The Role of Causality in Primary and Secondary Physics

[Johan0001]

With reference to the Delayed wheeler choice thought experiment.
I.E. deciding to detect the photon either from one side or the other side of a galaxy that has obscured the which way path of the photon emitted from a distant galaxy "behind" the "gravitational lensing galaxy".

Also with reference to the delayed quantum erasure experiment , where the signal photon has already been recorded before the idler photon has yet to complete its journey.

These experiments amongst others seem to imply Retrocausality (OR NOT?).
I.E. measurement recorded in the present affects a measurement recorded in the past.
Or in the case of the photon from the distant galaxy , the photon determines its path "once it has reached its destination".

If this (retrocausality) is NOT the case , what alternative explanations , could account for these findings.
I guess what I'm asking if we do NOT accept retrocausality , what are the alternatives?

If anybody has references , please post so that I can understand the alternatives.
Or what are the current theories regarding this phenomena?

Regards
Johan

 

[.Scott]

If you want to think of it as retrocausality, go ahead.
But it still does not provide a method for actually making a specific intentional change to past events.

There was a discussion thread just a couple of weeks ago:
https://www.physicsforums.com/threads/wheelers-delayed-choice-doesnt-change-the-past.935493/

The "conventional" way of looking at this is simply to work through the Math to discover what statistics should be expected. These QM rules do not describe the events as a function of time. They describe the totality of the experiment.

 

[Johan0001]

"If you want to think of it as retrocausality, go ahead."
Personally I do not , but what are the alternative interpretations.

I did follow the discussion you cited , they made reference to the "Block world" as being 'acasual' which I do not fully understand.
And a lot of discussion regarding interpretations.
But in general , there seems to be no explanation as to how the phenomenon occurs.

 

[Demystifier]

If you don't like retrocausality, then you can use almost any interpretation of QM to understand delayed choice. See
https://arxiv.org/abs/1703.08341 Sec. 5.1.

 

[vanhees71]

As all of physics also QT is a causal theory, and thus there is no retrocausation in any contemporary physical theory.

 

[.Scott]

"If you want to think of it as retrocausality, go ahead."
Personally I do not, but what are the alternative interpretations.

I did follow the discussion you cited, they made reference to the "Block world" as being 'acasual' which I do not fully understand.
And a lot of discussion regarding interpretations.
But in general, there seems to be no explanation as to how the phenomenon occurs.

From the "human" point of view, we can learn from a fixed past and prepare for a pliable future. And it's not just a human illusion. There really is a direction to time based on an ever-increasing entropy. But QM does not rely on that direction of time. Long after we have maxed out entropy and lost all possible track of time, QM will still prevail.

In the other thread, @Demystifier described classical physics as an x(t) exercise - describing the laws of physics as changes in position as a function of time.

The blockworld model abandons that x(t) bias. It simply states that the past, present, and future are always consistent with each other and the QM statistics.

 

[.Scott]

As all of physics also QT is a causal theory, and thus there is no retrocausation in any contemporary physical theory.

I think you need to take more care in the use of the term "retrocausality". It is certainly true that QM does not provide a means of choosing a new past and implementing that change. But when retro-causality is used to describe the results of delayed choice experiment, the statistics can be interpreted to demonstrate a past that is pliable to future events. That sense of retro-causality is a valid interpretation of QM.

 

[Johan0001]

But QM does not rely on that direction of time.

In my view , time is emergent with no universal reference , and it can only move forward.
If I play a movie in reverse time still moves forward.
So the above quote seems trivial to me or am I wrong?

What is not clear is that correlations rely on observations at different points in time.
Thus correlations are formed by observations. But at what point in time do they manifest.
If time does not matter then they were always there?
What other alternative can there be?Regards

 

[.Scott]

In my view, time is emergent with no universal reference, and it can only move forward.
If I play a movie in reverse time still moves forward.
So the above quote seems trivial to me or am I wrong?

What is not clear is that correlations rely on observations at different points in time.
Thus correlations are formed by observations. But at what point in time do they manifest.
If time does not matter then they were always there?
What other alternative can there be?

If you understand that there can be more than one way of describing the "why" behind the same final outcome, then you understand that there are different interpretations. You are taking time to be emergent. Could it be that the past, present, and future exist from the start? A sort of predestination? Would that contradict any of these experiments - or your emergent view of time? If not, then that is an alternative interpretation.

 

[currojimenez]

Take a look to Quantum non locality and relativity from Tim Maudlin. He study in deep those kind of issues

 

[vanhees71]

I think you need to take more care in the use of the term "retrocausality". It is certainly true that QM does not provide a means of choosing a new past and implementing that change. But when retro-causality is used to describe the results of delayed choice experiment, the statistics can be interpreted to demonstrate a past that is pliable to future events. That sense of retro-causality is a valid interpretation of QM.

I'd call it "postselection". You simply take an irreducible measurement protocol and choose a partial ensemble due to the coincidence information on the idler and signal photons. So indeed nothing is changed in the past but you postselect a partial ensemble from the full ensemble to "erase the which-way information and restor4e the double-slit fringe pattern" for the partial ensemble due to this postselection. If you choose the complementary subensemble you also erase which information and get the double-slit anti-fringe pattern. See the paper by Walborn et al.

 

[Johan0001]

If you understand that there can be more than one way of describing the "why" behind the same final outcome, then you understand that there are different interpretations. You are taking time to be emergent. Could it be that the past, present, and future exist from the start? A sort of predestination? Would that contradict any of these experiments - or your emergent view of time? If not, then that is an alternative interpretation.

By contradiction I assume you mean that the interpretation does not deviate from the mathematical probabilistic approach that accurately predicts the ''clicks" measured in these delayed choice experiments?

Then I understand your reasoning for different alternative interpretations.

​

Where could I get hold of these publications, to further my understanding, any help would be appreciated?

See the paper by Walborn et al

Take a look to Quantum non locality and relativity from Tim Maudlin. He study in deep those kind of issues

 

[vanhees71]

The Walborn et al paper is freely available from the arXiv

https://arxiv.org/abs/quant-ph/0106078

 

[Lord Jestocost]

I have no idea when the term “retrocausality” was coined. Maybe, the term expresses the conceptual difficulties one is running in when assuming that a strictly causal scheme is the way of treating phenomena in Nature.

“Causation and Time’s Arrow. Cause and effect are closely bound up with time’s arrow; the cause must precede the effect. The relativity of time has not obliterated this order. An event in Here-Now can only cause events in the cone of absolute future; it can be caused by events in the absolute past; it can neither cause nor be caused by events in the neutral wedge, since the necessary influence would in that case have to be transmitted with a speed faster than light. But curiously enough this elementary notion of cause and effect is quite inconsistent with a strictly causal scheme. How can I cause an event in the absolute future, if the future was predetermined before I was born? The notion evidently implies that something may be born into the world at the instant Here-Now, which has an influence extending throughout the future cone but no corresponding linage to the cone of absolute past. The primary laws of physics do not provide for any such one-way linkage; any alteration in a prescribed state of the world implies alterations in its past state symmetrical with the alterations in its future state. Thus in primary physics, which knows nothing of time’s arrow, there is no discrimination of cause and effect; but events are connected by a symmetrical causal relation which is the same viewed from either end.
Primary physics postulates a strictly causal scheme, but the causality is a symmetrical relation and not the one-way relation of cause and effect. Secondary physics can distinguish cause and effect but its foundation does not rest on a causal scheme and it is indifferent as to whether or not strict causality prevails.” (Eddington, The Nature of the Physical World)

 

[Johan0001]

Primary physics postulates a strictly causal scheme, but the causality is a symmetrical relation and not the one-way relation of cause and effect. Secondary physics can distinguish cause and effect but its foundation does not rest on a causal scheme and it is indifferent as to whether or not strict causality prevails.”

If we assume 'Primary' and 'Secondary' physics , there needs to be a transition/cutover point defined.
At what point would primary physics change from a " time symmetrical relation" to a "One way relation , of cause and effect"?

Perhaps where decoherence has smeared out to such an extent that one cannot practically reverse the entropy of the total prepared system , back to
its previous state? Thus creating the "irreversible arrow of time " and our evolving universe.