Classical physics vs quantum physics

[new6ton]

Is classical physics independent from quantum physics?

Or is classical physics an approximation derived from quantum physics?

Is it dependent on interpretations? What quantum interpretations support the latter above?

 

[jedishrfu]

I think folks look at classical physics as describing the everyday world of slow speeds and small distances pretty well. It is the power behind our engineering works.

Quantum Mechanics addresses questions where Classical Mechanics failed miserably. One of The first critical questions, was why electrons can orbit a nucleus while not giving off electromagnetic waves and crashing into the nucleus. QM provided the answer and a new way to look at nature.

Checkout some of the older threads below for more insight. This question has been asked many times before.

 

[new6ton]

I think folks look at classical physics as describing the everyday world of slow speeds and small distances pretty well. It is the power behind our engineering works.

Quantum Mechanics addresses questions where Classical Mechanics failed miserably. One of The first critical questions, was why electrons can orbit a nucleus while not giving off electromagnetic waves and crashing into the nucleus. QM provided the answer and a new way to look at nature.

Checkout some of the older threads below for more insight. This question has been asked many times before.

I know. But do you think:

1. Is classical physics independent from quantum physics?

2. Is classical physics an approximation derived from quantum physics?

Here is the subtleness:

If you believe in 1. The basis of hilbert space is independent of classical physics.

If you believe in 2. The basis of hilbert space is the primary or fundamental and classical physics an approximation derived from quantum mechanics. This is the position of Demystifier. I read this in the archives in one of Demystifier old posts yesterday and lost track of the threads.

I just want to know what is the position of other physicists regarding the above distinctions?

 

[jedishrfu]

We are not all physicists here and so our opinion is our own.

Perhaps if you take the time to read the related threads listed below you'll get an answer to your question.

We know 1 is false since QM extends and replaces CM in the subatomic world and we know that 2 is true that classical phenomena are often explainable from their underlying quantum phenomena.

However, its also true that QM has elements that CM cannot explain.

 

[bahamagreen]

Quotes attributed to Bohr...

“The very nature of the quantum theory ... forces us to regard the space-time coordination and the claim of causality, the union of which characterizes the classical theories, as complementary but exclusive features of the description, symbolizing the idealization of observation and description, respectively.”
― Niels Bohr

"Every description of natural processes must be based on ideas which have been introduced and defined by the classical theory."
― Niels Bohr

“Physics is not about how the world is, it is about what we can say about the world.”
― Niels Bohr

“In our description of nature the purpose is not to disclose the real essence of the phenomena but only to track down, as far as possible, relations between the manifold aspects of our experience.”
― Niels Bohr

“Everything we call real is made of things that cannot be regarded as real.”
― Niels Bohr

“The Stone Age didn't end because the World ran out of stones.”
― Niels Bohr

 

[Nugatory]

1. Is classical physics independent from quantum physics?

People did classical physics for centuries before anyone noticed quantum phenomena, and even today many people study classical physics in great depth without going anywhere near any quantum mechanics. So in that sense, yes, it is possible and even natural to consider classical physics as independent of quantum physics.

2. Is classical physics an approximation...

Yes. It’s a good one, so good that for most of the 18th and 19th centuries it was presumed to be exact.

...derived from quantum physics?

No. It was not derived, it was discovered through observation and analysis long before quantum mechanics was discovered. However, once we know about QM we can demonstrate that classical mechanics emerges from it. That tells us that QM is a more complete theory, working everywhere that classical mechanics does and also in some places where it doesn’t. It doesn’t change the fact that classical physics is and always has been a well-founded discipline in its own right.

 

[new6ton]

People did classical physics for centuries before anyone noticed quantum phenomena, and even today many people study classical physics in great depth without going anywhere near any quantum mechanics. So in that sense, yes, it is possible and even natural to consider classical physics as independent of quantum physics.
Yes. It’s a good one, so good that for most of the 18th and 19th centuries it was presumed to be exact.No. It was not derived, it was discovered through observation and analysis long before quantum mechanics was discovered. However, once we know about QM we can demonstrate that classical mechanics emerges from it. That tells us that QM is a more complete theory, working everywhere that classical mechanics does and also in some places where it doesn’t. It doesn’t change the fact that classical physics is and always has been a well-founded discipline in its own right.

If classical mechanics emerges from quantum mechanics, then the basis of hilbert space is more fundamental. So in the case of observables like position or spin. Then the position or spin basis is more fundamental. Is this correct?

 

[Nugatory]

If classical mechanics emerges from quantum mechanics, then the basis of hilbert space is more fundamental. So in the case of observables like position or spin. Then the position or spin basis is more fundamental.

It’s hard to agree or disagree as long as the discussion is using undefined terms like “more fundamental”.

 

[new6ton]

It’s hard to agree or disagree as long as the discussion is using undefined terms like “more fundamental”.

fundamental - something that can't be reduced to something simpler

If quantum mechanics is fundamental. Then the basis or hilbert space that represents something is more fundamental. "More fundamental" in the sense that comparing it to classical physics. Classical physics is derived from quantum physics.

The consequence is that Hilbert space that represents something is the more basic. So this Hilbert space that represents something is more basic than Newtonian mechanics which is just coarse graining of QM.

Demystifier position (pun unintended) is that the position basis of objects is more fundamental. Without the position or spin basis, objects won't even exist or have spins. Others disagree. So I want to see the comments or positions of others regarding this.

 

[PeterDonis]

fundamental - something that can't be reduced to something simpler

If quantum mechanics is fundamental.

We don't know that quantum mechanics as we have it today is fundamental in this sense. There might be another simpler theory underlying quantum mechanics that we just haven't discovered yet. (The search for a theory of quantum gravity can be thought of as a search for such a theory.)

"More fundamental" in the sense that comparing it to classical physics. Classical physics is derived from quantum physics.

Yes, this is true. And the opposite is not true; you can't derive quantum physics from classical physics.

Demystifier position (pun unintended) is that the position basis of objects is more fundamental.

Where are you getting this from? Please give a specific reference.

 

[new6ton]

We don't know that quantum mechanics as we have it today is fundamental in this sense. There might be another simpler theory underlying quantum mechanics that we just haven't discovered yet. (The search for a theory of quantum gravity can be thought of as a search for such a theory.)
Yes, this is true. And the opposite is not true; you can't derive quantum physics from classical physics.
Where are you getting this from? Please give a specific reference.

I read a hundred threads yesterday and still looking for the exact passage. But this one is part of it:

"
There is actually a simple explanation why all measurements can ultimately be reduced to measurements of positions. This is because all measurements require a macroscopic apparatus, for which decoherence determines the preferred basis. But the decoherence-induced preferred basis is the position basis, due to the fact that interactions are local in the position space. "

Source https://www.physicsforums.com/threa...d-positions-at-all-times.499976/#post-3310760

In another page. It's about what would occur if there was no preferred basis of position. I'm aware that classical positions of a ping pong ball is the expectation value of the center of mass position of the ping pong ball. But without preferred basis of position, would the ping pong ball even exist?

I'm still searching for the relevant passages. Thank you.

 

[new6ton]

I read a hundred threads yesterday and still looking for the exact passage. But this one is part of it:

"
There is actually a simple explanation why all measurements can ultimately be reduced to measurements of positions. This is because all measurements require a macroscopic apparatus, for which decoherence determines the preferred basis. But the decoherence-induced preferred basis is the position basis, due to the fact that interactions are local in the position space. "

Source https://www.physicsforums.com/threa...d-positions-at-all-times.499976/#post-3310760

In another page. It's about what would occur if there was no preferred basis of position. I'm aware that classical positions of a ping pong ball is the expectation value of the center of mass position of the ping pong ball. But without preferred basis of position, would the ping pong ball even exist?

I'm still searching for the relevant passages. Thank you.

I can't find the exact passages but would like to know the following basic first:

If atoms don't have preferred basis, could molecules even form?
How does an atom behave without any preferred basis of position?

The answer is not clear because even if the atom doesn't have position, it has interactions so how does the preferred basis of position affect the behavior of atoms or molecules?

 

[PeterDonis]

I read a hundred threads yesterday and still looking for the exact passage. But this one is part of it

This is talking about a particular interpretation of QM. You can't conclude anything about actual physics from a particular interpretation, since all interpretations make the same predictions for all experimental results.

If atoms don't have preferred basis, could molecules even form?
How does an atom behave without any preferred basis of position?

The answer is not clear because even if the atom doesn't have position, it has interactions so how does the preferred basis of position affect the behavior of atoms or molecules?

This "preferred basis" is part of a particular intepretation. But the existence of atoms and molecules is an experimental fact. So the questions you're asking aren't answerable, because you're mixing up the claims of a particular interpretation with experimental facts.

Also, none of this changes in any way the fact that classical physics can be derived as an approximation from quantum physics, but not the other way around. The "preferred basis of position" you refer to is not the same thing as "classical physics".

 

[Elias1960]

"
There is actually a simple explanation why all measurements can ultimately be reduced to measurements of positions. This is because all measurements require a macroscopic apparatus, for which decoherence determines the preferred basis. But the decoherence-induced preferred basis is the position basis, due to the fact that interactions are local in the position space. "
Source https://www.physicsforums.com/threa...d-positions-at-all-times.499976/#post-3310760

Unfortunately, that thread is no longer open for replies else I would object there.

Spoiler: off-topic remark about this

(which seems to be the standard for old threads, IMHO a very bad idea)

In fact, decoherence is not sufficient to define a preferred basis out of nothing, it requires some additional structure as already given. If you have, in real experiments, already a real measurement device, a real system you measure and if you have clarified what you consider as the environment (that means, everything else, which is relevant for the experiment only because of its ability to distort it), then decoherence works fine. If you have simply a Hamilton operator, it is not sufficient to give you a preferred basis.

As an example that decoherence does not give much without additional assumptions, I would recommend considering quantum condensed matter theory. Mathematically, it is not that much different from QFT. Then, in this analogy, the "positions of particles" would be the positions of phonons. Not the positions of the atoms of the condensed matter. How would one, in the large distance limit, find out using decoherence, that the phonon positions are nothing preferred in comparison with the positions of atoms (which in this analogy would be the field ontology)?

 

[PeterDonis]

which seems to be the standard for old threads, IMHO a very bad idea

Complaints about the mechanics of Physics Forums should be directed to the moderators in either a separate thread in the Feedback and Announcements forum, or by private conversation. Such complaints are off topic in an ordinary thread.

 

[new6ton]

This is talking about a particular interpretation of QM. You can't conclude anything about actual physics from a particular interpretation, since all interpretations make the same predictions for all experimental results.
This "preferred basis" is part of a particular intepretation. But the existence of atoms and molecules is an experimental fact. So the questions you're asking aren't answerable, because you're mixing up the claims of a particular interpretation with experimental facts.

Also, none of this changes in any way the fact that classical physics can be derived as an approximation from quantum physics, but not the other way around. The "preferred basis of position" you refer to is not the same thing as "classical physics".

Let's describe pure experimental facts without any interpretations.

Positions from center of mass of objects can occur even if the atoms don't have any positions. So it's ok to say that interactions (or quantum fields) that don't have positions were able to produce classical positions of objects. Is this right?

But then when one detects in the detector of the double slit experiment. The detection has positions. What produce the positions in the detector? Quantum field interacting with quantum fields only?

But why is there position when quantum fields interact with quantum fields? Is position apriori from the math or is position ad hoc or arbitary in the course of quantum fields?

 

[PeterDonis]

Let's describe pure experimental facts

If you're going to do that you need to describe the actual experiments and their accuracy.

Positions from center of mass of objects can occur even if the atoms don't have any positions.

How are these positions determined experimentally? With what accuracy?

when one detects in the detector of the double slit experiment. The detection has positions.

How are these positions determined experimentally? With what accuracy?

why is there position when quantum fields interact with quantum fields?

Quantum fields are theoretical entities; nobody directly observes them. I thought you said you wanted to talk about pure experimental facts.

 

[Demystifier]

How would one, in the large distance limit, find out using decoherence, that the phonon positions are nothing preferred in comparison with the positions of atoms (which in this analogy would be the field ontology)?

To talk about decoherence of a phonon, one should study how phonon interacts with its environment. In condensed matter physics one often writes down the Hamiltonian which describes interaction of phonons with electrons. But this Hamiltonan has a simple form in the k-space, not in the position space (see e.g. http://www.phys.ufl.edu/~pjh/teaching/phz7427/7427notes/ch4.pdf ). When the Hamiltonian is rewritten in the position space, one can see that the interaction is non-local. Of course, this is an effective Hamiltonian, not a fundamental one, so it is not in direct contradiction that the fundamental interactions of the Standard Model are local. But this effective Hamiltonian shows that the position basis is not a natural basis for phonon measurements. To measure the phonon position, one should devise a different kind of interaction. Perhaps it could be some interaction of phonons with local impurities, which might lead to something like Anderson localization, but for phonons. Such a localized interaction could serve as a phonon detector, so one could perform a 2-slit experiment with a single phonon. One would then have a phonon that travels through both slits, after which the phonon position is measured. The distribution of positions would show characteristic interference lines.

 

[vanhees71]

The qualitative reason to understand this is simple: Phonons are "quasiparticles". The physics of them, i.e., the "thing" observable in the lab (in this case in everyday life) is that they are vibrations (i.e., sound waves) of a piece of macroscopic matter, not some point-like object that you'd call a particle in everyday life.

The name "quasiparticle" comes about, because you can treat, e.g., the lattice vibrations of a solid in a way which is mathematically very similar to a desciption of (free) particles in QFT. The art in many-body theory is to find the right effective degrees of freedom to describe a phenomenon for a situation (if you are lucky at or close to thermal equilibrium) in terms of an ideal gas of these effective degrees of freedom. E.g., a modern way to understand the famous Debye theory of specific heat of solids at low temperature (where it explains the deviation from the classical Dulong-Petite rule) is to describe the corresponding lattice vibrations in terms of an ideal gas of (bosonic) quasiparticles.

Then, as @Demystifier describes, you can use perturbation theory to describe also electrons in this solid using standard the scattering formalism in terms of Feynman diagrams as interactions of electrons with phonons. This may give also rise to more surprising things like superconductivity which is described by other quasiparticles called Cooper pairs (named after the theory by Bardeen, Cooper, and Schriefer). Also here the Cooper pairs are not localized objects but rather paired electrons of opposite spin at opposite diameters of the fermi sphere, i.e., again a very specific collective excitation.

 

[new6ton]

l t

If you're going to do that you need to describe the actual experiments and their accuracy.
How are these positions determined experimentally? With what accuracy?
How are these positions determined experimentally? With what accuracy?
Quantum fields are theoretical entities; nobody directly observes them. I thought you said you wanted to talk about pure experimental facts.

I don't understand why you kept asking "How are these positions determined experimentally? With what accuracy?". In the photoelectric effect, the positions of the photons must be coincident to the positions of the electrons for there to be ejections.

I just want to know if positions are apriori like space being part of spacetime or it is a result of some kind of symmetry breaking where there is no position when some symmetry is recovered.

Now let's talk theoretically and not just present experimental facts which are not yet final or complete. Someday. If we can perform experiments to remove the position aspect of atoms. Does it mean the ping pong ball is still there because classical position is a result of interactions between atoms and not due to positions between atoms? Or in a multiverse version with different laws of physics where position basis in the formula don't exist for example. Does it mean classical positions can still exist without position basis? This is not mixing intepretations and experimental facts. I want to think about a universe where Many Worlds without position basis were experiment facts.

You may say not to think of such when it is not part of our world. I just want to learn the conceptual understanding and flexibility one can arrive by applying to different scenarios that can enable one to learn the concepts better. Hence kindly address the above and not just say to only think about it if our universe was really described by Many worlds without position basis. Learning is understanding things even hypothetically. Thank you.

 

[PeterDonis]

In the photoelectric effect, the positions of the photons must be coincident to the positions of the electrons for there to be ejections.

How accurately do we know the positions of the electrons? Here's a hint: in quantum mechanics, electrons never have positions known to infinite accuracy, i.e., narrowed down to a single point. That's impossible.

I just want to know if positions are apriori like space being part of spacetime or it is a result of some kind of symmetry breaking where there is no position when some symmetry is recovered.

This doesn't even make sense.

Now let's talk theoretically

Personal speculations are off limits here, and that's what the rest of your post is.

 

[PeterDonis]

The OP question has been answered and the thread has become personal speculation. Thread closed.