I Conflict between the Block Universe and Bell Tests?

[Lynch101]

TL;DR 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?

 

[PeterDonis]

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.

 

[evi7538]

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?

 

[Lynch101]

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 . Is it indeterministic?

 

[Lynch101]

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.

 

[PeterDonis]

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.

 

[PeterDonis]

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.

 

[PeterDonis]

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.

 

[Lynch101]

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.

 

[PeterDonis]

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.

 

[PeterDonis]

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.

 

[Demystifier]

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.

 

[Lynch101]

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.

 

[Lynch101]

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?

 

[Demystifier]

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.

 

[Lynch101]

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?

 

[PeterDonis]

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).

 

[PeterDonis]

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.

 

[Lynch101]

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.

 

[PeterDonis]

Would it be correct to say that the Block Universe is incompatible with any indeterministic interpretation of QM then?

Yes.

 

[Lynch101]

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.

 

[PeterDonis]

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.

 

[Lynch101]

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.

 

[Demystifier]

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.

 

[Demystifier]

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.

 

[PeterDonis]

I don't think it is necessary true.

More precisely: the Block Universe as an interpretation of classical, non-quantum special relativity is deterministic, because classical, non-quantum special relativity is itself deterministic. (So is classical, non-quantum general relativity, for that matter, so you could apply a Block Universe interpretation to any curved spacetime solution in GR.)

Imagine, for instance, that the world is a random set of points (events) on the spacetime manifold.

We're not allowed to just "imagine" anything we like in this discussion. We are only allowed to imagine things that are consistent with whatever laws of physics we are using. If we are using the laws of classical, non-quantum relativity, then we are only allowed to imagine scenarios that are deterministic--initial data on any spacelike hypersurface determines the entire spacetime.

 

[Demystifier]

More precisely: the Block Universe as an interpretation of classical, non-quantum special relativity is deterministic, because classical, non-quantum special relativity is itself deterministic. (So is classical, non-quantum general relativity, for that matter, so you could apply a Block Universe interpretation to any curved spacetime solution in GR.)

Let me just note that I applied block universe to black hole information paradox
http://de.arxiv.org/abs/0905.0538
which involves GR and QFT.

 

[Lynch101]

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.

It would still allow for only one possible outcome for Bell tests, however. This would presumably mean that it isn't compatible with indeterministic QM, no?

 

[Demystifier]

It would still allow for only one possible outcome for Bell tests, however. This would presumably mean that it isn't compatible with indeterministic QM, no?

I would say no. Or more precisely, it depends on what do you mean by "possible" and "indeterministic". Here is an example. Suppose that coin flipping is a random process and suppose that the outcome of this process turned out to be heads. Is there only one possible outcome in this case? Is the outcome "heads" indeterministic?

 

[Lynch101]

I would say no. Or more precisely, it depends on what do you mean by "possible" and "indeterministic". Here is an example. Suppose that coin flipping is a random process and suppose that the outcome of this process turned out to be heads. Is there only one possible outcome in this case? Is the outcome "heads" indeterministic?

It is possibly my misunderstanding of your previous post, coupled with how I understand the Block Universe but my understanding of the Block Universe is that the Universe would comprise all events on an objects world line. If these events are just random points in Block Spacetime, perhaps this would negate the determinism of the Block Universe but it wouldn't negate the fact that the event labelled "outcome of the Bell test" is "eternally etched in the Block". The outcome is already "out there" so to speak.

Does that make sense?

 

[Demystifier]

If these events are just random points in Block Spacetime, perhaps this would negate the determinism of the Block Universe but it wouldn't negate the fact that the event labelled "outcome of the Bell test" is "eternally etched in the Block". The outcome is already "out there" so to speak.

Does that make sense?

Yes it does.

 

[Lynch101]

Yes it does.

Well I guess the law of averages works then

 

[RUTA]

While the outcomes of any particular measurement in QM are indeterministic, their distribution in the classical context of 4D spacetime is totally deterministic. When you have so many such quantum exchanges, say of photons, that you might expect the contribution to the stress-energy tensor will bear on the metric per GR, you simply use the corresponding classical field (per the Faraday tensor in the case of photons) in the action for Einstein's equations. Whether or not you consider 4D spacetime to be "real" is irrelevant. The deterministic QM distribution is 4D meaning we're necessarily talking about averages of events distributed over space and time. See these Insights for example: Bell States and Conservation of Spin Angular Momentum and Answering Mermin's Challenge with Wilczek's Challenge.

 

[eltodesukane]

Of course there is conflict between the Block Universe and Bell Tests.
The Block Universe is a classical concept for a classical Universe.
The real Universe is not classical, so classical physics fundamentally doesn't apply to the actual real Universe.
(but classical physics is still very useful and a very good within its domain of validity)

 

[msumm21]

The Block Universe is an interpretation of SR, and SR is a deterministic theory

Wouldn't we say SR is not deterministic because my past light cone at any time is larger than it was at an "earlier time." So, according to SR, I have no way to know if I will be bombarded with X-rays in the next instant.

Maybe I'm using the wrong definition of deterministic? At least SR makes it impossible for any observer to predict the future, right?

 

[PeterDonis]

Wouldn't we say SR is not deterministic because my past light cone at any time is larger than it was at an "earlier time." So, according to SR, I have no way to know if I will be bombarded with X-rays in the next instant.

Your not knowing what you will observe in the next instant has nothing to do with whether what you will observe in the next instant is fully determined by initial conditions.

SR makes it impossible for any observer to predict the future, right?

Only because, to predict the future in a deterministic spacetime, you need initial data on an entire spacelike 3-surface, and the only data any observer can actually have is in their past light cone, which will never cover an entire spacelike 3-surface.

 

[msumm21]

Your not knowing what you will observe in the next instant has nothing to do with whether what you will observe in the next instant is fully determined by initial conditions.

OK, I was thinking that may be the definition of deterministic. However, if we reject the block universe, aren't we saying that there aren't real, fixed initial conditions? Not sure, still thinking...

 

[PeterDonis]

if we reject the block universe, aren't we saying that there aren't real, fixed initial conditions?

No. We're just pointing out that no observer can ever know a complete set of initial conditions for the entire universe. But any observer can know a complete set of initial conditions for what happens at his current present event, since the only set of initial conditions required for that event is what happens in its past light cone. He just doesn't know, at that event, a sufficient set of initial conditions to predict what happens in his future, since those conditions will include events that are outside his current past light cone.

 

[msumm21]

No. We're just ...

It seems to me that accepting a fixed, real time slice is accepting BU. Surely there's not one special time slice that's fixed and real, but others are not. Right?

 

[PeterDonis]

It seems to me that accepting a fixed, real time slice is accepting BU.

What is a "fixed, real time slice" and where did I talk about "accepting" one?

 

[RUTA]

There seems to be some confusion over the term “block universe.” Let me try to clarify what that means.

We all experience Now as unique from past experiences or experiences that are yet to happen (I won’t qualify all such statements with “I assume”). We tend to believe that we only exist (are only “real”) Now, i.e., my past self no longer exists and my future self has yet to exist. I assume there are other things that coexist with me at each Now, e.g., my twin brother who lives 600 miles away has a Now of his own and his Now coexists with my Now. When we turned 50 our 49 year-old selves no longer existed and our 51 year-old selves were yet to exist. We use this reasoning out of necessity nearly every day. If I want to call my brother I first ask myself, “What is my brother doing Now?“ because I don’t want to call if I believe he is sleeping or at work, for example. Not only is it perfectly reasonable to assume the validity of a shared set of Nows with other people, but it is absolutely necessary in dealing with objects at large distances. If I work for NASA and I’m in charge of a probe circling Jupiter 40 light-min away, I need to know where it is at any given Now, so I know where it will be 40 min from that Now when it receives my next orbital command. When I talk to engineers in Australia who will take over command of that spacecraft in 12 hours, we all share the same notion of our set of overlapping Nows, albeit with different time stamps for our different time zones. Of course, it could be that our probe has been struck by a meteor and it doesn’t exist where I think it does at some Now — I won’t know it has been struck for at least 40 min. But, that’s simply a result of the finite speed of light that carries information to me from the probe. That is, it’s a fact about my knowledge of the probe at some Now, not about whether or not the probe actually coexists with me (is “equally real”) at any Now. The probe doesn’t experience Now, but it’s meaningful to think about it as only existing (as only being “real”) at each Now. If everyone agrees on a shared collection of Nows, that would constitute a global “Now slice“ of spacetime. That is Presentism and that accords nicely with our everyday experience.

Unfortunately, that intuitive notion cannot hold if everyone measures the same speed of light regardless of their motion relative to the source, which is an empirical fact. That empirical fact is itself very counterintuitive. If we are at rest with respect to each other and I throw a ball away from me and towards you at 5 m/s relative to me, then you measure the speed of the ball to be 5 m/s relative to you. If I’m in a car moving at 30 m/s towards you and I throw a ball away from me and towards you at 5 m/s relative to me (and the car), then you measure the speed of the ball to be 35 m/s relative to you. That’s intuitive. But, if I do the same with a flashlight, you always measure the speed of the light from my flashlight to moving towards you at c. Even if I’m moving towards you at 0.5c, in which case intuition tells us you should measure the light from my flashlight to be moving towards you at 1.5c, you will still measure its speed to be c.

As a consequence of that fact, observers moving relative to each other will no longer agree on what events are simultaneous (are “equally real”), i.e., on what events occupy a “Now slice.” Thus, if everyone continues to attribute “existence” only to those events in spacetime that share their Now slice at any given moment of time in their existence, e.g., my attributing existence to the version of my twin brother that is the exact same age as me, then there is some frame of reference in which my Now coexists with a younger (or older) version of my brother. Thus, that special feeling of Now that I experience along my worldline in spacetime exists equally everywhere along my worldline as far as other observers are concerned. Here is 9-min video showing the relativity of Now slices (aka the relativity of simultaneity) that results from the light postulate. That is the block universe and it violates our everyday experience.

Of course, that doesn’t entail that our dynamical experience of time is ”an illusion” as the video implies. You can rather believe that the block universe is simply a tool for coordinating, explaining, and predicting our experiences. See Section 5 of this paper by Mermin, for example. That’s an “interpretative” aspect of block universe.

 

[Sunil]

The word "theory" is not correct here. These are interpretations of QM, not theories.

No. Pilot wave (or dBB) theory is a different theory. It gives quantum predictions only in particular states, namely in quantum equilibrium.

 

[PeterDonis]

Pilot wave (or dBB) theory is a different theory. It gives quantum predictions only in particular states, namely in quantum equilibrium.

The Bohmian interpretation includes the requirement of quantum equilibrium, which is why it is normally called an interpretation.

However, I agree that you can make a more general theory, different from standard QM, out of it by dropping the requirement of quantum equilibrium. Of course, this theory will then make all kinds of predictions that do not match experimental results.

 

[Kolmo]

Where is it actually proven that it gives the same results for all observables. I've seen the proofs that with quantum equilibrium we get the same results for single particle position measurements, but I've never seen the proof of the general case.

 

[Sunil]

That's done in Bohm's original paper.

Bohm, D. (1952). A suggested interpretation of the quantum theory in terms of "hidden" variables, Phys.Rev. 85(2), 166-193

And this is the main contribution of this paper. That the results for position measurements are fine was known already before by de Broglie 1927 or so. But de Broglie was unable to show that for non-position measurements this works fine too.

 

[Kolmo]

That's done in Bohm's original paper.

Bohm, D. (1952). A suggested interpretation of the quantum theory in terms of "hidden" variables, Phys.Rev. 85(2), 166-193

And this is the main contribution of this paper. That the results for position measurements are fine was known already before by de Broglie 1927 or so. But de Broglie was unable to show that for non-position measurements this works fine too.

I've read that before, but it seems to give some nice details about energy and scattering not a description for general operators. Also I've read a few criticisms of it, such as the inability to replicate multi-time correlations. Is there a presentation of the equivalence for general operators in some modern text?

 

[Sunil]

I've read that before, but it seems to give some nice details about energy and scattering not a description for general operators.

Hm. I read at page 180 "Let us now consider an observation designed to measure an arbitrary (hermitean) "observable" Q, associated with an electron."

 

[Demystifier]

Where is it actually proven that it gives the same results for all observables. I've seen the proofs that with quantum equilibrium we get the same results for single particle position measurements, but I've never seen the proof of the general case.

See e.g. the paper linked in my signature.