Who is Ballentine and why is he important in the world of quantum mechanics?

[Kontilera]

Im in no position to answer what a quantum field is, but questions such as this one might often lead astray to rather philosophical inquiries. If thats not what is asked for, more concrete questions might help the understand the concept. Such as:

1. How does it transform under coordinate change?
2. If it can be considered as a mapping, beween which spaces does it map?
3. What physical information can be calculated from a quantum field?

 

[vanhees71]

It must be nearly stationary during the time of observation. For less stationary sources with a known evolution law one gets a 1-point function heavily smeared in time.

Well, then you don't like this year's physics Nobel prize?

 

[DrChinese]

The evolution law of the universe is known only to God. But it is assumed to exist and to be deterministic and observer independent. In this sense it is objective. Observer form their own approximate models of this evolution law.

Wow, those assumptions fly in the face of almost every known quantum phenomena, whether quantum fields or particles or systems. I guess I'm surprised to hear this coming from you. Maybe I haven't read enough of your posts in this thread to understand your context, and if so, my apologies.

a) There is no currently known cause of apparently indeterministic outcomes of quantum measurements, so why would anyone assume there is such a cause?

b) Every prediction made on entangled systems (such as Bell or GHZ) is strictly observer dependent; and in fact the only relevant components of that prediction lie in a future measurement context freely* chosen by... observers. How does that point to objectivity? The field - whatever you may think it is - is not a factor in that calculation at all. (Obviously, any contribution it makes must cancel out, and can effectively be ignored completely.)

c) I won't even mention the usage of the word "evolution" in regards to quantum (as opposed to classical) systems or fields - other than to ask: How the passage of time (whether a nanosecond or a millennium) change anything? Time doesn't seem to a factor (in general, obviously there are exceptions where time is an explicit dependency).*Apparently.

 

[PeroK]

The evolution law of the universe is known only to God.

He should have been invited to the Solvay Conference!

 

[A. Neumaier]

Wow, those assumptions fly in the face of almost every known quantum phenomena, whether quantum fields or particles or systems. I guess I'm surprised to hear this coming from you. Maybe I haven't read enough of your posts in this thread to understand your context, and if so, my apologies.

The context is my thermal interpretation of quantum physics.
This is a deterministic theory in which the traditional stochastic observables of quantum mechanics are denied to be observables. Instead the N-point functions of quantum field theories, which have a deterministic evolution law, are identified as the beables of quantum field theory in the sense of Bell.

a) There is no currently known cause of apparently indeterministic outcomes of quantum measurements, so why would anyone assume there is such a cause?

Because one can hold the philosophical view that there is no effect without a cause. Then having found no cause only means one has not understood enough.

1000 years ago all what we now call science was unknown, and no causes were known for what we now understand as being caused. Why should it today be different compared to 1000 years in the future?

b) Every prediction made on entangled systems (such as Bell or GHZ) is strictly observer dependent; and in fact the only relevant components of that prediction lie in a future measurement context freely* chosen by... observers. How does that point to objectivity?

In a deterministic universe, free choices are only apparent. Indeed, all our free choices are actually determined by our motivations, experiences, goals, feelings, fits of the moment, etc.!

The field - whatever you may think it is - is not a factor in that calculation at all. (Obviously, any contribution it makes must cancel out, and can effectively be ignored completely.)

The field carries whatever causes the detection events. Without fields no transmission of information.

c) I won't even mention the usage of the word "evolution" in regards to quantum (as opposed to classical) systems or fields - other than to ask: How the passage of time (whether a nanosecond or a millennium) change anything? Time doesn't seem to a factor (in general, obviously there are exceptions where time is an explicit dependency).

Time is a parameter in the evolution of a quantum state, hence the state changes with time (unless the state is stationary).

 

[A. Neumaier]

He should have been invited to the Solvay Conference!

He is outside the universe, but determined the Solvay Conference to happen in the form it actually happened....

 

[A. Neumaier]

Well, then you don't like this year's physics Nobel prize?

How does that relate to my statement that you quoted?

 

[vanhees71]

I mean, in the work leadking to this year's physics Nobel prize, one indeed observes the time evolution of electrons in atoms/molecules on the atto-second resolution level.

 

[A. Neumaier]

In classical physics we prepare one electromagnetic field and can measure it anywhere with a single measurement, provided the intensity of the field is large enough. The smaller the intensity the large the exposure time needed for an accurate measurement.

The same holds verbatim in quantum field theory: We prepare one electromagnetic field and can measure it anywhere with a single measurement, provided the intensity of the field is large enough. The smaller the intensity the large the exposure time needed for an accurate measurement.

It must be nearly stationary during the time of observation. For less stationary sources with a known evolution law one gets a 1-point function heavily smeared in time.

If you think only statistically then you need to use this disguise. But If you think in terms of 1-point functions stationarity is not needed and one just has different smearing functions.

I mean, in the work leading to this year's physics Nobel prize, one indeed observes the time evolution of electrons in atoms/molecules on the atto-second resolution level.

This does not contradict my claims in this thread. For clarity I provided again the context for these claims, namely measuring the intensity of a field.

The observation of the short time dynamics of single electrons is quite different - it is a stochastic process where observation is very noisy. But the extracted information is again based on a statistical evaluation of the correlation function and its interpretation through the short time Fourier transform of a 2-point function. The latter is the true observable carrying the information of interest; the stochastic trajectory is just a means for arriving at this information.

 

[DrChinese]

Because one can hold the philosophical view that there is no effect without a cause. Then having found no cause only means one has not understood enough.

1000 years ago all what we now call science was unknown, and no causes were known for what we now understand as being caused. Why should it today be different compared to 1000 years in the future?

Even holding such a philosophical view (which admittedly is shared by many), you must admit the reasoning is essentially circular. No evidence is still no evidence, regardless of hope for the future.

I would agree it is reasonable (and in some ways useful) assumption. But it is the requirement that the causes precede the effect that gives me pause. I realize that requirement will be part and parcel of any interpretation with "thermal" in the name. But some degree of time symmetry in physics seems a relevant consideration too. Nonlocal contextuality has been demonstrated in so many recent experiments, and that seems to fly in the face of traditional concepts of causality. Ordering just doesn't matter, and in a deterministic universe, you'd think it would.

If you changed your "1000 years" to "100 years", your analogy might appear quite different (which is probably why you chose 1000 and not 100). By 1935, it was argued that QM was complete and there were no underlying "causes" to be found. 88 years later, it is even more evident - and certainly not less - that there is no meaningful root cause of quantum indeterminacy*. *The Bohmians got the jump on everyone by saying it is everywhere and now (but manifestly nonlocal), but is unknowable even in principle. Somehow their "unknowable in principle" seems to be a good bet no matter what interpretation we end up with.

 

[A. Neumaier]

Even holding such a philosophical view (which admittedly is shared by many), you must admit the reasoning is essentially circular. No evidence is still no evidence, regardless of hope for the future.

The thermal interpretation is positive evidence for the possibility of interpreting QFT deterministically and causally.

some degree of time symmetry in physics seems a relevant consideration too.

There is no evidence at all that Nature is time symmetric, but overwhelming evidence for the opposite. Time symmetry is simply a theoretical assumption made, supplemented by heuristics for nevertheless having a causal arrow.

Nonlocal contextuality has been demonstrated in so many recent experiments, and that seems to fly in the face of traditional concepts of causality. Ordering just doesn't matter, and in a deterministic universe, you'd think it would.

Since the thermal interpretation is nonlocal contextual, there is no conflict here.

If you changed your "1000 years" to "100 years", your analogy might appear quite different (which is probably why you chose 1000 and not 100). By 1935, it was argued that QM was complete and there were no underlying "causes" to be found. 88 years later, it is even more evident - and certainly not less - that there is no meaningful root cause of quantum indeterminacy*.

I found meaningful causes in the chaoticity of the hierarchical dynamics of the N-point functions of QFT.

This is just as meaningful as the causes for randomness in classical mechanics.

 

[PeterDonis]

Time symmetry is simply a theoretical assumption made, supplemented by heuristics for nevertheless having a causal arrow.

I wouldn't say it's an assumption--it's a property of what have turned out to be our best current theories, that their equations are time symmetric. Since, as you say, we observe that Nature itself is not time symmetric, we then have to figure out how time symmetric equations can produce outcomes that aren't time symmetric, which then leads to the heuristics you mention.

 

[A. Neumaier]

I wouldn't say it's an assumption--it's a property of what have turned out to be our best current theories, that their equations are time symmetric.

Even time-symmetric equations do not imply that Nature is time-symmetric and only our perception of it is asymmetric.

To get sensible physics one has to assume causal asymmetry in addition to the time-symmetric equations! Causality is at the very root of theoretical physics!

Both in classical and quantum field theory, time symmetry is explicitly broken by choosing the retarded solutions as the ones that have physical reality, and ignoring the advanced solutions. This is done on the basis of the law of causality, that nonlocal causes never conspire to have exclusively local effects.

 

[PeterDonis]

Even time-symmetric equations do not imply that Nature is time-symmetric and only our perception of it is asymmetric.

If the equations are just approximations, yes, that's true; the more fundamental equations they are approximations of could be time asymmetric.

To get sensible physics one has to assume causal asymmetry in addition to the time-symmetric equations!

More precisely, if one grants that the equations are time-symmetric, one has to make use of the fact that time-symmetric equations can still have time-asymmetric solutions--as long as the solutions occur in pairs, each the time reverse of the other. And then one has to explain why we live in one solution of such a pair instead of the other. I don't know if this is what you mean by "assume causal asymmetry"; your next paragraph suggests that it might be.

 

[A. Neumaier]

If the equations are just approximations, yes, that's true; the more fundamental equations they are approximations of could be time asymmetric.

Independent of whether the equations are just approximations, the solutions of linear hyperbolic differential equations (such as the wave equation) form a vector space that is the direct sum of a space of retarded solutions and a space of advanced solutions. Causality is indispensible to explain why only the retarded solutions and neither purely advanced nor mixed solutions occur in Nature.

This cannot be explained by the common argument that we simply experience the direction in which entropy increases.

More precisely, if one grants that the equations are time-symmetric, one has to make use of the fact that time-symmetric equations can still have time-asymmetric solutions--as long as the solutions occur in pairs, each the time reverse of the other.

This would still allow superpositions of retarded and advanced solutions as long as the retarded one dominates.

And then one has to explain why we live in one solution of such a pair instead of the other.

But we have to explain why these unphysical superpositions never occur! I don't know of any other explanation than to assume causality.

 

[PeterDonis]

This would still allow superpositions of retarded and advanced solutions as long as the retarded one dominates.

Hm, I see; if you have linear equations the solution set is not limited to the "pure" ones, so to speak.

My personal "solution" would be that the ultimate fundamental equations are not linear. But I don't know if that option is being investigated.

 

[A. Neumaier]

My personal "solution" would be that the ultimate fundamental equations are not linear. But I don't know if that option is being investigated.

In general, the linear equations define the 1-particle spaces upon which the quantum field theories are erected. The equations are necessarily linear, otherwise one has no superposition principle. But the latter is essential for the particle interpretation in scattering processes.

For Fermions one gets a Dirac equation. But only positive energy solutions are permitted for the superpositions. Positive energy is a causality constraint.

 

[er404]

My personal "solution" would be that the ultimate fundamental equations are not linear. But I don't know if that option is being investigated.

https://arxiv.org/abs/2312.01992

In this work, we review and extend a version of the old attempt made by Louis de broglie for interpreting quantum mechanics in realistic terms, namely the double solution. In this theory quantum particles are localized waves, i.e, solitons, that are solutions of relativistic nonlinear field equations. The theory that we present here is the natural extension of this old work and relies on a strong time-symmetry requiring the presence of advanced and retarded waves converging on particles.

 

[Fra]

But we have to explain why these unphysical superpositions never occur! I don't know of any other explanation than to assume causality.

If one looks from the observer side (which you don't in the same way).

How fit would a physical agent/observer be in order to maintain and encode picture that contains what most of us think of "unphysical things"?

Whould such an observer be stable? It would likely take an incredibly complex amount of processing to make up for the "poor ansatz". So selection pressure is to adjust the ansatz and gain stability.

So the alternative to a "constraint" is to consider the probability for agent survival if some have strange ways to encode the same data, that may be "logically possibly" but at face with a more natural or more economical interpretation of the data. In this view one would presume that among the crazy options, the distribution would be sharpply peaked around the "physically reasonable", not because the other ones are not possible, but because they are inefficient for nature.

This is how I interpret the KG to Dirac transition as well. Lets suppose, that an observer acually DID observer something going backwards in time? (whatever it means). the question is still, would the agent likely INFER that it was something going backwards in time, or would the discovery of a new particle simply by more natural? The latter is how I see it. Why and in what sense it is more "natural", is something to be clarified and makde more precise of course. But fitness is an possible intuitive angle.

(this would then indirectly relate back to the dynamics of hte "background spacetime", as the background spacetime on which QM is formed, might be emergent in a similar way)

/Fredrik

 

[vanhees71]

This does not contradict my claims in this thread. For clarity I provided again the context for these claims, namely measuring the intensity of a field.

The observation of the short time dynamics of single electrons is quite different - it is a stochastic process where observation is very noisy. But the extracted information is again based on a statistical evaluation of the correlation function and its interpretation through the short time Fourier transform of a 2-point function. The latter is the true observable carrying the information of interest; the stochastic trajectory is just a means for arriving at this information.

Of course. If you measure the intensity of the em. field prepared in single- or few-photon Fock states, it's also a very noisy observation.

 

[A. Neumaier]

Of course. If you measure the intensity of the em. field prepared in single- or few-photon Fock states, it's also a very noisy observation.

But too noisy to extract any useful information, hence it cannot be called a measurement of the intensity of the field, which would produce numerical values for the intensity.

 

[vanhees71]

Of course, it's very useful information to register a photon with a photon detector. It's the only useful information you can get about a single photon. "Intensity" for a single photon has of course the meaning of a probability (density) for detecting a photon at a given region in space at a given time.

 

[physika]

Of course, it's very useful information to register a photon with a photon detector. It's the only useful information you can get about a single photon. "Intensity" for a single photon has of course the meaning of a probability (density) for detecting a photon at a given region in space at a given time.

Casually

https://www.azooptics.com/News.aspx?newsID=28527
.....

 

[A. Neumaier]

Of course, it's very useful information to register a photon with a photon detector. It's the only useful information you can get about a single photon. "Intensity" for a single photon has of course the meaning of a probability (density) for detecting a photon at a given region in space at a given time.

No. For a single photon, probability is operationally meaningless.

 

[vanhees71]

So you say all quantum-optics experiments with single photons are meaningless? That doesn't make sense. What you can know about a single photon operationally is the detection probability at a given time and place, given its state.

 

[gentzen]

So you say all quantum-optics experiments with single photons are meaningless? That doesn't make sense. What you can know about a single photon operationally is the detection probability at a given time and place, given its state.

He takes "single photon" literally. What makes no sense is his cryptic way of restarting that old discussion just now. No idea what he wants to achieve. Yes, you use words and concepts too much in the "you know what I mean" way, but this cryptic way of taking words literally won't change your mind either.

(We can have that discussion another time, but not today, or tomorrow, or ...)

 

[vanhees71]

Nowadays we indeed can take "single photon" literally since the quantum opticians can prepare true single-photon states (e.g., using parametric down conversion and using one of the entangled photons to herald the other one used for experiments). I don't know, what he means by the claim that probabilities for single photons were meaningless. The only thing, however, we have in QT for single quanta are probabilities. These are also not meaningless in an operational sense, because it simply means that when repeating the experiment with many equally prepared photons (operationally defining the "state" as the corresponding preparation procedure) you get in the limit of infinitely many such experiments the probability distribution for registering the photon at the place of the detector.

 

[PeterDonis]

So you say all quantum-optics experiments with single photons are meaningless?

No, he's saying that you can't use a single instance to test a statistical prediction, and QM's predictions are statistical.

 

[vanhees71]

Ok, but that's self-evident and not specific to photons.

 

[kurt101]

Nonlocal contextuality has been demonstrated in so many recent experiments, and that seems to fly in the face of traditional concepts of causality. Ordering just doesn't matter, and in a deterministic universe, you'd think it would.

And yet there is always a simple causal explanation for these experiments where order does not matter that you seem to ignore.

For instance, in the entanglement swapping experiment that you often cite as evidence for violating causality there is a simple explanation as to why photons 2 & 3 can tell you whether photons 1 & 4 were entangled. When photon 1 is measured, through the mechanism of entanglement it can provide all the information about the measurement angle to photon 2. When photon 4 is measured, through the mechanism of entanglement it can provide all of the information about the measurement angle to photon 3. So when photons 2 & 3 come together they have all the information about the measurements being used in the experiment to decide whether photons 1 & 4 were entangled or not.

 

[PeterDonis]

there is always a simple causal explanation

The problem with this view is that your so-called "simple causal explanation" is stated assuming a particular time ordering of the measurements. But the actual correlations in the experiments are the same regardless of the time ordering of the measurements.

In other words, for it to be the case (as it in fact is) that "order does matter", your "simple causal explanation" would have to allow "causes" that happen either spacelike separated from their "effects", or after their "effects" (i.e., future timelike or null separated), or that two events can be causally connected without there being any fact of the matter about which is the "cause" and which is the "effect". There are no other choices, and all of them are unpalatable. Strictly speaking, they aren't logically impossible (although they do imply a concept of "causation" which is highly at variance with all such concepts currently in the literature), but their unpalatability means you can't get away with calling your claimed explanation "simple". It's not.

 

[vanhees71]

It's true that the "time ordering" of the measurements on the far distant parts of entangled systems doesn't make a difference, which clearly shows that there is no causal influence of one of the measurements on the other. This is the more clear if the "measurement events" (i.e., the events when the measurement results are stored to the measurement protocols at the far distant places) are space-like separated.

Of course there's a cause for the observed correlations between these outcomes, which is of course the preparation of the system in the entangled state, and this preparation process is clearly time-like (or light-like for photons) separated to both measurements. There's no violation of causality by construction in standard relativistic QFTs due to the imposed microcausality constraints on all operators that represent local observables. Particularly the Hamilton density commutes with any local observable-operator at space-like separated arguments.

 

[kurt101]

The problem with this view is that your so-called "simple causal explanation" is stated assuming a particular time ordering of the measurements. But the actual correlations in the experiments are the same regardless of the time ordering of the measurements.

In other words, for it to be the case (as it in fact is) that "order does matter", your "simple causal explanation" would have to allow "causes" that happen either spacelike separated from their "effects", or after their "effects" (i.e., future timelike or null separated), or that two events can be causally connected without there being any fact of the matter about which is the "cause" and which is the "effect". There are no other choices, and all of them are unpalatable. Strictly speaking, they aren't logically impossible (although they do imply a concept of "causation" which is highly at variance with all such concepts currently in the literature), but their unpalatability means you can't get away with calling your claimed explanation "simple". It's not.

Fair enough. In retrospect I would have used the word "plausible" instead of "simple".

My point is that all orderings of the entanglement swapping experiment have plausible causal explanations.

So what is more likely?
That a causal explanation is not possible.
Or we have not figured out why the order doesn't matter.

 

[vanhees71]

The order doesn't matter, because there's no need for a causal effect of one measurement on the other. This should be clear from the fact that the temporal order of these measurements is irrelevant, and correlations do not necessarily imply causal connections.

In fact, at least within the minimal statistical interpretation, the correlations are present because of the preparation of the photons in an entangled state before any measurement has been done on these photons. E.g., if you take two entangled photons from parametric down conversion (e.g., type II, where the polarization state is in the singlet, antisymmetric state and thus so must also be the momentum state to get a symmetric state under exchange of the photons as it must be), the single-photon polarizations are maximally uncertain, i.e., each of the photons is ideally unpolarized, neverthereless when measuring the polarization in the same direction for each photon you get a 100% (anti-)correlation: if you find photon 1 as H-polarized, then you find photon 2 as V-polarized and vice versa. It is completely irrelevant in which temporal order you measure the single-photon polarizations, i.e., photon 2 doesn't get V-polarized because of the measurement on photon 1 (which you can ensure by registering the photons at spacelike separated events) but the correlation was there due to the preparation in the entangled state.

 

[jtbell]

Actress Sally Field once said of her brother, physicist Rick Field, "He invented something called Field theory."

There's a paper that examines the imperturbability of an elevator operator in a Marshall field.