Is causality challenged by quantum theory's reliance on indeterminacy?

[Borean]

I think that there is a difference between saying that the physics is causal/acausal and saying the same of the universe.
If there is a cause for the photon to pass through the polariser, then it is not included within standard QM. So, we can say that QM is acausal. There may or may not be a cause, but it isn't described by QM.

What do you mean? The Bohmian model isn't QM?

There are other models, such as the Bohmian interpretation for which there is a cause, and full knowledge of the initial state would allow you to decide the outcome beforehand. The problem with this is that it is generally assumed to satisfy a "quantum equilibrium" hypothesis, in which case the initial state can never be known exactly and you still can't predict the outcome any better than standard quantum mechanics. For this reason, it is often considered to be an interpretation rather than a new model. I personally wouldn't recommend the Bohmian interpretation, I mention it here just as an example.

It sounds like the Bohmian model exactly admits to agnosticism/indeterminacy in the human epistemic realm. A more likely explanation, in my opinion.

 

[Dale]

QM is not acausal. A causal system has a state which depends only on past or present values. An acausal system depends at least on some future values and possibly on past or present values. An anticausal system depends only on future values.

There is nothing in standard QM that implies any dependence on future values, so QM is causal. The randomness does not imply acausality unless it somehow depends on future values of the state.

 

[Borean]

Can anyone tell me what is the cause of the phenomenon known as quantum tunnelling?

And additionally, the cause of the phenomenon of vacuum fluctuations or virtual particles?

Could anyone answer this?

 

[Borean]

QM is not acausal. A causal system has a state which depends only on past or present values. An acausal system depends at least on some future values and possibly on past or present values. An anticausal system depends only on future values.

There is nothing in standard QM that implies any dependence on future values, so QM is causal. The randomness does not imply acausality unless it somehow depends on future values of the state.

Interesting insight.

 

[Pio2001]

Hello,
Here is the current state of affairs to my knowledge.

Let's use the definitions given by Borean : indeterminacy means that there are some events whose cause we don't know, and acausality that there are some events that have no cause at all.

It's been common practice to name "hidden variable" an unknown cause that would give birth to the values that quantum mechanics doesn't predict in the measurment process. So if hidden variables doesn't exist, then the universe is acausal.

In 1964, Bell proves that quantum mechanics is in contradiction with local hidden variables (hidden variables whose effect travels slower or at the speed of light).
In 1967, Kochen and Specker prove that no hidden variables can be carried by the measured system and bear all prédictions about their possible measured properties. If hidden variable exist, local or not, they depend at least party, on the measurment setup. They are contextual.
In 1969, Bell's theorem was generalized by Clauser, Horne, Shimony and Holt to contextual hidden variables.

In 1982, Alain Aspect's experiment shows that quantum mechanics predictions in Bell and CHSH theorems are fullfilled. Therefore if the universe is causal, then it is non-local.

The Kochen-Specker theorem have been tested too. The results of a recent experiment by Roos, Blatt et al have just been published in Nature. It confirms the inexistence of non-contextual hidden variables with great accuracy.

Another recent experiment by Gröblacher et al ruled out a whole class of non-local hidden variable interpretations : http://www.arxiv.org/PS_cache/arxiv/pdf/0704/0704.2529v1.pdf
I'm not sure which class exactly, but it seems to me that they are interpretations without entanglement.

So we're left with some philosophiocal problems. Experiments give us the choice between
-Leaving out causality : some events occur without any cause.
-Leaving out locality : the cause of an event can lie in its future.
-Leaving out realism : there is no reality. There is just information gathered by observers.

The Copenhagen interpretation makes both the first and third choices, as well as Rovelli's relational interpretation.
Cramer's transactional interpretation makes the second choice. As did Böhm-De Broglie's pilot wave interpretation (but they didn't know it before Bell's theorem).
Everett many-world's interpretations are usually not developped enough to get a precise position in this choice. JesseM, in this forum, used to describe an extension of Everett's interpretation that would get rid of these three problems ( but introducing the existence of many worlds of course) : https://www.physicsforums.com/showthread.php?t=206291#11

 

[Borean]

Hello,
Here is the current state of affairs to my knowledge.

Let's use the definitions given by Borean : indeterminacy means that there are some events whose cause we don't know, and acausality that there are some events that have no cause at all.

It's been common practice to name "hidden variable" an unknown cause that would give birth to the values that quantum mechanics doesn't predict in the measurment process. So if hidden variables doesn't exist, then the universe is acausal.

In 1964, Bell proves that quantum mechanics is in contradiction with local hidden variables (hidden variables whose effect travels slower or at the speed of light).
In 1967, Kochen and Specker prove that no hidden variables can be carried by the measured system and bear all prédictions about their possible measured properties. If hidden variable exist, local or not, they depend at least party, on the measurment setup. They are contextual.
In 1969, Bell's theorem was generalized by Clauser, Horne, Shimony and Holt to contextual hidden variables.

In 1982, Alain Aspect's experiment shows that quantum mechanics predictions in Bell and CHSH theorems are fullfilled. Therefore if the universe is causal, then it is non-local.

The Kochen-Specker theorem have been tested too. The results of a recent experiment by Roos, Blatt et al have just been published in Nature. It confirms the inexistence of non-contextual hidden variables with great accuracy.

Another recent experiment by Gröblacher et al ruled out a whole class of non-local hidden variable interpretations : http://www.arxiv.org/PS_cache/arxiv/pdf/0704/0704.2529v1.pdf
I'm not sure which class exactly, but it seems to me that they are interpretations without entanglement.

So we're left with some philosophiocal problems. Experiments give us the choice between
-Leaving out causality : some events occur without any cause.
-Leaving out locality : the cause of an event can lie in its future.
-Leaving out realism : there is no reality. There is just information gathered by observers.

The Copenhagen interpretation makes both the first and third choices, as well as Rovelli's relational interpretation.
Cramer's transactional interpretation makes the second choice. As did Böhm-De Broglie's pilot wave interpretation (but they didn't know it before Bell's theorem).
Everett many-world's interpretations are usually not developped enough to get a precise position in this choice. JesseM, in this forum, used to describe an extension of Everett's interpretation that would get rid of these three problems ( but introducing the existence of many worlds of course) : https://www.physicsforums.com/showthread.php?t=206291#11

It seems you don't agree with each other.

I can only guess, then, that there is no consensus?

To the causalists who said that causality is preserved in quantum theory: how do you answer the above post?

 

[DrChinese]

My point was that even if we don't know or can't know the cause of a given thing, that in no way means there is no cause for given thing.

That is accurate. I believe that it is generally accepted that we do not know which of these is true. In other words, there could be a hidden (and non-local) cause - i.e. Bohmian/pilot wave interpretations. There could also be retro-causality. Or there could be no cause at all.

Clearly, we may discover these causes at a later date. However, it is telling that regardless of whether we are talking about photon spin or radioactive decay, there is not the slightest hint that there exists an underlying cause.

 

[Dale]

It seems you don't agree with each other.
...
To the causalists who said that causality is preserved in quantum theory: how do you answer the above post?

It appears to be a semantic argument. The disagreement is on the definition of "acausal". Pio2001 (and you?) are using a non-standard definition, so it is not surprising that there is disagreement. Semantic arguments usually degenerate like that.

 

[Borean]

It appears to be a semantic argument. The disagreement is on the definition of "acausal". Pio2001 (and you?) are using a non-standard definition, so it is not surprising that there is disagreement. Semantic arguments usually degenerate like that.

Inded it seems that "causal" and "acausal" tends to be used as an exaggeration of, very simply "unpredictable" or "indeterminable".

Physicists may be good at doing physics, but they are very bad at being sensitive about the metaphysics and philosophy implicit in their own statements.

 

[ueit]

Tthere could be a hidden (and non-local) cause - i.e. Bohmian/pilot wave interpretations. There could also be retro-causality. Or there could be no cause at all.

There could also be a hidden and local cause.

Clearly, we may discover these causes at a later date. However, it is telling that regardless of whether we are talking about photon spin or radioactive decay, there is not the slightest hint that there exists an underlying cause.

To have a "hint" you need a theory that proposes a hidden variable. Just by looking at a bunch of atoms popping out would not give you any hint, one way or another. If you accept Bohm's theory, you have your "hint".

On the other hand, EPR experiments prove IMO that there must be a cause (local or non-local) behind the individual experimental results. If there were no cause there could be no correlation, just statistical noise.

 

[Fra]

In a sense I'm quite pragmatic on this issue.

What difference is it to the context, between "there is a cause but I don't know about it", and there is no cause?

Unless these causal relations are known, business as usual is what we have, and our actions reflect that fact they we know of no cause. Which means we act just as if there is no particular cause.

In my preferred interpretation of QM, the action of a system is in accordance to it's state of information about it's environment. The systems action is indifference to any hidden causal rules. Most papers on the EPR jumps from no information, to a ergodic hypothesis with a choice of microstructure partition with equiprobably microstates the unknown. This is also (ergodic) information, that is added without physical motivation.

The pragmatic standpoind I have here is, what affects the action of a given system or observer? There is a kind of locality here in information space, that the action of a system is indifferent to information that it doesn't have. It respons to information at hand, and causal laws at hand. The fact that in the future, new laws of causation might be found doesn't impact present actions.

I think this is the key to make sense out of QM weirdness. It does away with a great deal of realism in the sense that a systems action really doesn't depend on "what is REALLY out there", it depends only upon what information the system has about what is possibly out there. It is momentarlity irrelevant wether this information is "right" or "wrong", however information that isn't consistent with the environment will imply a backreaction, which imples physical forces on the system causing it to "revise it's state", and simply "revise it's opinon in the light of new inforamtion".

To ask if we can predict when unexpected new information arrives is a silly question. By construction we can't. That's the undecidability all frogs are destined to live with.

/Fredrik

 

[ueit]

In a sense I'm quite pragmatic on this issue.

What difference is it to the context, between "there is a cause but I don't know about it", and there is no cause?

There is a big difference. You can have evidence for a cause without being able to point it. In fact, I think there is a confusion here between causality and determinism. Any quantum process has a cause and this cause has to do with the energy balance. That's why some nuclei are unstable while others not. So, the cause of radioactivity is the fact that the system after decay has less energy than the initial system.

I think this is the key to make sense out of QM weirdness. It does away with a great deal of realism in the sense that a systems action really doesn't depend on "what is REALLY out there", it depends only upon what information the system has about what is possibly out there. It is momentarlity irrelevant wether this information is "right" or "wrong", however information that isn't consistent with the environment will imply a backreaction, which imples physical forces on the system causing it to "revise it's state", and simply "revise it's opinon in the light of new inforamtion".

How can the information be "wrong"? What do you mean by that?

To ask if we can predict when unexpected new information arrives is a silly question. By construction we can't. That's the undecidability all frogs are destined to live with.

I have no idea what you are talking about.

 

[Fra]

There is a big difference. You can have evidence for a cause without being able to point it.

If we have physical evidence that can be quantified then I have no objection. A cause need not be determinisic. It can be probabilistic or fuzzy. just like there are deductive and inductive inferences.

But sometimes, people tend to impose "possible causations" from some realist minded reasoning, that is not really backed up by physical evidence. That's all I objected to.

How can the information be "wrong"? What do you mean by that?

This may be a nonstandard phrasing indeed. But I elaborated my view on this in this thread "The role of false info in the Copenhagen Int"
https://www.physicsforums.com/showthread.php?t=301893

I borrowed the notion "false info" from that thread.

I have no idea what you are talking about.

This was just a general comment on the common objection to the information interpretation that the wave collapse is nothing but an information update.

/Fredrik

 

[ueit]

This may be a nonstandard phrasing indeed. But I elaborated my view on this in this thread "The role of false info in the Copenhagen Int"
https://www.physicsforums.com/showthread.php?t=301893

I borrowed the notion "false info" from that thread.

I've looked into that thread. Do you imply that an atom can lie? What would an atom lie about?

 

[Fra]

I've looked into that thread. Do you imply that an atom can lie? What would an atom lie about?

I would never put it like that but what I do imply is something along the line that an atoms information about it's environment, can be destabilising itself. A stable atom is an atom whose state is in equilibrium, or in consistency with it's environment.

The "right" state from the point of view of the atom, would be the state in which it's stable.

One can imagine the analogy of an atom beeing wrong, as one beeing prepared in a different environment and then suddently thrown in a new environment. Then, it will still act as if it is in the former environement until the atom has interacted in the new environment in and received reactions. the result of this is either breakdown or destabilisation of the atom, or just an internal state revision.

So my point in that other thread is that in my "interpretation", which is nto strict copenhagen as I pointed out, the role of false information is just a state that is contradiction or not in equilibrium with the LOCAL environment. There are no global measures on correctness.

So if we would put it like a atom can lie, then yes it can lie in the above sense, but it is highly improbable to maintain the lie in a given environment. A lie isn't stable, since the lie is in my by an inconsisntecy that translates into a tension and a physical force.

You can have an ice cube in boiling water, it is not a contradiction per see, but it won't last very long. It's transient only.

/Fredrik

 

[Borean]

Or there could be no cause at all.

The problem I have with this proposition is that, if we define "causality" is the "principle that events need certain necessary and sufficient conditions to come into existence" and we define "cause" or the effect of that cause as the actuality of living up to any necessary and sufficient conditions, then, if you abandon this principle, you are saying that "events need not to live up to any conditions to occur; any conditions are sufficient for the events actuality and no conditions are more necessary for the actuality of any event than any other condition".

And if that is true, then indeed even probablistic causation/prediction would be impossible. Then even quantum mechanics would be false.

If that was the case, the universe would behave very differently, even on the microphysical/quantum level, because EVERY condition would then be affirmed as sufficient for the actuality of EVERY event, and that would mean that every event IS actual no matter what conditions there are and actually so at any point in time and space.

It seems meaningless to make such affirmations when we know that is NOT the case.

 

[DrChinese]

The problem I have with this proposition is that, if we define "causality" is the "principle that events need certain necessary and sufficient conditions to come into existence" and we define "cause" or the effect of that cause as the actuality of living up to any necessary and sufficient conditions, then, if you abandon this principle, you are saying that "events need not to live up to any conditions to occur; any conditions are sufficient for the events actuality and no conditions are more necessary for the actuality of any event than any other condition".

And if that is true, then indeed even probablistic causation/prediction would be impossible. Then even quantum mechanics would be false.

If that was the case, the universe would behave very differently, even on the microphysical/quantum level, because EVERY condition would then be affirmed as sufficient for the actuality of EVERY event, and that would mean that every event IS actual no matter what conditions there are and actually so at any point in time and space.

It seems meaningless to make such affirmations when we know that is NOT the case.

I am not asserting that there is no cause to probablistic behavior, but that may easily be the case.

But I disagree with your statements above. There are theoretical reasons to believe that all possibilities do occur (MWI interpretation is one example). Also, it is pretty clear that the probability waves do act as if they are real. Collapse is then somewhat like the ball randomly landing in the roulette slot - without any specific precipitating cause at all.

 

[Borean]

There are theoretical reasons to believe that all possibilities do occur (MWI interpretation is one example).

That is not a "reason". There are theoretical claims that all possibilities occur. But per Occams Razor, I would think there are much simpler and more minimalistic explanations for the same empirical data.

But anyway, the many-worlds interpretation doesn't live up to abandoning the causal principle I mentioned. It doesn't live up to the claim that "EVERY condition is sufficient for the actuality of EVERY event, and that every event IS actual no matter what conditions there are and therefore actual always and everywhere". Because that clearly doesn't apply in the one world we know - this one, and therefore, they have to invoke the separation of worlds, and those worlds clearly don't live up to this claim individually, either.

It is a completely different way in which "all possibilities occur", which doesn't live up to abandoning that there are certain constraints for necessary and sufficient conditions for events to be actual and take place.

 

[DrChinese]

That is not a "reason". There are theoretical claims that all possibilities occur. But per Occams Razor, I would think there are much simpler and more minimalistic explanations for the same empirical data.
...

Well, it would be a good hint (in favor of causality) if you could set up the same scenario and get the same result every time. That is a typical indication that cause and effect is at work. But that just doesn't happen much - that old uncertainty thing gets in the way. So I really don't think you have your finger on Occam here: you are assuming more, not less.

Besides, per Bell, you end up back at non-local interpretations if you want determinism/hidden variables/causality/etc. So is that minimalistic in your book?

 

[Borean]

Well, it would be a good hint (in favor of causality) if you could set up the same scenario and get the same result every time.

That assumes that you can ever set up two identical scenario, just because you are dealing with the same kind of particle or event. I don't assume this. To the contrary, you can never set up the same scenario, because scenario will always be separated either in time or space.

Besides, per Bell, you end up back at non-local interpretations if you want determinism/hidden variables/causality/etc. So is that minimalistic in your book?

I don't know. Are you saying that the copenhagen interpretation denies non-locality? As far as I know, phenomena such as quantum entanglement are rather widely held.

In any case, we don't need nonlocality for causality if we discount our basic ability to replicate the same scenario (and therefore make valid causal predictions). In that case, all we need to stop assuming is determinacy.

As far as I've discerned, there will always be different energy balances in different quantum particles and events, and in the quantum vacuum. It surprises me no one has considered the vacuum.

We already have good reasons to assume it isn't empty, with the discovery of things such as dark energy, dark matter, gravitational fields, and other entities we aren't aware about. So how could we say that particles erupt spontaneously from the vacuum "without any cause" and receiving their energy from "nowhere", or particles decay for "no reason", when clearly the vacuum is filled with energy and disturbing forces which could causally explain this without our determinacy?

 

[DrChinese]

1. That assumes that you can ever set up two identical scenario, just because you are dealing with the same kind of particle or event. I don't assume this. To the contrary, you can never set up the same scenario, because scenario will always be separated either in time or space.

2. I don't know. Are you saying that the copenhagen interpretation denies non-locality? As far as I know, phenomena such as quantum entanglement are rather widely held.

In any case, we don't need nonlocality for causality if we discount our basic ability to replicate the same scenario (and therefore make valid causal predictions). In that case, all we need to stop assuming is determinacy.

3. As far as I've discerned, there will always be different energy balances in different quantum particles and events, and in the quantum vacuum. It surprises me no one has considered the vacuum.

We already have good reasons to assume it isn't empty, with the discovery of things such as dark energy, dark matter, gravitational fields, and other entities we aren't aware about. So how could we say that particles erupt spontaneously from the vacuum "without any cause" and receiving their energy from "nowhere", or particles decay for "no reason", when clearly the vacuum is filled with energy and disturbing forces which could causally explain this without our determinacy?

1. You must admit this is circular reasoning. You assume causality because all events are unique...?

2. You can't have local causality, per Bell.

3. Isn't that just moving the goal posts? You are again coming back to the idea that causality may be correct, once we find the "missing" cause. I already believe that as a possibility.

 

[Borean]

1. You must admit this is circular reasoning. You assume causality because all events are unique...?

No, not at all.

I am only saying that I reject the premise that you can succesfully replicate "two identical scenario".

That premise would be a requirement before you could expect two scenario to be fully "causally" identical, or indeed to conclude acausality if the scenario were not, seemingly, "causally" identical.

By rejecting the premise, I neither have to expect that two scenario are causally identical (given causality), or conclude acausality when I find that they are not causally identical.

2. You can't have local causality, per Bell.

What I said already presumes that claim.

3. Isn't that just moving the goal posts? You are again coming back to the idea that causality may be correct, once we find the "missing" cause. I already believe that as a possibility.

No, that is just elaborating on the reasons _WHY_ there can never be two causally identical scenario (and therefore the premise of Bells experiment is faulty).

 

[DrChinese]

No, that is just elaborating on the reasons _WHY_ there can never be two causally identical scenario (and therefore the premise of Bells experiment is faulty).

Well, you are now entering the world of the philosophical (not a bad thing, and I happen to like this particular area). But science is about explaining patterns and pattern exceptions. Useful theories find commonality in events that are similar in specific ways, even if not identical.

Further, I quite agree that causality may not hold because no 2 events are identical. (Since by all logic, you could require the complete state of the universe to figure into any outcome.) But that does not change the fact that physics is full of USEFUL theories in which our knowledge is relatively minimal regarding the initial state.

More relevantly, it changes Bell's conclusion not at all. It simply means that no physical theory can be proposed which is local realistic. Because there can be no pattern matches for such a theory during experiments as strong as those for QM. (I.e. the cos^2 rule for entangled photons.)

 

[Borean]

Well, you are now entering the world of the philosophical (not a bad thing, and I happen to like this particular area). But science is about explaining patterns and pattern exceptions. Useful theories find commonality in events that are similar in specific ways, even if not identical.

It's not actually a "philosophical" invention of mine.

It's a premise for Bell's findings to hold true.

 

[DrChinese]

It's not actually a "philosophical" invention of mine.

It's a premise for Bell's findings to hold true.

I disagree here. The question is whether a local realistic theory can produce the statistical predictions of QM. Bell says it cannot. Causality is not an assumption per se.

 

[Borean]

I disagree here. The question is whether a local realistic theory can produce the statistical predictions of QM. Bell says it cannot. Causality is not an assumption per se.

Thats not what I meant.

I will quote what I already said (to your post actually):

DrChinese:
Well, it would be a good hint (in favor of causality) if you could set up the same scenario and get the same result every time.

Borean:
That assumes that you can ever set up two identical scenario, just because you are dealing with the same kind of particle or event. I don't assume this. To the contrary, you can never set up the same scenario, because scenario will always be separated either in time or space.

 

[DrChinese]

Thats not what I meant.

I will quote what I already said (to your post actually):

DrChinese:
Well, it would be a good hint (in favor of causality) if you could set up the same scenario and get the same result every time.

Borean:
That assumes that you can ever set up two identical scenario, just because you are dealing with the same kind of particle or event. I don't assume this. To the contrary, you can never set up the same scenario, because scenario will always be separated either in time or space.

Probably a case of nested quoting, because I don't have much problem with this. And reviewing your earlier comment further, I agree with your idea that vacuum fluctuations could be a source of indeterminacy.