I Quantum Ball and Cup - Thought Experiment

[gentzen]

Certain collapse interpretations appear to say that, in truth, the system went in both directions but spontaneously collapsed to a single position.

Can you be more specific, which concrete collapse interpretation you actually have in mind here?

But maybe that is the wrong question. Why do you want to understand QM in terms of some experiment with "nearly" no inherently quantum features? My impression is that you will only ever get interpretations of classical probability from such an experiment. So is your "real" question what the different interpretations would say, if you apply them in a context where classical probabilities would be sufficient?

 

[Lynch101]

Ok, but QM predictions of probabilities are not facts. So if you want to focus on facts, you shouldn't even be talking about predictions of probabilities.

I didn't say it was pointless. I said it was irrelevant to the particular thing you appeared to be focusing on. Which, as noted above, is a prediction of probabilities, not a fact.

Ok, fine, if that's what you want to ask about, you should just ask about it. There's no need to go wandering off into irrelevancies about past light cones.

"Fact" might not have been the clearest term to use, apologies. Would mathematical facts (as opposed to observational facts) be a better choice of words perhaps, or theoretical information? In interpretations of QM discussion is often on the interpretation of the statistical predictions. It's in that context I am trying to interpret what the probabilistic interpretations tell us, in conjunction with what an aspect of relativity tells us.

In that sense, I don't think the past light cone is irrelevant since it is that information which forms the basis of the question being asked. It's a question of how to interpret that information.

Yes. The particle always has one, deterministic trajectory. However, as I have already noted, there is no accepted relativistic version of Bohmian mechanics. So if you're asking about relativistic QM, there is not an accepted Bohmian interpretation of that.

Just to focus in on how we can interpret the probabilistic interpretations here, because it provides some context for the question re: QFT. In this case, the probabilistic predictions tell us that, in truth, there isn't really a genuine possibility of measuring the particle in either position. It is our information about the system which is incomplete.

No. Collapse interpretations, meaning interpretations that say collapse is a real physical process, say that about collapse of the wave function. There are no "positions" independent of the wave function. There are no hidden particles as there are in Bohmian mechanics.

This is obviously a bit less intuitive but I wouldn't necessarily envisage hidden particles. Am I right in saying, in these collapse theories the wave function is a representation of the physical system delocalised in space; that, in some sense, the system is not localised to a single position but is physically spread out covering multiple positions?

My understanding is that, in this case, there is no missing information and the probabilistic predictions are a function of the genuine possibility of measuring the system in any of the positions (with non-zero probability). To account for why we only ever measure the position of the system in a single position (as opposed to all the non-zero positions) these interpretations say the system, randomly and physically, "collapses" to a single position. This entails some form of physical FTL causation which cannot be used for signalling.

Not in any meaningful sense, since collapse intepretations still have to satisfy the no signaling theorem in QM.

It would be meaningful in the sense that it is the reason [according to those collapse theories] we measure the system in a single position as opposed to all the non-zero predicted positions. It would be meaningful in the sense that it is a physical process which plays a causal role in the Universe. It wouldn't be exploitable, but it does have more than just a superficial meaning, I would say.

a photon passing through a beam splitter and having 50% probability of being detected by each of two detectors, one in each output arm of the beam splitter. In such an experiment, we already know that only one of the two detectors will register a photon on each run, so there is literally no possibility at all of a photon being detected at two spacelike separated events.

You don't even need relativity or any rule about "no FTL travel" for this; it's literally required just by the experimental setup itself.

At least this much I do get None of them say that both detectors will register a measurement event.

Looking at this in terms of the different interpretations [excluding the past light cone], while I wouldn't call them intuitive, I can make some sense of what Bohmian Mechanics and certain collapse theories say. I'm just less clear on what QFT says.

BM appears to say that both detectors don't truly have a genuine possibility of registering the measurement event. It's our incomplete information which makes it appear that way. This explains why we only end up with a single measurement outcome.

Certain collapse theories appear to say there is a genuine possibility of measuring at A or B but random, physical, FTL collapse of the wave function explains why we only get a single measurement event.

I'm not entirely clear how QFT explains the single measurement outcome. Unlike BM, it doesn't appear to imply that the probabilistic predictions are the result of incomplete information. FWICG, it says there is a genuine possibility of measuring the system at either location. This sounds similar to certain collapse theories, but the collapse theories explain this through FTL collapse of the physically delocalised system (represented by the wave function). Is there a similar explanation associated with QFT?

The subsequent question, with regard to the past light cone is just an extension of the question of whether there was a genuine possibility of measuring the system at all non-zero positions. Given that some of them lie outside the past light cone, how can we interpret that information?

Second, the supposed rule you give here for relativistic interpretations isn't a matter of interpretation at all. It's a matter of the basic math of relativistic QM, i.e., QFT. And what the basic math of QFT tells you is: there is no way to even formulate this question because there is no way of identifying "the same particle" at different events in QFT. QFT is a theory of fields, not particles. "Particles" are just particular field states with particular observable manifestations. If you have two measurement events, event A measuring "one electron here" and event B measuring "one electron here", there is no way to tell that those two electrons are "the same" electron. Particles don't have little identifiers on them that keep track of them. All you can say is that there was an electron present at event A and an electron present at event B. You cannot say they were "the same" electron. (And you cannot say they were not the same electron either.)

I'm not necessarily talking about separate events, as opposed to the probability of a measurement event. It's more a question of whether there was a genuine possibility of measuring the field state at all of the non-zero locations (similar to what collapse interpretations might say); or no genuine possibility (similar to BM).

My understanding is that QFT says there was a genuine possibility, but it's different from collapse interpretations. Collapse interpretations put forward a relatively clear (if unintuitive) explanation as to the origin of the probabilities and the single measurement outcome. I'm not clear on what the alternative explanation is according to QFT.

Causality in QFT is more subtle than that. QFT actually does not have a rule that anything that happens at a given event can only be caused by something in the past light cone of that event. All QFT says is that measurements at spacelike separated events must commute, i.e., their results must not depend on the order in which they are made. (This rule makes obvious sense since the time ordering of spacelike separated events is frame dependent, so there is no invariant fact of the matter about which one occurs first.) QFT does not forbid causal connections between spacelike separated events, as long as what happens at those events does not depend on their ordering.

It's probably my misinterpretation, but does this not allow for FTL causation, if the causal influence can come from outside the past light cone?

 

[Lynch101]

Can you be more specific, which concrete collapse interpretation you actually have in mind here?

I don't recall the particular names of the collapse theories. It's mostly been from discussions on here. PD has clarified a general idea with regard to certain collapse interpretations, so I would take it as a more general case of "any collapse interpretation to which the above applies".

But maybe that is the wrong question. Why do you want to understand QM in terms of some experiment with "nearly" no inherently quantum features? My impression is that you will only ever get interpretations of classical probability from such an experiment. So is your "real" question what the different interpretations would say, if you apply them in a context where classical probabilities would be sufficient?

I'm not sure I understand. The double-slit experiment does have quantum features. I'm trying to get an understanding of how to interpret the information that some of the positions predicted for the system lie outside the past light cone of the final measured position.

 

[DrChinese]

1. Certain collapse interpretations appear to say that, in truth, the system went in both directions but spontaneously collapsed to a single position. This spontaneous collapse involved some sort of FTL propagation.

2, Does the non-zero probability mean there was a genuine possibility the system could have been measured outside the past light cone of the measurement event, or does it simply represent a lack of information on our part?

1. What is propagating FTL? In your examples with the beam splitter, I don't see anything that qualifies. The "nonlocal" effect is the absence of a detection "there" when there is a detection "here", if you want to call that nonlocal. But nothing propagates.

2. Again, I don't see what is outside of any past light cone in your example.

On the other hand, there is no requirement that quantum effects are constrained to a past light cone. You can have entangled photon pairs in which the individually detected photons have never appeared in the other's past light cone.

 

[Lynch101]

1. What is propagating FTL? In your examples with the beam splitter, I don't see anything that qualifies. The "nonlocal" effect is the absence of a detection "there" when there is a detection "here", if you want to call that nonlocal. But nothing propagates.

You're right apologies. Collapse interpretations say that the causal influence is instantaneous (FTL), don't they? As opposed to "propagating" at some finite FTL speed.

2. Again, I don't see what is outside of any past light cone in your example.

There is a position that had a non-zero probability assigned to it, which lies outside the past light cone. This is what I'm looking to interpret.

A BM-like explanation might say there wasn't really a genuine possibility of measuring it in that position. The probability represents incomplete information on our part. A collapse-like explanation might say there was a genuine possibility of measurement but instantaneous-action-at-a-distance explains why there was a genuine possibility it could have been measured outside the past light cone.

Is there a BM- or collapse-like explanation for QFT? One which says how it was a genuine possibility to make a measurement at all of the non-zero positions or how it was not genuinely possible?

On the other hand, there is no requirement that quantum effects are constrained to a past light cone. You can have entangled photon pairs in which the individually detected photons have never appeared in the other's past light cone.

I think this is slightly different. In Peter's example there is only a single photon.

 

[PeterDonis]

It's in that context I am trying to interpret what the probabilistic interpretations tell us, in conjunction with what an aspect of relativity tells us.

I've already answered that. Go back and read what I said about causality and spacelike separated measurements commuting in QFT. Again, that is all independent of any interpretation.

Most of the literature on QM interpretations, unfortunately, uses non-relativistic QM as a framework, so it doesn't discuss the issue you are raising with how QM interpretations interact with relativity. So all we really have is the interpretation-independent rules I've already described.

 

[PeterDonis]

I don't think the past light cone is irrelevant

It's irrelevant in the particular scenario you have chosen. So if you want to make it relevant, you'll need to find another scenario that illustrates why.

 

[PeterDonis]

the probabilistic interpretations here,

in these collapse theories

Certain collapse theories

Please give specific references for which interpretations/theories you are talking about. (And note that some "theories", like the GRW stochastic collapse model, are not interpretations of QM, they are different theories that make different predictions about some experimental results from those of standard QM.) In order to say anything about what particular interpretations do or do not say, we need to have some kind of valid reference as a basis for discussion. That is part of the rules of this subforum.

 

[PeterDonis]

I'm not entirely clear how QFT explains the single measurement outcome.

QFT is not an interpretation of QM. It's relativistic QM. It doesn't "explain" single measurement outcomes at all, any more than basic non-relativistic QM, independent of any interpretation, does. QFT just makes predictions.

Please read my previous post about the lack of QM interpretation literature that uses QFT, instead of non-relativistic QM, as a framework.

 

[PeterDonis]

does this not allow for FTL causation, if the causal influence can come from outside the past light cone?

What it allows is not well described by the term "FTL causation", since it cannot be used to send any signals or otherwise influence any outcomes. The no signaling theorem is still true in QFT. QFT basically just says that, since the actual rule is that spacelike separated measurements commute, it is impossible to say that spacelike separated measurements cannot cause each other. But it is also impossible to say that they can cause each other. QFT simply makes no claim either way. As I noted in my previous post, QFT is not an interpretation of QM. It's just relativistic QM. It doesn't make claims about metaphysical concepts like "cause": it just makes predictions. The "spacelike separated measurements commute" rule (and its converse, that timelike or null separated measurements do not have to commute) is the closest thing QFT has to "causality".

 

[PeterDonis]

I don't recall the particular names of the collapse theories. It's mostly been from discussions on here.

What discussions? Do those discussions give any references?

 

[PeterDonis]

A BM-like explanation might say there wasn't really a genuine possibility of measuring it in that position.

It's rather strange to say that Bohmian mechanics does not imply "FTL" here, since Bohmian mechanics explicitly claims that a change in the wave function at some point can instantaneously (i.e., "FTL") influence a particle anywhere else in the universe.

 

[gentzen]

I'm not sure I understand. The double-slit experiment does have quantum features. I'm trying to get an understanding of how to interpret the information that some of the positions predicted for the system lie outside the past light cone of the final measured position.

In the end, we try to help you. But after some time, just saying that what you propose is wrong, or that you misunderstood this or that doesn't seem to provide much additional benefit for you. At least that is my impression. So I tried to ask instead what it is that you currently try to learn. And I made an "educated guess" what it might be. The "experiment" proposed in your original question was not yet concerned with interactions between quantum theory and relativity, and not with a double-slit experiment either.

So my basic question is: how can we help you to advance that next step that you currently want to take?

 

[Lynch101]

QFT is not an interpretation of QM. It's relativistic QM. It doesn't "explain" single measurement outcomes at all, any more than basic non-relativistic QM, independent of any interpretation, does. QFT just makes predictions.

Please read my previous post about the lack of QM interpretation literature that uses QFT, instead of non-relativistic QM, as a framework.

Ah, I see. Thanks for that clarification. I may have been conflating QFT with the minimal statistical interpretation. I thought they were one and the same thing, or at least said the same thing.

QFT is not an interpretation of QM. It's relativistic QM. It doesn't "explain" single measurement outcomes at all, any more than basic non-relativistic QM, independent of any interpretation, does. QFT just makes predictions.

Please read my previous post about the lack of QM interpretation literature that uses QFT, instead of non-relativistic QM, as a framework.

You've already mentioned this with regard to BM and collapse theories, but are there any particular interpretations which attempts to explain single measurement outcomes that have a relativistic version?

It's irrelevant in the particular scenario you have chosen. So if you want to make it relevant, you'll need to find another scenario that illustrates why.

I think we might be disagreeing over semantics here because it is from the double-slit experiment that the information comes, so it can't be irrelevant to that scenario. Do you perhaps mean that it is meaningless?

What discussions? Do those discussions give any references?

Hold on, I'll ask.

Which collapse interpretations were you referring to here?

No. Collapse interpretations, meaning interpretations that say collapse is a real physical process, say that about collapse of the wave function. There are no "positions" independent of the wave function. There are no hidden particles as there are in Bohmian mechanics.

It's rather strange to say that Bohmian mechanics does not imply "FTL" here, since Bohmian mechanics explicitly claims that a change in the wave function at some point can instantaneously (i.e., "FTL") influence a particle anywhere else in the universe.

I didn't, I said that there was no genuine possibility that it could have been measured at the other detector (because the particle has one deterministic trajectory).

To use an analogy for the example you suggested (and to represent my understanding).

Let's there are two bags and we are told that we will find a ball in one of them. We don't know in which bag we'll find the ball, so we ascribe a 50% probability to finding the ball in either bag A or B.

A BM-like interpretation says that we don't really have a genuine chance of finding the ball in either bag because the ball is always, only in one bag. It's incomplete information that leads us to ascribe the probability.

A physical collapse interpretation says that the physical system (as represented by the wave function) is physically in both bags, so there is a genuine chance of finding the ball in either. When we reach into the bag the wave function collapses and we either find the ball or we don't. But, there was a genuine possibility of finding it because the physical system was in both bags and there was an instantaneous (FTL) collapse into a single position i.e. the ball.

I was thinking that QFT might offer a different interpretation of what happens, but I was mistaken in my assumption that it was an interpretation like the others.Are there other interpretations that attempt to explain how we end up with the ball in just one of the bags?

 

[Lynch101]

In the end, we try to help you. But after some time, just saying that what you propose is wrong, or that you misunderstood this or that doesn't seem to provide much additional benefit for you. At least that is my impression. So I tried to ask instead what it is that you currently try to learn. And I made an "educated guess" what it might be. The "experiment" proposed in your original question was not yet concerned with interactions between quantum theory and relativity, and not with a double-slit experiment either.

I do appreciate that and through dialogue I was shown a preconception on my part, with regard to QFT being an interpretation of QM.

So my basic question is: how can we help you to advance that next step that you currently want to take?

I'm trying to get a handle on the different interpretations of QM. I have an understanding of what the Bohmian interpretation says and what some physical collapse interpretations say. I'm familiar with the names of some other interpretations but not necessarily what they say is occurring in individual experimental runs.

The analogy I posted above (apologies for the repeat, I didn't want to reference another post). In the example PD suggested, we can look at something that has a 50% probability.

Let's say there are two bags and we are told that we will find a ball in one of them. We don't know in which bag we'll find the ball, so we ascribe a 50% probability to finding the ball in either bag A or B.

A BM-like interpretation says that we don't really have a genuine chance of finding the ball in either bag because the ball is always, only in one bag. It's incomplete information that leads us to ascribe the probability.

A physical collapse* interpretation says that the physical system (as represented by the wave function) is physically in both bags, so there is a genuine chance of finding the ball in either. When we reach into the bag the wave function collapses and we either find the ball or we don't. But, there was a genuine possibility of finding it because the physical system was in both bags and there was an instantaneous (FTL) random collapse into a single position i.e. the ball.Do all interpretations involve either physical FTL collapse or BM-like definite positions (with some other form of FTL occurrence)?

 

[Morbert]

Do all interpretations involve either physical FTL collapse or BM-like definite positions (with some other form of FTL occurrence)?

Antirealist interpretations involve neither.

 

[PeterDonis]

are there any particular interpretations which attempts to explain single measurement outcomes that have a relativistic version?

Not that I'm aware of. As I said, the QM interpretation literature has generally used non-relativistic QM as a framework, unfortunately.

it is from the double-slit experiment that the information comes

What information? Where? Please give a reference. You can't just wave your hands and say the double slit experiment demonstrates some particular thing you have in your head. You need to back up such assertions. So far you haven't.

I said that there was no genuine possibility that it could have been measured at the other detector (because the particle has one deterministic trajectory).

No. you don't even know that, because you don't know the precise deterministic trajectory even after the measurement result is observed. The measurement result doesn't tell you the precise, exact position of the particle, and therefore does not pick out a single deterministic trajectory for the particle. There is a finite measurement error involved. And there will be possible deterministic trajectories within that finite measurement error that do pass through the other detector, because in Bohmian mechanics (since the only accepted version of it is non-relativistic), the deterministic particle trajectories are not limited to the speed of light.

To use an analogy

Your analogy is flawed because it is based on, at the very least, an incorrect understanding of Bohmian mechanics (see above). I'll save comments on collapse interpretations for a separate post.

 

[PeterDonis]

Which collapse interpretations were you referring to here?

I was defining what the general term "collapse interpretation" means (according to my best understanding) in that quote. But that in itself does not tell you how collapse interpretations would interpret a scenario you made up that doesn't even involve QM (as others have already pointed out). I certainly was not making claims about "collapse interpretations" at this level of detail:

A physical collapse* interpretation says that the physical system (as represented by the wave function) is physically in both bags, so there is a genuine chance of finding the ball in either. When we reach into the bag the wave function collapses and we either find the ball or we don't. But, there was a genuine possibility of finding it because the physical system was in both bags and there was an instantaneous (FTL) random collapse into a single position i.e. the ball.

You need to back up this kind of detailed claim with a reference.

In fact, this discussion in general is reaching the point where it is not productive to continue because we are just talking about vague generalities instead of specific interpretations that have actually been proposed in the literature, with specific information about what those interpretations say about scenarios like the one you describe in the above quote. In short, if you want to know what particular interpretations say, you need to go read the literature in which those particular interpretations are proposed and described and used to analyze scenarios. If you have questions about what you read, by all means post them here.

 

[Lynch101]

Antirealist interpretations involve neither.

What do antirealist interpretations say is occurring during individual runs of the experiment?

 

[Morbert]

What do antirealist interpretations say is occurring during individual runs of the experiment?

An antirealist interpretation would say e.g. the probability that your measurement apparatus will record a ball in bag 1 is p1.

 

[PeterDonis]

What do antirealist interpretations say is occurring

Generally, "antirealist" means that asking what "is occurring" is a misguided question, since it presupposes realism.

 

[Lynch101]

I appreciate your patience but I think we might be talking past each other here because I don't think I'm saying anything that you haven't already affirmed above.

What information? Where? Please give a reference. You can't just wave your hands and say the double slit experiment demonstrates some particular thing you have in your head. You need to back up such assertions. So far you haven't.

The information is that some of the positions which were predicted (in the double-slit experiment) lie outside the past light cone of the measurement event.

I'm not suggesting anything new. As far as I can tell, you've already affirmed this to be the case.

Meaning, individual impacts of particles on the detector screen? Yes, it could be [the case that some of the predicted positions lie outside the past light cone].

Parentheses added by me to include context of the question being addressed.

You initially raised an issue pertaining to the use of the word "fact" which I amended to exclude the idea of "observational fact" and instead refer to theoretical information. You have dismissed this as irrelevant, but I think you might meaningless because the information in question - which you have already affirmed - forms the basis of the question. So it is relevant to the question.

The information is there from the predictions and from relativity. The question is how it might be interpreted.

No. you don't even know that, because you don't know the precise deterministic trajectory even after the measurement result is observed. The measurement result doesn't tell you the precise, exact position of the particle, and therefore does not pick out a single deterministic trajectory for the particle. There is a finite measurement error involved. And there will be possible deterministic trajectories within that finite measurement error that do pass through the other detector, because in Bohmian mechanics (since the only accepted version of it is non-relativistic), the deterministic particle trajectories are not limited to the speed of light.

Your analogy is flawed because it is based on, at the very least, an incorrect understanding of Bohmian mechanics (see above). I'll save comments on collapse interpretations for a separate post.

Ah I see, thank you for that clarification. What is the interpretation here of "trajectories that do pass through the other detector"; does this mean that that the system moves through the detector but doesn't register as a measurement event?

 

[Lynch101]

I was defining what the general term "collapse interpretation" means (according to my best understanding) in that quote. But that in itself does not tell you how collapse interpretations would interpret a scenario you made up that doesn't even involve QM (as others have already pointed out). I certainly was not making claims about "collapse interpretations" at this level of detail:

Do different [physical] collapse interpretations offer different explanations of what occurs in the double-slit experiment?

You need to back up this kind of detailed claim with a reference.

In fact, this discussion in general is reaching the point where it is not productive to continue because we are just talking about vague generalities instead of specific interpretations that have actually been proposed in the literature, with specific information about what those interpretations say about scenarios like the one you describe in the above quote. In short, if you want to know what particular interpretations say, you need to go read the literature in which those particular interpretations are proposed and described and used to analyze scenarios. If you have questions about what you read, by all means post them here.

I'll take a look at some of the [physical] collapse theories. You mentioned GRW, are there many others?

Do they all say something different about the physical collapse of the system in the double-slit experiment?

 

[Lynch101]

An antirealist interpretation would say e.g. the probability that your measurement apparatus will record a ball in bag 1 is p1.

And the probability that the ball is in bag 2 would be p2?

 

[Lynch101]

Generally, "antirealist" means that asking what "is occurring" is a misguided question, since it presupposes realism.

It might be worth making a distinction here between assuming the realism of the mathematics and assuming realism simpliciter.

Asking "what is occurring" assumes realism simpliciter but not necessarily the realism of the mathematics. It might be that the mathematics doesn't give us the complete picture.

But I would think assuming realism simpliciter is a necessity, no? Otherwise there would be nothing to interact with the measurement apparatus to give the measurement event. Are there antirealist interpretations that assume realism simpliciter is an incorrect assumption?

 

[Morbert]

And the probability that the ball is in bag 2 would be p2?

No. The probability that the measuring device will register a ball in bag 2 would be p2.

 

[PeterDonis]

The information is that some of the positions which were predicted (in the double-slit experiment) lie outside the past light cone of the measurement event.

Not positions, measurement events. Positions aren't points in spacetime, and it's meaningless to talk about positions as they relate to light cones of points in spacetime.

The fact that, in the kinds of scenarios you are describing, possible measurement events can be spacelike separated is true, but in the absence of relativistic formulations of various QM interpretations, it's outside the scope of this forum to speculate about what various interpretations might make of this in the light of relativity. If you can find a reference that discusses it, that's different, but so far you have given no references at all.

 

[PeterDonis]

What is the interpretation here of "trajectories that do pass through the other detector"

It should be obvious: trajectories in space that pass through the region of space occupied by the other detector.

does this mean that that the system moves through the detector but doesn't register as a measurement event?

Particle trajectories in and of themselves, in Bohmian mechanics, say nothing whatever about measurement events. If you already know that a particle was detected at detector A and not at detector B, then of course you know that, if the particle's Bohmian trajectory passed through detector B, it did not result in a measurement event at detector B. But there is no way to know whether or not that actually happened. Exact particle trajectories in BM are not measurable, because, as I've already said, all real measurements have finite error.

 

[PeterDonis]

Do different [physical] collapse interpretations offer different explanations of what occurs in the double-slit experiment?

I'll take a look at some of the [physical] collapse theories. You mentioned GRW, are there many others?

Do they all say something different about the physical collapse of the system in the double-slit experiment?

I don't have the literature in front of me and I'm not going to take the time to look it up. If you want questions like these answered, you should be looking up the literature.

 

[Lynch101]

No. The probability that the measuring device will register a ball in bag 2 would be p2.

All of the interpretations, in effect, say this, don't they?

CMIIW, but BM says that particles follow deterministic trajectories and always have a definite position, which is why we end up with a single measurement event. The deterministic particle trajectories are not limited to the speed of light and the neither are influences at the level of the pilot wave.

In this case , we have probabilistic interpretations as a result of incomplete information about the system.

Collapse interpretations, meaning interpretations that say collapse is a real physical process, say There are no hidden particles as there are in Bohmian mechanics. Instead, collapse of the wave function is a physical process which involves instantaneous (FTL) localisation.

In this case , we have probabilistic interpretations as a result of the fundamental randomness of nature, which occurs during the physical collapse process.

While these are not very intuitive and defy our classical biases, they offer an explanation as to how we get individual measurements from the probabilistic predictions.

Is there a comparable explanation associated with antirealist interpretations?

 

[Lynch101]

Not positions, measurement events. Positions aren't points in spacetime, and it's meaningless to talk about positions as they relate to light cones of points in spacetime.

Do [the relevant] measurements not give us values for position?

Either way, we don't necessarily need to talk about positions, if it is controversial to do so. We can talk about the possibility of a measurement event (on the system in question). In this context we would say that some of the possible measurement events (on the system in the double-slit experiment) lie outside the past light cone of the actual measurement event (on the system in question). This is just information we can gather from QM predictions and relativity.

It is this information which I am seeking interpretation of. It can't be irrelevant since it is plainly obvious and forms the basis of the question being asked. We might suggest that it doesn't mean anything, but that would be questionable given that both pieces of information alone can be interpreted to have meaning.

The fact that, in the kinds of scenarios you are describing, possible measurement events can be spacelike separated is true, but in the absence of relativistic formulations of various QM interpretations, it's outside the scope of this forum to speculate about what various interpretations might make of this in the light of relativity. If you can find a reference that discusses it, that's different, but so far you have given no references at all.

When we talk about possible measurement events being spacelike separated, are we talking about two separate measurements on entangled photon pairs, for example? If so, that's not the case I am talking about.

I'm talking about the double-slit experiment where there is only a single measurement event and the past light cone of that single event.

Have we, perhaps, been talking past each other?

 

[Lynch101]

It should be obvious: trajectories in space that pass through the region of space occupied by the other detector.Particle trajectories in and of themselves, in Bohmian mechanics, say nothing whatever about measurement events. If you already know that a particle was detected at detector A and not at detector B, then of course you know that, if the particle's Bohmian trajectory passed through detector B, it did not result in a measurement event at detector B. But there is no way to know whether or not that actually happened. Exact particle trajectories in BM are not measurable, because, as I've already said, all real measurements have finite error.

Sorry, more precisely, it's the "pass through the other detector" that I wasn't clear on. You've qualified it somewhat more here by saying pass through the region of space occupied by the other detector.

I would have thought that, if the trajectory of the particle brought it into contact with the detector then it would necessarily result in a measurement event. I thought that was just a basic feature of BM which made it appealing to some as an answer to "the measurement problem". But this seems to suggest otherwise.

 

[Lynch101]

I don't have the literature in front of me and I'm not going to take the time to look it up. If you want questions like these answered, you should be looking up the literature.

I'll take a look at some of the literature, I just figured it would be worth asking. You mentioned GRW, would that be an example of a physical collapse interpretation?

 

[Lord Jestocost]

I recommend the entry "Collapse theories" by Giancarlo Ghirardi and Angelo Bassi on the "The Stanford Encyclopedia of Philosophy".
https://plato.stanford.edu/entries/qm-collapse/

 

[Morbert]

All of the interpretations, in effect, say this, don't they?

All interpretations accept that the statistical predictions of QM are reproduced in data generated by experiment. You asked if all interpretations involve either physical FTL collapse or BM-like definite positions. Antirealist positions involve neither.

Is there a comparable explanation associated with antirealist interpretations?

Antirealists reject the idea that quantum theories are explained by some thoroughly intelligible ontology of microscopic systems. Instead the intelligibility of microscopic systems is found in those questions which can be resolved by experiment.

 

[Lynch101]

I recommend the entry "Collapse theories" by Giancarlo Ghirardi and Angelo Bassi on the "The Stanford Encyclopedia of Philosophy".
https://plato.stanford.edu/entries/qm-collapse/

Excellent. Thank you LJ. I never thought to look to the SEoP for physics information before.

 

[Lynch101]

Antirealists reject the idea that quantum theories are explained by some thoroughly intelligible ontology of microscopic systems. Instead the intelligibility of microscopic systems is found in those questions which can be resolved by experiment.

There is an important distinction to be made here between rejecting the idea of an ontology of microscopic systems altogether, and rejecting the intelligibility of that ontology.

I've read some positions that take the former position and argue for idealism, although I'm not sure how coherent they are. The latter position, to my mind, seems to imply some form of [forever] hidden information which would mean that a complete** theory of "the physical reality*" is impossible.

Is there an alternative interpretation/conclusion to that?

*"the physical reality" being the physical experimental set-up as opposed to the mathematical description of it.

**complete being where "every element of the physical reality has a counterpart in the theory" (as per EPR). In this case the microscopic ontology would not be represented in the mathematics.

 

[Morbert]

There is an important distinction to be made here between rejecting the idea of an ontology of microscopic systems altogether, and rejecting the intelligibility of that ontology.

I've read some positions that take the former position and argue for idealism, although I'm not sure how coherent they are. The latter position, to my mind, seems to imply some form of [forever] hidden information which would mean that a complete** theory of "the physical reality*" is impossible.

Is there an alternative interpretation/conclusion to that?

*"the physical reality" being the physical experimental set-up as opposed to the mathematical description of it.

**complete being where "every element of the physical reality has a counterpart in the theory" (as per EPR). In this case the microscopic ontology would not be represented in the mathematics.

This is looping back on the completeness discussions so I'll just reiterate that antirealism frames QM as a complete physical theory and refer you to those earlier discussions.

 

[Lynch101]

This is looping back on the completeness discussions so I'll just reiterate that antirealism frames QM as a complete physical theory and refer you to those earlier discussions.

I wouldn't say it's looping back on them but expanding on them. You raised the issue of intelligibility previously. The point above is a clarification of the distinction between denying the ontology of the microscopic system altogether and denying the intelligibility of that ontology.

Perhaps questions about [anti-realist] interpretations will inevitably return to the question of completeness.

 

[PeterDonis]

Do [the relevant] measurements not give us values for position?

Not exact ones, no. Please re-read "all measurements have finite error" again and again until you understand what it means.

Either way, we don't necessarily need to talk about positions, if it is controversial to do so.

If you don't talk about exact positions of the hidden particles in Bohmian mechanics, you can't really talk about Bohmian mechanics at all, since those exact (hidden) particle positions are the key ingredient of BM.

 

[PeterDonis]

I would have thought that, if the trajectory of the particle brought it into contact with the detector then it would necessarily result in a measurement event.

Why? What reference have you read about BM that leads you to believe this?

Once again: you really, really, really need to base your claims about what various interpretations say on actual references about those interpretations. You should not just wave your hands and try to figure out what the interpretations say on your own.

 

[PeterDonis]

In this context we would say that some of the possible measurement events (on the system in the double-slit experiment) lie outside the past light cone of the actual measurement event (on the system in question). This is just information we can gather from QM predictions and relativity.

Yes. But "possible events" are not necessarily actual events, and before you go applying QM plus relativity to "possible events", you should first go look to see whether relativity itself says anything at all about "possible events" that don't become actual. (Hint: it doesn't.)

It is this information which I am seeking interpretation of.

So why don't you go read what the various papers in the literature on the various interpretations say about this?

If your answer is that, well, the various papers in the literature on the various interpretations don't really say anything about it, perhaps you should rethink your belief that this question is a meaningful question.

In fact, I can turn the question around: why do you think this question is a meaningful question?

When we talk about possible measurement events being spacelike separated, are we talking about two separate measurements on entangled photon pairs, for example?

No. I am talking about the cases you have described (a single photon passing through a beam splitter, and a single photon going through a double slit apparatus).

 

[Lynch101]

Not exact ones, no. Please re-read "all measurements have finite error" again and again until you understand what it means.

If you don't talk about exact positions of the hidden particles in Bohmian mechanics, you can't really talk about Bohmian mechanics at all, since those exact (hidden) particle positions are the key ingredient of BM.

I appreciate your exactness in your replies because it helps to further my understanding, so again, thank you for that. In this case, however, I think we might be talking past each other. My misinterpretation of certain features of BM is a primary factor in this. I don't think we need to talk about the exact position of the particle because I'm talking about the probabilistic predictions of measurement events and the interpretation of those positions.

In fact, I can turn the question around: why do you think this question is a meaningful question?

Because I think the probabilistic predictions of QM call for interpretation and, in relativity, the location of an event in relation to the past light cone has meaning.

The interpretation I am trying to get at can, perhaps, be broken into two parts:
1) In what sense is it a genuine possibility for a measurement event to occur at any of the predicted,
non-zero, measurement event positions?
2) In what sense was there a genuine possibility of measuring the system at a measurement event
position which was/is outside the past light cone of the actual measurement event?In considering the question of the possibility of the system being measured at a position outside the past light cone, of the actual measurement, we might consider a macroscopic example, a spaceship say - solely to attempt to demonstrate the idea.

If we don't know where a spaceship is we might make probabilistic predictions as to where we might find it. Eventually, we measure the location of the spaceship (to within some finite error). If we look back at our predictions we will see that some of the probabilistic predictions lie outside the past light cone. How would we interpret this information?

Incomplete
Someone might suggest that the reason we made such probabilistic predictions in the first place is because our information was incomplete. In truth, some of the positions we predicted weren't genuinely possible, given the actual trajectory of the spaceship and the limiting factor of no-FTL travel.

Simple FTL
Someone else might come along and say, actually, the spaceship might be able to travel faster than the speed of light and so there was a genuine possibility of measuring the position of the spaceship somewhere outside the past light cone.

Collapse
Someone else might say, actually, the "spaceship system" is delocalised in space and when we make a measurement it randomly collapses into a single measurement event. This collapse happens instantaneously (FTL) so there was a genuine possibility of measuring the spaceship somewhere outside the past light cone of the actual measurement event.

BM-like*
Someone else might say, actually, our information is incomplete AND the spaceship can also travel FTL. Not only this, but when the spaceship triggers the measurement device it can result in the spaceship being measured at a spacelike separated position. This process occurs instantaneously (FTL). In this case there was a genuine possibility that the spaceship could have been measured at any of the non-zero predicted positions.

Are there other possible interpretations which don't imply either incompleteness, or some form of FTL causal influence?*This can be modified to make it a more accurate representation. The instantaneous (FTL) causal influence would carry the explanatory power.

 

[PeterDonis]

The interpretation I am trying to get at

...needs to be "got at" by going and looking at the literature, not by vague discussions in this forum with no basis in the literature. That includes "getting at" what Bohmian mechanics says.

Thread closed.

 

[PeterDonis]

we might consider a macroscopic example

As one other note: if you want to understand QM, you need to look at experiments that need QM for their explanation. Using examples that only need classical physics for their explanation, like the behavior of spaceships, is pointless. Please bear that in mind.