I The quantum wave is not real?

[rolnor]

TL;DR Summary

The quantum wave is not real?
https://compass.onlinelibrary.wiley.com/doi/10.1111/phc3.12611

I have read in the included reference that the quantum wave is not real in the sense we mean with for example the electromagnetic waves. This can be inferred when looking at the double slit experiment. It seems that this experiment has been looked at as if the quantum wave as is a real wave, like the electromagnetic wave, but it is not as I understand it?

 

[pines-demon]

TL;DR Summary: The quantum wave is not real?
https://compass.onlinelibrary.wiley.com/doi/10.1111/phc3.12611

I have read in the included reference that the quantum wave is not real in the sense we mean with for example the electromagnetic waves. This can be inferred when looking at the double slit experiment. It seems that this experiment has been looked at as if the quantum wave as is a real wave, like the electromagnetic wave, but it is not as I understand it?

Just a couple of thoughts: (1) the interpretation of quantum mechanics is an open problem, different interpretations postulate different ideas about it but produce the same predictions (2) when discussing interpretations one has to be careful with the word "realism" and "realistic interpretations", there is a close relative in this discussions called "local realism" that is about something else entirely. The best word here would be to ask if the wavefunction is "ontic".

Edit: note also that the article says that it will focus on interpretations where the wavefunction is real.

 

[gentzen]

It may come as no surprise that philosophers of physics generally hate the point of view. It wasn't in their coursework when they were first discussing philosophy and it impinges on philosophies that aren't part of the standard curriculum of philosophy of physics. And to my ear, their main complaint is: "Wah, wah, wah, you don't believe wave functions are real, they're clearly real, so we're not going to pay attention to QBism".

The (representative) philosophers of physics on the slide, from left to right are: Wayne C. Myrvold, David Z. Albert, David Wallace, Ruth E. Kastner, Harvey R. Brown, and Tim Maudlin.

Chris Fuchs claims that just because he doesn't believe wave functions are real, philosophers won't listen to him. However, the problem for the philosophers is probably rather that he doesn't even seem to care what is real, independent of whether it is the wave function, particles, or measurement results.

If he would just deny that the wavefunction is real, he certainly wouldn't be alone. But what can it even mean that the wavefuntion is not real? My guess that is that you can try to ask the same question in the context of non-deterministic computation to better understand this:

Both non-deterministic computation and quantum computation give rise to a compact closed category. But how can you interpret a compact closed category? For non-deterministic computation, it is basically a collection of equations, local ones, or rather locally verifiable ones, where “local” can take on many meanings, depending on context. Obviously, such equations don’t have a preferred time-direction, and also no preferred “causality direction”.

Take two simple examples, namely non-deterministic finite automata, and non-deterministic push-down automata. An analog question for the finite automata would be whether the set of states reachable for a given initial segment of an input string is real. You can answer yes, if you want. However, then you should also try to ask the same question for the push-down automata. Now you are not just talking about a set of finite states of the automata, but about set of pairs of finite automata state and a state of the stack. You can still declare that this set is real, but other interpretations can become more attractive now, where that set it is not real.

 

[Peter Morgan]

TL;DR Summary: The quantum wave is not real?
https://compass.onlinelibrary.wiley.com/doi/10.1111/phc3.12611

I have read in the included reference that the quantum wave is not real in the sense we mean with for example the electromagnetic waves. This can be inferred when looking at the double slit experiment. It seems that this experiment has been looked at as if the quantum wave as is a real wave, like the electromagnetic wave, but it is not as I understand it?

That's behind a paywall. There's a preprint on arXiv, https://arxiv.org/abs/1810.07010

 

[martinbn]

If he would just deny that the wavefunction is real, he certainly wouldn't be alone. But what can it even mean that the wavefuntion is not real?

For me it is the other way around. What does it mean for the wave function to be real? It clearly is not the same thing as the electromagnetic field is real in classical electrodynamics.

 

[pines-demon]

Chris Fuchs claims that just because he doesn't believe wave functions are real, philosophers won't listen to him. However, the problem for the philosophers is probably rather that he doesn't even seem to care what is real, independent of whether it is the wave function, particles, or measurement results.

For the same reason, physicists do not listen to him either.

OP can look for psi-ontic/psi-epistemic interpretations much has been said about it, but Qbism, specifically, falls very rapidly into solipsism.

 

[pines-demon]

For me it is the other way around. What does it mean for the wave function to be real? It clearly is not the same thing as the electromagnetic field is real in classical electrodynamics.

You could think of Bohmian mechanics, were there are particles and pilot-waves.

 

[rolnor]

Just a couple of thoughts: (1) the interpretation of quantum mechanics is an open problem, different interpretations postulate different ideas about it but produce the same predictions (2) when discussing interpretations one has to be careful with the word "realism" and "realistic interpretations", there is a close relative in this discussions called "local realism" that is about something else entirely. The best word here would be to ask if the wavefunction is "ontic".

Thanx, I am thinking about the double slit experiment, the fact that the photons does give an interference pattern is due to the electromagnetic field waves, then one can destroy this by measuring the single photon, but this is not affecting the electromagnetic wave, but the quantum wave, so its seems one is dealing with two different phenomena in the same experiment, and these are not interconnected, or are they?

 

[martinbn]

You could think of Bohmian mechanics, were there are particles and pilot-waves.

But the pilot waves are not real as the electromagnetic ones. They do not propagate in space as time goes on.

 

[gentzen]

For me it is the other way around. What does it mean for the wave function to be real? It clearly is not the same thing as the electromagnetic field is real in classical electrodynamics.

If you have a well isolated small quantum system at nearly zero temperature, prepared in a nearly pure state, its wavefunction becomes a very useful and concise description. In fact, you could even consider a small number of such well isolated small quantum systems, and consider controlled interactions between them. The wavefunctions and the tensor product of their Hilbert spaces remains a very good description. It is fine to call the wavefunctions "real" in such a context.

The same can be seen for the finite state automata. The set of reachable states for ... is a very good description in that context, and can be called "real". The trouble arises from generalizing from this specific context, and I think it is easier to understand this for non-determistic pushdown automata. There are still good descriptions of them, like the ones employed for example by the CYK algorithm, but the "simple blackbox" set of all reachable state is no longer such a good description.

 

[pines-demon]

Thanx, I am thinking about the double slit experiment, the fact that the photons does give an interference pattern is due to the electromagnetic field waves, then one can destroy this by measuring the single photon, but this is not affecting the electromagnetic wave, but the quantum wave, so its seems one is dealing with two different phenomena in the same experiment, and these are not interconnected, or are they?

Electromagnetic waves are made of photons!

The thing is that if you have a macroscopic electromagnetic wave (made of many photons) and perform the double slit experiment, you can ask in any point of space what amplitude of the wave is. You can put a screen and see the whole interference pattern.

When you do the same with a single photon, putting a screen will result in a single photon being detected in some point of the screen and then you cannot detect it anymore. Performing the same experiment many times will result in different dots in the screen, aggregating all of the dots you will recover the interference pattern.

The weird part is that the photon behaves "as if" it is a wave before it arrives to the screen. That's what interpretations try to answer. Quantum mechanics proposes a wave function in its mathematical formalism, the wave function cannot be measured directly, if that wavefunction is ontic (exist independent of the experiment and observers) or if it just a formalism to understand what is going on, is up to interpretation.

 

[pines-demon]

But the pilot waves are not real as the electromagnetic ones. They do not propagate in space as time goes on.

How it does not? If start with a gaussian wavepacket (free particle), it will spread with time...

 

[martinbn]

How it does not? If start with a gaussian wavepacket (free particle), it will spread with time...

If you have more than one particle, the wave function is not a function of space and time.

 

[martinbn]

If you have a well isolated small quantum system at nearly zero temperature, prepared in a nearly pure state, its wavefunction becomes a very useful and concise description. In fact, you could even consider a small number of such well isolated small quantum systems, and consider controlled interactions between them. The wavefunctions and the tensor product of their Hilbert spaces remains a very good description. It is fine to call the wavefunctions "real" in such a context.

Of course you can call them "real", with the appropriated redefinition of "real". But they are not real in the same sense as the electromagnetic waves.

 

[pines-demon]

If you have more than one particle, the wave function is not a function of space and time.

Sure there are striking differences between EM waves and wavefunctions, but the wave function could be real in some sense. In many-worlds, there is only Hilbert spaces so that's no issue.

 

[martinbn]

Sure there are striking differences between EM waves and wavefunctions, but the wave function could be real in some sense. In many-worlds, there is only Hilbert spaces so that's no issue.

Yes, in some sense anything can be real, but the OP said:

I have read in the included reference that the quantum wave is not real in the sense we mean with for example the electromagnetic waves.

 

[pines-demon]

Yes, in some sense anything can be real, but the OP said:

Sure but many things are strikingly different to EM waves and we do not call them less real (I could argue that water waves are more real). The paper is more specifically about this epistemic vs ontic-like defintions.

Edit: the article barely mentions the electric field.

 

[rolnor]

Electromagnetic waves are made of photons!

The thing is that if you have a macroscopic electromagnetic wave (made of many photons) and perform the double slit experiment, you can ask in any point of space what amplitude of the wave is. You can put a screen and see the whole interference pattern.

When you do the same with a single photon, putting a screen will result in a single photon being detected in some point of the screen and then you cannot detect it anymore. Performing the same experiment many times will result in different dots in the screen, aggregating all of the dots you will recover the interference pattern.

The weird part is that the photon behaves "as if" it is a wave before it arrives to the screen. That's what interpretations try to answer. Quantum mechanics proposes a wave function in its mathematical formalism, the wave function cannot be measured directly, if that wavefunction is ontic (exist independent of the experiment and observers) or if it just a formalism to understand what is going on, is up to interpretation.

Thanx. But it seems you affect the electromagnetic wave by measuring, the electromagnetic wave is what gives rise to the pattern? Not the QM-wave? Does the measuring affect the QM-wave and this, in turn, affect the electromagnetic wave?

 

[pines-demon]

But it seems you affect the electromagnetic wave by measuring, the electromagnetic wave is what gives rise to the pattern? Not the QM-wave? Does the measuring affect the QM-wave and this, in turn, affect the electromagnetic wave?

This is tricky. There are semantical ambiguities:

you affect the electromagnetic wave by measuring,

what do you mean by affect?

the electromagnetic wave is what gives rise to the pattern. Not the QM-wave?

what do you mean by give rise?

Please provide examples.

 

[rolnor]

This is tricky. There are semantical ambiguities:

what do you mean by affect?

what do you mean by give rise?

Please provide examples.

1.You make the interference pattern go away, the single photon is "just" a particle

2. The photon behaves as if it goes through both slits and gives rise to an interference pattern, this is because it can be seen as electromagnetic waves, but when you measure/detect the photon, the pattern disappears, does this mean the photon looses its electromagnetic wave-properties? If so, does that mean that the measure/detection, which affects the QM-wave, affects the electromagnetic wave properties?

 

[anuttarasammyak]

TL;DR Summary: The quantum wave is not real?
https://compass.onlinelibrary.wiley.com/doi/10.1111/phc3.12611

I have read in the included reference that the quantum wave is not real in the sense we mean with for example the electromagnetic waves. This can be inferred when looking at the double slit experiment. It seems that this experiment has been looked at as if the quantum wave as is a real wave, like the electromagnetic wave, but it is not as I understand it?

What are examples of real and not real subjects in physics ? Real seems category error in physics to me.

 

[gentzen]

What are examples of real and not real subjects in physics ? Real seems category error in physics to me.

A measured current through a wire or a measured voltage difference between two contacts is real. An electric potential on the other hand is not real by itself, because only potential differences are important. Similarly, the global phase of a wavefunction is often not important. This can be fixed by instead using a density matrix.

Let me use the density matrix to construct another example of something not real: One can construct a measure on the space of wavefunctions from a density matrix, which reproduces the density matrix as the description of that ensemble of wavefunctions. If one repeatedly draws wavefunctions from that ensemble, then does some simulations with it, and collects some statistics based on it, then even so the collected statistics might describe something real, the individual drawn/sampled wavefunctions are not.
Alternatively, you could also compute the collected statistics by computing an eigenvalue decomposition of the density matrix, computing with the eigenvectors as wavefunction, and then weighting the results by the eigenvalues. Again, the wavefunctions which occur here are not real.

 

[Ben vdP]

Well, you can get 10 different answers from 10 different persons.


Conceptually you could see the wavefunction as the impact the quantum particle has on its environment,
loosely put, or its indirect presence that is felt everywhere. In EM you feel the presence of charge through the em-field.

This is a different interpretation than Kopenhagen. In above interpretation you do not need anything probabilistic.
No chance of finding particle here, chance of finding particle there; the idea that Einstein had rejected.
Instead it is everywhere and the wavefunction describes how the particles "influence" is spread out and the square the intensity.
But you do not need to go that far though, conceptionally.

Consider that a quantum particle should not been seen in isolation.
It is in constant interaction with the space around it and interacting with other quantum particles exchanging virtual particles.
And also there is self-interaction. The quantum particle is an integral part of all of that.
It is not something that sits at a certain point and there are no consequences elsewhere.

There is some similarity with the bouncing ball system of vibrating droplets on the surface of a fluid. In that model, that came out of
fluid dynamics, you also have a combination of waves and particles.
And when you perform the double slit experiment on the droplets you are getting identical results as with quantum particles,
due to the waves. Afaik self-interaction played a major role. This model might give you some ideas on how to view quantum particles.

 

[martinbn]

A measured current through a wire or a measured voltage difference between two contacts is real. An electric potential on the other hand is not real by itself, because only potential differences are important. Similarly, the global phase of a wavefunction is often not important. This can be fixed by instead using a density matrix.

In this sense the wave function is not real, it cannot be measured.

 

[WernerQH]

What are examples of real and not real subjects in physics ? Real seems category error in physics to me.

What is considered real is important (physics is about the real world around us!), but it is subject to change. Every physicist considers the electric field real, but it once was a completely abstract concept describing stress in a hypothetical material medium (the ether). Everybody seems to consider real what he views as the essential elements of his world picture. The ether is no longer real, although for Maxwell light waves without a medium carrying them were unthinkable.

I share the view that "the" wave function should not be considered real. For one thing, it does not live in real space, but in an abstract configuration space. What I find even more problematic is the idea that something supposedly evolving continuously and deterministically according to Schrödinger's equation affords a faithful description of the randomness and abruptness that we observe in the real world around us.

 

[pines-demon]

Every physicist considers the electric field real, but it once was a completely abstract concept describing stress in a hypothetical material medium (the ether).

Not sure, I know of a debate about that happened a few years ago. On one hand sure it is accepted as having more real presence than the electromagnetic potentials, but on the other hand it is not technically what can be measured. Note that you construct the field by imposing that the force on a charge is $F=qE$, so without charges there is no force. It only exists if you have more than one charge, even EM waves can be seen as the delayed force of one charge over another. Again, I am not arguing for or against it but we have to set the definitions here before we can call something real. There is a paper by OP and other definitions often used to define this for the wave function.

 

[bhobba]

The answer is interpretive. Gleason's theorem (see Wikipedia article) suggests it is just a calculational aid.

That said, Quantum Field theory is a more fundamental theory than basic QM taught in beginner textbooks. I believe the Quantum field is real (it has energy, etc.). The quantum wave has been replaced with the quantum field that permeates all space. Particles are like knots in the Quantum Field. IMHO, at this more basic level, it is not an issue one way or the other.

Thanks
Bill

 

[bhobba]

But what can it even mean that the wavefuntion is not real?

It is just a calculational aid, like probability. Gleason suggests to me that it is precisely what it is.

Now, the deeper theory, QFT, is a different matter. The quantum field, having energy, etc., is real. For example, E=MC2 says mass is a form of energy, and I do not know anyone who thinks mass is not 'real'. I suppose such a view could be justified, but without a good reason, since most stuff out there has mass, it is a strange worldview.

To me, QM is just an approximation to QFT. Discussions of interpretational issues should, IMHO, focus much more on QFT than QM. Also, there is Wienberg's Folk Theorem that any theory at large distances is a QFT.

Thanks
Bill

 

[bhobba]

Does this mean the photon loses its electromagnetic wave properties?

It simply means there has been an interaction with the quantum field the photon is part of. In QFT, observations are interactions with the underlying Quantum Field. Zee looks at it as a source and sinks in his book, Quantum Field Theory as Simply as Possible (recommended). A better way of looking at the double slit experiment than wave particle duality is:

https://arxiv.org/abs/quant-ph/0703126

Art Hobson in the American Journal of Physics (81(3), 211–223 (2013)) had the following paper published

https://arxiv.org/abs/1204.4616

I agree with its conclusion - adopting the field view from the start (but not detailing it - that unfortunately is more advanced than a beginner's textbook, although Zee's book is very approachable) makes QM less mysterious. Of course, some interpretational issues remain - but IMHO QM is clearer.

Thanks
Bill

 

[rolnor]

The answer is interpretive. Gleason's theorem (see Wikipedia article) suggests it is just a calculational aid.

That said, Quantum Field theory is a more fundamental theory than basic QM taught in beginner textbooks. I believe the Quantum field is real (it has energy, etc.). The quantum wave has been replaced with the quantum field that permeates all space. Particles are like knots in the Quantum Field. IMHO, at this more basic level, it is not an issue one way or the other.

Thanks
Bill

Thanx, this makes sense. Its interesting that it is so hard to define whats "real", I do understand that something that has energy is real.

 

[weirdoguy]

Discussions of interpretational issues should, IMHO, focus much more on QFT than QM.

This! I don't quite understand why people focus so much on non-relativistic theory. Relativity can turn all of this efforts on its head.

 

[pines-demon]

Thanx, this makes sense. Its interesting that it is so hard to define whats "real", I do understand that something that has energy is real.

Energy makes EM real does not seem to be a good argument, I can also write the Lagrangian of particle with the wavefunction in it, does the wave function carry energy?

 

[gentzen]

This! I don't quite understand why people focus so much on non-relativistic theory. Relativity can turn all of this efforts on its head.

Maybe because QFT somehow invites "everybody" to make overarching statements, which are not defensible in their stated generality?

Fine, you don't need to say much. Just clearly replace the statement

Schrodinger's equation is simply the low energy approximation of the Lorentz-invariant Klein-Gordon equation of quantum field theory

by something which is less disparaging towards nonrelativistic QM. As I said, the Klein-Gordon equation only applies to bosons, and this is not a limitation of nonrelativistic QM. It is true that nonrelativistic QM has trouble creating or destroying particles, and hence has a hard time with photons, because they are massless.

Please also see my point. I have invested huge efforts to understand nonrelativistic QM good enough to be able to somehow use it in my daily job. I have some knowledge of QFT, but far less than of QM. It is fine for me to read your references to Zee, or Demystifier's paper. But the way this discussion here went feels very bad to me.

In this discussion here, we have for example the following statement:

The quantum field, having energy, etc., is real. For example, E=MC2 says mass is a form of energy, and I do not know anyone who thinks mass is not 'real'.

So you just mention the words "quantum field", and somehow this makes our trouble with explaining what we mean by "real" suddenly go away?

 

[martinbn]

In this discussion here, we have for example the following statement:

Now, the deeper theory, QFT, is a different matter. The quantum field, having energy, etc., is real. For example, E=MC2 says mass is a form of energy, and I do not know anyone who thinks mass is not 'real'.

So you just mention the words "quantum field", and somehow this makes our trouble with explaining what we mean by "real" suddenly go away?

I think, by real he means having an objective existence, that it is part of the territory and not the map. I think those are constantly being confused or people are not trying to write more clearly.

 

[bhobba]

Thanks; this makes sense. It's interesting that it is so hard to define what's "real." I do understand that something that has energy is real.

It is a philosophical problem that, by forum rules, can not be discussed here. It is recognised, however, that in the quantum interpretations section, it will occasionally be brushed up against, so some leeway is given.

For most applied math/science, the external reality we all experience is objective. As one physicist (Victor Stenger) said, it is what kicks back. Indeed, when we kick a rock, it kicks back, so generally, it is considered real. It has other properties like mass, which you feel when you lift or move it (eg kicking back). From E=MC^2, energy is then real. EM Fields (classical or quantum) are real; due to Noether's theorem, they have energy.

Other things are not real, e.g., probability, which depends on your interpretation (Bayesian or Frequentist). That is not real.

Observations, ie things that occur and can be measured, are real in QM. They are included in the observations observable. This is the starting point of QM and can be justified from a simple model. I will not provide details unless people want to see them. This is the first axiom of QM as found in Ballentine. The second axiom (the state and the Born Rule ) follows (with some caveats) from the first. Ballentine develops QM just from those axioms. The reality of the state is interpretation-dependent. I believe it is just a calculational device to help calculate probabilities, which are not real. The following paper explains that the state is not the limiting case of QFT (antiparticles are the issue):

https://arxiv.org/abs/1712.06605

Obviously, for ordinary QM, the fields still exist, but they are not the quantum state of ordinary QM. For most practical purposes, they are the same; as I said, antiparticles are the issue, but they are usually neglected.

Thanks
Bill

 

[bhobba]

I think, by real he means having an objective existence, that it is part of the territory and not the map. I think those are constantly being confused or people are not trying to write more clearly.

Exactly, and well said.

Thanks
Bill

 

[bhobba]

Energy makes EM real does not seem to be a good argument, I can also write the Lagrangian of particle with the wavefunction in it, does the wave function carry energy?

Interesting point. Since I think the wave-function is not real, when using the path integral approach, one gets the Lagrangian of an actual particle; it must be related to the quantum field. As the paper I linked to explains, they are very close in ordinary QM, The problem is that they must also include antiparticles, which they usually do not. The quantum state is an abstract approximation, but it is enough to (via Noether) give energy, momentum, etc. What determines the Lagrangian must be valid for the conclusion, e.g., having energy to be valid.

Thanks
Bill

 

[WandaS]

2. The photon behaves as if it goes through both slits and gives rise to an interference pattern, this is because it can be seen as electromagnetic waves, but when you measure/detect the photon, the pattern disappears, does this mean the photon looses its electromagnetic wave-properties? [...]

The interference pattern has nothing to do with electromagnetic waves, but with indeterminism and superposition, I think here's the misunderstanding.

I would completely forget the EM waves here, you can do the same experiment with electrons too. The position of an electron is given by its wave function, at a wave's peak the probability to find it is highest, and least at its trough. The wave function is so a "probability wave".

Prior to a measurement, an electron has no definite position, but is in all positions allowed by the wave packet at once, it is in superposition of all possible states or positions. When we measure its position, we'll find it within the wave packet, but what position exactly is totally random; with most probability at its peaks, but could also be at some other place within the wave. But we can't say that it was at this place already before the measurement, as prior to the measurement its position is not determined.

It's as if the measurement "chooses" a random position out of the possible ones, without reason and therefore in an impredictable manner. That's why it's called "collapse" of the wave function.

But how can we be sure that an electron wasn't already at a definite position (but was in superposition) before the measurement?
Because of the interference pattern we see at the double slit experiment:

If the electrons had at each moment a definite position (we just don't know), we would see two stripes at the detection screen, like when we throw balls, or like when just one of the slits is open.
The interference pattern is the result of the "probability waves": two peaks add to a higher peak (highest probability), a peak and a trough cancel each other out, resulting in the bright and dark stripes of the pattern.

(If we shoot one electron at a time, we also see an interference pattern: it's as if the electron goes through both slits at once (superposition), creating the same pattern at the screen.)

As to the reality of the wave, the Copenhagen Interpretation says something like:
Only what can be measure can be seen as real, and therefore the wave is not an element of reality in its strict sense (but a mathematical tool for making probabilistic predictions).

 

[anuttarasammyak]

What are examples of real and not real subjects in physics ? Real seems category error in physics to me.

That said, is information including wave function, its entanglement, entropy ( and temperature ? ) widely regarded to be not real ones in physics ?

[EDIT]
Wave function, which is an infromation but not real entity, follows physical equation, i.e. Shoredinger equation. Distribution function which is an information but not real entity in physics follows Liouville equation. Even they are not real, the information develop according to physical equation, which is amazing for me.

 

[bhobba]

That said, is information including wave function, its entanglement, entropy ( and temperature) widely regarded as not real in physics?

Wave function - reality is interpretation-dependent
Entanglement - very real, as Bell showed
Temperature - real

Probability, based on the so-called Cox axioms, is quantified plausibility, so it is not real (it does not 'kick back'):

https://en.wikipedia.org/wiki/Cox's_theorem

There is a. branch of math known as rigorous probability theory where everything is proven from the so-called Kolmogorov Axioms. Books like A First Look at Rigorous Probability Theory by J Rosenthal take this approach. Using just the Kolmogorov axioms, no position is taken on reality - it is just a mathematical system like group theory.

Ross - Introduction to Probability Models takes an intuitive approach in moving from the Kolmogorov axioms to applications

The classic Feller - An Introduction to Probability Theory and Its Applications takes a middle ground.

Thanks
Bill