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

[Demystifier]

My understanding of this thread in broad terms is that orthodox QM does not demand that all measurables have well-defined values at all times and that you believe that this makes QM incomplete and inevitably leads to philosophical inconsistencies.

Not exactly. I can accept a theory in which some values are not always defined, as far as the theory clearly specifies sufficient and necessary conditions for the values to become defined. The usual condition that it happens when those are "measured" is just not clear enough, unless one also makes a clear definition of measurement. It is this typical lack of clarity that makes it incomplete, which does not necessarily make it inconsistent. The inconsistencies usually appear in the second step, when people who are well trained in physics and math try to do philosophy.

 

[Demystifier]

An observable has a determined value, if with 100% probability a measurement of this observable results in this value.

EPR started from the same postulate and, by assuming also locality, derived that QM is incomplete. I know that you disagree with EPR, but I don't see any consistent way to disagree with them given those two premises.

 

[vanhees71]

The usual condition is not that the observables are measured but that the system is prepared in a state such that the observable in question takes a determined value. It is utmost important to distinguish between preparations and measurements, i.e., to distinguish between states and observables. Otherwise you must run into inconsistencies when discussing quantum theory.

 

[martinbn]

EPR started from the same postulate and, by assuming also locality, derived that QM is incomplete. I know that you disagree with EPR, but I don't see any consistent way to disagree with them given those two premises.

But they said clearly what they mean by incomplete. In that sense QM is incomplete no question about it. Was that a useful definition?

 

[vanhees71]

EPR started from the same postulate and, by assuming also locality, derived that QM is incomplete. I know that you disagree with EPR, but I don't see any consistent way to disagree with them given those two premises.

I disagree with EPR, because local relativistic QFT is a model, where locality and indeterminism is fully consistently discribed. For me EPR is completely refuted by the demonstration of the violation of Bell's inequalities.

 

[martinbn]

I disagree with EPR, because local relativistic QFT is a model, where locality and indeterminism is fully consistently discribed. For me EPR is completely refuted by the demonstration of the violation of Bell's inequalities.

I don't understand in which part you disagree with EPR?

EPR say that if you can predict with 100% certainty a value of a dynamical variable the theory should account for it, otherwise the theory is incomplete. QM clearly is incomplete in that sense. Bell showed something that implies that if you complete the theory to include all those values, the completed theory will be non-local. To me all this means that you shouldn't try to EPR-complete a theory. And if a theory is EPR-incomplete so what! It does seem like a demand on the theory that comes form classical prejudice anyway.

 

[vanhees71]

I think this describes best what I mean: "And if a theory is EPR-incomplete so what!" You can rephrase my opinion as that relativistic local (!) QFT, as the most comprehensive and accurate description of nature we have today and precisely predicts quantitatively how Bell's inequalities are violated, shows that Nature can be better described by an EPR-incomplete theory.

Whether there is a then indeed necessarily nonlocal EPR-complete theory as "complete" in describing Nature as relativistic local QFT or not, I don't know. I also don't know, why this should be a "better" theory than QFT. I also don't think that Bohmian mechanics is in any way to be preferred to standard non-relativistic QM, because it's more complicated and doesn't describe more phenomena than the standard theory.

 

[CelHolo]

Whether there is a then indeed necessarily nonlocal EPR-complete theory as "complete" in describing Nature as relativistic local QFT or not, I don't know

To be honest given the near century we've had of not needing such a theory while increasing our understanding of nature while remaining in the "EPR-incomplete" theory and all the no-go theorems against the EPR style theories, it's fruitless to keep seeking an EPR theory.

 

[Demystifier]

I disagree with EPR, because local relativistic QFT is a model, where locality and indeterminism is fully consistently discribed. For me EPR is completely refuted by the demonstration of the violation of Bell's inequalities.

That's an utter nonsense. To agree with EPR means to agree that their assumptions logically imply their conclusions. No experiment can refute a validity of a logical argument. A logical argument can only be refuted by another logical argument. The logical assumptions of EPR are QM and locality, which you accept. Their logical conclusion is incompleteness, which is not refuted by experimental violation of Bell's inequalities.

 

[Demystifier]

But they said clearly what they mean by incomplete. In that sense QM is incomplete no question about it. Was that a useful definition?

Yes it was. When combined with the Bell's result, now we know that if we want to complete QM in this sense, we must abandon locality (in the EPR-Bell sense).

 

[martinbn]

Yes it was. When combined with the Bell's result, now we know that if we want to complete QM in this sense, we must abandon locality (in the EPR-Bell sense).

How is that useful?

 

[martinbn]

That's an utter nonsense. To agree with EPR means to agree that their assumptions logically imply their conclusions. No experiment can refute a validity of a logical argument. A logical argument can only be refuted by another logical argument. The logical assumptions of EPR are QM and locality, which you accept. Their logical conclusion is incompleteness, which is not refuted by experimental violation of Bell's inequalities.

There is one more ingredient to EPR, the definition of incompleteness. He (vanhees71) can disagree with that definition.

 

[PeroK]

That's an utter nonsense. To agree with EPR means to agree that their assumptions logically imply their conclusions. No experiment can refute a validity of a logical argument. A logical argument can only be refuted by another logical argument. The logical assumptions of EPR are QM and locality, which you accept. Their logical conclusion is incompleteness, which is not refuted by experimental violation of Bell's inequalities.

So, hypothetically, we get E, P & R back; show them Bell's inequality; show them a modern experimental set-up of Bell's inequality (I think they'd be very excited at this point); run the experiment; and:

@vanhees thinks they'd go away disappointed.

You think they'd say "I told you so"?

 

[martinbn]

So, hypothetically, we get E, P & R back; show them Bell's inequality; show them a modern experimental set-up of Bell's inequality (I think they'd be very excited at this point); run the experiment; and:

@vanhees thinks they'd go away disappointed

You think they'd say "I told you so"?

I think that none of them will say "Oh, nature is non-local, relativity is wrong, we must adopt a preferred frame theory."

 

[Demystifier]

How is that useful?

It's like usefulness of the impossibility of perpetuum mobile. We don't longer waste time on thinking about the impossible.

 

[CelHolo]

It's like usefulness of the impossibility of perpetuum mobile. We don't longer waste time on thinking about the impossible.

I think they mean what is the point of seeking an EPR complete theory?

 

[martinbn]

It's like usefulness of the impossibility of perpetuum mobile. We don't longer waste time on thinking about the impossible.

You mean it was useful for Bell to show that we cannot do what Einstein was hoping, so let's stop looking for hidden variable theories. But I question the usefulness of the definition at the time of EPR to show that QM needs modifications.

 

[Demystifier]

I think that none of them will say "Oh, nature is non-local, relativity is wrong, we must adopt a preferred frame theory."

What's your guess, what Einstein would say? I'm pretty much convinced that Einstein would seek an ontological interpretation. Perhaps he would be interested in my version of Bohmian mechanics without a preferred frame.

 

[martinbn]

What's your guess, what Einstein would say? I'm pretty much convinced that Einstein would seek an ontological interpretation. Perhaps he would be interested in my version of Bohmian mechanics without a preferred frame.

Einstein was aware of Bohmian mechanics and didn't see the point. If I remember correctly his opinion is that BM is a cheap trick.

 

[Demystifier]

@martinbn any comments on my #46?

 

[Demystifier]

Einstein was aware of Bohmian mechanics and didn't see the point. If I remember correctly his opinion is that BM is a cheap trick.

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

 

[martinbn]

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

I don't know. He might continue his unified field theory research, he might accept that nature is quantum mechanical and not what he expected it to be.

 

[martinbn]

So you are saying that the state always exists, while values of the observables exist only when they are measured, is that right? But it creates a lot of additional questions:
1. Why do values not exist before measurement?
2. How the values know that there is a measurement out there?
3. What's the precise definition of measurement?
4. Can measurement be derived from something more fundamental, or is measurement a primitive concept?
5. Does a value (randomly created in a measurement) have influence on the state?
6. If the answer to 5. is "yes", does this influence violate unitarity, linearity, locality and/or the Schrodinger equation?

First of all not every question is worth posing and trying to answer. Second having questions is not a bad thing. It just means that there will definitely be more progress in the future. What I find bad practice is: if you cannot answer a question to make up an answer and pretend you've solve the problem. It is better to admit that the problem is hard and open at the moment. Look at mathematics. Some problems took centuries to solve.

1. Why should they? May be that is how nature works. All experiments seem to suggest that.
2. This is stupid. The values don't know anything! Perhaps you need to rephrase your question.
3. What is the precise definition of 90% of the concepts in physics?
4. Sure, but then there will be other primitive concepts.
5. Same as 3. What do you mean?
6. N/A

 

[vanhees71]

That's an utter nonsense. To agree with EPR means to agree that their assumptions logically imply their conclusions. No experiment can refute a validity of a logical argument. A logical argument can only be refuted by another logical argument. The logical assumptions of EPR are QM and locality, which you accept. Their logical conclusion is incompleteness, which is not refuted by experimental violation of Bell's inequalities.

EPR is not a logical conclusion but an assumption on the behavior of Nature.

 

[vanhees71]

So, hypothetically, we get E, P & R back; show them Bell's inequality; show them a modern experimental set-up of Bell's inequality (I think they'd be very excited at this point); run the experiment; and:

@vanhees thinks they'd go away disappointed.

You think they'd say "I told you so"?

I don't know, whether EPR would be convinced or not, but I think it's rather on the side of the proponents of standard quantum theory saying "I told you so" ;-)).

 

[Demystifier]

Second having questions is not a bad thing. It just means that there will definitely be more progress in the future. What I find bad practice is: if you cannot answer a question to make up an answer and pretend you've solve the problem. It is better to admit that the problem is hard and open at the moment.

4. Sure, but then there will be other primitive concepts.

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.

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).

 

[vanhees71]

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

 

[PeroK]

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.

And, perhaps, the acid text would be gravity. 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 - but not to BMG.

In any case, that's the sort of debate that I (as someone who cares little for philosophy) would be interesrted it. All this about ontology is pointless, compared to finding a fundamental theory of gravity.

 

[vanhees71]

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.

 

[Demystifier]

In any case, that's the sort of debate that I (as someone who cares little for philosophy) would be interesrted it. All this about ontology is pointless, compared to finding a fundamental theory of gravity.

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

And by the way, the Bohmian interpretation (with field ontology instead of particle ontology) offers an elegant solution of the problem of time in quantum gravity.