Graduate Are quantum fields real objects in space?

[martinbn]

Copenhagen has evolved and changed since Bohr and Einsteins time:
https://arxiv.org/pdf/1511.01069.pdf

These days modern Copenhagenists seem to be switching to Decoherent Histories they call Copenhagen done right just as Feynman did at the end. Gell-Mann thinks it's basically MW without the many worlds ie just one world.

That seems to be the emerging consensus interpretation - but there are many others about - Copenhagen is still the most generally held view according to Sean Carrol:
http://www.preposterousuniverse.com/blog/2013/01/17/the-most-embarrassing-graph-in-modern-physics/

Thanks
Bill

Ok, there are versions of the interpretation. My question is according to which version the fields are not real objects in space. That was the statement that @atyy made.

 

[DarMM]

Ok, there are versions of the interpretation. My question is according to which version the fields are not real objects in space. That was the statement that @atyy made.

@atyy knows better, but some versions of Copenhagen like that of Rudolf Haag would just posit that an observer working in spacetime region $\mathcal{O}$ can measure quantities associated with the C*-algebra $\mathcal{A}(\mathcal{O})$ and that the C*-algebra has a basis consisting of observables of the form $e^{i\phi(f)}$.

This would just see observables as being formed from fields, i.e. one has a map $f: \phi \rightarrow \mathcal{A}(\mathcal{O})$, but without having fields as real objects in the theory.

 

[Demystifier]

Well, if the particle doesn't exist, then what do you measure? So, there is no particle, I guess it is vacuum. Then you measure the spin(of what, the vacuum?) and pop the particle exists, but only for a moment, only when measure, then it doesn't exist until the next measurement. That seems a very strange way of describing the situation. And I don't see that, nor anything that would suggest it, in the papers you cited.

I don't think you answered my question.

 

[Demystifier]

Thus approximate position and approximate momentum must exist before measurement, or all of our experiments do not make sense.

I disagree. For instance, beauty does not exist until one observes it (beauty is in the eyes of the beholder), yet it doesn't mean that observation of beauty does not make sense. Of course, the shape of the beautiful object exists before the observation, but the shape by itself is not beautiful.

Another, more physical example is the color. The EM wave has its wavelength even without observation, but it has a color only when someone observes it.

 

[martinbn]

@atyy knows better, but some versions of Copenhagen like that of Rudolf Haag would just posit that an observer working in spacetime region O\mathcal{O} can measure quantities associated with the C*-algebra A(O)\mathcal{A}(\mathcal{O}) and that the C*-algebra has a basis consisting of observables of the form eiϕ(f)e^{i\phi(f)}.

This would just see observables as being formed from fields, i.e. one has a map f:ϕ→A(O)f: \phi \rightarrow \mathcal{A}(\mathcal{O}), but without having fields as real objects in the theory.

How is this related to whether the fields are real objects in space or not?

I don't think you answered my question.

Do you mean this question?

What exactly are the quantities (or qualities) of the particle that do exist without measurement?

What do you mean by quantities and qualities? It exists and its state can be fully described by a vector in a certain vector space.

 

[martinbn]

But you didn't answer my question. If the particle doesn't exist, does it mean that we have vacuum i.e. empty space?

 

[Demystifier]

It exists and its state can be fully described by a vector in a certain vector space.

OK, now you answered my question so I can proceed. Essentially, you identify the particle with the corresponding state in the Hilbert space. The particle is the state in the Hilbert space. Fine, now let us see what are the consequences of this statement.

Consider a state which before the measurement is a Gaussian wave function with a very large width $\Delta x$ in the position space. So far so good. But now assume that we perform the measurement of the particle position. This means that the wave function changes to a new wave function with a new width $\delta x$ ,where $\delta x\ll \Delta x$. This change is called the wave function collapse. But the collapse is non-local, it happens faster than light. And that's the problem.

A way out of this problem is to say that collapse is not a real physical event, but only the update of our knowledge. However, you are not allowed to use that argument, because you essentially said that the wave function is the particle. This means that the collapse is a real physical event, and not only an update of knowledge.

Do you agree with my sequence of arguments? Do you accept that measurement involves a nonlocal collapse as a real physical event? Do you find such kind of nonlocality problematic?

 

[DarMM]

How is this related to whether the fields are real objects in space or not?

I don't understand, if the fields are not taken as really existing, but just taken as mathematical method for generating basis elements for an observable algebra, then they're not real objects in space right? It's directly related to it by saying they're not real. I genuinely don't understand, it seems fairly clearly related.

 

[DarMM]

I disagree. For instance, beauty does not exist until one observes it

Does this not ignore those who are objectively handsome/beautiful such as us Advisors?

 

[Demystifier]

But you didn't answer my question. If the particle doesn't exist, does it mean that we have vacuum i.e. empty space?

If you think that it is not easy to make sense of interpretation in which particle does not exist until measured, I perfectly agree with you. One needs to work hard to make sense of it. As a result of such a hard work, I wrote the paper http://de.arxiv.org/abs/1112.2034

 

[bhobba]

Does this not ignore those who are objectively handsome/beautiful such as us Advisors?

Maybe that's why I became a mentor - my mirror tells me different every morning - my shaver even started to shut down when I used it. Disconcerting actually - but I eventually sorted the shaver out - took it to the place I bought it from who puled it to pieces - even my whiskers are against me - they clogged it up despite me cleaning it every day - it turns out you have to pull it to pieces and get rid of bits in the things that spin and foul it.

Thanks
Bill

 

[martinbn]

OK, now you answered my question so I can proceed. Essentially, you identify the particle with the corresponding state in the Hilbert space. The particle is the state in the Hilbert space. Fine, now let us see what are the consequences of this statement.

No, not at all. The particle is not the state. Take for example classical mechanics. The particle is described by six numbers. The particle is not a six-tple of numbers.

I don't understand, if the fields are not taken as really existing, but just taken as mathematical method for generating basis elements for an observable algebra, then they're not real objects in space right? It's directly related to it by saying they're not real. I genuinely don't understand, it seems fairly clearly related.

You are using the same word in two different ways, which causes the confusion. The fields are the physical objects that exists in space (and time), which we want to study. The operators, which you also call fields, are the mathematical description, and of course they don't exist, it is meaningless to say that they do.

If you think that it is not easy to make sense of interpretation in which particle does not exist until measured, I perfectly agree with you. One needs to work hard to make sense of it. As a result of such a hard work, I wrote the paper http://de.arxiv.org/abs/1112.2034

But, there is no such interpretation. At least so far you havn't shown one. In all your citations there wasn't even a hint that particles/fields don't exist. And you didn't answer my question. If the particle doesn't exist, do we have empty space?

 

[DarMM]

Maybe that's why I became a mentor

Well you mentors are on a whole other level, not merely good looking but so much so that us mortals can only grasp it dimly.

 

[DarMM]

You are using the same word in two different ways, which causes the confusion. The fields are the physical objects that exists in space (and time), which we want to study. The operators, which you also call fields, are the mathematical description, and of course they don't exist, it is meaningless to say that they do.

No, I'm not using it in two sense. In Haag's description there is nothing physically real obeying field equations, hence there are no physically real fields. There is only the algebra of local observables. Fields only appear as one method of constructing the algebra.

Other methods exist:
https://arxiv.org/abs/1005.2656

 

[bhobba]

Well you mentors are on a whole other level, not merely good looking but so much so that us mortals can only grasp it dimly.

Yea right. We are so good looking we have an area we have to look at to discuss posts that need looking at. I am sure its meant to age us so whatever looks we have are soon gone - assuming they are there to begin with. The things we do so people have a reliable source of science/math/engineering . Seriously it is nice helping people with science stuff in a different way than just contributing to threads - I really enjoy it.

Thanks
Bill

 

[DarMM]

Yea right. We are so good looking we have an area we have to look at to discuss posts that need looking at. I am sure its meant to age us so whatever looks we have are soon gone - assuming they are there to begin with.

No sympathy from me, you know it takes six of us Advisors to carry you lot in one of your golden palanquins?

(Alright I'll stop dragging the thread off topic now! )

 

[martinbn]

No, I'm not using it in two sense. In Haag's description there is nothing physically real obeying field equations, hence there are no physically real fields. There is only the algebra of local observables. Fields only appear as one method of constructing the algebra.

I think the problem in communication comes in the use of the phrase "physically real". Here is the question, is there something out there or do we have empty space? If there is something, we need to give it a name, say a particle or a field. That is what is physically real, that is what exists. That thing can be described mathematically by say a mathematical field, that obeys the equations. Of course the mathematical object doesn't exist, but the there is something that does, which is also called a field.

I don't understand why this whole busyness is so hard to understand. Even with my poor writing skills, it should be clear.

 

[Demystifier]

No, not at all. The particle is not the state. Take for example classical mechanics. The particle is described by six numbers. The particle is not a six-tple of numbers.

But whatever mathematical property follows from those six numbers, we say it is a property of the particle itself. For instance, if the particle equations of classical mechanics have a property of nonlocality, as e.g. in Newton theory in gravity, we say that particles themselves obey nonlocal laws.

So in that sense, do you agree that your view of particle in QM implies non-locality? If not, why not?

And you didn't answer my question. If the particle doesn't exist, do we have empty space?

In a sense, yes. We have the wave function, but not a physical object associated it. This is like "classical mechanics" in which we have the Hamiltonian $H(x,p)$, but not a particle with the trajectory $X(t)$.

 

[martinbn]

But whatever mathematical property follows from those six numbers, we say it is a property of the particle itself. For instance, if the particle equations of classical mechanics have a property of nonlocality, as e.g. in Newton theory in gravity, we say that particles themselves obey nonlocal laws.

So in that sense, do you agree that your view of particle in QM implies non-locality? If not, why not?

Wait, what has that to do with the discussion? Locality or non-locality is a separate question. The question is are the fields/particles real in space?

In a sense, yes. We have the wave function, but not a physical object associated it. This is like "classical mechanics" in which we have the Hamiltonian H(x,p)H(x,p), but not a particle with the trajectory X(t)X(t).

This seems very inconsistent. If we have empty space, then what is the difference between one particle and two? It's just empty space in both cases. In the example of classical mechanics what is the Hamiltonian of? How can you not have a particle?

 

[DarMM]

I think the problem in communication comes in the use of the phrase "physically real". Here is the question, is there something out there or do we have empty space? If there is something, we need to give it a name, say a particle or a field.

In Haag and other versions of Copenhagen what is taken to exist are events in macroscopic objects like detection devices. Fields are simply tools used to assists in computing correlations between these events.

What actually does exist they are silent on. Though Bohr, Omnés and Haag take a similar view that the fundamentally stochastic nature of QM and results like Bell's theorem (obviously not in the case of Bohr) indicate a limit in the applicability of human mathematics to nature and thus the fundamental stuff is incomprehensible.

So the reason one has no hidden variables is because the "stuff" doesn't admit a mathematical description.

That thing can be described mathematically by say a mathematical field, that obeys the equations

In this view the physically real things do not obey field equations.

 

[martinbn]

In Haag and other versions of Copenhagen what is taken to exist are events in macroscopic objects like detection devices. Fields are simply tools used to assists in computing correlations between these events.

What actually does exist they are silent on. Though Bohr, Omnés and Haag take a similar view that the fundamentally stochastic nature of QM and results like Bell's theorem (obviously not in the case of Bohr) indicate a limit in the applicability of human mathematics to nature and thus the fundamental stuff is incomprehensible.

So the reason one has no hidden variables is because the "stuff" doesn't admit a mathematical description.

Ok, now I understand, and it is perfectly fine. My problem is not with what actually exists, but with atyy's claim that it doesn't exist. I did say earlier that in my impression Copenhagen is silent on the issue. Demistifier said that some versions are not silent and say that the particle/field doesn't exist.

In this view the physically real things do not obey field equations.

Of course not. The baseball ball doesn't obey any equations. The functions that describe it obey the equations.

 

[Demystifier]

Wait, what has that to do with the discussion? Locality or non-locality is a separate question. The question is are the fields/particles real in space?

It's related, as seen e.g. in the EPR argument and the Bell theorem.

 

[bhobba]

I think the problem in communication comes in the use of the phrase "physically real". Here is the question, is there something out there or do we have empty space?

We do give it a name EM fields etc. I explained why classically most physicists consider them real - they carry, via Noether, momentum and energy which generally physicists think of as real. It's of course a deep philosophical question if they are, but if you are commonsenseical you tend to go down that path. As Weinberg said in his article about Kuhn (I must be frank I am no fan of Kuhn, Popper is better, but even he doesn't quite capture it as Feynman does) when he talks about reality, and wants to be careful, he says - whatever that is. Its just that physically most think of things like energy and momentum as real - again using whatever conception of real they hold to. After all mass is a form of energy, so fields can in principle be converted to mass, and if you do not think of mass as real, again under whatever you think real is, you are in very strange territory indeed (Penrose may be in that very territory) - I think physicists will generally not go that far. Now if classical EM are real and they are a limit of EM QFT fields, it's hard to think exactly at what point in taking that limit it becomes real, so generally speaking most would think them real. Remember though a QFT field is a field of quantum operators and we know they have a very real aspect - the eigenvalues are the possible outcomes of observations - and observations are very real. Some say QM is incomplete because we do not know exactly what is an observation - but that is getting off topic.

Thanks
Bill

 

[DarMM]

Ok, now I understand, and it is perfectly fine. My problem is not with what actually exists, but with atyy's claim that it doesn't exist. I did say earlier that in my impression Copenhagen is silent on the issue. Demistifier said that some versions are not silent and say that the particle/field doesn't exist.

Of course not. The baseball ball doesn't obey any equations. The functions that describe it obey the equations.

Well more so, if there are no objects describing them that obey field or particle equations you cannot really call them particles or fields. I mean those terms really only mean "a thing described by particle/field theories". So there is "stuff" but it's not particles or fields.

I'm not saying I agree with this, but I do think @atyy's description is right.

 

[martinbn]

Well more so, if there are no objects describing them that obey field or particle equations you cannot really call them particles or fields. I mean those terms really only mean "a thing described by particle/field theories". So there is "stuff" but it's not particles or fields.

That's just terminology, whether it is particles, fields or something else is secondary for this question. The point is that there is something.

I'm not saying I agree with this, but I do think @atyy's description is right.

Well, he claims that there isn't anything according to Copenhagen. He doesn't say that there is something, which isn't fields, and the fields aren't real, they are mathematical descriptions of something real. He says that if no one measures there is nothing.

 

[Demystifier]

If we have empty space, then what is the difference between one particle and two?

When we perform the measurement (which for no-reality interpretations is a misnomer, one should rather call it the experiment), we hear one or two clicks in the detector. That's the difference.

In the example of classical mechanics what is the Hamiltonian of? How can you not have a particle?

Mathematically it makes perfect sense to have a Hamiltonian as an object by its own. Physically, for a version of classical mechanics without particle trajectories see my https://link.springer.com/article/10.1007/s10702-006-1009-2

 

[Demystifier]

The point is that there is something.

The Bell theorem says that if there is something, then this something obeys nonlocal laws. And yet, if I remember correctly, in other threads you deny nonlocality. My point is that it is inconsistent to accept that both (i) there is something and (ii) this something obeys local laws.

 

[Demystifier]

But, there is no such interpretation. At least so far you havn't shown one. In all your citations there wasn't even a hint that particles/fields don't exist.

How about the following quotes of Mermin, taken from https://en.wikipedia.org/wiki/Relational_quantum_mechanics#History_and_development :
David Mermin has contributed to the relational approach in his "Ithaca interpretation."[8] He uses the slogan "correlations without correlata", meaning that "correlations have physical reality; that which they correlate does not", so "correlations are the only fundamental and objective properties of the world".

In the same paragraph on wikipedia:
The moniker "zero worlds"[9] has been popularized by Ron Garret[10] to contrast with the many worlds interpretation.

 

[DarMM]

The way I see Bell's theorem and the interpretive camps.

The assumptions of the theorem are:

1. Ontological Framework Axioms (Single World, No RetroCausality, No Superdeterminism)
2. Relativistic Causation, i.e. no physical effects that reach outside their relativistic light cone
3. Common cause. That is some event can be considered the cause of other events, i.e. C is a common cause of A,B if there correlations would be absent without C
4. Decorrelating Explanation. There is an event, conditioned on which the correlations between A,B vanish, hence it explains their correlation.

Non-Realist interpretations tend to drop number 4, that is they don't view the correlations in Bell's inequality as being explained by any event in spacetime. There can be causes (i.e. the device that prepares the Bell state), however that only allows the correlations to exist, it doesn't explain them.

Or more clearly, the preparation of the Bell state is necessary to find the correlations, but what actual achieves them is not a mechanistic (in the sense of admitting a mathematical description) process occurring in spacetime.

 

[atyy]

Well, he claims that there isn't anything according to Copenhagen. He doesn't say that there is something, which isn't fields, and the fields aren't real, they are mathematical descriptions of something real. He says that if no one measures there is nothing.

There is certainly nothing you are reading correctly.