I Nature Physics on quantum foundations

[Demystifier]

The editors of high impact journal Nature Physics explain why the field of quantum foundations is important for physics.
https://www.nature.com/articles/s41567-022-01766-x

 

[vanhees71]

Already the 1st paragraph tells me why the philosophical part of what they call "quantum foundations" really is pretty questionable.

Just 2 simple points:

Quantum theory is NOT weird but the most comprehensive theory about Nature we have today.

Wave-particle duality is no phenomenon but a theoretical concept that's outdated for about 100 years.

 

[Fra]

I share the view that quantum foundations are important, but I admit I share Vanhees impression that some of the things they raise are old things, somewhat outdated. That's not to say qm foundations are not important, I just don't see any of the modern arguments in that particular paper which seems unfair.

How about just holding back the judgement and the dismissal of foundational research until we have included gravity properly in our theory. It remains to see if the QFT framework (and indirectly QM foundations) comes out ontop or not.

/Fredrik

 

[WernerQH]

The editors of high impact journal Nature Physics explain why the field of quantum foundations is important for physics.
https://www.nature.com/articles/s41567-022-01766-x

I agree that there are loose ends. But I fail to see much progress:

Although a fresh view can invigorate any field, much of this work also manifests a disregard for the progress that has been made since quantum mechanics was established. The quantum foundations literature is the product of decades of careful thought about the issues involved in understanding and interpreting the physical world. As with any topic, a failure to constructively engage with existing work runs the risk of repeating earlier mistakes and misunderstandings.

Nothing fundamental has changed in the way we do quantum mechanical calculations. Now we have just more no-go theorems, peculiar inequalities, and a proliferation of interpretations, rather than having found the one and only natural interpretation of quantum theory. Most of the work on quantum foundations seems to have explored blind alleys. Much like the search for a mechanical model of the ether, which once was an obvious and important field of research. Maxwell himself might have realized that there is no need for an ether, if he had had more time. For his contemporaries electrodynamics (or rather the ether) had some "weird" features, that took four decades to get rid of. In the case of quantum theory it seems to take significantly longer to remove superfluous metaphysical baggage.

I think it is not quantum theory that is weird, but the way it is phrased / taught:

But the particle can at the same time be entangled with another particle located elsewhere such that the outcome of measuring one particle determines the state of the other.

The particle concept is problematic, and even talk about quantum particles doesn't remove its misleading connotations. My view is that quantum theory makes much more sense if it is formulated without reference to "particles" and "measurements".

 

[Demystifier]

My view is that quantum theory makes much more sense if it is formulated without reference to "particles" and "measurements".

How do you formulate the Born rule in different bases (say position basis and momentum basis) without measurements?

 

[WernerQH]

How do you formulate the Born rule in different bases (say position basis and momentum basis) without measurements?

Quantum theory makes predictions about events. It is customary to compute a probability amplitude and take the squared modulus as an extra step. But this dichotomy of unitary evolution and "measurements" is artificial. Schwinger fused them in a coherent (closed time path) formalism a long time ago. In his generalized quantum action principle the Born rule is built-in:

[...] if a system is suitably perturbed in a manner that depends upon the time sense, a knowledge of the transformation function referring to a closed time path determines the expectation value of any desired physical quantity [...]

[J.Math.Phys. 2, 407-432, (1960)]

 

[Demystifier]

But this dichotomy of unitary evolution and "measurements" is artificial.

Agreed, but when Schwinger talks about "perturbed" system in your quote above, doesn't he introduce a similar artificial dichotomy between perturbed and unperturbed?

 

[vanhees71]

This is just the introduction of the Schwinger-Keldysh time contour. It's a very important tool in non-equilibrium many-body QFT, but it's just standard QT. It's not adding anything to the interpretation than is contained in the minimal interpretation but is a mathematical technique to solve the equations of motion of Q(F)T.

 

[WernerQH]

Agreed, but when Schwinger talks about "perturbed" system in your quote above, doesn't he introduce a similar artificial dichotomy between perturbed and unperturbed?

It is widely believed that QFT hinges on perturbation theory. But a more general viewpoint is that of the path integral as providing a generating functional from which all quantities of interest (especially correlations) can, in principle, be derived by simple differentiation.

 

[Demystifier]

It is widely believed that QFT hinges on perturbation theory. But a more general viewpoint is that of the path integral as providing a generating functional from which all quantities of interest (especially correlations) can, in principle, be derived by simple differentiation.

In general, the correlation functions of observables at different times computed this way do not correspond to correlations observed by measuring observables at different times. This is because such a computation does not take into account a change of the state induced by measurement. This change of the state, known under the names information update, projection or collapse, is not a unitary transformation. How does Schwinger, or anyone else who uses a path integral formalism and claims that there is nothing special about measurements, account for this?

 

[Fra]

I think it is not quantum theory that is weird, but the way it is phrased / taught:

Yes, I think one could also say that it's nature that is weird. When corroborated theory is seen from the outdated stance that nature is NOT weird, it really gets weird. We can resolve this mismatch if we adopt a weird take on nature :)

/Fredrik

 

[WernerQH]

[...] such a computation does not take into account a change of the state induced by measurement. This change of the state, known under the names information update, projection or collapse, is not a unitary transformation.

I believe you have this backwards. What you are talking about is a theoretian's idealized measurement that has little to do with real experiments. Applied to a harmonic oscillator, a position measurement would imply adding an infinite amount of kinetic energy, if the imagined collapsed state really were an eigenstate of the position operator. Contrariwise, there is little reason to doubt that the correlation function $\langle x(t)x(0) \rangle$ computed for an unperturbed oscillator is a useful first approximation to measurement results. And of course this first approximation can be improved by including perturbing effects (as done by Schwinger).

 

[WernerQH]

I'd say, "measurement" is mere heuristics to justify how we compute quantum mechanical correlation functions. It shouldn't be thought of as a separate process. I think trying to "solve the measurement problem" is akin to the search for a "mechanical model" of the ether.

 

[geordief]

Wave-particle duality is no phenomenon but a theoretical concept that's outdated for about 100 years.

I am not attempting to contribute to this discussion-above my competence , but could I perhaps ask where I might read up on this particular point?

I was under the impression that the wave-particle duality was indeed a very strong mainstream scientific explanation and had never come across such a statement as yours up till now.

Maybe you could give me some kind of a pointer so that I could perhaps understand the point you are making?

(quite probably I have misunderstood ...)

 

[WernerQH]

I was under the impression that the wave-particle duality was indeed a very strong mainstream scientific explanation and had never come across such a statement as yours up till now.

That's also my view (that it's mainstream), although I wouldn't call it an explanation but rather a mere statement of the fact that the so-called quantum objects are neither particles nor waves, but share properties of both.

@vanhees71 is tired of explaining that photons are not "little bullets" (and he's right!)
I think his statement is a little exaggerated.

 

[Demystifier]

What you are talking about is a theoretian's idealized measurement that has little to do with real experiments.

It has absolutely nothing to do with idealization. Even realistic POVM measurements involve a non-unitary change of state upon measurement. Or do you disagree?

 

[WernerQH]

It has absolutely nothing to do with idealization. Even realistic POVM measurements involve a non-unitary change of state upon measurement. Or do you disagree?

Also POVM measurements are an idealization. I think it's not useful to talk about the "state" of a "system" at a particular instant of time. (We integrate over all possible states when we use Schrödinger's equation.) What quantum theory predicts is the probabilities of particular sequences of events (or "histories", if you like).

 

[geordief]

(If my question makes any sense)Is it impossible for any two elements of a quantum system to be at rest with respect to each other?

If so ,would that be a consequential observation?

 

[vanhees71]

I am not attempting to contribute to this discussion-above my competence , but could I perhaps ask where I might read up on this particular point?

I was under the impression that the wave-particle duality was indeed a very strong mainstream scientific explanation and had never come across such a statement as yours up till now.

Maybe you could give me some kind of a pointer so that I could perhaps understand the point you are making?

(quite probably I have misunderstood ...)

The wave-particle duality was an important heuristic idea before modern quantum mechanics has been discovered in 1925/26, and the main protagonists (among them Einstein and de Broglie) were well aware of the problems with this idea. It was solved with Born's probabilistic interpretation of the quantum states, i.e., that the modulus squared of the Schrödinger wave function is a probability distribution for the position of a particle, described by this wave function.

 

[vanhees71]

That's also my view (that it's mainstream), although I wouldn't call it an explanation but rather a mere statement of the fact that the so-called quantum objects are neither particles nor waves, but share properties of both.

@vanhees71 is tired of explaining that photons are not "little bullets" (and he's right!)
I think his statement is a little exaggerated.

Photons are of course another complication. For photons you cannot even define a position observable to begin with, and the only (very) successful forumulation of relativistic QT is local (microcausal) relativistic QFT. For a very clear discussion, see

B. Gin-ge Chen et al (ed), Quantum field theory lectures of Sidney Coleman, World Scientific (2019)

 

[Demystifier]

What quantum theory predicts is the probabilities of particular sequences of events (or "histories", if you like).

Are you suggesting that consistent histories interpretation is the way to go, is it how you avoid a reference to measurement?

Note that the usual formula for probability of a history involves products of unitary evolution operators and non-unitary projectors, which in the standard interpretation is interpreted in terms of a series of projections induced by measurements at different times.

 

[WernerQH]

Are you suggesting that consistent histories interpretation is the way to go, is it how you avoid a reference to measurement?

I'm not a fan of consistent/decoherent histories. My preference is a blend of the statistical and transactional interpretations, and ... GRW. At least the original GRW is (to my mind) rather ad hoc, but I like the idea that only events are real. A short coordinated "wiggling" of electrons in a detector, for example, constitutes what we could call the measurement of the polarization of a photon.

Note that the usual formula for probability of a history involves products of unitary evolution operators and non-unitary projectors, which in the standard interpretation is interpreted in terms of a series of projections induced by measurements at different times.

That's what I dislike about consistent histories: preferred variables / frameworks. It is too non-committal about the actual ontology, about what really happens. In the Schwinger-Keldysh formalism the "series of projections" arises naturally through creation and annihilation operators at adjacent points of the forward and backward time branch. For Schwinger the closed time path (with a "backward" branch) may have been a purely formal device, but I think that the backwards running time has physical significance, that physical events do occur in close pairs, that there are two world-sheets with opposite sense of time tacked together. The forward evolution of a $\ket{\text{ket}}$ according to $e^{-iHt}$ is only half the story. The Schwinger-Keldysh formalism includes the other half and the Born rule as well.

 

[gentzen]

I'm not a fan of consistent/decoherent histories.

Thanks, I already feared that Morbert or me would now have to clarify which of the things you mentioned must be interpreted differently from the histories perspective.

At least the original GRW is (to my mind) rather ad hoc, but I like the idea that only events are real.

... preferred variables / frameworks. It is too non-committal about the actual ontology, about what really happens.

For Schwinger the closed time path (with a "backward" branch) may have been a purely formal device, but I think that the backwards running time has physical significance, that physical events do occur in close pairs, that there are two world-sheets with opposite sense of time tacked together.

Sure, events are nice as ontology, because they provide a clear connection to spacetime, and are somewhat minimal. However, coherent states could be nice too, because they provide a clear connection to classical mechanics, and are "more compatible" with decoherence than purely spacetime based ontologies.

However, the reason why I comment on your choice of events as the preferred ontology is that I recently read the SEP article on Relational Quantum Mechanics (from Winter-2021). I was surprised, that it contained statements like "Facts as this one (“the particle is at x at time t”) are called “events”, or “quantum events”. Quantum theory is about events." I vaguely remembered having read Rovelli's paper from 1996, and that he was much more non-commital about the actual ontology back then. And after reading the section on the Frauchiger-Renner thought-experiment, I decided that Rovelli probably had accidentally overcommited to a specific ontology now (no idea when he changed his mind), and that this "overcommited" ontology will most probably be inconsistent.

Recently I read a comment on an article attacking that "new" ontology:

This paper refers to the version of RQM that existed before the introduction of "cross-perspective links" in arXiv:2203.13342, a change that amounts to saying, "Well, we didn't want all those 'relative facts' anyway."

I have no idea whether my "interpretation of the events" has anything to with what really happened.

But those events might still provide a "slight warning" about being too commital to your ontology, just because it makes sense to you and you intuitively like it.

 

[WernerQH]

However, the reason why I comment on your choice of events as the preferred ontology is that I recently read the SEP article on Relational Quantum Mechanics (from Winter-2021). I was surprised, that it contained statements like "Facts as this one (“the particle is at x at time t”) are called “events”, or “quantum events”. Quantum theory is about events."

RQM never had much appeal to me. I find "the particle is at x at time t” too unspecific a characterization of an event. The events that I have in mind are interactions of electrons and photons, for example. Through the fluctuation/dissipation theorem the photon emission rate in a medium can be expressed in terms of a Fourier integral over the current density fluctuations, hinting (to me, at least) at the possibility that the emission of a photon relates in the real world to two closely spaced, short-lived currents.

But those events might still provide a "slight warning" about being too commital to your ontology, just because it makes sense to you and you intuitively like it.

However, coherent states could be nice too, because they provide a clear connection to classical mechanics, and are "more compatible" with decoherence than purely spacetime based ontologies.

Thanks for your warning. But I can't make sense of coherent states as something "real" - it's more a piece of mathematics (holomorphic representation) to me. As to the connection to the classical world I think that the continuous world lines of classical particles have to be replaced in the quantum picture by dotted lines, the incessant interactions of an electron with the electromagnetic field. The world looks classical when you reduce the time resolution.

 

[gentzen]

Thanks for your warning. But I can't make sense of coherent states as something "real"

Oh, sorry. I should have made clearer what I meant by the warning, and what I meant by "overcommited" ontology. Events or flashes (like in GRW) are fine as ontology, as long as there are not "too many" (or as long as they are not "too precise"). Basically Bohmian mechanics is the only interpretation known to me that managed to have a maximally refined microscopic ontology without getting inconsistent or making different predictions from standard QM.

It is probably easier to see for GRW: The flashes are OK as ontology, but you can only have very few of them. But if you believe that you found "the right ontology", and you are unaware of the problems other interpretations encountered before, then you might believe that this is enough, so you go to a maximally refined ontology (because that removes vagueness), and thereby run into trouble.

 

[WernerQH]

The flashes are OK as ontology, but you can only have very few of them.

Why is that? I would expect them to occur at the zeptosecond scale ($10^{-21} {\rm s}$), corresponding to the electron mass. I looked at two papers by Tumulka, but they were quite different from what I have in mind. What kind of trouble do you anticipate?

 

[Fra]

Sure, events are nice as ontology, because they provide a clear connection to spacetime, and are somewhat minimal.

I am not sure I understand Werners line of thinking. I assume you both by event, refers to a "4D spacetime" event?

I just ask because I myself entertain the concept of an abstract detection event in the sense that an "observer/agen/IGUS" registers a distinguishable event. But where the structure of this abstract set of events (such as dimensionality of the set) are not necessarily primary, or assume, but instead emergent(say as chaos dimension complexity etc). Those type of "events" are in fact information updates, or elementa of measurements, and are the type of ontology I often think of.

/Fredrik

 

[gentzen]

Why is that? I would expect them to occur at the zeptosecond scale ($10^{-21} {\rm s}$), corresponding to the electron mass. I looked at two papers by Tumulka, but they were quite different from what I have in mind. What kind of trouble do you anticipate?

If you add a "non-emergent" ontology, then you risk getting a different theory, making "slightly" different predictions. Why? Probably because an ontology risks to make everything too exact, and to remove too much vagueness. Bohmian mechanics has its equilibrium distribution, to bring back the vagueness. Without something similar, you are often forced to "thin out" your ontology, to avoid making experimentally verifiable predictions that differ from standard QM.

 

[WernerQH]

Probably because an ontology risks to make everything too exact, and to remove too much vagueness.

We are probably talking past each other. Why should vagueness be an important ingredient? For me, QED is a fundamentally statistical theory. Does randomness constitute enough "vagueness"? I don't want to create a new theory. I think QED is perfect, and I only aim to see it more clearly as a theory of a special kind of point process in spacetime (actually two five-dimensional manifolds glued together - I'm lacking the proper mathematical term).

 

[ohwilleke]

The editors of high impact journal Nature Physics explain why the field of quantum foundations is important for physics.
https://www.nature.com/articles/s41567-022-01766-x

So much, said so well, and yet, with very little in the way of a bottom line or an action item. Mostly, what keeps it a clean and elegant, but ultimately useless discussion, is that it doesn't engage with competing proposed resolutions of the open questions and what is at stake if one or another of them is adopted.

 

[ohwilleke]

Quantum theory is NOT weird but the most comprehensive theory about Nature we have today.

Not really a fair criticism.

The fact that a theory is comprehensive and accurate description of Nature doesn't preclude it from being weird.

"Weird" is a word generally used in reference to "common sense" and a typical person's intuition about how the world works. Weird is a close synonym of "counterintuitive."

Every shred of physical intuition gained from daily life, some of it hard wired into our brains, is based on a classical worldview that works for Newtonian mechanics and Maxwell's equations, but fails miserably in the domain of quantum theory in half a dozen or so distinct respects. Yet many people aren't even seriously exposed to quantum theory for the first time until they are college students with STEM majors (assuming that they ever go to college and that they ever major in STEM).

Calling quantum theory "unnatural" would indeed have been inappropriate, but "weird" is right on target.

 

[Morbert]

"It is easy to dismiss research into the foundations of quantum mechanics as irrelevant to physicists in other areas. Adopting this attitude misses opportunities to appreciate the richness of quantum mechanics."
I'm ok with foundations work being irrelevant to most physicists. My interest in foundations wasn't because I thought foundations work could invigorate other research projects. Almost the opposite: I became interested in foundations in the hope that it could better articulate how physicists and chemists already understand quantum mechanics.

When QM is introduced axiomatically in undergrad, it's usually in terms of some procedure of unitary evolution + measurement + state reduction. But huge areas of physics and chemistry make use of QM without this procedural approach, and alternative interpretations can better overlap with these areas. (E.g. I find consistent histories interesting because it nicely complements the way many of us already think about electron transport calculations in transistors)

 

[gentzen]

We are probably talking past each other.

That is quite possible. My reaction to your "intended interpretation" was dominated by your reference to GRW (paired with my limited knowledge of why such objective collapse theories differ from standard QM):

My preference is a blend of the statistical and transactional interpretations, and ... GRW. At least the original GRW is (to my mind) rather ad hoc, but I like the idea that only events are real. A short coordinated "wiggling" of electrons in a detector, for example, constitutes what we could call the measurement of the polarization of a photon.

GRW is known to allow both a "flash ontology" (or "flashes ontology") and a "mass density ontology". (I guess that some objective collapse theories are commited to a "mass density ontology", i.e. theories like the Diósi–Penrose model.) So I guessed that your "events" would be similar to the "flashes" for GRW.

Why should vagueness be an important ingredient? ... Does randomness constitute enough "vagueness"?

The "flashes" for GRW have infinitely accurate spacetime coordinates. For GRW, even their randomness seems to be not enough to get rid of that excess accuracy again. But for Bohmian mechanics, the randomness is sufficient, so an unconditionally true answer to that question seems impossible.

The trouble with excess accuracy is that the information content of a system with a finite energy in a finite spacetime region should better not be infinite. It is convenient to work with real numbers for mathematical models, but their infinite accuracy forces you to have some mechanism (like "vagueness") to avoid that their infinite accuracy has experimentally observable consequences.

For me, QED is a fundamentally statistical theory. ... I don't want to create a new theory. I think QED is perfect, and I only aim to see it more clearly as a theory of a special kind of point process in spacetime

I guess that the mechanism for QFT to get rid of the excess accuracy (related to "point process in spacetime") is renormalization. You find "points of view" like the following in modern QTF1 lecture notes:

Many points of view; one is that it is our own fault: QFT is somewhat idealised; it assumes infinitely extended fields (IR) with infinite spatial resolution (UV); there is no wonder that the theory produces infinities. Still, it is better to stick to idealised assumptions and live with infinities than use some enormous discrete systems (actual solid state systems).

There is also a physics reason why these infinities can be absorbed somehow: Our observable everyday physics should neither depend on how large the universe actually is (IR) nor on how smooth or coarse-grained space is (UV).

 

[Fra]

Every shred of physical intuition gained from daily life, some of it hard wired into our brains, is based on a classical worldview that works for Newtonian mechanics and Maxwell's equations

Hmmm... I don't think it's how my brain is wired. My brain, and I think yours, makes use of inferences, abduction, lossy retention and actions influenced by subjective expectations that has been tuned by evolution, even though we may not think of it. These things are IMO in excellent harmony with quantun weirdness if you only embrace the inside observer view So I see good reasons why we WILL ultimately see how natural QM is, and we will look back and wonder how Newtons mechanics ever made sense

/Fredrik

 

[WernerQH]

The trouble with excess accuracy is that the information content of a system with a finite energy in a finite spacetime region should better not be infinite. It is convenient to work with real numbers for mathematical models, but their infinite accuracy forces you to have some mechanism (like "vagueness") to avoid that their infinite accuracy has experimentally observable consequences.

I don't subscribe to the view that information is physical. It belongs to our theories and models. And we can safely ignore excess digits.

I guess that the mechanism for QFT to get rid of the excess accuracy (related to "point process in spacetime") is renormalization.

Yes, it is useful (no, necessary!) to ignore the scales that are not relevant. We couldn't do physics otherwise.

You find "points of view" like the following in modern QTF1 lecture notes:

Thank you. Your posts always contain interesting pointers.

 

[A. Neumaier]

The events that I have in mind are interactions of electrons and photons, for example.

How do you turn this into an unambiguous definition of what events are? Without that, your talk is fuzzy words only...

manifolds glued together - I'm lacking the proper mathematical term

The term is ''double cover of standard spacetime''

 

[Peter Morgan]

Having looked through the above posts, in which the measurement problem and "collapse" appear repeatedly, as so often, I commend to people here the mathematics in my recent paper in JPhysA 2022, "The collapse of a quantum state as a joint probability construction", https://arxiv.org/abs/2101.10931 (the DOI for the published version may be found there, but there's very little difference), as a way to rethink the measurement problem.
I'll leave the paper mostly to tell its own story, though I emphasize it is very far from complete, but I'll mention here that it constructs in Section 4 what I call a "super-Heisenberg picture" that we can think of as empirically equivalent to the Heisenberg, interaction, and Schrödinger pictures (but not as unitarily equivalent because it absorbs unitary evolution and non-unitary "collapse" into a unitary evolution of the measurement operators so that the quantum state is completely static).

 

[OscarCP]

Not to discuss the foundations of QM, but just to recommend to anyone who might not know, but perhaps may be interested, to have a look at a Website called "World Science Festival" that I have recently discovered.
It has a large collection of lecture-long (one hour and a quarter, or longer) videos of lectures, interviews and panel discussions on topics in the main sciences, and tons of them on physics, cosmology, astrophysics and geophysics.
To give you an idea, in two occasions in the last two weeks I "attended" for free a lecture by Stanford's Leo Susskind on the Higgs boson, with a remarkable preliminary introduction to quantum fields, and another with a panel discussion on whether quantum physics is complete, incomplete or what? that included Utretcht University's Nobel laureate Gerard T'Hooft, who made some very interesting and a few also surprising remarks about "or what?"
The lectures, interviews and panel discussion explanations are usually conducted in a style that makes them accessible to the educated and interested layman, are mostly at the undergraduate intro level, but particularly in the discussions and interviews -- always conducted by a very knowledgeable host -- "bleeding edge" ideas tend to pop up along with the more basic stuff. And always with the personal views on the subject at hand of some top physicists.
For someone that did not know about this: in my opinion, worth giving it a try.

 

[Fra]

I don't subscribe to the view that information is physical. It belongs to our theories and models. And we can safely ignore excess digits.

My problem with this take on the "problem" described by gentzen is that

1) it is ambigous in what way you choose to ignore it, analogous to changing the order of limits at will. This is ad hoc procedure we all know from physics. By my point here is still not from the stringent math side, you also loose the conceptual and inferential clarify when doing this. We loose the ability to reason rationality because the input is a soup already, so it can be nothing but soup coming out.

2) I think it also inflates "theory space", if you have ignored these things for a long time (which we have done) then at some point in evolution you have completely lost track of what is happning, and you can no longer tell which complexions of theory space that have physical significance, and what is mathematical extrapolations. I think this makes improving the theory in a rational way harder. This has always disturbed me, and I think we could do a little bit better. We have some actual fine tuning to do, but it's tricky enough without trying to fine tune ghosts.

/Fredrik

 

[Morbert]

How do you turn this into an unambiguous definition of what events are? Without that, your talk is fuzzy words only...

I don't know if this serves @WernerQH 's purposes, but Froehlich gives a useful notion of events in QM, motivated by event algebras in probability theory:

https://arxiv.org/abs/1905.06603

 

[A. Neumaier]

I don't know if this serves @WernerQH 's purposes, but Froehlich gives a useful notion of events in QM, motivated by event algebras in probability theory:

View attachment 314130

https://arxiv.org/abs/1905.06603

Useful? Giving the name 'event' to some construct doesn't make it an event in any realistic sense.

What does it mean for such an event to happen, i.e., to be real? is there a trajectory of real events? Or are there events happening at the same time in different places? etc.

 

[ohwilleke]

Hmmm... I don't think it's how my brain is wired. My brain, and I think yours, makes use of inferences, abduction, lossy retention and actions influenced by subjective expectations that has been tuned by evolution, even though we may not think of it. These things are IMO in excellent harmony with quantun weirdness if you only embrace the inside observer view So I see good reasons why we WILL ultimately see how natural QM is, and we will look back and wonder how Newtons mechanics ever made sense

/Fredrik

You can't viscerally experience phenomena associated with quantum but not classical physics without scientific instrumentation. The scales are beyond those are senses are designed to experience. Our brains use Newtonian type assumptions as we walk, run, jump, fall, and view the world around us with our own eyes.

QM can be taught better to make it seem somewhat less weird, and can be more familiar, but Newtonian mechanics will always make sense because it is descriptive of almost everything we experience personally and perfectly adequate for the lion's share of practical application. (If there is snark in this answer, I apologize for missing it, it is hard to read tone in writing sometimes.)

 

[Morbert]

Useful? Giving the name 'event' to some construct doesn't make it an event in any realistic sense.

What does it mean for such an event to happen, i.e., to be real? is there a trajectory of real events? Or are there events happening at the same time in different places? etc.

According to Froehlich, if $\{\pi_\xi,\xi\in\chi\}$ is a potential event, then the event $\xi$ that actually occurs is the event that selects the state of the system after the occurrence (see equation 9 in the paper). It's useful insofar as he uses it to reformulate quantum dynamics as a stochastic branching process, where measurements are no longer fundamental. (Measurements according to Froehlich are special events that generate a large amount of entropy)

 

[Fra]

but Newtonian mechanics will always make sense because it is descriptive of almost everything we experience personally and perfectly adequate for the lion's share of practical application.

What I had in mind was far more intuitive to us humans than falling objects or ballthrowing.

I was thinking of human-human interactions and the laws of social interactions. Try to describe that in the Newtonian paradigm.

/Fredrik

 

[A. Neumaier]

According to Froehlich, if $\{\pi_\xi,\xi\in\chi\}$ is a potential event, then the event $\xi$ that actually occurs is the event that selects the state of the system after the occurrence (see equation 9 in the paper). It's useful insofar as he uses it to reformulate quantum dynamics as a stochastic branching process, where measurements are no longer fundamental. (Measurements according to Froehlich are special events that generate a large amount of entropy)

So the state of the system is something objective that changes if and only if an event happens? Thus the Schrödinger equation does not govern the change of states? What then?

 

[Fra]

You can't viscerally experience phenomena associated with quantum but not classical physics without scientific instrumentation.

You mean the detectors with irreversible pointers that we conceptually keep on the external/classical side of the cut? Those are the idealisation yes. But isn't the imperfectness in that ideal the problem? Where is the "classical side" in a quantized gravitational system? At the future infinity?

/Fredrik

 

[Morbert]

So the state of the system is something objective that changes if and only if an event happens? Thus the Schrödinger equation does not govern the change of states? What then?

The time evolution of the state would indeed not be governed by the Schroedinger equation, and would be objective and fundamentally stochastic. This stochastic process is accounted for with a filtration and a state space that Froehlich calls "the non-commutative spectrum" of the system.

Although the wavefunction is considered objective, what are considered real are the events. I'm not very familiar with GRW but I believe it also considers events to be what are real, and so the ETH approach presented by Froehlich might rigorise GRW.

 

[WernerQH]

I don't know if this serves @WernerQH 's purposes, but Froehlich gives a useful notion of events in QM, motivated by event algebras in probability theory:

https://arxiv.org/abs/1905.06603

Thank you for the reference. I've seen Fröhlich's paper before, and I sympathize with his philosophy, but my notion of event is much more primitive: a point in space-time where a field excitation is created or destroyed. For me the creation of a photon is a real physical event, but it does not happen in an instant. It is a composite event. In a medium with a refractive close to 1, the emissivity (${\rm W~m^{-2}~Hz^{-1}~sr^{-1}}$) can be written as $$
\epsilon = { \mu_0 \omega^2 \over 8 \pi^2 c} \sum_{\mu\nu} {\bf e}_\mu^* {\bf e}_\nu
\int_{-\infty}^{\infty} dt \int d^3x\ e^{-i(kx - \omega t)} \langle j_\nu(0,0) j_\mu(x,t) \rangle
$$ suggesting that an emission event actually comprises four primitive (strictly localized) events ($\Psi,\Psi^\dagger,\Psi,\Psi^\dagger$). Between the times (events) $j(0)$ and $j(t)$ the "state" of the system, whether the photon has been created or not, is in limbo. Evolution is not strictly Markovian. It takes a short, but non-zero time until a fact ("photon has been emitted") is established. And experiments are always reported post factum. :-)

The time evolution of the state would indeed not be governed by the Schroedinger equation, and would be objective and fundamentally stochastic.

Absolutely. Continuous and deterministic evolution according to Schrödinger's equation does not square with the graininess and randomness that we experience in the real world. I agree with Fröhlich that it is an intellectual scandal that it still seems necessary to graft extra "measurement" processes on the evolution of the real world.

 

[A. Neumaier]

The time evolution of the state would indeed not be governed by the Schroedinger equation, and would be objective and fundamentally stochastic. This stochastic process is accounted for with a filtration and a state space that Froehlich calls "the non-commutative spectrum" of the system.

Although the wavefunction is considered objective, what are considered real are the events. I'm not very familiar with GRW but I believe it also considers events to be what are real, and so the ETH approach presented by Froehlich might rigorise GRW.

What is the difference between real and objective? How can something nonreal be objective, and how can something nonobjective be real?

 

[Fra]

Froehlich gives a useful notion of events in QM, motivated by event algebras in probability theory:

View attachment 314130

https://arxiv.org/abs/1905.06603

This caught some of my attention, i will try to read it later. Thanks for the link! Part of what caught my attention is a claim that its not a pure interpretation but should yield different predictions and some other things such as stochastic evolution, it sounds like having similarities to my preferred views. But i need to read it all to make sure I don't jump into conclusions... will report back... it will take some time due to some traveling where reading is tricky.

/Fredrik