Conflict between the Block Universe and Bell Tests?

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Lynch101
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Summary:

Based on my understanding of the Block Universe interpretation of relativity, it appears to me as though there is a conflict between Bell tests and the Block Universe.
Usually questions I have of this nature are down to my limited understanding of the concepts, and this may be no different. Based on my understanding of the Block Universe interpretation of relativity, it appears to me as though there is a conflict between Bell tests and the Block Universe.

To give a brief outline of my understanding and where I think the conflict lies: my understanding of the the Block Universe is that it says that the locus of all events that make up our history, and that of every object, extend through spacetime as a world line. That is, the Universe is a block-like structure which includes the entire history of all objects, past, present, and future.

If we consider the world line of a Stern Gerlach plate, which is used in a Bell test. In a Block Universe it would seem that the exposure event is on the world line of the SG plate, which is extended in spacetime. That is, it would seem to necessitate that there is only ever one possible outcome in a Bell test, even prior to the running of the test.


Have I misunderstood the Block Universe, or Bell tests, or both?
 

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PeterDonis
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it would seem to necessitate that there is only ever one possible outcome in a Bell test, even prior to the running of the test.
The Block Universe is an interpretation of SR, and SR is a deterministic theory, so if you do not think QM is deterministic, then the Block Universe is obviously inconsistent with anything QM says, since SR itself is. But SR is a classical (i.e., non-quantum) theory, so it wasn't developed with QM in mind anyway, so you shouldn't expect an interpretation of SR to be consistent with QM.

If you think QM is deterministic, then "there is only ever one possible outcome" to any QM experiment, or indeed to any experiment whatever, anywhere in spacetime, since that's what determinism means. Things would simply have to be determined so that all of the QM experimental outcomes matched QM statistics.
 
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If you think QM is deterministic
Is there still any room for any deterministic formulation of QM in view of Bell's theorem?
Would any deterministic formulation of QM imply an existence of a set of variables that deterministically determine the state of any system? Wouldn't that contradict the Bell's theorem?
 
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Lynch101
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The Block Universe is an interpretation of SR, and SR is a deterministic theory, so if you do not think QM is deterministic, then the Block Universe is obviously inconsistent with anything QM says, since SR itself is. But SR is a classical (i.e., non-quantum) theory, so it wasn't developed with QM in mind anyway, so you shouldn't expect an interpretation of SR to be consistent with QM.

If you think QM is deterministic, then "there is only ever one possible outcome" to any QM experiment, or indeed to any experiment whatever, anywhere in spacetime, since that's what determinism means. Things would simply have to be determined so that all of the QM experimental outcomes matched QM statistics.
This might be too broad a categorisation, but I will try it anyway. Would the BU be compatible only with hidden variables theories e.g. Pilot Wave theory? Or are there other classes of deterministic theories?

Just reading back on the other thread, you mentioned that QFT doesn't share the determinism of non-quantum relativity theory. Normally, I would interpret this mean that QFT is indeterministic but I'm slowly starting to distrust my intuition in these matters :biggrin:. Is it indeterministic?
 
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Lynch101
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Is there still any room for any deterministic formulation of QM in view of Bell's theorem?
Would any deterministic formulation of QM imply an existence of a set of variables that deterministically determine the state of any system? Wouldn't that contradict the Bell's theorem?
Don't take my word for this, I'm just checking to see if my understanding is correct, so definitely wait for someone else to correct this.

I think the violation of Bell's inequality means one of the following assumptions [of Bell's theorem] must be false:
1. Realism
2. Locality
3. Local realism
4. Free Will

So, I think it rules out local hidden variables but not all hidden variables theories.


As I say, wait for someone to fact check that, because I'm just repeating what I've read.
 
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PeterDonis
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Is there still any room for any deterministic formulation of QM in view of Bell's theorem?
Sure. There are deterministic interpretations of QM: MWI and Bohmian, for example.
 
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PeterDonis
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Would any deterministic formulation of QM imply an existence of a set of variables that deterministically determine the state of any system?
Yes.

Wouldn't that contradict the Bell's theorem?
No, because the variables in question would be nonlocal, and Bell's theorem only rules out local hidden variables.
 
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PeterDonis
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Would the BU be compatible only with hidden variables theories e.g. Pilot Wave theory? Or are there other classes of deterministic theories?
The word "theory" is not correct here. These are interpretations of QM, not theories.

As I responded to @evi7538 , there are indeed deterministic interpretations of QM.

QFT doesn't share the determinism of non-quantum relativity theory
Please give a specific quote.
 
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Lynch101
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The word "theory" is not correct here. These are interpretations of QM, not theories.
As I responded to @evi7538 , there are indeed deterministic interpretations of QM.[/quote]
Are all deterministic theories of QM necessarily dependent on hidden variables, or are there other kinds?



Please give a specific quote.
It has, in the sense that Minkowski spacetime defines the 4-d geometry in which events have to fit. (Actually, you can also do QFT on other background spacetimes, not just Minkowski spacetime.) But it does not share the determinism of non-quantum relativity theory.
 
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PeterDonis
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Are all deterministic theories of QM necessarily dependent on hidden variables
They are not "deterministic theories". They are deterministic interpretations of QM. The theory is QM.

Whether such interpretations are necessarily dependent on hidden variables depends on what you mean by "hidden variables". There don't seem to be any in the MWI by any definition of "hidden variables" that I'm aware of.
 
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PeterDonis
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I would interpret this mean that QFT is indeterministic
It means that QFT is a quantum theory, a form of QM, so whether or not is deterministic depends on what interpretation of QM you adopt and whether that interpretation is deterministic or not. The basic math of QM, and of QFT, certainly doesn't seem to be deterministic, since it predicts probabilities of different possible experimental results, not definitely that one particular result will occur.

Non-quantum relativity theory, OTOH, is deterministic as a theory--the basic math is deterministic.
 
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That is, it would seem to necessitate that there is only ever one possible outcome in a Bell test, even prior to the running of the test.

Have I misunderstood the Block Universe, or Bell tests, or both?
You misunderstood block universe. According to it, there is only one outcome, but not before running the test.
 
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Lynch101
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It means that QFT is a quantum theory, a form of QM, so whether or not is deterministic depends on what interpretation of QM you adopt and whether that interpretation is deterministic or not. The basic math of QM, and of QFT, certainly doesn't seem to be deterministic, since it predicts probabilities of different possible experimental results, not definitely that one particular result will occur.

Non-quantum relativity theory, OTOH, is deterministic as a theory--the basic math is deterministic.
I see. Thank you Peter, you seem to give pretty clear answers without any bias in either direction. I think part of my issue is that [what little] I have learned has been from sources with a slight bias towards certain interpretations and so things tend to get stated in a very matter of fact way.
 
  • #14
Lynch101
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You misunderstood block universe. According to it, there is only one outcome, but not before running the test.
I'm wondering if we're saying the same thing but getting caught up on the use of the word "before", or are we saying different things?

My understanding of the BU was that all events on the world line of an object are extended in spacetime. In this sense, our past and future "co-exist" in the block with our present. This means that the future is inevitable because it is already written as opposed to it being unwritten but inevitable.

In this sense, the outcome does not temporally precede the running of the experiment, but at the moment of the beginning of the experiment, the outcome is already "set in stone" so to speak, it is already in the block.

If that makes sense?
 
  • #15
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My understanding of the BU was that all events on the world line of an object are extended in spacetime. In this sense, our past and future "co-exist" in the block with our present.
That's wrong, events are not extended in spacetime. It's true that past and future coexist in BU, but in a different sense.

Here is an example. Take a piece of paper and draw two dots. Those dots are not extended on paper, they are just dots. But those dots coexist, in the sense that both dots are there on the paper. That should be easy. The hard part is now to imagine that spacetime is like a piece of paper. Not that space is like a piece of paper (that would be easy to imagine), but that spacetime is like a piece of paper. More precisely, since the paper is 2-dimensional, you have to imagine that one dimension of the paper is space and another dimension of the paper is time. If you can imagine this, then that's the idea of BU in a nutshell.
 
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That's wrong, events are not extended in spacetime. It's true that past and future coexist in BU, but in a different sense.

Here is an example. Take a piece of paper and draw two dots. Those dots are not extended on paper, they are just dots. But those dots coexist, in the sense that both dots are there on the paper. That should be easy. The hard part is now to imagine that spacetime is like a piece of paper. Not that space is like a piece of paper (that would be easy to imagine), but that spacetime is like a piece of paper. More precisely, since the paper is 2-dimensional, you have to imagine that one dimension of the paper is space and another dimension of the paper is time. If you can imagine this, then that's the idea of BU in a nutshell.
My apologies, I really need to try to be more precise with my statements.

I understand your example with the paper but I should have said that the world line of an object is extended in spacetime and all events on that world line co-exist in the Block. To personalise it somewhat, if we think about ourselves, then it means that the events of our 10th birthday, our 30th birthday, and our 80th birthday all co-exist in the block. So, even now our 80th birthday (perhaps being presumptuous) is inevitable because it is already written in the block; as opposed to the alternative that it is inevitable but not written in the block.

That is how I have understood it. Essentially, it is a block structure because past, present, and future events co-exist to make it a block, as opposed to only present events being included in the structure of the universe.

Is that incorrect?
 
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  • #17
PeterDonis
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According to it, there is only one outcome, but not before running the test.
The Block Universe is deterministic (since it's an interpretation of a deterministic theory, classical non-quantum SR), so all outcomes to all experiments are determined. I'm not sure that is consistent with what you say here (it depends on what you mean by "before", which is probably a word to avoid in this context since words dealing with time relationships have to be used very carefully, if at all, in discussions like this).
 
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PeterDonis
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My understanding of the BU was that all events on the world line of an object are extended in spacetime.
This is true of any interpretation of SR. A worldline in SR is just a 1-dimensional timelike curve in a 4-dimensional geometry.

The BU says that all events everywhere in 4-d spacetime "exist". It doesn't change the math at all; it just makes a claim about how the label "exist" applies to events that are represented in the math.
 
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Lynch101
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This is true of any interpretation of SR. A worldline in SR is just a 1-dimensional timelike curve in a 4-dimensional geometry.

The BU says that all events everywhere in 4-d spacetime "exist". It doesn't change the math at all; it just makes a claim about how the label "exist" applies to events that are represented in the math.
Thanks Peter, I think I confused the issue with by using the word "before".

Would it be correct to say that the Block Universe is incompatible with any indeterministic interpretation of QM then? I'm presuming it would, on the basis that the BU is deterministic, but as I say, I am learning to be skeptical of my intuition.
 
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PeterDonis
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Would it be correct to say that the Block Universe is incompatible with any indeterministic interpretation of QM then?
Yes.
 
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Lynch101
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Yes.
Thank you for your patience with these very simplistic questions Peter, but I find that I can end up assuming too much based on certain info that I come across.

Is it also correct that a "growing block" interpretation too would be incompatible with indeterminate interpretations of QM, for the same reason? Or is there something in the fact that "the future is not yet set in stone/the block" that would potentially make it compatible?

It might be an easier question to ask if indeterminate interpretations of QM necessarily require presentism?

Apologies for bringing it back to those kinds of philosophical ideas, but I try to relate it back to familair ideas.
 
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PeterDonis
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Is it also correct that a "growing block" interpretation too would be incompatible with indeterminate interpretations of QM, for the same reason?
Depends on how the block is supposed to grow, i.e., whether the growth process is assumed to be deterministic or not. Which in turn depends on which theory this is supposed to be an interpretation of, non-quantum classical SR (which is deterministic) or QM (which is not necessarily deterministic).

It might be an easier question to ask if indeterminate interpretations of QM necessarily require presentism?
I don't see why they would. Even in non-deterministic interpretations of QM, once a measurement result has been recorded, it doesn't change. So such interpretations would be perfectly compatible, for example, with an interpretation in which events in one's past light cone were fixed and certain.
 
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Lynch101
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Depends on how the block is supposed to grow, i.e., whether the growth process is assumed to be deterministic or not. Which in turn depends on which theory this is supposed to be an interpretation of, non-quantum classical SR (which is deterministic) or QM (which is not necessarily deterministic).
Ah yes, I see. Thank you!


I don't see why they would. Even in non-deterministic interpretations of QM, once a measurement result has been recorded, it doesn't change. So such interpretations would be perfectly compatible, for example, with an interpretation in which events in one's past light cone were fixed and certain.
But in what sense you mean fixed and cer......I'm joking, I'm joking!!

Thank you Peter.
 
  • #24
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My apologies, I really need to try to be more precise with my statements.

I understand your example with the paper but I should have said that the world line of an object is extended in spacetime and all events on that world line co-exist in the Block. To personalise it somewhat, if we think about ourselves, then it means that the events of our 10th birthday, our 30th birthday, and our 80th birthday all co-exist in the block. So, even now our 80th birthday (perhaps being presumptuous) is inevitable because it is already written in the block; as opposed to the alternative that it is inevitable but not written in the block.

That is how I have understood it. Essentially, it is a block structure because past, present, and future events co-exist to make it a block, as opposed to only present events being included in the structure of the universe.
That's correct.
 
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The Block Universe is deterministic
I don't think it is necessary true. Imagine, for instance, that the world is a random set of points (events) on the spacetime manifold. Since it is random, I would not call it deterministic. But if I interpret spacetime manifold itself as a block spacetime, without a fundamental difference between future and past and without a notion of the flow of time, then I still have a block universe.
 
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