I Quantum stuff is neither particles nor waves

[Cobul]

When I asked my professors about quantum interpretations like whether it is particles or waves in the double slit experiment. They always say it is just quantum stuff which is neither particles nor waves, so one must not describe them at such. So why do we have so many interpretations. Why not just say it is quantum stuff (in addition to emphasizing shut up and calculate. I know all interpretations use the same math)?

Can you show some experiments showing it is not just quantum stuff but really have particle contents for instance?

 

[phinds]

Have you done any research on that? What have you found?

 

[Cobul]

Have you done any research on that? What have you found?

I have read up on the double slit experiments, stern-garlach experiments, Aspects, Bell-theorem. But many professors were not even aware of the quantum interpretations but just saying quantum stuff is neither particles nor waves. They are just quantum, period.

When I asked told them why collapse was random and not deterministic. They told me it was because of the Eigenfunctions or Eigenstates. Nothing out of ordinary.

So what is their perspective to think of that? What should I asked them to make aware of any discrepancies?

Maybe quantum stuff is really quantum and doesn't have to conform to any of our thinking. So why are you guys and others discussing about it?

 

[phinds]

So why are you guys and others discussing about it?

Discussing about it in what way? No one here will contend that quantum objects are particles in the classical sense.

 

[PeroK]

I have read up on the double slit experiments, stern-garlach experiments, Aspects, Bell-theorem. But many professors were not even aware of the quantum interpretations but just saying quantum stuff is neither particles nor waves. They are just quantum, period.

This is good advice. Modern physics has, however, stuck with the term particle. We have, for example, the standard model of particle physics, and electrons, quarks, neutrinos etc. are all classified as particles.

That said, these particles behave quantum mechanically and that includes behaviour that was previously (before the development of QM) seen as wavelike behaviour. Note the following:

1) Before QM electrons behaved sometimes like classical particles and sometimes like classical waves. This was called wave-particle duality.

2) Modern QM explains why particles behave as they do, using a uniform QM formalism based on the concept of a particle described by a wavefunction.

That should be the end of the story and we should no longer talk about wave-particle duality. But:

3) Modern popular science books and documentaries are obsessed with wave-particle duality and promote something of a myth that it is central to modern QM.

Note that none of the QM and QFT textbooks I have mention wave-particle duality (except as a historical footnote).

The best thing you can do is to try to forget everything you think you have learned about QM from popular science sources and start afresh with what your professors are teaching you: that is the real, academic subject, and not a popular science mish-mash of drama and misinformation.

For example, you can essentially forget you ever heard the term wave-particle duality, as it is nowhere to be found in the formalism of QM.

You may be misunderstanding the idea of a quantum interpretation, as this is not really what quantum interpretations are about. I'll ask to have this post moved to the regular quantum physics forum.

If you are curious, you can start to read about different quantum interpretations. Start here, say:

https://en.wikipedia.org/wiki/Interpretations_of_quantum_mechanics

Note that learning about different interpretations is not necessary for a first course in QM.

 

[PeterDonis]

I'll ask to have this post moved to the regular quantum physics forum.

I'm not sure that's warranted. Discussion based on a wrong idea about what quantum interpretations are still belongs in the interpretations forum, since it is still a very different discussion from discussion based on "basic" QM without adopting any interpretation at all.

 

[Cobul]

Discussing about it in what way? No one here will contend that quantum objects are particles in the classical sense.

So there is no mystery in the double slit experiment especially the one at a time electron emission. The usual argument is how could the particle interfere with itself. It's not a particle, but a quantum stuff. So the problem goes away.

Couldn't this be said for the rest of the experimental setups like the Stern-Gerlach experiments?

 

[PeterDonis]

It's not a particle, but a quantum stuff. So the problem goes away.

You might want to look into Bell's theorem, and experiments confirming that the Bell inequalities are violated, before concluding that there is no problem.

 

[Cobul]

This is good advice. Modern physics has, however, stuck with the term particle. We have, for example, the standard model of particle physics, and electrons, quarks, neutrinos etc. are all classified as particles.

That said, these particles behave quantum mechanically and that includes behaviour that was previously (before the development of QM) seen as wavelike behaviour. Note the following:

1) Before QM electrons behaved sometimes like classical particles and sometimes like classical waves. This was called wave-particle duality.

2) Modern QM explains why particles behave as they do, using a uniform QM formalism based on the concept of a particle described by a wavefunction.

That should be the end of the story and we should no longer talk about wave-particle duality. But:

3) Modern popular science books and documentaries are obsessed with wave-particle duality and promote something of a myth that it is central to modern QM.

Note that none of the QM and QFT textbooks I have mention wave-particle duality (except as a historical footnote).

The best thing you can do is to try to forget everything you think you have learned about QM from popular science sources and start afresh with what your professors are teaching you: that is the real, academic subject, and not a popular science mish-mash of drama and misinformation.

For example, you can essentially forget you ever heard the term wave-particle duality, as it is nowhere to be found in the formalism of QM.

You may be misunderstanding the idea of a quantum interpretation, as this is not really what quantum interpretations are about. I'll ask to have this post moved to the regular quantum physics forum.

If you are curious, you can start to read about different quantum interpretations. Start here, say:

https://en.wikipedia.org/wiki/Interpretations_of_quantum_mechanics

Note that learning about different interpretations is not necessary for a first course in QM.

I think it's related to quantum stuff. Your reference explained "An interpretation of quantum mechanics is an attempt to explain how the mathematical theory of quantum mechanics "corresponds" to reality. ".

Quantum stuff is math-like. And quantum stuff is reality. Why is there need for interpretations. This is if you are thinking of Newtonian dynamics. But everything is quantum or quantum stuff even the house we are living in.

 

[Cobul]

You might want to look into Bell's theorem, and experiments confirming that the Bell inequalities are violated, before concluding that there is no problem.

Been there. Done that. Bohr already explained the world was quantum stuff, so no problem even if there is correlations aspect-wise because everything is quantum. There is no objective world, as Bohr seemed to say.

 

[PeroK]

I think it's related to quantum stuff. Your reference explained "An interpretation of quantum mechanics is an attempt to explain how the mathematical theory of quantum mechanics "corresponds" to reality. ".

Quantum stuff is math-like. And quantum stuff is reality. Why is there need for interpretations. This is if you are thinking of Newtonian dynamics. But everything is quantum or quantum stuff even the house we are living in.

Let's look at three "interpretations" of Newton's Theory of Gravity and, in particular, the Earth and Moon system. An interpretation is a way to explain what happens, rather than simply solve some mathematical equations.

1) Force (action at a distance)

In this interpretation, the Earth and the Moon apply gravitational forces to each other. I.e. we "really" have action at a distance.

2) Gravitational Field gradient.

In this interpretation, the Earth creates a gravitational field, whose strength varies with distance. This field applies a force locally to the Moon in the direction of the negative gradient of the field.

3) Lagrangian and Action Minimalisation.

In this interpretation, the Moon tries out all paths and chooses a path that minimises the action - integral of Kinetic minus Potential energy.

So, what does the Earth-Moon system "really" do? Well, as long as all three produce the same answer in terms of the orbit, there's no way to distinguish them: does the force really exist? Does the gravitational field really exist? These are essentially interpretational questions.

Now, the interpretations of QM go much deeper and none of them is perfect, in the sense that there is something not to like about them all. They are all attempts - like the above - to give some "meaning" to the mathematics and to explain why you make certain calculations.

 

[Cobul]

Let's look at three "interpretations" of Newton's Theory of Gravity and, in particular, the Earth and Moon system. An interpretation is a way to explain what happens, rather than simply solve some mathematical equations.

1) Force (action at a distance)

In this interpretation, the Earth and the Moon apply gravitational forces to each other. I.e. we "really" have action at a distance.

2) Gravitational Field gradient.

In this interpretation, the Earth creates a gravitational field, whose strength varies with distance. This field applies a force locally to the Moon in the direction of the negative gradient of the field.

3) Lagrangian and Action Minimalisation.

In this interpretation, the Moon tries out all paths and chooses a path that minimises the action - integral of Kinetic minus Potential energy.

So, what does the Earth-Moon system "really" do? Well, as long as all three produce the same answer in terms of the orbit, there's no way to distinguish them: does the force really exist? Does the gravitational field really exist? These are essentially interpretational questions.

Now, the interpretations of QM go much deeper and none of them is perfect, in the sense that there is something not to like about them all. They are all attempts - like the above - to give some "meaning" to the mathematics and to explain why you make certain calculations.

The right answer to the above is General Relativity which explained Earth moon "gravity". And GR is pure math.

Likewise, QM is pure math. In the double slit experiment one "particle" at a time emission for instance. There is "no particle" in between emitter and detector. So Bohman Mechanics is not right. And Many Worlds not right either. The "particle" only appears at detector during a measurement. This is pure math applied like General Relativity. Problem solved. Doesn't all contend to this?

 

[PeroK]

The right answer to the above is General Relativity which explained Earth moon "gravity". And GR is pure math.

Likewise, QM is pure math. In the double slit experiment one "particle" at a time emission for instance. There is "no particle" in between emitter and detector. So Bohman Mechanics is not right. And Many Worlds not right either. The "particle" only appears at detector during a measurement. This is pure math applied like General Relativity. Problem solved. Doesn't all contend to this?

That's not right at all. Newtonian Gravity is a physical theory, so you can still ask how the theory is actually supposed to work. At least to illustrate a point.

In QM there is always a particle. Particles don't come in and out of existence only when you measure them. It's only their dynamic properties (position, momentum, angular momentum etc.) that are only well-defined by measurement. The particle's wave-function continuously evolves. If there really were no particle, there would be no wave-function.

Bohmian Mechanics and MWI predict the same results as other interpretations, so you cannot say they are definitely wrong.

Assuming you are learning QM at university, you have stacked up a number of fundamental misunderstandings that you really need to address.

 

[Cobul]

That's not right at all. Newtonian Gravity is a physical theory, so you can still ask how the theory is actually supposed to work. At least to illustrate a point.

In QM there is always a particle. Particles don't come in and out of existence only when you measure them. It's only their dynamic properties (position, momentum, angular momentum etc.) that are only well-defined by measurement. The particle's wave-function continuously evolves. If there really were no particle, there would be no wave-function.

This was why I said earlier that there was only quantum stuff and there were no particles nor waves. This is valid for interpretations too. That is. Quantum stuff is not particle nor waves, it's quantum stuff. Hence there is no problem about what goes on between emission and detection in double slit experiment, no "particle" takes fight or move. There is only quantum stuff that interacts. Why you don't seem to agree with concept?

Bohmian Mechanics and MWI predict the same results as other interpretations, so you cannot say they are definitely wrong.

Assuming you are learning QM at university, you have stacked up a number of fundamental misunderstandings that you really need to address.

 

[PeroK]

This was why I said earlier that there was only quantum stuff and there were no particles nor waves. This is valid for interpretations too. That is. Quantum stuff is not particle nor waves, it's quantum stuff. Hence there is no problem about what goes on between emission and detection in double slit experiment, no "particle" takes fight or move. There is only quantum stuff that interacts. Why you don't seem to agree with concept?

Quantum "stuff" is not standard terminology. It's just loose general terminology. The correct terminology is particle or quantum particle. See, for example:

https://en.wikipedia.org/wiki/Standard_Model#Particle_content

 

[Demystifier]

Why not just say it is quantum stuff

Because it raises many additional questions, without giving a clear answer. Some of those questions are:
What is the mathematical representation of the quantum stuff?
Is it the wave function, and if yes, how is it not a wave?
Is the quantum stuff there when nobody looks?

 

[PeterDonis]

Why is there need for interpretations.

You can learn all about the math of QM, how it makes predictions, and how well those predictions compare with experiment, without ever having to learn anything about QM interpretations. So if you don't like QM interpretations, you can just ignore them.

However, you actually aren't doing that. When you say, for example, this...

There is no objective world

...you are adopting an interpretation of QM. So you have just shown why there is a need for interpretations: because nobody is really able to just talk about the math of QM, how it makes predictions, and how well those predictions compare with experiment, and stop there. Everyone--even you, as I have just shown--feels an irresistible need to go beyond that. But there is no unique way to go beyond that--there are lots of different possible ways, which are mutually incompatible, and none of which can be tested by experiment since the experimental predictions stay the same for all of them. And that is why there are multiple QM interpretations.

 

[PeterDonis]

GR is pure math.

QM is pure math.

Any physical theory, if you just look at the theoretical model--the machinery that makes predictions--is pure math. But nobody just looks at the machinery that makes predictions. You can't, because you have to test the theory's predictions against actual experiments. And that means you need more than just the pure math. You need a correspondence between the math and actual observations.

 

[Cobul]

Quantum "stuff" is not standard terminology. It's just loose general terminology. The correct terminology is particle or quantum particle. See, for example:

https://en.wikipedia.org/wiki/Standard_Model#Particle_content

I got the word quantum stuff from Nick Herbert Quantum Reality. Although he didn't put any space between them (he called it "quantumstuff". The following are its attributes.

By dissolving the matter/field distinction, quantum physicists realized a dream of the ancient Greeks who speculated that beneath its varied appearances the world was ultimately composed of a single substance. Some philosophers said it was All Fire; some All Water. We now believe the world to be All Quantumstuff.

The world is one substance. As satisfying as this discovery may be to philosophers, it is profoundly distressing to physicists as long as they do not understand the nature of that substance. For if quantumstuff is all there is and you don't understand quantumstuff, your ignorance is complete.

(...)

It is hard to believe that the electron is physically smeared out across its realm of positional possibilities, because every time we measure it we never see a smeared electron, always a point particle. In each atom, however, something seems smeared out to fill the atom, an indescribable something we call the "probability cloud," "realm of positional possibilities," "electron wave function," or "quantumstuff" without really being very sure what we're talking about. Whatever it is, though, the whole world is made of it.

It would be tempting to dismiss the possibility wave of a single atom as an airy statistical fiction with no more reality than the dice odds for a single roll, but thesewaves of possibility have more tangible consequences than dice odds. Try, for instance, to push your hand through the nearest wall. Since atoms are mostly empty space, their electrons are too small to stop you. Only each atom's possibility wave pushes back at you. Pretty substantial, aren't they?

Quantumstuff or wavefunction or Hilbertspace is a new reality, a new substance. And to answer a poster series of questions:

Because it raises many additional questions, without giving a clear answer. Some of those questions are:

What is the mathematical representation of the quantum stuff?

Is it the wave function, and if yes, how is it not a wave?

Is the quantum stuff there when nobody looks?

The mathematical representation of quantumstuff is wavefunction or Hilbert space.

It is the wave function, it is not just a wave because it is quantumstuff, with attributes like particles and waves.

Yes, the quantum stuff is there when nobody looks. Remember when we sit on chair, our buttocks pushing against the atom's possibility wave or quantumstuff. And our buttocks don't have eyes, so it is there when nobody look.

Quantumstuff is the only interpretation that conforms to the math and reality. All else are imitations, like Bohmian Mechanics, Many Worlds, etc. which are trying to make it Newtonian.

 

[PeterDonis]

I got the word quantum stuff from Nick Herbert Quantum Reality.

This is a pop science book, not a textbook, and is not a valid basis for PF discussion. You need to be using textbooks or peer-reviewed papers as sources to actually learn the physics.

Quantumstuff is the only interpretation that conforms to the math and reality. All else are imitations, like Bohmian Mechanics, Many Worlds, etc. which are trying to make it Newtonian.

This might be your opinion, but I strongly suspect it is due to lack of information on your part, which in turn is due to your trying to learn physics from pop science sources instead of textbooks and peer-reviewed papers.

And with that, this thread is closed.