I What Makes Ontology Easy for Kids but Challenging for Quantum Physicists?

[Demystifier]

Again I've to ask, which are the meaningful questions QM can't answer?

See #113. Of course, they are not meaningful to you, but apparently some of them are meaningful to martinbn. Meaning is in the eye of the beholder.

 

[PeroK]

Then why do you write so much on this forum and so little on the BSM forum?

I get dragged into these threads against my better judgement.

 

[Demystifier]

All this about ontology is pointless, compared to finding a fundamental theory of gravity.

And all this about fundamental theory of gravity is pointless, compared to fighting the global pandemic crisis. And in the long run all this about global pandemic crisis is pointless, compared to fighting the global warming crisis. So what?

 

[PeroK]

And all this about fundamental theory of gravity is pointless, compared to fighting the global pandemic crisis. And in the long run all this about global pandemic crisis is pointless, compared to fighting the global warming crisis. So what?

I don't know much philosophy or Latin, but I recognise a non sequitur when I see one!

 

[Lord Jestocost]

So what kind of theory would Einstein seek? Wild guess is OK.

To my mind, orthodox quantum theory was and is mainly criticized because it questioned and still questions some personal ideological beliefs which some people cling to with ferocity. Therefore, I think that Einstein would seek a theory that would support the fundamental assumption of the materialistic ideology. As Heisenberg puts it in his book "Physics and Philosophy" (chapter: Criticism and Counterproposals to the Copenhagen Interpretation of Quantum Theory):

"These attempts can be divided into three different groups. ...

... The third group, finally, expresses rather its general dissatisfaction with the results of the Copenhagen interpretation and especially with its philosophical conclusions, without making definite counter proposals. Papers by Einstein, von Laue and Schrödinger belong to this third group which has historically been the first of the three groups.

However, all the opponents of the Copenhagen interpretation do agree on one point. It would, in their view, be desirable to return to the reality concept of classical physics or, to use a more general philosophic term, to the ontology of materialism.

This, however, is impossible or at least not entirely possible because of the nature of the atomic phenomena, as has been discussed in some of the earlier chapters. It cannot be our task to formulate wishes as to how the atomic phenomena should be; our task can only be to understand them." [bold by LJ]

 

[Lord Jestocost]

... When I talk about Bohmian mechanics, I don't "make up an answer and pretend I've solved the problem". Instead, I make an educated guess of a simplified model that might be on the right track and, after further development, perhaps one day might solve the problem.

I agree. This is the only way science can proceed. As Oliver Passon puts it in “What you always wanted to know about Bohmian mechanics but were afraid to ask”:

“The interpretation of quantum mechanics has been discussed ad nauseam and the engagement with it can be a frustrating and disappointing business. This subject matter continues to produce an endless stream of publications and nobody can reasonably expect this issue to be settled in the future. So much the worse, the different camps stand in fierce opposition and one gets the impression that this is an other obstacle for reaching substantial progress.” [bold by LJ]

 

[CelHolo]

If BM solves anything by re-establishing definite particle trajectories, then a fundamental theory of gravity ought to be a good candidate. If QM is incomplete in any useful, meaningful sense then that may be a stumbling block to QG

As far as I can tell all progress has been away from classical-like theories like BM. QFT made more concepts susceptible to complementarity/contextuality such as particle number and even the notion of particle, formulating it correctly required eliminating more non-operational concepts such as correctly considering what operators correspond to observables, precise analysis of detection events to remove infrared divergence.

I really think all indications are that progress will continue on these lines, as we see already in QG research with black hole complementarity, dualities and so on.

 

[martinbn]

So we agree that QM in its standard form is incomplete, in the sense that there are some meaningful questions that it doesn't answer.

Well, no, because we don't agree on what the meaningful questions are. You can always ask 'why' questions without an end. For me a theory is complete if it can answer any question that can in principle be realized as an experiment. If you shoot particles through a double slit, what do you see? How much energy...? In that sense QM is complete (in its domain of applicability, it obviously need extending when it comes to say gravity).

But then I need to tell you just one thing. When I talk about Bohmian mechanics, I don't "make up an answer and pretend I've solved the problem". Instead, I make an educated guess of a simplified model that might be on the right track and, after further development, perhaps one day might solve the problem. Admitting that the problem is open (which I admit) is not the same thing as not trying to solve it (which I try).

But you don't do that! Every time you run into a problem with well tested theories(say relativity) you use wishful thinking. Your answer is of the form "May be there is a way for relativity to be emergent at these scales but deep down it is just nonrelativistic space an time". To be clear by you I mean the plural, the whole BM community.

 

[Demystifier]

But you don't do that! Every time you run into a problem with well tested theories(say relativity) you use wishful thinking. Your answer is of the form "May be there is a way for relativity to be emergent at these scales but deep down it is just nonrelativistic space an time".

I say educated guess, you say wishful thinking. What exactly is the difference?

 

[Demystifier]

In that sense QM is complete (in its domain of applicability, it obviously need extending when it comes to say gravity).

Do you then think that your engagement on the quantum foundations forum is a waste of time? Or if not, what's your motivation for that?

 

[Fra]

What ontology does Qbism have? They claim there's meaning in a probability for a single event. I never could get what that meaning should be.

I'll throw in my understanding of this:

think it helps to distingish between descriptive probability (ie statistics) and the guiding probability (as in gambling). The statistical interpretation has no meaning for a single event, it has meaning only after a massive ensemble is formed. But the guiding probability has, it guides the action of the agents random walk, step by step. The ontology of guiding probability, is effectively the microstructure of the agent, which is forged from it's past interaction history.

All the above is of course hard to merge with current formalism without deforming it, this is why it comes out as unpalatable to some.

The ontology I make use of, I see as subjective, or relative to an agent. Two agents still need to interaction to compare their "ontologies".

/Fredrik

 

[AndreiB]

What ontology does Qbism have? They claim there's meaning in a probability for a single event. I never could get what that meaning should be.

They use a Bayesian concept of probability. It's an agent's degree of belief that event X will happen. Single events (like the outcome of a presidential election) can be ascribed a probability in this way, I see no problem with that.

I always thought an ontic interpretation assumes more than the existence of observations, i.e., there should be more meaning in a quantum state than a way to calculate probabilities, but precisely this is at odds with the way how the formalism is successfully related to nature as observed in real-world labs.

I think the term "ontic" is not used here in the sense that the theory has some ontology. "Ontic" interpretations are those where the quantum state itself is part of the ontology. Other interpretations are called "epistemic", and in those the quantum state is a derived concept (a calculation tool). Many worlds, Bohm or GRW are "ontic" whyle 't Hooft's cellular automaton interpretation or consistent histories interpretation are epistemic. In Bohm's theory, the wavefunction is seen as a sort of field. For 't Hooft is just a statistical representation of the true state of system which is the CA state. But all above theories/interpretations have an ontology.

 

[AndreiB]

It does say. It is called the state of the system. In classical physics the state consists of values of observables. In QM it does not.

I don't think that an "observable" (the outcome of an observation) in classical physics has to be the actual property that is part of the classical state. Even in classical physics the observation requires an interaction, so any measurement disturbs the system (Newton's third law).

 

[AndreiB]

All this about ontology is pointless, compared to finding a fundamental theory of gravity.

It's not pointless. In order to combine QM with GR you need the fundamental elements of both theories (their ontologies) to match. But first you need to decide what those fundamental elements are.

Questions like "was the particle there before it was measured" do matter in GR. They are not only of philosophical interest.

 

[AndreiB]

For me a theory is complete if it can answer any question that can in principle be realized as an experiment.

I think you need to add the logical implications of the experiments. The EPR argument proves that you need additional variables to make the theory local. And if it's non-local you still need some additional structure (like defining an absolute frame).

 

[martinbn]

I say educated guess, you say wishful thinking. What exactly is the difference?

I thought my example with relativity was making my point.

Do you then think that your engagement on the quantum foundations forum is a waste of time? Or if not, what's your motivation for that?

The motivation is the same as when I read theards (in other forums) about perpetual motion, logical errors in relativity and so on. It is clear that it is nonsense, but figuring out why or reading someone's response to such nonsense usually leads to a better understanding for me.

 

[vanhees71]

They use a Bayesian concept of probability. It's an agent's degree of belief that event X will happen. Single events (like the outcome of a presidential election) can be ascribed a probability in this way, I see no problem with that.I think the term "ontic" is not used here in the sense that the theory has some ontology. "Ontic" interpretations are those where the quantum state itself is part of the ontology. Other interpretations are called "epistemic", and in those the quantum state is a derived concept (a calculation tool). Many worlds, Bohm or GRW are "ontic" whyle 't Hooft's cellular automaton interpretation or consistent histories interpretation are epistemic. In Bohm's theory, the wavefunction is seen as a sort of field. For 't Hooft is just a statistical representation of the true state of system which is the CA state. But all above theories/interpretations have an ontology.

Physics is about observations in Nature and particularly quantative measurements in the lab. Any probabilitistic theory (including classical statistics) thus must be epistemic, because then all there is is the statistical meaning of probabilities.

Of course you can use a Bayesian interpretation of probabilities to use probability theory as a way to make decisions, e.g., whether after some probability analysis you decide to gamble at the casino or not, but this has nothing to do with physics. In physics all you can decide is which observable(s) you want to observe and how to construct a measurement device to do so. Then you can model this setup within QT and test the probabilistic predictions against your experimental data on ensembles.

 

[AndreiB]

Of course you can use a Bayesian interpretation of probabilities to use probability theory as a way to make decisions, e.g., whether after some probability analysis you decide to gamble at the casino or not, but this has nothing to do with physics.

I don't find QBism convincing either. I only wanted to point out that even such a non-realist interpretation needs an ontology. Here is a paper dealing with the probabilities in QBism:

Quantum probabilities as Bayesian probabilities​

Carlton M. Caves, Christopher A. Fuchs, Ruediger Schack
Phys. Rev. A 65, 022305 (2002)

https://arxiv.org/abs/quant-ph/0106133

 

[Demystifier]

It is clear that it is nonsense, but figuring out why or reading someone's response to such nonsense usually leads to a better understanding for me.

Thank you for finally explaining it to me, I will have it in mind the next time you ask me something.

 

[martinbn]

Thank you for finally explaining it to me, I will have it in mind the next time you ask me something.

Let me ask you the same. Do you then think that your engagement on the quantum foundations forum is a waste of time? Or if not, what's your motivation for that?

 

[Demystifier]

Let me ask you the same. Do you then think that your engagement on the quantum foundations forum is a waste of time? Or if not, what's your motivation for that?

I will not answer you, because you will think it's a nonsense anyway.

 

[martinbn]

I will not answer you, because you will think it's a nonsense anyway.

You have often responded like this, what do you expect me to think!

 

[Demystifier]

You have often responded like this, what do you expect me to think!

I expect you to think that all I say is nonsense, so why bother.

 

[martinbn]

I expect you to think that all I say is nonsense, so why bother.

I am not the only one reading. You are refusing everyone an answer to what your motivation is! But anyway, I will not bother you anymore.

 

[Demystifier]

You are refusing everyone an answer to what your motivation is!

Nobody else asked me that.

 

[Fra]

probabilities to use probability theory as a way to make decisions, e.g., whether after some probability analysis you decide to gamble at the casino or not, but this has nothing to do with physics.

I think this is superficial association, if this is all there is to it, I would agree with you. But I, and probably others, think there is a deeper way to the understanding. The gambling is an analogy, it's not supposed to be taken literally that agents "gamble". The analogy is IMO rather than physical interactions is a random walk, no one is making an aware "bet", the "bets" are rather natural and unavoidable. But the gambling analogy is still useful as a thinking tool, it's not to suggest that physical interaction are like speculations.

/Fredrik

 

[gentzen]

The gambling is an analogy, it's not supposed to be taken ...

It is more than just an analogy, gambling and game theory were the origin of probability theory, and gambling is still the hydrogen atom of probability theory.

 

[Fra]

It is more than just an analogy, gambling and game theory were the origin of probability theory, and gambling is still the hydrogen atom of probability theory.

Sure, I just commented on what I was guessing was the objection that - somehow "conscious agents" - are involved in the gambling. It's not how I see it, just like "conscious observers" are not what CI is about. I have an evolutioanry perspective, which I see as as "natural" form of gambling, or "spontaneous random walks", which can be seen as a form of gambling, with traits of self organisation. I don't have a problem with the gaming analogy myself, but just like some confuse CI interpretation with conscious observers, I think the gaming perspective can also be seen differently.

/Fredrik

 

[CelHolo]

Of course you can use a Bayesian interpretation of probabilities to use probability theory as a way to make decisions, e.g., whether after some probability analysis you decide to gamble at the casino or not, but this has nothing to do with physics. In physics all you can decide is which observable(s) you want to observe and how to construct a measurement device to do so. Then you can model this setup within QT and test the probabilistic predictions against your experimental data on ensembles

If you're interested, and this doesn't really have much to do with QBism, this is handled in Bayesian theory by de Finetti's theorem, which replicates your intuition here. Basically if you have an ensemble with members labelled $i = 1 \ldots n$ each with a possible outcome for some observation $x_i$ and you assume members of the ensemble can be exchanged:
$P(x_{1}, \ldots x_{i}, \ldots, x_{j}, \ldots , x_{n}) = P(x_{1}, \ldots x_{j}, \ldots, x_{i}, \ldots , x_{n})$, i.e. it is not important that one member or another gave a particular outcome, then as $n \rightarrow \infty$ you have:
$P(x_{1}, \ldots , x_{n}) \approx \int{P(\rho(x))d\rho}$
where $\rho(x)$ is the probability distribution for the outcomes, i.e. a model, and $P(\rho)$ is the probability of that model being correct.

In plainer terms under the assumption that you are dealing with a large ensemble of identically prepared systems then beliefs/credences about the outcomes are standard hypothesis testing where one is verifying models $\rho(x)$.

So it's built into Bayesian theory that testing on large identical ensembles is testing scientific models, but it also allows you to do interesting things like capture errors when you have only medium sized ensembles, i.e. when you're "halfway" between pure "gambling" like in a casino or horse-racing for one shot events and the other extreme with scientific hypothesis testing on large ensembles. In this way your point about the difference between a casino and experimental ensemble is made rigorous as being two different limiting cases of the general theory of probability.

 

[Interested_observer]

But the fundamental theory of gravity won't be found by such kinds of speculations, and I fear not even by pure theoretical-physics work. Without any guidance from observations, there's perhaps no chance to get an idea, how such a quantum theory of gravitation might look like.

It may not be quantum.