was that humans often seek to understand things with analogies and generalizations from prior experience. The QM atom IS a better descriptor of reality than the Rutherford atom.

I think the fundamental basis for the contradiction in the original comment (QM tells you about reality/QM tells you nothing about reality) is that QM does tell about reality, but does so in non-intuitive ways, that don't fit our expectations that we generalize from macroscopic experiences.

There are many threads started with the fundamental proposition that QM things should fit our intuition better.

I am not saying that QM is 100% non-intuitive. But often people want the math to be explainable in words that make "common sense". Learning classical physics is often enlightening. QM is sometimes inexplicable rules.

A Rutherford model of the atom is obviously incorrect. But the thought process was clear. Given data that tells you most of the microscopic volume is non-interfering, but some acts like a veritable solid ... an intuitive way to order things in your model is with a nucleus, and space around it where the electrons go, LIKE a solar system. (Which clearly has a problem with radiating energy and crashing).

Maybe I am giving too much of intuition and analogies ... the contradiction of this thread my be an entirely different one.

 

[vanhees71]

Surely intuition is important to find new theories, as this example paradigmatically shows. Only 2 years after Rutherford's discovery Bohr came with his ad-hoc "solution" of the problem by just declaring some classical trajectories as "non-radiating" by his quantization conditions. Then Sommerfeld gave a complete mathematical formulation of this theory, but then the trouble started: It turned out to work for hydrogen as far as the spectral lines were concerned. By a mathematical curiosity, due to the high symmetry of the "Kepler problem", Sommerfeld could even get the fine structure right by using the Bohr quantization condition to the relativistic equations of motion, even without introducing spin, which at this time was indeed apparently not necessary.

However, the next logical step, applying the theory to other atoms than hydrogen, the Bohr theory failed. Alreadhy the next more complicated He atom didn't fit. What also didn't fit was the Zeeman effect except for the part of it than called the "normal Zeeman effect", and it was considered "normal" only, because it seemed to work with the quasiclassical methods of old quantum theory. The socalled anomalous Zeeman effect and also the related finding of the Stern Gerlach effect (with only two instead of the expected three lines) was again explained by some handwaving. That's another mathematical curiosity, because there were made two mistakes as we know today: 1st if the Stern Gerlach result were due to orbital angular momentum, indeed there should be at least 3 lines in the splitting of the silver-atom beam. 2nd nobody had an idea of spin 1/2 and a gyro factor of about 2 of the electron, although the latter was in principle known around this time. Famously Einstein and de Haas missed the opportunity to measure the gyro factor 2 in their famous experiment named after them, because Einstein persuaded de Haas not to consider the hints of a gyro factor larger than 1 in the quantitatively not so unique results, but it was not much later corrected by Barnett who found the correct factor of around 2.

At the end both the SG effect and the "normal and anomalous" Zeeman effect(s) was of course completely resolved by the correct quantum-mechanical theory of angular momentum including spin with the gyro factor of 2 resulting from the Dirac equation.

 

[physika]

So in one sense, the math doesn't tell you about "reality"

I agree, the case of QM with/without hilbert space.

...

 

[Sunil]

As for QM being solely mathematical, if so, it would be "merely conveying information" and if that is the case, it would violate Pusey's "no go theorem" which argues if the state is merely information it must contradict QM (See Pusey et al. (2012) Nature Phys. 8: 475 ).

The PBR theorem is wrongly named, because psi-ontology is a misguiding name.

First of all, it does not even claim to cover purely epistemic interpretations. It presupposes realism, and claims only that the wave function should be part of the reality. And even this is misguided, because they confuse reality of the whole universe with the reality of the observed quantum system.

An epistemic interpretation also fulfills the "psi-ontology" condition, because the state is defined by the preparation procedure, and the facts about the preparation - the measurement device and its pointer position - is obviously something real, and if two wave functions are different, they will have to be prepared by different such real measurement devices or having now different pointer position. Thus, the wave function is described by real objects. A triviality - if one does not confuse the ontology of the particular system studied with the ontology of the whole universe.

The PBR theorem contains nothing which would be able to make this difference. So they prove a triviality (the preparation devices and preparation measurement results are real and differ for different wave functions) and suggest something else (the hidden variables of the system should contain the wave function).

For an interpretation with a completely epistemic wave function but real trajectories see

Caticha, A. (2011). Entropic Dynamics, Time and Quantum Theory, J. Phys. A 44 , 225303, arxiv:1005.2357

 

[Morbert]

An epistemic interpretation also fulfills the "psi-ontology" condition, because the state is defined by the preparation procedure, and the facts about the preparation - the measurement device and its pointer position - is obviously something real, and if two wave functions are different, they will have to be prepared by different such real measurement devices or having now different pointer position.

Does a wavefunction imply a unique preparation procedure or measurement procedure though?

 

[Sunil]

Does a wavefunction imply a unique preparation procedure or measurement procedure though?

Not necessarily. Many different preparation procedures can define the same wave function. For the non-overlap condition we need only the reverse: That the same preparation procedure gives the same wave function. An overlap would be a preparation procedure which prepares two different wave functions for the same system.

(Note also that there will be a lot of wave functions which cannot be prepared by any preparation procedure.)

 

[Morbert]

Not necessarily. Many different preparation procedures can define the same wave function. For the non-overlap condition we need only the reverse: That the same preparation procedure gives the same wave function. An overlap would be a preparation procedure which prepares two different wave functions for the same system.

(Note also that there will be a lot of wave functions which cannot be prepared by any preparation procedure.)

I think I understand. If we only consider the microscopic system being measured, then a wavefunction $\psi$ fixes a property associated with the projector $|\psi\rangle\langle\psi|$, but not the other way around, since a wavefunction not orthogonal to $\psi$ could still assign some nonzero probability to the property fixed by $\psi$ .

But if we consider a Hilbert space that includes the actually existing lab where the preparation takes place, then the wavefunction $\psi$ fixes the property $|\psi\rangle\langle\psi|\Pi_{L_\psi}$, where $\Pi_{L_\psi}$ is the projector associated with any preparation of $\psi$. This time $\psi$ is fixed by $\Pi_{L_\psi}$, since the preparation of any other wavefunction $\phi$ fixes a property $|\phi\rangle\langle\phi|\Pi_{L_\phi}$ which is orthogonal to $|\psi\rangle\langle\psi|\Pi_{L_\phi}$

tl;dr By considering the entire lab and associating preparation procedures with projectors, we can define probability distributions over procedures that don't overlap and associate them with our original wavefunctions.

 

[dx]

Bohr was deeply convinced about his view of quantum mechanics based on the idea of complementarity, but he was prepared for the possibility that Einstein might be right in some sense. As he liked to emphasise, even cultures and personalities can be complementary, and "the opposite of a deep truth may also be in some sense a deep truth" Bohr's personality was I guess in some sense complementary to Einstein's personality.

The discussions, to which I have often reverted in my thoughts, added to all my admiration for Einstein a deep impression of his detached attitude. His favoured use of picturesque phrases such as "ghost waves guiding the photons" showed a profound humour behind his piercing remarks. For such endeavours of seeking the proper balance between seriousness and humour, Einstein's own personality stands as a great example, and when expressing my belief that logical order to a large extent allows us to avoid deep truth, I hope it will be taken in his spirit

 

[vanhees71]

Indeed, while Einstein always wrote texts in clear prose, Bohr always expressed himself in a pretty fuzzy way, and of course Einstein was right in critisizing some parts and flavors of the Copenhagen interpretation, particularly those flavors which include a quantum-classical cut and an instaneous collapse. Of course Einstein was wrong in his assumption that there's a local hidden-variable theory in agreement with QT. This has only been clarified with Bell's theoretical work and all the successful experimental "Bell tests", with results all in agreement with QT and not with local deterministic HV models. We can't know, how Einstein would have reacted to this overwhelming evidence and also how he'd thought about modern relativistic local QFTs.

 

[A. Neumaier]

It is quite nice to have a mental image that reflects ordinary things we already understand. QM really misses the boat on a LOT of that. Things are just incredibly weird, when compared to ordinary macroscopic things. And some things that attempt to be analogous, such as "spin" really don't quite match.

QM does tell about reality, but does so in non-intuitive ways, that don't fit our expectations that we generalize from macroscopic experiences.

there must be something we could consider as real, even if we do not understand it, and our failure to understand surely cannot be a guiding rule for physics. Further, if we take expectation values, following Ehrenfest's theorem, it is surely as real as classical physics.

Most of the weirdness of quantum mechanics comes from the way the math is interpreted - namely by putting far too much emphasis on pure states and Born's rule. With my
thermal interpretation of quantum physics, which works in the Ehrenfest picture of quantum mechanics that treats quantum expectations as real, most weirdness is gone.